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Geological significance and geochronology of Paleoproterozoic mafic dykes of Xiaoqinling Gold District, southern margin of the North China craton
Abstract
Lode gold deposit is the most economically important type of gold deposits worldwide. It commonly occurs in Archean to Paleoproterozoioc metamorphic terrains and has close spatial relationship with mafic dykes. However, it is still in dispute whether mafic dykes and gold mineralization are genetically related. In this paper, we present LA-ICPMS zircon U-Pb and biotite 40Ar-39Ar ages of four mafic dikes samples collected from three major gold deposits (Dongchuang, Dahu and Qiangma deposits) in the Xiaoqinling gold district, southern margin of the North China craton. Four samples yielded a consistent zircon LA-ICPMS age (207Pb/206Pb 1819 ± 10 Ma) and a slightly younger 40Ar/39Ar plateau age (1719.0 ± 21.0 Ma) of biotite. Our results suggest that: (1) numerous mafic dikes in the Xiaoqinling gold district formed in Paleoproterozoioc, in an extensional setting after the collision between the eastern and western blocks of the North China craton at ca. 1.85 Ga; (2) previous whole rock K-Ar and Rb-Sr ages (187.6-75.9 Ma) which are significantly younger than our ages represent thermal perturbation (or cooling ages) caused by Mesozoic tectonic-thermal event. Recent geochronological studies show that the majority of gold deposits formed in Early Cretaceous (130-120 Ma) in the Xiaoqinling district. Therefore, the mineralization event had no relationship with Paleoproterozoic mafic dykes although they were spatially associated. However, this study does not rule out the possibility of existence of Mesozoic mafic dykes in the Xiaoqinling area, which may be very important for the understanding of the metallogeny and regional tectonic setting.
... Ga A-type granites (e.g., Guijiayu and Yuantou granites; Deng et al., 2016bDeng et al., , 2019 and ~ 1.84-1.80 Ga mafic-intermediate dikes (e.g., dolerite, lamprophyre, and diabase; Wang et al., 2008;Bi et al., 2011;Li et al., 2020). Zircon U-Pb analyses have revealed that the majority of leucosomes within migmatites of the Xiaoqinling area were formed during ~ 1.91-1.78 ...
... Ga, respectively (e.g., Zhao et al., 2015;Li et al., , 2022Diwu et al., 2020;Wang et al., 2022;this study). The transition to a post-collisional extensional setting was complete by ~ 1.82 Ga (e.g., Wang et al., 2008Wang et al., , 2020Bi et al., 2011;Li et al., 2012Li et al., , 2020Deng et al., 2016bDeng et al., , 2019; this study). Zircon REE patterns and internal and external morphologies are also shown to indicate the behaviors of leucosome zircons during partial melting. ...
... Intensive late Paleoproterozoic magmatism in the Taihua Complex ( Fig. 12b) is recognized mainly in the Xiaoqinling area and represented by the intrusion of granite, diorite, lamprophyre, and diabase during ~ 1.88-1.80 Ga (Wang et al., 2008(Wang et al., , 2017bBi et al., 2011;Li et al., 2012Li et al., ,2020Yu et al., 2013;Deng et al., 2016bDeng et al., , 2019Jia et al., 2020). A relative probability plot of zircon U-Pb ages of igneous rocks shows that the Taihua Complex records an important magmatic event at ~ 1.82 Ga (Fig. 12b), typified by simultaneous emplacement of numerous mafic dikes and A-type granites (Bi et al., 2011;Li et al., 2012Li et al., , 2020Deng et al., 2016b). ...
... A great amount of late Paleoproterozoic to Mesoproterozoic intrusive magmatism had also developed in the southern margin of the NCC, including mafic dike swarms (e.g., Peng et al., 2004Peng et al., , 2005Peng et al., , 2008Wang et al., 2008;Hu et al., 2010;Bi et al., 2011;Peng, 2015), syenites and monzonites (e.g., Ren et al., 2000;Liu, 2011) and granitoid rocks (e.g., Hu and Lin, 1988;Lu, 1989;Lu et al., 2003;Bao et al., 2009;Wang et al., 2013;Deng et al., 2015), as well as extensive late Mesozoic granitic and mafic rocks (e.g., Gao et al., 2014a,b). Importantly, some rocks show an affinity to A-type granitoid, with wide emplacement ages from the late Paleoproterozoic to early Mesoproterozoic, such as the $1.62 Ga Longwangzhuang granite (Fig. 1b, Hu and Lin, 1988;Lu, 1989;Lu et al., 2003;Bao et al., 2009;Wang et al., 2013), the $1.60 Ga Maping granite (Fig. 1b, Liu, 2011;Deng et al., 2015) and the $1.53 Ga Zhangjiaping granite (Fig. 1b, Yan and Xu, 1984; our unpublished data). ...
... Although the coeval mafic igneous rocks have been recognized recently in the Xiaoqinling area, southern margin of the NCC (e.g., Wang et al., 2008;Bi et al., 2011), their higher e Nd (t = 1.80 Ga) values (À4.45 to +6.20) than those of the Guijiayu A-type granite (e Nd (t = 1.80 Ga) = À5.82 to À5.65) indicate that the latter could not have been derived directly by fractional crystallization of the former. More importantly, all the granites from the Guijiayu pluton characterized by high Rb/Sr ratios (0.41-1.1) and metaluminous to weakly peraluminous natures (Fig. 9) are inconsistent with the chemical signatures of mantle-derived A-type granite (e.g., Creaser et al., 1991;Turner et al., 1992;King et al., 1997;Patiño Douce, 1997;Shellnutt and Zhou, 2007). ...
... The required high melting temperatures to produce the Guijiayu A-type granite could have been offered by the underplating of mafic magmas (King et al., 1997;Shellnutt et al., 2011;Zhao et al., 2013;Zhou et al., 2014) (see the next part 6.3). In fact, such coeval mafic magmatism has been revealed recently in the study area as mentioned before, such as the $1.80 Ga mafic dykes in the Xiaoqinling gold district (e.g., Wang et al., 2008;Bi et al., 2011). ...
The occurrence of extensional-related A-type granites with a high-K nature in some early Precambrian blocks hints a possible link with the orogeny, and may imply the ending of the orogenic processes and the beginning of craton stabilization. The newly recognized late Paleoproterozoic A-type granite, the Guijiayu pluton composed of monzogranites in the Xiaoqinling area of the southern margin of the North China Craton (NCC), is a promising locus to investigate the late Paleoproterozoic tectonic evolution of the NCC. LA-ICP-MS zircon U-Pb dating shows that the Guijiayu pluton was emplaced at ?1.80 Ga. Petrologically, the Guijiayu granites contain perthite, annite and calcic amphibole but are lack of alkali mafic minerals. Geochemically, all the rocks are enriched in silicon and total alkali, and depleted in MgO, CaO and P2O5, with high FeOt/MgO, TiO2/MgO and 10,000Ga/Al ratios and high HFSEs (i.e., Zr, Nb, Ce, Y) concentrations, as well as negative Eu, Sr, P and Ti anomalies. They are also characterized by high melting temperatures and metaluminous to weakly peraluminous natures. Taken together, the Guijiayu granites show an affinity to aluminous A-type granite. Their geochemical characteristics (e.g., obviously negative Eu and Sr anomalies), along with the low zircon ?Hf(t) values (?13.9 to ?7.0) and whole-rock ?Nd(t) values (?5.81 to ?5.65), indicate that the Guijiayu aluminous A-type granite was derived from partial melting of the crustal calc-alkaline granitoids dominated by the granodiorite of the Taihua Group in the study area. Combined with the regional data, especially for the identification of the clockwise P-T paths of Paleoproterozoic metamorphism in the southern margin of the NCC, even coeval granites and regional retrograde metamorphism in other areas in the NCC, it is most likely that the peak of continent-continent collision had finished in the study area at least before ?1.80 Ga, and that the generation of the Guijiayu aluminous A-type granite was related to the underplating of mafic magmas in a post-collisional regime.
... These rocks formed in the Paleoproterozoic (Diwu et al., 2014) and have been subjected to amphibolite-facies metamorphism in the Paleoproterozoic (Li et al., 2007). The Archean rocks were intruded by Paleoproterozoic pegmatite (Li et al., 2007), Proterozoic and Mesozoic granitic intrusions Ding et al., 2011;Hu et al., 2012;Zhao et al., 2012), and Paleoproterozoic and Early Cretaceous mafic dikes (Wang et al., 2008;Mao et al., 2010;Bi et al., 2011;Li et al., 2022). ...
... The quartz-vein-style Dahu Au-Mo deposit is hosted by biotite plagiogneiss, amphibole plagiogneiss, and amphibolite of the Archean Taihua Group (Fig. 2), which was intruded by mafic dikes and granite porphyries. A zircon LA-ICP-MS U-Pb age of 1816 ± 14 Ma was reported for a mafic dike (Bi et al., 2011). The granite porphyry dikes are locally cut by mineralized quartz veins, but their exact age is unknown. ...
The southern margin of the North China craton hosts a number of quartz vein-style deposits of the unusual Au-Mo ore spectrum, including the Dahu deposit in the Xiaoqinling region. Pristine euhedral monazite and rutile crystals from the Dahu Au-Mo mineralized veins yield LA-ICP-MS weighted mean ²⁰⁶Pb/²³⁸U ages of 212.1 ± 1.7 Ma (1s, MSWD = 1.13) and 209.7 ± 2.7 Ma (2s, MSWD = 1.7), respectively. These ages point to a Late Triassic gold mineralization event in the Xiaoqinling region, aside from the regionally well-recognized Early Cretaceous gold mineralization event. Tectonic setting, mineral association, elemental and isotopic signatures, and fluid characteristics of the Dahu deposit suggest that the Late Triassic Au-Mo mineralization represents a carbonatite-associated hydrothermal system, with ore fluids exsolved from a late-stage carbonatite system. Mineralogically, the Dahu deposit is enriched in hydrothermal minerals common to carbonatite systems, such as carbonates, sulfates, and REE minerals. Trace element and isotope compositions of vein minerals also show similarities to those of carbonatites. Monazite displays a pronounced LREE-enriched and smooth pattern (no Eu anomaly), with εNd(t) values ranging from -14.9 to -13.3. Rutile is characterized by high Nb concentrations (6 to 18 wt.%) and extremely high Nb/Ta ratios of 236 to 2044. Fluid inclusions in the Dahu Au-Mo mineralized veins and the regionally distributed coeval carbonatites are all dominated by low to moderate salinity H2O-CO2 fluids that contain sulfates as trapped solids/daughter minerals. Our study suggests that the southern margin of the North China craton has potential for the exploration of polymetallic hydrothermal mineralization related to hidden carbonatites.
... Subsequently, the aforementioned three Paleoproterozoic orogenic belts evolved into a postcollisional extensional setting, as partly illustrated by widespread mafic dike swarms (1.82-1.76 Ga; Wang et al., 2004;Peng et al., 2005;Bi et al., 2011a), voluminous volcanic rocks mainly in the southern portion of the Trans-North China orogen (1.80-1.62 Ga; Lu and Li, 1991;Li et al., 1995;Zhao et al., 2004), and a number of A-type and alkaline granite intrusions (1.88-1.74 ...
... Ga; Geng et al., 2004;Li and Zhao, 2007;Zhao and Zhou, 2009;Shi et al., 2017;Li et al., 2018), and mafic dike swarms (1.82-1.76 Ga; Wang et al., 2004;Peng et al., 2005;Bi et al., 2011a) in and surrounding the Trans-North China orogen (Fig. 1). These late Paleoproterozoic magmatic rocks formed in a postcollisional extensional setting following collision of the Western and Eastern blocks, a process that led to the formation of the Trans-North China orogen and final stabilization of the North China craton (Zhao et al., 2002b;Zhao and Zhou, 2009;Shi et al., 2017). ...
The North China craton was stabilized in the late Paleoproterozoic but experienced significant removal of ancient lithospheric keel in the late Mesozoic that resulted in the formation of numerous world-class gold deposits with combined reserves of more than 7000 t of gold. However, it remains uncertain whether the North China craton contains older gold deposits formed during generation and final stabilization of the craton. Here, we show that the Shanggong gold deposit (105 t Au at 5.31 g/t) on the southern margin of the North China craton formed in the late Paleoproterozoic during the collision between the Eastern and Western blocks that led to formation of the Trans−North China orogen and final stabilization of the craton. The Shanggong deposit is hosted in amphibolite-facies rocks of the Neoarchean to early Paleoproterozoic Taihua Group and overlying volcanic rocks of the late Paleoproterozoic Xiong’er Group. Gold mineralization is structurally controlled by NE-striking faults and occurs in four segments: the Liuxiugou, Hugou, Shanggong, and Qiliping segments. The ores consist mainly of quartz-ankerite-sulfide stockworks and sulfide disseminations in hydrothermally altered wall rocks. Gold is mostly contained in arsenian pyrite that is variably associated with minor sphalerite, galena, and chalcopyrite. Ore-related alteration assemblages comprise mainly quartz, ankerite, K-feldspar, sericite, and tourmaline. Both the stockworks and mineralized alteration assemblages contain hydrothermal accessory minerals, including monazite, apatite, and rutile. Paragenetic relations and textural data show that these accessory phases precipitated synchronously with gold-bearing sulfides. Laser ablation−inductively coupled plasma−mass spectrometry spot analyses of monazite and apatite from the Shanggong segment yielded reproducible U-Pb dates of 1747 ± 20 Ma (2σ, mean square of weighted deviates [MSWD] = 0.46) and 1788 ± 200 Ma (2σ, MSWD = 11.3), respectively. These dates are indistinguishable within errors from an apatite U-Pb date of 1743 ± 79 Ma (2σ, MSWD = 1.6) at the Liuxiugou segment and a rutile U-Pb date of 1804 ± 52 Ma (2σ, MSWD = 0.77) at the Hugou segment. These new dates suggest that the Shanggong deposit formed at ca. 1.80−1.74 Ga, coeval with or immediately after formation of the Trans−North China orogen and final stabilization of the North China craton. Sulfides from the Shanggong gold deposit have δ34S values ranging from −18.5‰ to −6.9‰, whereas the coexisting ankerite has δ13CPDB of −6.81‰ to −1.61‰ and δ18OSMOW of 15.70‰−17.62‰. The stable isotope data are distinctively different from values of the Early Cretaceous gold deposits in the southern North China craton, indicating contrasting hydrothermal systems responsible for these two categories of gold deposits. The results presented here, combined with independent geologic evidence, allow Shanggong to be the first confirmed Paleoproterozoic orogenic gold deposit in the North China craton. Recognition of Paleoproterozoic orogenic gold mineralization provides significant new insights into the gold metallogeny of the well-endowed North China craton and has implications for future gold exploration along the three Paleoproterozoic orogenic belts in this craton.
... These rocks probably formed in the Neoarchean and have been subjected to amphibolite-facies metamorphism during the Paleoproterozoic (Zhou et al. 1998;Ni et al. 2003;Li et al. 2007). The Archean rocks were intruded by Paleoproterozoic pegmatites (Li et al. 1996;Li et al. 2007), Proterozoic and Mesozoic granitic intrusions (Wang et al. 2010;Hu et al. 2012;Li et al. 2012a, b), and Paleoproterozoic and Early Cretaceous mafic dikes (Wang et al. 2008;Bi et al. 2011). Early Cretaceous plutons are widely exposed across the region. ...
... The > 1,200 gold-bearing quartz veins are hosted mainly by the Taihua rocks and are controlled by small-to medium-scale EW-striking faults. Gold mineralization is characterized by multiple stages of gold deposition, each with contrasting Au-Ag-Te-Bi mineral assemblages (e.g., Bi et al. 2011;Jian et al. 2014;Liu et al. 2019;Chang et al. 2020), and gold occurring as Au-(Ag)-tellurides or native gold. Gold-bearing mineral assemblages are observed as inclusions or fracture-fillings in pyrite and quartz, as well as interstitial to pyrite and quartz crystals. ...
Low-temperature polymetallic melts can scavenge gold from aqueous fluids and may be instrumental in the formation of some hydrothermal lode gold deposits. However, this process is yet to be fully appreciated, because of the difficulty in recognizing textural evidence for preexisting melts and a lack of awareness that metallic melts can persist in hydrothermal systems down to very low temperature. The Xiaoqinling gold district, central China, belongs to the world-class East Qinling gold province in the North China Craton. Samples of pyrite from the S60 quartz vein at Xiaoqinling record evidence of multistage gold mineralization, with abundant Au-rich polymetallic droplets trapped as primary melt inclusions in pyrite. We examine this pyrite using a combination of nanoscale secondary ion mass spectrometry, electron probe microanalysis, and scanning electron microscopy. We present textural evidence left by migration of melt inclusions, which can be used to distinguish previously trapped polymetallic melt inclusions from mineral inclusions. We propose that polymetallic melt inclusions trapped early in the crystallization history of the host mineral could migrate to the crystal surface and enter into contact with aqueous fluids. The same effect can also be achieved by fracturing of the host mineral. In this way, relatively low volumes of polymetallic melts can continue to scavenge gold from the multiple pulses of ore fluids that are commonly involved in the formation of large gold deposits.
... The Archean rocks were intruded by pegmatite sills and dikes in the Paleoproterozoic (1955± 30 Ma: H.M. Li et al., 2007b, mafic dikes in the Paleoproterozoic (1851 ± 18 to 1719 ± 21 Ma: T.H. Wang et al., 2008;Bi et al., 2011b;J.W. Li et al., 2012b) and Early Cretaceous (130 ± 4 to 126 ± 1 Ma: T.H. Wang et al., 2008;J.W. Li et al., 2012b), and granitic plutons in the Proterozoic and Mesozoic. Mesozoic granitic intrusions formed during the Late Triassic and the Early Cretaceous, and are widely exposed in the Xiaoqinling region. ...
... More than 50 mafic dikes are present, and they dominantly strike northwest, dip northeast, and vary in strike length and width from 10 to 1,100 m and 0.5 to 50 m, respectively (Z.M. Yang et al., 1995). Bi et al. (2011b) reported a zircon LA-ICP-MS U-Pb age of 1816 ± 14 Ma for a mafic dike. Several granite porphyry dikes are present. ...
The Dahu Au-Mo deposit is located in the Xiaoqinling region and belongs to the Qinling-Dabie orogen of central China. The quartz vein-style Dahu Au-Mo deposit (31 t Au, average grade: 4.7 g/t; 30,000 t Mo, average grade: 0.13 %) formed in four stages, i.e., an early quartz-K-feldspar stage (I), a pyrite-molybdenite stage (II), a sulfide-telluride-sulfosalt-gold stage (III), and a late carbonate-barite stage (IV). Abundant tellurides and sulfosalts occur in stage III: altaite, tellurobismuthite, buckhornite, hessite, petzite, calaverite, aikinite, kupcikite, lindstromite, salzburgite, wittichenite, and tetradymite. The system is characterized by carbono-aqueous fluids of low to moderate salinity and high oxygen fugacity. Total homogenization temperatures of the H2O-CO2 fluid inclusions in stage I and stage III quartz range from 230 degrees to 440 degrees C and 198 to 320 degrees C, respectively. The variation in CO2/H2O ratios and microthermometric data of the H2O-CO2 fluid inclusions results from a combination of fluid unmixing and sequential trapping of fluid inclusions, but also from postentrapment reequilibration. Abundant minerals are present as solid phases in H2O-CO2 fluid inclusions in stage I quartz: Cu1.6.5S, covellite, chalcopyrite, bornite, molybdenite, pyrite, colusite, anhydrite, and celestine. delta O-18(qartz) values of stage I quartz range from 10.2 to 12.0 parts per thousand, with calculated delta O-18(fluid) values ranging from 5.6 to 7.4 parts per thousand. The delta D values of fluid inclusion waters (thermal decrepitation) in stage I quartz range from -117 to -54 parts per thousand. The low and scattered delta D values reflect a mixture of ore-forming fluids trapped by the H2O-CO2 fluid inclusions at deep crustal levels and postmineralization fluids, possibly meteoric waters, trapped by secondary aqueous fluid inclusions at higher crustal levels during uplift of the Xiaoqinling core complex. Four molybdenite samples yield an Re-Os isochron age of 206.4 +/- 3.9 Ma (MSWD = 0.88), suggesting ore formation in the Late Triassic. Metal signature, mineral association, fluid characteristics, and isotope data of the Dahu deposit, together with the tectonomagmatic setting, imply that the ore-forming fluids were derived from an unexposed and oxidized Magmatic system, which likely stems from partial melting of enriched lithospheric mantle beneath the North China craton in a postcollisional extensional setting.
... These rocks probably formed in the Neoarchean and have been subjected to amphibolite-facies metamorphism in the Paleoproterozoic Ni et al. 2003;Li et al. 2007). The Archean rocks were intruded by Paleoproterozoic pegmatite/granite (Li et al. 1996(Li et al. , 2007, Proterozoic and Mesozoic granitic intrusions Hu et al. 2012;Li et al. 2012b), and Paleoproterozoic and Early Cretaceous mafic dikes (Wang et al. 2008;Bi et al. 2011). Jurassic magmatism, although widespread in eastern China, has rarely been reported in the Xiaoqinling gold district. ...
We present in situ LA-ICP-MS U-Pb dating of xenotime and monazite in assemblages with native gold and Au (Ag) tellurides from the Xiaoqinling lode gold district in central China. Composite xenotime and monazite grains formed through coupled dissolution-reprecipitation reactions reveal two discrete gold mineralization events. The first gold mineralization event, recorded by monazite (158.6 ± 3.3 Ma, Tera-Wasserburg lower intercept age) and xenotime cores (157.11 ± 0.83 Ma, weighted mean 206Pb/238U age), is characterized by the mineral assemblage of lingbaoite (AgTe3)-sylvanite ([Au,Ag]2Te4)-stützite (Ag5–xTe3)/native tellurium-sylvanite-stützite. The second gold mineralization event, recorded in the rims of xenotime (135.46 ± 0.93 Ma, weighted mean 206Pb/238U age), is characterized by the mineral assemblage of native gold-calaverite (AuTe2)-petzite (AuAg3Te2)-tellurobismuthite (Bi2Te3). Our study implies that the large-scale Jurassic mineralization event in eastern China, related to flat subduction of the paleo-Pacific plate beneath the eastern China continent, also caused widespread gold mineralization in the Qinling-Dabie Orogen, in addition to production of its world-class porphyry Mo deposits. The fact that only a few Jurassic gold mineralization ages have been reported before, may be due to the lack of suitable geochronometers to record the earlier Jurassic hydrothermal processes, which have been overprinted by the better-recognized Early Cretaceous gold mineralization event. This study also presents a rare example of xenotime compositional alterations and resetting of U-Pb ages induced by low to moderate salinity carbono-aqueous fluids at low temperatures. The textural relationships between gold minerals in contact with such composite xenotime crystals demonstrate that they could have precipitated before, coeval with, or after the dated domains. Since low to moderate salinity carbono-aqueous fluids are commonly involved in the formation of lode gold deposits, it is crucial to examine xenotime textures and recognize potential alteration textures before carrying out isotopic dating of xenotime collected from these deposits. Without prior compositional and textural characterization, attempts to date such composite crystals could yield mixed dates and meaningless ages.
... 1.85-1.80 Ga and 139-126 Ma (Bi et al., 2011a;Li et al., 2012b;Ren, 2012;Wang et al., 2008a;Zhao et al., 2010). Trace element and Sr-Nd-Pb isotope analyses suggest that the Early Cretaceous mafic dikes were sourced from subcontinental lithospheric mantle previously metasomatized by subducted oceanic plates (Wang et al., 2008b(Wang et al., , 2008c. ...
The North China Craton is well endowed with numerous Early Cretaceous gold deposits that are mainly hosted in Archean to early Paleoproterozoic metamorphic rocks and late Mesozoic granitoid intrusions intruding the former. Whether or not gold was sourced from those metamorphic rocks remains hotly debated. Here we present trace element and S-Pb isotope data of pyrite from the Neoarchean to early Paleoproterozoic metamorphic rocks in the Xiaoqinling district of southern North China Craton and late Paleoproterozoic pegmatites derived from the former rocks, in an attempt to provide new insights into the gold sources for Early Cretaceous gold lodes genesis in this district. Pyrite from the metamorphic rocks typically occurs as inclusions in rock-forming minerals or disseminated grains roughly distributed along the foliation of the rocks, whereas pyrite from the pegmatites usually occurs as disseminations closely associated with quartz and magnetite. The textural data suggest that pyrite in the metamorphic rocks and pegmatites are of metamorphogenic and magmatogenic origins, respectively. LA-ICP-MS spot analyses show that pyrite both from the metamorphic rocks and pegmatites has Au contents mostly below the detection limit, with minor analyses revealing Au marginally higher than the detection limit. Considering that Au is moderately incompatible and highly siderophile, the very low gold contents in pyrite allow us to infer that the metamorphic rocks contain negligible gold. Pyrite grains from the metamorphic rocks and pegmatites have broadly consistent δ³⁴S values ranging from −2.7 to 31.5 (mean = 17.6) per mil and 6.3 to 17.9 (mean = 14.3) per mil, respectively. Similarly, they have comparable, highly radiogenic lead isotopes, which are 16.859–24.889 for ²⁰⁶Pb/²⁰⁴Pb, 15.355–15.694 for ²⁰⁷Pb/²⁰⁴Pb, and 37.480–43.848 for ²⁰⁸Pb/²⁰⁴Pb. The sulfur and lead isotopes of pyrites from the metamorphic rocks and pegmatites are distinct from those of Early Cretaceous gold deposits hosted by these rocks. Taken together, the pyrite trace element and S-Pb isotope data presented here preclude the Precambrian metamorphic basement rocks as an important gold source for the extensive Early Cretaceous gold mineralization in the Xiaoqinling district and, by inference, other major Early Cretaceous gold provinces across the North China Craton.
... The Mesoproterozoic Guandaokou Group consists of low-grade metamorphosed terrigenous clastic rocks and marine (Cao et al., 2017). Intrusive rocks are widespread in the southern NCC margin, including the Archean gneissic tonalite-trondhjemitegranodiorite (TTG) suite (e.g., Taiyuling and Wengchapu; Zheng et al., 2020a, b), Paleoproterozoic-Mesoproterozoic granites (Yuantou and Huayangchuan A-type granites, Xiaohe gneissic granite; Deng et al., 2019;Xue et al., 2018;Li, 2011) and mafic dyke swarms (Bi et al., 2011;Wang et al., 2008), Mesozoic carbonatite dykes (Huangyanchuan and Huanglongpu; Zheng et al., 2020a, b), and granitoid batholiths (Huashan and Laoniushan; Yang et al., 2019;Hu et al., 2012). Major regional structures in the southern NCC margin include a series of nearly EW-trending faults, superimposed by NE-trending secondary faults and fractures (Zheng et al., 2020a, b), which control the distribution of some deposits. ...
The Huayangchuan ore belt is located in the western segment of Xiaoqinling Orogen in the southern margin of the North China Craton (NCC), and hosts voluminous magmatism and significant U−REE−Mo−Cu−Fe polymetallic mineralization. However, geochronological framework of the various mineralization phases in this region is poorly understood. Here, we present new Re−Os isochron ages on magnetite from the Caotan Fe deposit (2 675 ± 410 Ma, MSWD = 0.55), and on pyrite from the Jialu REE deposit (2 127 ± 280 Ma, MSWD = 1.9) and Yuejiawa Cu deposit (418 ± 23 Ma, MSWD = 11.5), and Re−Os weighted average model age on pyrite from the Taoyuan Mo−U deposit (235 ± 14 Ma, MSWD = 0.17). These ages, combined with regional geology and mineralization ages from other deposits, suggest that mineralization in the Huayangchuan ore belt lasted from the Neoarchean to the Late Mesozoic. The mineralization corresponds to regional tectono-magmatic events, including the Neoar-chean alkali magmatism (REE mineralization), Paleoproterozoic plagioclase-amphibolite emplacement (Fe mineralization), Paleoproterozoic pegmatite magmatism (U mineralization), Paleozoic Shangdan oceanic slab subduction-related arc magmatism (Cu mineralization), Early Mesozoic Paleo-Tethys Ocean subduction-related arc magmatism (Mo−U mineralization), and Late Mesozoic Paleo-Pacific oceanic plate subduction direction change-related Mo(-Pb) mineralization. We proposed that the Huayang-chuan ore belt has undergone prolonged metallogenic evolution, and the magmatism and associated mineralization were controlled by regional geodynamic events.
... Field observation shows that the mafic dikes in the mine are partly associated with gold veins, which are generally hosted in the same structures and may locally crosscut each other (Li et al., 2012b). Recent in-situ zircon U-Pb dating results show that the district scale mafic dikes formed in two episodes at 1851 ± 18 to 1813 ± 17 Ma and 140 ± 4 to 126 ± 1 Ma, respectively (Wang et al., 2008a;Bi et al., 2011a;Li et al., 2012b;Ren, 2012). ...
The Xiaoqinling district, situated at the southern North China Craton, is the second largest gold province in China. The genesis of gold deposits in this district, however, has been debated for decades because of lacking unequivocal constraints on mineralization age and gold source(s). In this study, we present ore-related hydrothermal rutile U-Pb dating, pyrite trace element compositions, and S-Pb isotopes of the Qiangma Au deposit that is one of the largest gold deposits in the Xiaoqinling district, to provide new constraints on genesis of the regional gold mineralization. The Qiangma Au deposit is formed by four paragenetic stages consisting of milky quartz—coarse-grained pyrite, grey quartz—fine-grained pyrite, grey quartz—polymetallic sulfides, and quartz-calcite. The gold mineralization is accompanied by intensively hydrothermal alteration assemblage comprising quartz, sericite, sulfides, and calcite. Hydrothermal rutile texturally intergrown with gold-bearing pyrite and sericite yields a U-Pb age of 128.1 ± 7.4 Ma (2σ, MSWD = 0.9), representing the timing of gold mineralization at Qiangma. This age is consistent with the ages of regional felsic to mafic magmas and metamorphic core complexes as results of late Mesozoic tectonic reactivation of the North China Craton. Petrographic investigation and LA-ICP-MS trace element analysis show that gold mainly occurs as native gold with minor amounts of lattice gold within pyrite, which has no correlation with As and Te. Pyrite from the four stages have sulfur isotopes mainly between 1.8 to 5.8 ‰ (mean of 3.7 ‰), suggesting a magmatic affiliation. Pyrite from quartz-polymetallic sulfides stage has lead isotopes of 17.51-17.71 for ²⁰⁶Pb/²⁰⁴Pb, 15.39-15.49 for ²⁰⁷Pb/²⁰⁴Pb, and 38.01-38.53 for ²⁰⁸Pb/²⁰⁴Pb, which are distinctly different from those of Mesoproterozoic clastic and carbonate rocks and late Mesozoic granites, but are broadly consistent with late Mesozoic subcontinental lithospheric mantle derived mafic dikes. Collectively, our new lines of evidence suggest that the Qiangma Au deposit, as well as some gold deposits in the Xiaoqinling district, is most likely related to mantle-derived magmas generated in an extensional setting related to thinning and destruction of subcontinental lithospheric mantle keel beneath the North China Craton.
... These rocks probably formed in the Neoarchean and have been subjected to amphibolite-facies metamorphism in the Paleoproterozoic (Zhou et al., 1998;Ni et al., 2003;Li et al., 2007). The Archean rocks were intruded by Paleoproterozoic pegmatite (Li, S.M., et al., 1996;Li, H.M., et al., 2007), Proterozoic and Mesozoic granitic intrusions (Wang et al., 2010;Ding et al., 2011;Hu et al., 2012;, and Paleoproterozoic and Early Cretaceous mafic dikes (Wang et al., 2008;Zhao et al., 2010;Bi et al., 2011a). ...
We present petrographic and microthermometric evidence for precipitation of Au-Ag-Te-rich melt directly from hydrothermal fluids and subsequent entrapment as primary melt inclusions within pyrite from quartz veins of the Xiaoqinling lode gold district, southern margin of the North China craton. We propose the formation of Au-Ag-Te-rich melt through adsorption-reduction mechanisms on pyrite and subsequent growth of the melt nuclei via direct scavenging of metals from fluids. Because neither initial formation nor later growth of the melt require saturation of the ore fluid with respect to the constituent metals, this mechanism offers a new understanding of the enrichment of low-abundance ore components, such as gold. Our model may thus partly explain the discrepancy between the high gold solubilities reported from experimental studies and the much lower gold concentrations usually measured in natural fluids. This study also implies that Au-Ag-Te-rich melt has probably gone unrecognized in other lode gold deposits in which Au-Ag tellurides are present.
... Diabase, gabbro, and lamprophyre dikes are widespread in the Xiaoqinling district. Zircon U-Pb dating results reveal two episodes of mafic dike intrusion at 1813 ± 17 to 1851 ± 18 Ma and 126 ± 1 to 140 ± 4 Ma, respectively (Wang et al., 2008a;Bi et al., 2011;Li et al., 2012b;Ren, 2012). In most gold mines, the Early Cretaceous dikes generally show crosscutting relations to mineralized quartz-sulfide veins (Li et al., 2012b). ...
The Hongtuling Au-Mo deposit on the southern margin of the North China Craton consists of Au veins and minor Mo veins that formed at ∼130 Ma and ∼204 Ma, respectively. Mo mineralization is dominated by molybdenite that is associated with K-feldspar, quartz and calcite, whereas Au mineralization occurs mostly as auriferous quartz-sulfides veins associated with alteration assemblages consisting of pyrite ± sericite ± quartz. Laser ablation inductively coupled plasma mass spectrometry trace element analysis shows that pyrite from Mo veins (Py-Mo) is relatively enriched in Mo and Pb, whereas pyrite from Au veins (Py-Au) contains much higher Au, Ag, Te, Cu, Pb, and Ni. Sulfide minerals and celestine from Mo veins range in δ³⁴S values from -13.4 to -8.89 ‰ (n = 24) and 5.26 to 7.76 ‰ (n = 8), respectively. Extrapolation of sulfur isotopes of coexisting sulfides and celestine to zero sulfide-celestine fractionation on Δ³⁴SClt-Py versus δ³⁴S diagram shows that the hydrothermal fluid from which molybdenite precipitated has δ³⁴S of 2.49 ± 3.93 ‰. In comparison, Py-Au yields δ³⁴S values of -6.23 ∼ 0.16 ‰, with a mean of -2.60 ‰ (n = 20). The Py-Mo and Py-Au have significantly different lead isotopic compositions, which form two distinct groups on the ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb diagram, with Py-Au plotting close to the lead growth curve of the subcontinental lithospheric mantle of the North China Craton at ∼ 130 Ma.
Combined with previous studies, our new trace element and S-Pb isotope data suggest that Mo mineralization at Hongtuling is most likely related to magmas derived from partial melting of Late Archean to Paleoproterozoic metamorphic basement rocks under a post-collisional setting related to the collision between the North China Craton and Yangtze Craton, whereas Au mineralization is genetically associated with mantle-derived magmas generated in an extensional setting due to thinning and destruction of subcontinental lithospheric mantle keel beneath the North China Craton. As such, the Triassic Mo and Early Cretaceous Au veins can both be classified as magmatic-hydrothermal mineralization but are affiliated with two distinct systems. This study highlights the utilization of pyrite trace element and isotopic compositions in distinguishing multiple mineralization events in a single deposit.
... 2.1-1.85 Ga; Zhao et al., 2009;Li et al., 2011b) and mafic dikes of different periods Mao et al., 2010;Bi et al., 2011a;Li et al., 2012b;Ren, 2012). Most of the mafic dikes were emplaced during the Paleoproterozoic and the Paleozoic and have been strongly deformed and altered, while minor mafic dikes were formed in the Early Cretaceous Ren, 2012). ...
The Xiaoqinling gold field, located along the southern margin of the North China craton, is the second largest gold producer in China, which comprises more than 1,200 auriferous quartz veins with a proven gold reserve of at least 800 tons. Previously, the absolute age of the gold metallogenesis in this area has not been well defined due to the lack of suitable dating minerals. This study presents new in situ laser ablation-inductively coupled plasma-mass spectrometry U-Pb ages of coexisting hydrothermal monazite and rutile for the Fancha gold deposit in this area, which yielded 206Pb/238U ages of 127.5 ± 0.7 Ma (n = 65, mean square of weighted deviates [MSWD] = 1.8) and 129.7 ± 4.3 Ma (n = 37, MSWD = 1.4), respectively. Both ages overlap within analytical uncertainty at the 2σ level of significance, suggesting that both gold-bearing veins were emplaced at ca. 128 Ma. Mineralogical observations indicate that the monazite and rutile precipitated simultaneously with gold from the hydrothermal fluid. Our new data, combined with recently published monazite age, define a more precise gold episode, demonstrating that the gold endowment of the Xiaoqinling area was formed during a relatively brief period at ca. 130 to 127 Ma. We suggest that auriferous fluids were generated as a result of interactions between the enriched mantle and the lower crust, which was driven by westward flat slab subduction of the Paleo-Pacific plate during the late Mesozoic. The peak of lithospheric thinning during the postsubduction may have led to the rapid release of gold from the fertilized mantle. Consequently, the large number of gold-bearing veins in the Xiaoqinling area may ultimately be related to the tectonic evolution and mantle fluid processes that occurred during Early Cretaceous lithospheric extension.
... Additionally, mafic dikes are widespread in the Xiaoqinling region, and they include mainly diabase, diabase gabbro, sillite and lamprophyre. Available zircon LA-ICP-MS U-Pb and biotite 40 Ar/ 39 Ar geochronological data indicate that some of these dikes have Paleoproterozoic ages of 1846 ± 16 to 1753 ± 30 Ma (Bi et al., 2011b), whereas others have Early Cretaceous ages of~126 Ma (obtained by zircon SHRIMP U-Pb dating) (Wang et al., 2008). ...
The Jinqu Au deposit, located in central Xiaoqinling gold field along the North China Craton’s southern margin, has total reserve of 21 t of Au with 4.23 g/t Au on average. It consists cccmainly of ore-bearing auriferous quartz veins in amphibolite facies metamorphic rocks of the Archean Taihua Group. The deposit’s mineral paragenesis and ore-forming processes have never been studied. This paper reports its mineral paragenesis, which includes sulfides (pyrite, galena, chalcopyrite, pyrrhotite, sphalerite, molybdenite and bornite), oxides (rutile, scheelite, hematite, magnetite), tellurides (sylvanite [(Au, Ag)2Te4], hessite [Ag2Te], petzite [Ag3AuTe2], krennerite [Au3AgTe8], calaverite [AuTe2], stützite [Ag5−xTe3], empressite [AgTe], rucklidgeite [(Pb, Bi)3Te4], altaite [PbTe], volynskite [AgBiTe2], tetradymite [Bi2Te2S], lingbaoite (AgTe3) and melonite [NiTe2]) and native elements (tellurium and gold). Based on our data, the associated ore-forming processes can be deduced as: iron oxides → (CO2(g) and H2S(g) loss, ƒTe2/ƒS2 ratio increase) sulfides, Au-Ag-tellurides → (condensation of H2Te vapor) Pb-Bi-tellurides → (mixing with meteoric water) carbonate. It has been indicated by thermodynamic modelling that formation of Au-Ag-Te-bearing minerals occurred under physico-chemical conditions of logƒTe2 values from − 7.28 to − 3.21, logƒO2 values from − 26.67 to − 26.44, logƒH2S values from − 1.34 to 0.34, logƒS2 values from − 9.53 to − 5.64, logαAu⁺(aq)/αAg⁺(aq) values from − 4.72 to − 4.55 and pH values from 3.33 to 4.20 at 365℃ and logƒTe2 values from − 10.53 to − 6.62, logƒO2 values from − 31.99 to − 30.98, logƒH2S values from − 0.65 to − 0.36, logƒS2 values from − 13.62 to − 7.20, logαAu⁺(aq)/αAg⁺(aq) values from − 6.42 to − 6.21 at 335℃. The changes in physico-chemical conditions and in precipitation of Au-Ag-Te-bearing minerals were due to boiling or to condensation of Te-rich magmatic gas into ore-forming fluids. These reflect evolution from an early oxidizing to a later relatively reducing mineralization environment. Therefore, compared with the Dahu Au-Mo deposit and with the Yangzhaiyu Au deposit, which exist in the southern and northern parts of the Xiaoqinling region, respectively, the ore paragenesis and ore-forming physico-chemical conditions of the Jinqu Au deposit were different such that its characteristics are similar to low-sulfidation Au deposits.
... It is also an important mineral province, hosting a variety of ore deposits [1,2]. The Mesozoic granites and hydrothermal mineralization systems in the Xiong'er region (southern NCC margin) have been widely studied [3][4][5][6][7][8][9][10][11]. The regional Au and Mo mineralization was coeval with the first Mesozoic magmatic phase there (late Jurassic-early Cretaceous, ca. ...
The Longmendian Ag–Pb–Zn deposit is located in the southern margin of the North China Craton, and the mineralization occurs mainly in quartz veins, altered gneissic wallrocks, and minor fault breccias in the Taihua Group. Based on vein crosscutting relations, mineral assemblages, and paragenesis, the mineralization can be divided into three stages: (1) quartz–pyrite, (2) quartz–polymetallic sulfides, and (3) quartz–carbonate–polymetallic sulfides. Wallrock alteration can be divided into three zones, i.e., chlorite–sericite, quartz–carbonate–sericite, and silicate. Fluid inclusions in all Stage 1 to 3 quartz are dominated by vapor-liquid two-phase aqueous type (W-type). Petrographic and microthermometric analyses of the fluid inclusions indicate that the homogenization temperatures of Stages 1, 2, and 3 are 198–332°C, 132–260°C, and 97–166°C, with salinities of 4.0–13.3, 1.1–13.1, and 1.9–7.6 wt% NaCleqv, respectively. The vapor comprises primarily H2O, with some CO2, H2, CO, N2, and CH4. The liquid phase contains Ca2+, Na+, K+, SO42−, Cl−, and F−. The sulfides have δ34S=–1.42 to +2.35‰ and 208Pb/204Pb=37.771 to 38.795, 207Pb/204Pb=15.388 to 15.686, and 206Pb/204Pb=17.660 to 18.101. The H–C–O–S–Pb isotope compositions indicate that the ore-forming materials may have been derived from the Taihua Group and the granitic magma. The fluid boiling and cooling and mixing with meteoric water may have been critical for the Ag–Pb–Zn ore precipitation. Geological and geochemical characteristics of the Longmendian deposit indicate that the deposit is best classified as medium- to low-temperature intermediate-sulfidation (LS/IS) epithermal-type, related to Cretaceous crustal-extension-related granitic magmatism.
... Additionally, mafic dikes are widespread in the Xiaoqinling region, and they include mainly diabase, diabase gabbro, sillite and lamprophyre. Available zircon LA-ICP-MS U-Pb and biotite 40 Ar/ 39 Ar geochronological data indicate that some of these dikes have Paleoproterozoic ages of 1846 ± 16 to 1753 ± 30 Ma (Bi et al., 2011b), whereas others have Early Cretaceous ages of~126 Ma (obtained by zircon SHRIMP U-Pb dating) (Wang et al., 2008). ...
The quartz vein-style Dahu Au-Mo deposit, with an average grade of 6.8 g/t for 38 tons Au, and an average grade of 0.24% for 100,000 tons Mo, is situated in the Xiaoqinling region in the northernmost part of Qinling orogen in China. It is characterized by enrichment of native gold, molybdenite, telluride and Bi-sulfosalt. The current understanding of telluride and Bi-sulfosalt mineralization in the Dahu Au-Mo deposit is very poor, and this paper is the first systematic study of these minerals in this deposit. The Au-Mo ores occur mainly as veins and stockworks as well as disseminated in quartz veins, and they are hosted by biotite-plagioclase gneiss, amphibole-plagioclase gneiss, and amphibolite of the Archaeozoic Taihua Group. Four paragenetic stages of mineralization are recognized that reflect early formation of sulfide minerals and subsequent precipitation of telluride minerals: (I) quartz-K-feldspar-pyrite-molybdenite, (II)quartz-pyrite-molybdenite, (III) sulfide-telluride-sulfosalt-gold, and (IV) carbonate-barite. The telluride minerals are mainly altaite, tellurobismuthite, and tetradymite, and to a minor extent calaverite, petzite, hessite, and stützite. Some Bi-sulfosalt minerals also exist in the deposit, including aikinite, lindströmite, saddlebackite, krupkaite, and hammarite. Of these minerals, saddlebackite, krupkaite and hammarite are reported in this deposit for the first time herein. The Au-Ag-Te-Bi-S minerals substantially formed in the Dahu Au-Mo deposit under conditions of T = 260 °C, P ≈ 100 MPa, pH = 3.9 to 7.9, logƒO2 = -40.9 to -33.8, logƒTe2 = -12.5 to -7.5, logƒS2 = - 13.5 to -8.2, logƒH2S = -2.4 to -0.3 and logαAu+(aq)/αAg+(aq) = -6.7 to -6.2. The deposit formed from an ore-forming fluid dominated by HTe- or/and Te22-, in which Au was transported in the forms of Au(HS)2- and/or an unknown aqueous Te species (e.g., AuTe2-). Furthermore, during the mineralization stages of the Dahu deposit, the ore-forming fluid evolved from an initial oxidation type to a later relative reduction type, and the telluride minerals, Bi-sulfosalt minerals and native gold were formed in a relatively reduced condition. The Au-Ag-Te-Mo-Bi metal associations in the Dahu Au-Mo deposit were derived from a magmatic-hydrothermal system during the Late Triassic.
... The Taihua Supergroup is composed of the lower unit of gneisses, amphibolites, and tonalitictrondhjemitic-granodioritic gneisses (TTGs) and the upper unit of gneiss, amphibolite, marble, and quartzite (Zang, Zhang, Yuan, & Xiao, FIGURE 1 (a) Simplified geological map of eastern China and major tectonic units of the North China Craton (modified after (Zang, Zhang, Yuan, & Xiao, 2001); (b) geologic map of the Xiaoqinling gold district (modified from N. Li, Chen, Fletcher, & Zeng, 2011) [Colour figure can be viewed at wileyonlinelibrary.com] 2001; G. W. Zhang, Yu, Dong, & Yao, 2000). The major intrusions are represented by the Paleoproterozoic (1819 ± 10 Ma)-Mesozoic (126 ± 4.8 Ma) dikes (Bi, Li, & Li, 2011;X. Y. Gao, Zhao, Bao, & Yang, 2014;Hou, Santosh, Qian, Lister, & Li, 2008;B. ...
The Chen'er gold deposit hosted in the Precambrian Taihua Supergroup metamorphic rocks is one of the typical gold deposits in the Xiaoqinling region along the southern margin of the North China Craton. The metallic mineral assemblage in this deposit includes sulfides, tellurides, native gold, electrum, and native bismuth. The telluride minerals are composed of calaverite, petzite, hessite, altaite, joseite; except for hessite and matildite, the other tellurides and native bismuth are first reported in this study. Fluid inclusion studies in the auriferous quartz reveal three types: aqueous, aqueous–carbonic, and carbonic inclusions. The homogenization temperatures range from 184 to 364°C, and salinities are in the range of 1.74‐ to 11.47‐wt.% NaCl equiv. At temperatures of 275°C, the logfTe2 and logfS2 of the fluid are estimated as −16.13 to −8.19 and −12.74 to −4.80, respectively, and at temperatures of 305°C, the pH and logfO2 of the fluid are estimated as 3.61 to 5.69 and −34.31 to −27.81, respectively. It is inferred that Au(HS)2− and Au(HS)0 are the predominant gold‐bearing complexes, and tellurium was transported as Te22−. Based on thermodynamic models of logfO2(g) versus pH, we suggest that the precipitations of native gold and electrum in the early and main mineralization stages are different, with the oxygen and hydrogen isotope values of auriferous quartz suggesting mixing of magmatic water with meteoric water.
... Hu et al., 2012). Mafic dikes and pegmatites are widespread in the district and formed in two episodes at ∼1.9 to 1.8 Ga and 140-125 Ma (Li et al., 2007a;Wang et al., 2008;Zhao et al., 2010;Bi et al., 2011;Li et al., 2012b;Deng et al., 2013;Zhao et al., 2015). ...
The Xiaoqinling district, the second largest gold producer in China, is situated on the southern margin of the North China Craton and immediately to the north of the Qinling Orogen. The timing and genesis of the gold deposits have been hotly debated because the area has undergone multiple tectono-magmatic events since the stabilization of the North China Craton in the early Paleoproterozoic. Whether they are genetically related to the Triassic orogeny that formed the Qinling Orogen or associated with the early Cretaceous tectonic reactivation of the craton remains controversial. Here we present monazite U-Pb and molybdenite Re-Os dating results from the Hongtuling vein deposit in the eastern part of the Xiaoqinling district to provide new insights into the age and tectonic setting of gold mineralization. The Hongtuling deposit consists of gold veins at the top and minor molybdenum veins at the bottom, with molybdenum veins being locally crosscut by gold veins. Molybdenum veins mainly consist of calcite, quartz, K-feldspar, molybdenite, and pyrite with minor amounts of galena, and a variety of accessory minerals. Gold veins are dominated by quartz and pyrite, with minor to trace amounts of chalcopyrite, galena, sphalerite, and sericite. Monazite grains are well developed both in molybdenum and gold veins, and are intergrown with molybdenite and pyrite, respectively. Monazites from gold vein contain lower REEs contents, more prominent negative Eu anomalies, and higher Th/U ratios than those from molybdenum vein. The textural and geochemical evidence indicates that monazites from both veins precipitated from contrasting ore-forming fluids. Laser ablation ICP-MS U-Pb dating of monazite from the molybdenum vein yields a ²⁰⁶ Pb/ ²³⁸ U age of 203.5 ± 8.1 Ma (MSWD = 0.23, 2σ), which is statistically indistinguishable from the molybdenite Re-Os age of 204 Ma. In contrast, monazite from the gold vein has a significantly younger ²⁰⁶ Pb/ ²³⁸ U age of 130.4 ± 5.3 Ma (MSWD = 0.99, 2σ). Results presented here, combined with independent studies, suggest that the molybdenum and gold veins of the Hongtuling deposit, and by inference the other deposits in the district, formed in two discrete mineralization events in the late Triassic and early Cretaceous. The molybdenum mineralization is most likely related to the post-collisional magmatism associated with the Qinling orogeny, whereas gold mineralization occurred in an extensional setting associated with the decratonization of the North China Craton.
... These rocks probably formed in the Neoarchean and have been subjected to amphibolite-facies metamorphism in the Paleoproterozoic [43][44][45]. The Archean rocks were intruded by Paleoproterozoic pegmatite [45,46], Paleoproterozoic and Early Cretaceous mafic dikes [47][48][49], and Proterozoic and Mesozoic granitic intrusions. Mesozoic granitic intrusions formed during the Late Triassic (228-215 Ma: [50,51]) and the Early Cretaceous (146-131 Ma: [21,[51][52][53][54][55][56]) and are widely exposed in the Xiaoqinling region (Figure 1(a)). ...
The recently developed technique of ultraviolet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICP-MS) combined with a freezing cell is expected to improve the analysis of CO 2-rich fluid inclusions by decreasing their internal pressure and avoiding the common problem of uncontrolled explosive fluid release on ablation. Here, we report the application of this technique through the case study of CO 2-rich fluid inclusions from the quartz vein-style Au-Mo deposit of Dahu in the Xiaoqinling region of central China. The concentrations of were analyzed in 124 (not all for Al and Ca) fluid inclusions, which have low to moderate salinity and multiphase composition (liquid H 2 O + liquid CO 2 ± vapor CO 2 ± solids). The Dahu fluids are dominated by Na and K. The concentrations of Mo are always below the detection limit from 0.005 to 2 ppm (excluding values obtained from fluid inclusions with accidentally trapped solids). The Dahu ore fluids differ from metamorphic fluids in compositions and most likely represent two separate pulses of spent fluids evolved from an unexposed and oxidized magmatic system. The UV-fs-LA-ICP-MS analysis of fluid inclusions in a frozen state improves the overpressure problem of CO 2-rich fluid inclusions during laser ablation. The transformation of gaseous and liquid CO 2 into the solid state leads to a significant decline in the internal pressure of the fluid inclusions, while femtosecond laser pulses generate a minimal heat input in the sample and thus maintain the frozen state during ablation. Transient signals of CO 2-rich fluid inclusions obtained in this study typically had one or multiple peaks lasting for more than 15 seconds, without an initial short signal spike as obtained by ns-LA-ICP-MS analysis of CO 2-rich fluid inclusions at room temperature.
... The required heat to produce the Huayangchuan A-type granite could have been provided by the underplating of mafic magmas (King et al., 1997;Shellnutt and Iizuka, 2011). In fact, such coeval mafic magmatism has been revealed recently in the study area as mentioned before, such as the 1.80 Ga mafic dykes in the Xiaoqinling gold district (Bi et al., 2011). The covariant εHf(t) versus δ 18 O diagram (Fig. 11) indicates that there were ancient crust with mantle contribution to the Huayangchuan granites. ...
Paleoproterozoic A-type granites are widely outcropped in the North China Craton (NCC), particularly in the Trans-North China Orogen. However, their genesis and tectonic significance remain obscure. Here we report systematic studies on geochronology and geochemical characteristics of A-type granite in Huayangchuan, south margin of the NCC. The samples are enriched in total alkali (K2O + Na2O > 8.97 wt%), and depleted in MgO (0.84–0.93 wt%), CaO (1.28–1.90 wt%) and P2O5 (0.18–0.20 wt%), with high FeOT/MgO (5.69–6.67). They are characterized by high Zr + Y + Nb + Ce values (1293–1392 ppm) and 10,000 × Ga/Al ratios (3.14–3.35), which are typical characteristics of A-type granite. The Huayangchuan A-type granite can be further classified as A1-type subgroup based on particular geochemical features, e.g., low Y/Nb (0.87–1.00) and Yb/Ta (0.88–1.10). High precision zircon U–Pb dating of the A-type granite by secondary ion mass spectrometry (SIMS) yields Paleoproterozoic ²⁰⁷Pb/²⁰⁶Pb ages of 1829.5 ± 2.5 Ma. The low zircon ɛHf(t) values (−6.97 to −10.45), along with zircon Hf model age of 2.7–2.9 Ga, indicate that the Huayangchuan A-type granite was derived from partial melting of the ancient continental crust with contribution of enriched mantle components. The low zircon δ¹⁸O composition (4.00 to 6.78‰) indicates that the zircons were crystallized from low δ¹⁸O magmas, which derived from the crust metasomatized by low δ¹⁸O mantle fluids or melts. The E-W trend A1-type granitic plutons in the NCC are generally outcropped in a rift tectonic regime, which is consistent with the development of the mantle plume in the Xiong'er district. The large volume of basaltic magmas, generated by mantle plume head, underplated the lower continental crust and formed the Huayangchuan A-type granite.
... The diabasic to granitic dykes include lamprophyre, doleritic and dioritic porphyry, and granodioritic porphyry. The geochemistry and petrogenesis of these dykes have been the focus of several investigations (e.g.,Bi, Li, & Li, 2011, Guo et al., 2004, Hu et al., 2001Sun, Hu, & Ling, 2000;Yang, Chung, Zhai, & Zhou, 2004), which highlight the relationship among the basement rocks, plutons, and various types of dykes as an important factor to understand the genesis of gold and other metallic mineralization. The " Dehua‐Youxi‐Yongtai gold triangle " in Fujian province, Southeast China, is the region of several gold deposits (Chen, 2011), with a close relationship between porphyritic dykes and gold mineralization. ...
The widespread tectonic and magmatic events in Southeast China during the Mesozoic were accompanied by large‐scale metallogenesis. Here, we investigate the petrology, geochemistry, and zircon U–Pb age of the porphyry dykes in relation to the gold mineralization at Dongyang deposit in the Fujian province. The geochemical data show silica‐rich and Mg‐ and Ca‐poor A‐ type granite affinity for the dykes. The rocks have high concentrations of LREE Q2 and LILE and relatively low contents of HREE and HFSE. They display (La/Yb) N and (La/Sm) N values in the range of 5.7 to 31.4 and 6.0 to 8.3, respectively, with negative anomalies of Ba, Nb, Ta, Sr, Zr, Eu, and Ti and positive Ce anomaly. The source magma for the dykes was derived from a mixture of partially melted Al‐rich crustal material, with an input of mantle components. We present sensitive high‐resolution ion microprobe zircon U–Pb data from the dykes, which show emplacement ages in the range of 158 to 162 Ma, coinciding with the onset of the Yanshanian magmatism in Southeast China. Integrating the data obtained from studies on geology, geochronology and petro‐geochemistry, we suggest that the dykes played role in controlling the gold mineralization, and the magmatism and gold mineralization are the response of continental lithosphere thinning in Southeast China triggered by the subduction of the ancient Pacific Ocean Plate.
... SHRIMP U-Pb dating of magmatic zircon by Wang et al. (2008) gave ages of 126.9±4.8 and 128.6±4.7 Ma which may date another stage of mafic dyke intrusion. However, Bi et al. (2011) reported a zircon LA-ICP-MS 207 Pb/ 206 Pb age of 1 819±10 Ma (1σ) by analyzing four mafic dyke samples in the Xiaoqinling area. Thus they suggested that numerous mafic dykes in the Xiaoqinling area should have formed in Palaeoproterozoic, in an extensional setting after the collision between the eastern and western blocks of the NCC at ca. 1.85 Ga, and they suggested that the previous whole rock K-Ar and Rb-Sr ages which are significantly younger should represent thermal perturbation caused by Mesozoic tectonic-thermal event. ...
In this study, we carried out petrography, zircon U-Pb geochronology and Hf isotopic analyses on a granitic pegmatite dyke in the Xiaoqinling area in southern margin of the North China Craton (NCC). Our study suggests that the pegmatite dyke likely crystallized from a volatile-rich pegmatitic magma. Different from most other pegmatite elsewhere, zircon from this pegmatite dyke does not contain unusually high U and Th concentrations and suffered no evident radioactive damage, therefore we successfully obtained a zircon U-Pb dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), which yields an average 207Pb/206Pb age of 1 814±6 Ma, representing the intrusive age of the granitic pegmatite dyke. Zircon ɛ
Hf(t) values are between −8.3 and −3.0, corresponding to Hf depleted mantle model ages from 2 649 to 2 991 Ma with an average of 2 881 Ma. These data indicate that this granitic pegmatite dyke may have been derived from partial melting of Meso-Neoarchean metamorphic rocks from the Xiaoqinling basement. Granitic pegmatite magma may have emplaced within the Taihua Group wall rocks during the last stage of the middle to high grade metamorphism. Furthermore, according to the petrographic observation, the 1.81 Ga pegmatite dyke and the 1 800–1 750 Ma Xiong’er Group rocks were not undergone middle to high grade metamorphism, indicating 1.81 Ga as the termination of the latest regional metamorphism in the southern margin of the NCC.
... However, many report semi-quantitative compositional data obtained by energy-dispersive X-ray detector. Recent studies documented the first occurrence of the unnamed phase PbBi 4 Te 4 S 3 (Lipovetskiy et al. 1978, Cook et al. 2007 in China (Zhao et al. 2011), and presented electron microprobe compositions for tellurobismuthite, hessite, altaite, calaverite, and petzite (Bi et al. 2011a, Li et al. 2012. ...
The Xiaoqinling vein-style gold district (reserves of >630 t Au) has a very pronounced tellurium signature. The Yangzhaiyu gold deposit is one of the largest gold deposits in this district. The ore-mineral assemblage consists of pyrite, chalcopyrite, galena, bornite, sphalerite, gold, and native tellurium together with a wide variety of telluride and Bi-sulfosalt minerals. The telluride mineral assemblage comprises Phase A, sylvanite [(Au,Ag)2Te4], hessite [Ag2Te], petzite [Ag3AuTe2], calaverite [AuTe2], stützite [Ag5-xTe3], rucklidgeite [PbBi2Te4], altaite [PbTe], volynskite [AgBiTe2], tellurobismuthite [Bi2Te3], tetradymite [Bi2Te2S], and buckhornite [AuPb2BiTe2S3]. The Bi-sulfosalt minerals observed are Phase B, aikinite [PbCuBiS3], felbertalite [Cu2Pb6Bi8S19], and wittichenite [Cu3BiS3]. We observed two unnamed phases: Phase A and Phase B. Phase A, with the empirical formula of AgTe3, occurs in small intergrowth aggregates with sylvanite and chalcopyrite, and locally with bornite, galena, and altaite. Phase A and its associated minerals occur along healed micro-fractures in pyrite. It has a distinctly golden reflection color, without discernable reflectance pleochroism or anisotropy. AgTe3 was synthesized in 1982, but its natural occurrence has not been reported to date. Phase B, with the empirical formula of Cu20FePb11Bi9S37, was observed in an intergrowth aggregate together with hessite, galena, chalcopyrite, bornite, and buckhornite. It has a pinkish grey reflection color, without discernable reflectance pleochroism or anisotropy. The telluride and gold mineralization took place at or below ca. 220 ° C and evolved towards decreasing fTe2. The ore-forming fluids and components of the Yangzhaiyu gold deposit may ultimately stem from refertilized upper mantle, either directly from devolatilization or from a magmatic system resulting from partial melting.
... Several granitic veins are emplaced into granitic gneiss. Recent sensitive highresolution ion microprobe (SHRIMP) and laser ablationinductively coupled-mass spectrometry (LA-ICP-MS) U-Pb zircon ages of the Taihua Complex indicate that the complex was formed during the Paleoproterozoic [1][2][3][4]. ...
Xiaoqinling District is an important gold-producing area in China. It ranks second to Jiaodong with regard to gold deposits. The uprising period of the Wenyu granitic pluton and the wall-rocks of the deposit, as well as the mineralizing depth and reserved place of gold ore bodies, are significant to ore exploration. Fission-track (FT) analysis of zircons and apatites of granitic rocks from the Wenyu granitic pluton shows that apatite FT (AFT) data modeling indicates a rapid cooling rate of 20°C/Ma from 138 to 120 Ma after emplacement at 138 Ma. Thermal evolution and inversion curves suggest a secondary phase of fast cooling and uprising from 45 to 35 Ma, and 35 Ma, respectively, with a cooling rate of 6.7°C/Ma and a denudation quantity of ∼4.3 km. The last cooling phase took place from
... These dykes range in composition from diabasic to granitic, including lamprophyre, doleritic and dioritic porphyry, and granodioritic porphyry. The geochemistry and petrogenesis of these dykes have been the focus of several previous studies (e.g., Bi et al., 2011;Cheng et al., 1998;Guo et al., 2004;Hu et al., 2001;Sun et al., 2000;Yang et al., 2004). The relationship among the basement rocks, plutons and various types of dykes is important to understand the genesis of the gold and other metallic mineralization in these regions. ...
... These dykes range in composition from diabasic to granitic, including lamprophyre, doleritic and dioritic porphyry, and granodioritic porphyry. The geochemistry and petrogenesis of these dykes have been the focus of several previous studies (e.g., Bi et al., 2011;Cheng et al., 1998;Guo et al., 2004;Hu et al., 2001;Sun et al., 2000;Yang et al., 2004). The relationship among the basement rocks, plutons and various types of dykes is important to understand the genesis of the gold and other metallic mineralization in these regions. ...
The Trans-North China Orogen (TNCO), a Paleoproterozoic suture that amalgamates the Western and Eastern Blocks of the North China Craton (NCC), witnessed extensive magmatism and metallogeny during Mesozoic, associated with intraplate tectonics and differential destruction of the cratonic lithosphere. Here we investigate a suite of porphyry dykes surrounding the Mapeng batholith in the Fuping Complex within the TNCO in relation to the Mesozoic gold and molybdenum mineralization. The major element chemistry of these dykes show a range of SiO2 (57.92 to 69.47 wt.%), Na2O (3.20 to 4.77 wt.%), K2O (3.12 to 4.60 wt.%) and MgO (0.51 to 3.67 wt.%), together with high concentration of LREE and LILE, and relatively low contents of HREE and HFSE. The rocks display (La/Yb)(N) = 13.53-48.11, negative Nb, Ta, Th, U and Zr anomalies, and distinctly positive Ba, K and Sm anomalies. The mineralogy and geochemistry of the porphyry dykes indicate the rocks to be high-K calc-alkaline, and I-type, with adakitic features similar to those of the adjacent Mapeng batholith. The source magma for these rocks was derived from a mixture of reworked ancient continent crust and juvenile mantle materials. The zircon U-Pb data from these rocks show ages in the range of 124 to 129 Ma, broadly coinciding with the emplacement age of the Mapeng intrusion. The inherited zircons of ca. 2.5, 2.0 and 1.8 Gain the dykes represent capture from the basement rocks during melting. The zircon Lu-Hf isotopic compositions show negative epsilon(Hf)(t) values varying from -27.8 to -11.3, with Hf depleted model ages (t(DM)) ranging from 1228 Ma to 1918 Ma and Hf crustal model ages (t(DM)
... Ga, with minor amounts of diabase dykes formed during 128.6 ± 4.7−126.9 ± 4.8 Ma (Bi et al., 2011;Wang et al., 2008). ...
... In addition, a number of granitic and a few diabase dikes intrude the Early Cretaceous Plutons. Previous K-Ar dating suggested that many mafic dikes were emplaced in the 182 to 67 Ma interval (Ni et al., 1994;Liu and Jin, 1996); recent in-situ zircon U-Pb analyses, however, demonstrate that a majority of these mafic dikes were emplaced at ca. 1.85 to 1.80 Ga, with minor amounts of diabase dikes formed in the Early Cretaceous Bi et al., 2011). ...
The Yangzhaiyu gold deposit is one of numerous lode gold deposits in the Xiaoqinling district, southern margin of the North China Craton. Gold mineralization is hosted in Neoarchean to early Paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein-proximal alteration zone. Ore-related hydrothermal alteration is dominated by sericite + quartz + sulfide assemblages close to gold veins, and biotite + quartz + pyrite ± chlorite ± epidote alteration generally distal from mineralization. Pyrite is the predominant sulfide mineral, locally coexisting with minor amounts of chalcopyrite, sphalerite, and galena. Gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with Au-bearing tellurides, mainly petzite and calaverite coexisting with hessite, tellurobismuthite, and altaite. Fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures (175°-313°C) from aqueous or aqueous-carbonic fluids with moderate salinity (5.14 wt% NaCl equiv). δ 34S values of sulfide minerals range mainly from 2.0 to 4.4%, whereas auriferous quartz vein samples have δ18O values of 12.4 to 9.6%, with calculated δ 18OH 2O values of 6.0 to 3.2%. Gold-related pyrite grains yield elevated 3He/ 4He ratios (1.51-0.32 Ra) relative to crustal reservoirs and mantle-like 20Ne/ 22Ne and 21Ne/ 22Ne ratios (9.90-9.68 and 0.029, respectively). The stable and noble gas isotopes thus suggest deep-seated, most likely magmatic and mantle-derived, sources for the ore fluids, sulfur and, by inference, other components in the ore system. 40Ar/ 39Ar dating of ore-related sericite and biotite separates indicates two episodes of gold genesis at 134.5 to 132.3 and 124.3 to 123.7 Ma. The mineralization ages overlap zircon U-Pb ages of 141.0 ± 1.6 to 125.8 ± 1.4 Ma (2σ) for the Wenyu and Niangniangshan monzogranite Plutons and a number of mafic to intermediate dikes intruding these Plutons, all being proximal to the Yangzhaiyu gold deposit. The synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur. The geochronologic data also suggest that gold veining took place billions of years after the stabilization of the North China Craton and associated metamorphism in the Late Archean to early Paleoproterozoic. This contrasts sharply to lode gold deposits in other Precambrian cratons that formed predominantly in Late Archean to Paleoproterozoic, temporarily and genetically related to regional high-grade metamorphism and compressional or transpressional tectonism. Available data have demonstrated that the North China Craton was reactivated in the late Mesozoic, as marked by voluminous igneous rocks, faulted-basin formation, high crustal heat flow, and widespread metamorphic core complexes in the eastern part of the craton. It is thus suggested that the Yangzhaiyu gold deposit, together with other deposits of similar ages in the Xiaoqinling district, were products of this craton reactivation event. Lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy, fluid, and sulfur required for the formation of the gold deposits.
... The Xiaoqinling district and adjacent areas also host many dolerite, diabase, and lamprophyre dikes, which were emplaced in two separate episodes at ca. 1.85-1.80 Ga and 140-120 Ma (Mao et al., 2010;Bi et al., 2011a;Li et al., 2012b;Ren, 2012). ...
Lode gold deposits in Precambrian cratons represent the world's major gold source and were mostly generated during formation and stabilization of the cratons. However, there is an extraordinary exception in the North China craton (NCC), where lode gold deposits formed after prolonged stabilization of the craton. Molybdenite Re–Os and hydrothermal sericite and biotite 40Ar/39Ar dating of major gold deposits from the Xiaoqinling district, southern NCC, bracket their emplacement in the range of 154.1±1.1 to 118.9±1.2 Ma (n=23), postdating formation of the craton by more than 1.7 billion years. Fluid inclusions extracted from gold-bearing pyrite have elevated 3He/4He ratios (1.52–0.22 Ra) and mantle-like Ne isotopes (20Ne/22Ne=10.02−9.22 and 21Ne/22Ne=0.033−0.027), indicating presence of mantle-derived fluids in the ore system. Measured δ34S of pyrite and δD and δ18O of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic/mantle source for sulfur and ore fluids. Gold deposits of similar ages also widely occur in the eastern and northern margins of the NCC, which, together with those in the Xiaoqinling district, have a total reserve of ∼2500 t gold, forming the only known giant late Mesozoic gold province in the world's Precambrian cratons. These deposits formed coevally with extensive felsic to mafic magmatism, development of intracontinental rift basins, and exhumation of metamorphic core complexes across the eastern NCC, events interpreted as indicating thinning and destruction of the lithosphere beneath the craton. Rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids, sulfur and, by inference, other ore components to form the giant gold provinces.
... The pre-ore dikes are mostly Paleoproterozoic in age (1.84-1.82 Ga), as constrained by SHRIMP and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating Bi et al. 2011). ...
The Mesozoic Yangzhaiyu lode gold deposit is situated in the southern edge of the North China craton. Gold mineralization is hosted in Archean amphibolite facies metamorphic rocks, and consists mainly of auriferous quartz veins. Pyrite is the predominant sulfide mineral, with minor amounts of chalcopyrite, sphalerite, and galena. Based on morphology and paragenesis, there are three generations of pyrite, termed as first generation (G1), second generation (G2), and third generation (G3). They have distinct contents, occurrences, and distribution patterns of gold. The coarse-grained, euhedral G1 pyrite contains negligible to low levels of gold, whereas both invisible and visible gold are present in the fine- to medium-grained G2 pyrite that is characterized by abundance of microfractures and porosities, forming a foam-like texture. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) depth profiles indicate that invisible gold occurs either as solid solution or as nanoparticles of gold-bearing tellurides in the G2 pyrite. Visible gold is widespread and present as irregular grains and stringers of native gold mostly along grain boundaries or filling microfractures of pyrite, likely resulting from remobilization of invisible gold once locked in the G2 pyrite. The G3 pyrite, invariably intergrown with chalcopyrite, sphalerite, and galena, contains the highest levels of invisible gold. There is a positive correlation between Au, Ag, and Te, indicating that gold occurs as submicroscopic Au-bearing telluride inclusions in the host minerals. Whenever gold, either invisible or visible, is present, As is always below or only marginally higher than the detection limit of LA-ICP-MS. This indicates that As played an insignificant role in gold mineralization. Tellurides are widespread in the auriferous quartz veins, consisting mainly of petzite, calaverite, hessite, altaite, and tellurobismuthite. Native gold commonly occurs as intergrowths with tellurides. Textural evidence indicates a precipitation sequence, in a temporal order, of calcaverite, petzite, altaite, tellurobismuthite, and hessite. Little amount of sulfide phases has been found in association with the tellurides, indicating that tellurides were deposited under low S fugacity (fS
2
) and/or high Te fugacity (fTe
2
) conditions. The textural relationships, when combined with fluid inclusion microthermometric data of auriferous quartz veins and tellurides thermodynamic data, permit estimation for logfTe
2
during telluride formation, which are −6.8 to −10.8 at 300°C and −9.6 to −17.6 at 250°C. Available geochronological and geochemical data suggest that Te was most likely derived from the late Mesozoic magmatic rocks widespread in the Xiaoqinling district and other parts of the southern North China craton, which were emplaced broadly contemporaneous with gold mineralization at Yangzhaiyu. This study highlights the role of Te and tellurides as important gold scavengers in As-deficient ore fluids.
Paleoproterozoic A-type granites are widely distributed in the southern margin of the North China Craton (SNCC), providing important information for understanding the Paleoproterozoic tectonic regimes in this area. This paper reports newly obtained whole-rock compositions and zircon U-Pb ages for the Tieluping syenogranite porphyry (TLP) and Huoshenmiao alkali granite porphyry (HSM) in the SNCC. Zircons from the TLP and HSM have U-Pb ages of 1 805 ± 12 and 1 792 ± 14 Ma, respectively. These ages are taken to date the emplacement of these intrusions. They had high total alkali contents (K2O + Na2O > 7.13 wt.%), with high 10 000 × Ga/Al ratios (3.06–3.41) and Zr + Y + Nb + Ce values (709 ppm–910 ppm) as well as high zircon saturation temperatures (864–970 °C), indicative of A-type granite affinities. High Y/Nb (1.75–3.32), Ce/Nb (7.72–9.72), and Yb/Ta (2.89–5.60) ratios suggested that TLP and HSM belonged to the A2-type granite. The negative whole rock εNd(t) values (−8.4 to −6.6) and negative zircon εHf(t) values (−15.9 to −6.3) confirmed that TLP and HSM were likely generated by the partial melting of an ancient continental crust. The εHf(t) (−7.4 to +4.0) values of inherited zircons in the TLP suggested that they were derived from the partial melting of Archean basement rocks. Considering the geochemical similarity of the 1.80 Ga A-type granitoids in the SNCC, we propose that the TLP and HSM were formed in a post-collisional regime that was likely associated with the break-off of the Paleoproterozoic subducted slab. Upwelling of the asthenosphere provided huge heat to generate the regional 1.80 Ga A-type granite in the SNCC.
A-type granites are produced commonly in extensional settings and they are important proxies to track breakup of continent. Here we report systematic studies on geochronology and geochemical features of A-type granites in Songxian–Ruyang region, southern margin of the North China Craton (NCC), in order to constrain their petrogenesis and tectonic implications. Zircons from these granites show oscillatory zoning and high Th/U ratios (0.12 to 1.26), indicating a magmatic origin. LA–ICP–MS zircon U–Pb dating shows that they were emplaced at 1813 to 1789 Ma, indicating that they were formed in late Paleoproterozoic. Geochemically, these rocks are high in SiO2 (67.51–74.42 wt%) and total alkalis (Na2O + K2O = 7.76–10.19 wt%), and low in MgO (0.09–1.31 wt%), CaO (0.09–1.11 wt%) and P2O5 (0.12–0.26 wt%) contents, with negative Nb, Ta, Sr, P and Ti anomalies. They are characterized by high FeOT/(MgO + FeOT), 10000 Ga/Al ratios (1.57–3.15) and Zr + Nb + Ce + Y contents (566–835 ppm). They are also characterized by high calculated Zr saturation temperatures (892–918 °C), high Ti-in-zircon temperatures (693–912 °C) and metaluminous to weakly peraluminous natures, which are typical features of aluminous A-type granites, further belonging to the A2 granites. The low zircon εHf(t) values (–13.8 to –8.6), along with the low whole rock εNd(t) values (–8.62 to –6.10) and low apatite εNd(t) values (–8.31 to –6.56), indicate that the Songxian–Ruyang A-type granites were derived from partial melting of the calc-alkaline tonalitic–trondhjemitic–granodioritic gneisses in Taihua Complex, and they were formed in post-collisional extensional setting. Combined with coeval regional granitoids, the south margin of the NCC is in a continuous extensional setting during 1.8–1.6 Ga, and the Songxian–Ruyang A-type granites can represent a response of the breakup of Columbia supercontinent.
Gold deposits in the Xiaoqinling province, southern margin of the North China Craton, are hosted by Neoarchean amphibolite-facies rocks of a metamorphic core complex. Over 630 t of gold resource was concentrated in the narrow-uplifted basement, which was classified into three (northern, middle, and southern) major ore belts. Gold-related scheelite grains from the Qinnan and Qiaoshangzhai (northern belt), Tonggou (middle belt), and Yangzhaiyu (southern belt) deposits were analyzed using a combination of cathodoluminescence (CL) imaging and LA-ICP-MS trace element analysis to decipher their metallogenesis. Our new results indicate that: (1) the Xiaoqinling scheelites are characterized by high Sr content and varying internal texture, replicating fluid-rock reactions. Oscillatory zoning and inhomogeneous trace element compositions suggest a dynamic hydrothermal system with fluctuating chemical properties; (2) the REEs of Xiaoqinling scheelites were mainly controlled by the substitution mechanism 2Ca²⁺ = REE³⁺ + Na⁺. All scheelite grains yielded typical bell-shaped REEN patterns, with negative and positive Eu anomalies occurring in samples from the Qinnan-Qiaoshangzhai and Tonggou-Yangzhaiyu deposits, respectively. Meanwhile, the scheelite Mo concentration in the northern belt (>100 ppm) is much higher than that in the middle and southern belts (<20 ppm), suggestive of significant variations in the oxygen fugacity (fO2). The spatial association of the northern ore belt with the Taiyao detachment fault may imply an increase in the fluid fO2 by percolating meteoric waters; (3) the obtained scheelite ⁸⁷Sr/⁸⁶Sr ratios show a broadly increasing trend from north to south (are 0.71480 to 0.71626 for Qinnan, 0.72784 to 0.72920 for Qiaoshangzhai, 0.71864 to 0.76251 for Tonggou, and 0.72760 to 0.74781 for Yangzhaiyu). This trend may depict the ore-fluid flow direction, i.e., the Sr isotopic signature shifted towards a more radiogenic composition with increasing distance from north to south due to fluid reaction with the Taihua Group (amphibolite- to granulite-facies metamorphic rock pile).
The Xiaoqinling district, located in the southern margin of the North China Craton, is one of China’s most significant gold districts (resources of >630 t Au). A complex series of overprinting Mesozoic tectonic and related hydrothermal events associated with the metallogenic evolution in this region has led to controversial interpretations and models. Here we present new geological, mineralogical, ore texture, geochronological, and in-situ S isotope data from the Dahu Au-Mo, Qinnan Au-Mo, and Tonggou Au deposits from this region which indicate that the Mo and Au were introduced in two discrete mineralization events. The Mo-bearing mineral assemblage mainly comprises quartz, anhydrite, pyrite, molybdenite, and rutile that yielded an age of 216.1 ± 9.7 Ma at Dahu. In-situ LA-ICP-MS δ³⁴S values for the molybdenite and related pyrite cluster tightly at -10 to -9‰. On the other hand, the Au-bearing mineral assemblage mainly comprises quartz, pyrite, galena, sphalerite, chalcopyrite, native gold, and Te (±Bi) minerals. Hydrothermal rutile and monazite associated with auriferous pyrite yielded ages of 124.7 ± 3.9 Ma (Dahu), 121.5 ± 1.3 Ma (Qinnan), and 122.1 ± 2.0 Ma (Tonggou). The in-situ δ³⁴S values of gold-related sulfides fall within a range of -4 to +3‰. Although Mo and Au ores are both present in the same ore veins, their mineral assemblage, mineralization age, fluid properties and sources are different. Textural relationship shows that the earlier Mo-bearing quartz-anhydrite veins were overprinted by later auriferous fluids. Several lines of evidence support a multiphase mineralization process of Mo and Au, which are genetically linked to the post-collisional extension of the Qinling orogen and the Paleo-Pacific post-subduction tectonics, respectively. The episodic hydrothermal mineralization in Xiaoqinling represents a protracted and complex lode system.
The Laowan Au−Ag−Te deposit (containing>100t Au), located at the center of the Qinling−Dabie orogenic belt in central China, represents a breakthrough in gold prospecting in the Tongbai district. A tunnel geological survey and microscope-based mineral identification, combined with EMPA/EDS mapping and LA-ICP-MS analysis, were used in this study to determine the mineral types present. The sequence of formation and symbiosis were determined using the Au-hosting mineral geochemical element typomorphism to provide a basis for Au enrichment and the sources of material and ore−forming fluids. The results indicate that: (1) Tellurides were widespread in the main metallogenic auriferous pyrites, consisting of petzite, calaverite, tellurobismuthite, tetradymite, volynskite, and hessite. The vertical zonation of different telluride combinations indicates that mineral distribution shows the vertical zoning characteristics of upper Ag and lower Bi. (2) Petrographic analyses indicate that 5 generations of pyrite formed during the ore-forming stage (Py1, Py2, Py3, Py4, and Py5). LA-ICP-MS analytical data showed that Py3 and Py4 were rich in Au−Ag−Bi−Te, with a correlation between gold and tellurium. Auriferous pyrites were rich in both As and Te, and the enrichment of Au was unrelated to As. Te is an important scavenger of gold, the effect of As is not obvious. (3) From 250°C to 200°C, logfTe2 and logfS2 decreased from −13.0 to −8.3 and −14.7 to −8.6 to −15.2 to −11.2 and −16.7 to−10.5, respectively. fTe2, fS2 and temperature were the conditions controlling variations in the telluride combination. (4) The occurrence of tellurides, the correlation of Au−Ag−Bi−Te elements and enrichment of Te and Bi, combined with previous isotopic geochemical data, indicate that the ore-forming fluids and metals might be stemmed from the mantle-derived magmatic system, which could have provided materials, sufficient heat energy, volatiles, and components for the gold mineralisation.
The North China Craton (NCC), which underwent multiple subduction and lithospheric destruction during the Mesozoic, is well known for hosting some of the world-class gold deposits. In many cases, Mesozoic mafic dykes are associated with the gold deposits, although there has been no systematic studies aimed to integrate the characteristics of these dykes in different regions of the NCC. Here we investigate diabase and lamprophyre dyke suites in the Xiaoqinling region along the southern margin of the NCC, and compile the published geochronological, geochemical and isotopic data for regional mafic dykes and gold deposits from the major gold belts including Jiaodong, Xiaoqinling and central NCC. Zircon U-Pb data show the mafic dykes in Xiaoqinling were emplaced in two episodes at 219–227 Ma and 121–137 Ma, and their εHf(t) values (ranging from −2.1 to 3.8 and −13.6 to 1.6 respectively) suggest increasing input of lithospheric materials in the Early Cretaceous dykes. The compiled geochemical data on the mafic dykes from different regions in the NCC show that the magmatic evolution involved olivine and clinopyroxene fractional crystallization with limited crustal contamination and subduction-related fluid metasomatism. The geochemical and isotopic signature suggests multiple sources for the parent magma of the mafic dykes ranging from convective asthenospheric mantle to ancient lithospheric mantle, with the magma source characterized by enriched mantle (EM2 or EM2+ EM1), and that signatures might be potential for gold mineralization. Further, most dykes are coeval and cospatial with gold deposits in the NCC. The geochronological data show that small amount of both Mesozoic dykes and gold deposits formed in Triassic whereas the peak formed in Cretaceous following slab roll back of the Pacific plate subducting from the east during 145–110 Ma. We envisage that the tectonic extension associated with Pacific slab subduction contributed to the magmatism and gold mineralization in the NCC.
Dyke swarms formed in extensional settings and are important proxies to track mantle source characteristics as well as continent and supercontinent break-up. The late Paleoproterozoic mafic-intermediate dykes in the Xiaoqinling region, southern margin of the North China Craton (NCC), provide a window to investigate the late Paleoproterozoic magma sources and tectonic evolution. Although many studies have been conducted on this region, the timing of late Paleoproterozoic extension and tectonic setting remain debated. Here we present results from LA-ICP-MS zircon U-Pb geochronology, Lu-Hf isotopic analyses and geochemistry of these dykes, together with an evaluation of the published geochronological and geochemical data for the regional dyke activity. The zircon U-Pb data show emplacement ages of 1751-1841 Ma for the dykes and compiled age data indicate widespread late Paleoproterozoic dyke emplacement during 1.85-1.75 Ga, with the majority of formed at 1.84-1.80 Ga. The geochemical characteristics, along with zircon εHf(t) values (showing two distinct ranges: 0.7-7.4 and -5.1-0), indicate multiple magma sources for the 1.84-1.80 Ga dykes from asthenosphere and depleted subcontinental lithosphere sources followed by interaction and mixing with continental crust. Geochemically, the dykes are enriched in Ba, K and P and depleted in Nb, Ta and Sr. The geological and geochemical features of these dykes are consistent with their formation within a post-collisional extension setting. We also evaluate the significance of these dykes in terms of paleogeographic correlations within the Columbia supercontinent through correlations with the North China Craton, Indian Shield and West Australia Craton.
Late Mesozoic granitoids are widely emplaced in the Qinling Orogen on the southern margin of North China Craton (NCC), and can provide important clues on the Late Mesozoic regional geodynamic setting. The Late Jurassic Laoniushan granitic pluton is located in the western end of the southern NCC margin. Zircon U–Pb dating for the granodiorite, quartz diorite, and biotite monzogranite of the pluton yielded 152.5 ± 1.6 Ma, 148.7 ± 1.2 Ma, and 149–146 Ma, respectively. Compilation of the single zircon ages revealed magmatic peaks at ca. 146, 152, and 163 Ma, suggesting multiphase Late Jurassic magmatism. These granitoids are high‐K calc‐alkaline, I‐type, and metaluminous to weakly peraluminous. The rocks show chondrite‐normalised LREE enrichments, HREE depletions and flat HREE patterns, together with (slightly) negative Eu, Nb, Ta, P, and Ti anomalies. The Laoniushan granodiorite and biotite monzogranite have similar REE patterns to the quartz diorite, but the former two have much smaller Eu anomalies than the latter. The negative zircon εHf(t) (−32.6 to −13.7), εNd(t) (−20.7 to −14.3) and crustal‐like whole‐rock trace‐elemental ratios (e.g., Nb/Ta, Th/U, Nb/U, and Ce/Pb), together with the low Mg (MgO = 0.18–2.02 wt%, Mg# = 20–34), Cr (7.00–39.0 ppm), and Ni (0.80–4.00 ppm) contents, suggests that the Late Jurassic granitoids were probably derived from the partial melting of ancient crustal rocks with minor mantle‐derived input. The granodiorite and quartz diorite have similar average εHf(t) (−26.5 and −26.7, respectively), εNd(t) (−20.5 and −20.2, respectively), TDM2(Hf) (2880 and 2885 Ma, respectively) and TDM2(Nd; 2607 and 2578 Ma, respectively), whereas the biotite monzogranite has higher average εHf(t) (−17.1) and εNd(t) (−15.3), and younger TDM2(Hf) (2281 Ma) and TDM2(Nd) (2176 Ma). This suggests that the biotite monzogranite has different magma sources and shallower magma formation depths compared to the granodiorite and quartz diorite. The latter two were likely formed by the partial melting of continental crustal rocks (notably the Taihua Group basement rocks), which was triggered by mantle underplating led by asthenospheric upwelling. The Late Jurassic Laoniushan granitoids may have formed in an extensional setting caused by the Palaeo‐Pacific Plate subduction beneath Eastern China. Commencement of Late Jurassic Laoniushan granitic magmatism likely marked the incipient of lithospheric thinning along the southern NCC margin. Considering also the regional geological evolution, we conclude that although the Laoniushan pluton exhibits spatial–temporal relationships with the nearby Mo (–U) and REE deposits, there is still no unambiguous metallogenic link with these deposits.
The Fancha gold deposit is one of the representative “lode type” deposit in the Xiaoqinling goldfield. Four-stage mineralization process were identified, namely, I) the barren quartz stage; II) the pyrite-dominated stage; III) the quartz-polymetallic sulfides stage; and IV) the quartz-carbonate stage. Correspondingly, pyrite crystal can be divided into three generations, i.e. Py1, Py2 and Py3 according to their different textures and paragenesis during the first three mineralization stages. In order to better understand the ore genesis, an integrated analysis on Re-Os-He-Ar-S-Pb multi-isotopes was carried out for each mineralization stage to delineate the timing of the mineralization and the origins of the ore-forming materials in detail. Gold-bearing pyrite yields a Re-Os isochron age of 124.3 ± 2.6 Ma (MSWD = 1.9), overlapping with the previous hydrothermal mica ⁴⁰Ar/³⁹Ar ages, suggesting that gold mineralization occurred in the Early Cretaceous. The ³He/⁴He (R/Ra) and ⁴⁰Ar/³⁶Ar values of the Py2 are 0.68 to 1.17 and 656.55 to 7384.2, and those of the Py3 are 0.20 to 0.33 and 647.67 to 8913.55, respectively, which reveal a significant contribution of mantle component in the ore-forming fluids, and the content of crust fluid is increasing in the later evolution. In-situ δ³⁴S values of the sulfides and tellurides in the auriferous quartz veins, including the pyrite (Py1, Py2 and Py3), chalcopyrite, sphalerite, galena, bismuthinite, and tetradymite, display a relatively narrow range (−4.9 to 3.6‰) consistent with magmatic sulfur, which are quite different from those of the pyrite in surrounding rock (−7.8 to −9.3‰). In-situ Pb isotopic compositions of the galena and bismuthinite of stage III are homogeneous with values of ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁶Pb/²⁰⁴Pb being 37.482–37.508, 15.392–15.523, 16.995–17.006, and 39.279–39.287, 15.588–15.590, 18.283–18.288, respectively, indicating a mixed Pb sources with obvious proportions of mantle. Combined with previous data, we suggest that the ore-forming fluids and metals of the Fancha gold deposit stemmed directly from the regional mantle-derived magmatic hydrothermal system, which is coupled with the large-scale lithospheric extension and thinning in the eastern North China Craton during the Early Cretaceous.
The Longwangzhuang pluton is a typical example of Paleoproterozoic A-type granite intrusions at the southern margin of the North China Craton. This pluton is composed of arfvedsonite granite and minor aegirine–augite granites. Samples from both granite types display similar zircon U-Pb ages with 207U-206Pb ages of 1612 ± 19 Ma [mean square weighted deviation (MSWD) = 0.66] and 1609 ± 24 Ma (MSWD = 0.5), respectively. The granites exhibit similar high silica (SiO2 = 71.1–73.4 wt.%), high alkaline (Na2O + K2O = 8.10–9.26 wt.%, K2O/Na2O > 1), and low Al2O3 (11.8–12.8 wt. %) contents and metaluminous to weakly peraluminous bulk chemistry. The chemical variations of the Longwangzhuang pluton suggest the effects of mineral fractionation. In addition, all samples show typical characteristics of A-type granites, such as high 10000Ga/Al ratios (4.10–7.28), high FeOtot/(FeOtot + MgO) ratios (0.88–0.99), and high Zr (484–1082 ppm), Ce (201–560 ppm), and Y (78–156 ppm) contents. The εNd(t) values and the (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t ratios of the arfvedsonite granite samples vary from −4.6 to –5.3, 15.021 to 17.349, 15.241 to 15.472, and 33.206 to 36.905, respectively, and those for the aegirine–augite granite sample amount at −0.2, 14.421, 15.175, and 33.706. The distinct and variable Nd and Pb isotope values indicate the presence of heterogeneous protoliths. Based on its geochemistry, its low initial Pb isotope ratios, and its enrichment in Nd isotopes, we infer that the Longwangzhuang A-type granite is the partial melting product of basement rocks such as the Taihua Group gneisses accompanied by some involvement of juvenile material from the mantle. Together with published data from other Paleoproterozoic A-type granite plutons exposed at the southern margin of the craton, our findings suggest that these granites had a similar origin. Furthermore, geochemically, they can be divided into two groups: A2-type, which formed earlier (~1.8–1.6 Ga), and A1-type, which formed later (~1.6–1.5 Ga). Combining this information with the variations in whole-rock Nd and zircon Hf isotopic composition at ca. 1.6 Ga, we propose that tectonic transformation from post-orogenic to anorogenic magmatism occurred at the southern margin of the North China Craton at that time.
The Dahu lodegold deposit is situated in the Xiaoqinling district, hosted in the east-west-trending shear zones, and characterized by auriferous quartz vein-type. Based on the morphology and paragenetic assemblage, three generations of pyrite (termed as first-generation pyrite (Py1), Py2, and Py3) can be classified. The Py2 can be further identified as brittle fractured pyrite (Py2-B) and ductile deformed pyrite (Py2-D). The different generations of pyrites feature distinct contents and distribution patterns of Au and other trace elements. The coarse-grained, euhedral Py1, disseminating in the early formed pyrite-poor quartz veins, contained negligible to extremely low levels of Au and associated trace elements, probably indicating a slow growth rate from a hydrothermal fluid unsaturated with respect to Fe, S, and Au and other trace elements, such as Cu, Pb, Bi, and Te, during the early orestage. Py2, occurring as laminations in the auriferous quartz veins, consistently contained significant amounts of Au, Bi, and Te, with a strong positive correlation between these elements, probably indicating a rapid growth rate and concentrated precipitation process of pyrite due to a hydrothermal fluid supersaturated with respect to Fe, S, Au, and other trace elements during the pyrite-rich auriferous quartz veining. Py3, which precipitated in the polymetallic sulfide orestage, contained a small amounts of Ag and a dramatically decreased content of Au, Bi, Te, and ratios of Bi/Ag. The results of electron back-scattered diffraction analysis on the Py2 revealed that Py2-B was characterized by predominantly brittle deformation with a limited local plastic deformation by dislocation glide on a simple slide system, whereas the Py2-D displayed a significant crystal plastic deformation through a lattice rotation on two separate 〈1 0 0〉 axes and local dislocation creep. Microscopic petrography and laser ablation inductively coupled plasma mass spectrometry analyses revealed that the brittle fracturing-dominated Py2-B generally displayed homogenous distribution patterns of Au and other trace elements and scarce visible gold grains, whereas the plastically deformed Py2-D generally exhibited an inhomogeneous distribution pattern of trace element and commonly contained fine-grained irregular-shaped visible native gold and other sulfide inclusions in the low-angle boundaries and dislocation sites within Py2-D, possibly suggesting the significant influence of ductile deformation on remobilization and reprecipitation of Au and associated trace elements once locked in Py2. The corrosion of Au-Te-Bi-bearing Py2 by the late polymetallic sulfide assemblage led to a liberation of invisible Au and other trace elements and reprecipitation of visible native gold and other Bi–Pb minerals. The results indicate that brittle deformation exhibited no essential effects on the crystal structure and chemical component of pyrite, whereas the plastic deformation can cause the readjustment of crystal lattices and liberation of impurities (e.g., Au, Te, Bi, and Cu) from the crystal lattices of pyrite. The plastic deformation and dissolution–remobilization of pyrite are the important mechanisms responsible for the visible discrete gold grain formation in auriferous quartz veins. Macroscopically, a preferred constant E–W extending orientation and typical lamination structure of the auriferous quartz veins suggest a channeled fluid flowing along the closed and connected fractures under a syn-mineralization S–N oriented extensional tectonic regime related to the Xiaoqinling metamorphic core complex. Here, we report that the microstructures of ductile deformation and brittle fracturing of the auriferous quartz veins are probably indicative of the continuous shearing activation of the ore-hosting normal fault.
The Fancha lode gold deposit is hosted in the Neoarchean Taihua Group metamorphic rocks in the Xiaoqinling gold field, on the southern margin of the North China Craton. Pyrite, as the predominant sulphide mineral and Au‐hosting mineral, was investigated using a combination of ore microscopy and in‐situ laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Three types of pyrite were identified during the ore‐forming process, that is, the coarse euhedral grains in milky quartz veins (Py1), the fine to medium, euhedral to subhedral grains in light grey quartz (Py2), and the fine grains intergrown with abundant sulphide minerals in light grey quartz (Py3). The trace elements exhibit systematic different concentrations among three types of pyrite, implying different crystallization processes. The Py1 grains have the lowest concentrations of Au and other accompanying elements such as Ag, Te, Bi, Cu, Pb, and Zn, and the trace elements of pyrite in wall rock are similar to those of Py1. The Py2 and Py3 grains show consistently high contents of Au, Ag, Te, Bi, Cu, Pb, and Zn. The uneven distribution of gold and positive correlation between Au, Ag, Te, and Bi within Py2 and Py3 grains, indicating that gold mainly occurs as submicroscopic inclusions of Au–Ag–Te–Bi assemblages in the pyrite. This interpretation is supported by the microscopic observation that large quantities of telluride and Bi‐sulfosalt minerals contact directly with gold. Observed Te–Bi minerals include petzite, sylvanite, hessite, calaverite, rucklidgeite, altaite, volynskite, tellurobismuthite, buckhornite, tetradymite, krupkaite, and bismuthinite, and native bismuth is also found. Bi (Te) melts, as an excellent gold scavenger, have played a certain role in governing gold distribution patterns and leading to high concentrations during the mineralization process in the Fancha gold deposit. The Te–Bi mineral assemblages also reveal that the gold was precipitated under variable redox conditions with log fTe2 ranged from <−11.0 to −6.4, and log fS2 ranged from <−11.8 to −8.6 at 300°C, vertically upward from the deep fluid source. Combined with available geochronological and geochemical data, enrichment of Te and Bi indicates that the ore‐forming fluids and metals might be stemed from a mantle‐derived magmatic system.
The giant Huayangchuan U–Nb–Pb deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit in the southern margin of the North China Craton (NCC), central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area, but their ages and petrogenesis are not well constrained. The exposed granitic rocks are mainly biotite monzogranite porphyry, granite pegmatite, granodiorite, and biotite monzogranite with zircon U–Pb ages of 1808 ± 10 Ma, 1807 ± 14 Ma, 233 ± 1.4 Ma, and 132 ± 0.6 Ma, respectively. The Paleoproterozoic biotite monzogranite porphyry belongs to shoshonite and metaluminous series, showing enrichment of LREE and LILE and depletion of HREE and HFSE, with high Zr + Y + Nb + Ce values and Ga/Al ratios, which are consistent with A-type granite. Whereas, the contemporaneous granite pegmatite dykes with weak mineralization are also cala-alkaline to shoshonite and peraluminous series, enriched in Rb, Ba, and LREE, and depleted in Nb, Ta, Ti and HREE. The shoshonite and weakly peraluminous Triassic granodiorite is slightly enriched in LREE with flat HREE patterns, enriched in Ba and Sr and depleted in Nb, Ta, P, and Ti, with similar geochemical characteristics to adakite-like rocks. The Early Cretaceous biotite monzogranite is characterized by LREE enrichment and flat HREE patterns, belonging to shoshonite and metaluminous to weakly peraluminous I-type granite, with U and LILE enrichment, and HFSE-depleted. The high initial ⁸⁷Sr/⁸⁶Sr ratios and enriched Nd (εNd(t) = −17.5 to −17.1) and Hf (εHf(t) = −33.2 to −14.6) isotopes reveal that the Huayangchuan granitic rocks are obviously sourced from crustal-derived magmas. Combined with regional geology and this study, we proposed that: (1) the Paleoproterozoic biotite monzogranite porphyry and granite pegmatite were generated from ancient lower crust during post-collisional extension setting; (2) the granodiorite was likely sourced from partial melting of thickened lower crust with pelagic sediments materials addition during the Triassic although the mineralization-related carbonatite having similar age with granodiorite may derive from mantle; and (3) the Early Cretaceous biotite monzogranite was mainly sourced from the partial melting of lower crust induced by the underplating of the mafic magma. We suggest that the first phase of magma occurred in the Huayangchuan district during the Paleoproterozoic following the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen. Since the Mesozoic, the ongoing northward subduction of the Yangtze Craton (YZC) resulted in the crust thickening, developing the Triassic granitic magmatism and major mineralization associated with carbonatite. The widespread Cretaceous granite and deposit in the southern margin of NCC indicate that intracontinental extension and lithospheric thinning occurred in response to the tectonic regime transition from NS-trending to EW-trending subduction.
The Dongjikou pyroxene sysnite, appearing as irregular ovoid, intruding into the Palaeoproterozoic high-grade metamorphic rocks of Taihua group, and developing gneissic schistosity in Huayangchuan region, Xiaoqinling, is the only known Caledonian rock body. Detailed petrography and field work studies on Dongjikou pyroxene sysnite indicate that inner parts of the rock body, according to the different structural characteristics, can be divided into the earlier-formed fine-grained pyroxene syenite, developing optalic boeder and a little later-formed medium-grained porphyritic pyroxene syenite, developing chilled border. The mineral association in both types of rocks is composed of plagioclase, potassium feldspar, pyroxene, amphibole, quartz, etc. The phenocrysts are mainly composed of pyroxene and plagioclase. Pyroxene and amphibole have experienced the retrogressive metamorphism. LA-ICP-MS zircon U-Pb ages of the fine-grained pyroxene syenite and medium-grained porphyritic pyroxene syenite are (214.8±3.1) Ma(MSWD=0.48) and (212.9±3.1) Ma(MSWD=5.6) respectively, which represents that the formation age of the rock is Late Triassic instead of the Caledonian period. The isotopic dating results show that the Caledonian magmatism event was extremely weak or inexistent in Xiaoqinling area. According to isotopic dating in this paper and a systematical summary of the previous research on the geological characteristics, it can be concluded that the age of the Dongjikou pyroxene sysnite is very close to that of the Indosinian magmatic and metallogenic event in Xiaoqinling area. It's concerned that the Indosinian magmatism is related to the transition regime from oceanic slab subduction to intercontinental collision. Moreover, intensive crust-mantle interactions caused upwelling of the deep material and triggered the gold enrichment.
The Taihua Group, exposed in the southernmost terminal Trans-North China Orogen (TNCO), is important for deciphering the mysterious evolution processes of the southern margin of the North China Craton during Proterozoic, and determination of the collision time of the eastern and western blocks of the North China Craton. Petrological study on three kinds of migmatites formed by anatexis from the Taihua Group in Xiaoqinling area, West Henan, indicates that the transformation of migmatites caused by anatexis is uneven, which is obviously limited by the composition of protolith, structure of rocks and other properties. LA-ICP-MS U-Pb dating of the anatexis zircons shows that the protolith crystallized in the Paleoproterozoic and the anatexis occurred at about 1.8 Ga. The migmatites and contemporaneous mafic dike swarms and A-type granite in the study area were generated in a post-orogenic, extensional environment following continent-continent collision at 1.85 Ga between the eastern and western blocks of the North China Craton.
Precise age determination and source tracing of the granitic and pegmatitic gneisses of the Taihua Complex in the Xiaoqinling area are important to address the debates on the timing and processes of the amalgamation of the Western and Eastern Blocks along the Trans-North China Orogen. This paper conducted zircon dating of metamorphic rocks from the Xiaoqinling area. The results demonstrate that the collision between the Western and Eastern Blocks occurred in the Paleoproterozoic. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) zircon uranium–lead (U–Pb) analyses yielded consistent results. The Early Paleoproterozoic (the first period) age of 2346 ± 28 Ma was determined for granitic gneiss in the Taihua Complex and 2328 ± 14 Ma for the Xiaohe granite pluton that intruded into the granitic gneiss. The gneissose pegmatitic granite and granitic veins that intruded into the granitic gneiss yielded LA-ICP-MS zircon ages of 1881 ± 24 and 1866 ± 19 Ma, respectively. These Paleoproterozoic ages strongly indicate that the ocean between the Eastern and Western Blocks were completely subducted at ∼1.85 Ga. The ɛHf(t) values of the zircons from the granitic gneiss can be divided into a positive group (0.44 to 3.77) and a negative group (−3.11 to −1.27). The two-stage hafnium (Hf) model ages (TDM2(Hf)) corresponding to the positive values are between 2672 and 2887 Ma. The ɛHf(t) values of the zircons from the Xiaohe granite range from −2.49 to 2.58, with the Hf isotope model ages ranging from 2742 to 3022 Ma. Thus, the Xiaohe granite could have resulted from the partial melting of the Late Archean lower crust caused by subduction and the presence of juvenile mantle material. The primary magma of the granitic gneiss of the Paleoproterozoic Taihua Complex was derived from the differentiation of the depleted Late Archean mantle source with a small portion derived from the remelting of the subducted lower crust.
The40Ar-39Ar age method is employed in this work to analyze the auriferous altered rocks from the middle-deep section of the Q875 gold-quartz
vein in the Xiaoqinling area, and the results show that the main gold deposition of the Q875 occurred in ca. 128–126 Ma. As
a typical gold-bearing quartz vein in this gold-rich area, the age data obtained from the Q875 also constrain on the metallogenic
time of the lode gold deposits developed in the same geological settings. This geochronological study supplies new evidence
for further understanding the timing of gold mineralization, the genesis of gold deposits and the geodynamic settings in Xiaoqinling
area.
The Tanami region of northern Australia has emerged over the last two decades as the largest gold-producing region in the Northern Territory. Gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks, and by sulfide-rich replacement zones within iron formation. Although limited, geochronological data suggest that most mineralization occurred at about 1,805–1,790 Ma, during a period of extensive granite intrusion, although structural relationships suggest that some deposits predate this period. There are three main goldfields in the Tanami region: the Dead Bullock Soak goldfield, which hosts the world-class Callie deposit; The Granites goldfield; and the Tanami goldfield. In the Dead Bullock Soak goldfield, deposits are hosted by carbonaceous siltstone and iron formation where a late (D5) structural corridor intersects an early F1 anticlinorium. In The Granites goldfield, deposits are hosted by highly sheared iron formation and are interpreted to predate D5. The Tanami goldfield consists of a large number of small, mostly basalt-hosted deposits that probably formed at a high structural level during D5. The D5 structures that host most deposits formed in a convergent structural regime with σ
1 oriented between E–W and ENE–WSW. Structures active during D5 include NE-trending oblique thrust (dextral) faults and ESE-trending (sinistral) faults that curve into N- to NNW-trending reverse faults localized in supracrustal belts between and around granite complexes. Granite intrusions also locally perturbed the stress field, possibly localizing structures and deposits. Forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid-crust. In areas characterized by the N- to NW-trending faults, orebodies also tend to be N- to NW-trending, localized in dilational jogs or in fractured, competent rock units. In areas characterized by ESE-trending faults, the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during E–W- to ENE–WSW-directed transpression. Many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation. Ore deposition occurred at depths ranging from 1.5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430°C, similar to lode–gold fluids elsewhere in the world. These fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow, although it is also possible that the fluids were derived from coeval granites. Lead isotope data suggest that lead in the ore fluids had multiple sources. Hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids. Gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks. Fluid unmixing is caused by three different processes: (1) CO2 unmixing caused by interaction of ore fluids with carbonaceous siltstone; (2) depressurization caused by pressure cycling in shear zones; and (3) boiling as ore fluids move to shallow levels. Deposits in the Tanami region may illustrate the continuum model of lode–gold deposition suggested by Groves (Mineralium Deposita 28:366–374, 1993) for Archean districts.
Research of Sr, Nd, Pb isotope compositions of the meso-basic dykes in the Xiaoqinling and Xiong' ershan area in the southern edge of North China Craton, shows that (87Sr/86Sr)i and (143Nd/ 144Nd)i of the dykes is 0. 712310-0. 735100 and 0. 511160-0. 512066, respectively, with an average value of 0. 722117 and 0. 511436 separately. The dykes have εSr(t)and εNd(t)value in 113. 06-436. 61 and -7. 9-25. 57 in the time scale of 130Ma calculated. Sr, Nd isotope compositon show the characteristics of extremely enriched magma sources. 206Pb/207Pb, 207 Pb/204 Pb, 208Pb/204Pb of the dykes vary betweem 16. 1921-19. 8149, 15. 3245 -15. 7540, and 36. 8607-41. 6251,respectively, with averaging 17. 7067, 15. 6638 and 38. 9494, indicating a distinct radiogentic lead feature. Comparsion of dykes with regional Taihua Formation and Xiong' er strata as well as Mesozoic granite shows that the source of dyke magma is of mixing features of crustal and deep sea sediments, and magatic material for dykes is of affinity to the Yangtze Plate. So it is proved that the Qinling orogenic belt stands for a geodynamic evolution process in which the Yangtze Plate subducted towards the southern margin of North China Craton. Combined with deep geophysical information in the Qinling orogen, the authors think that activation of the Pacific tectonic domain is a fundamental factor for tectonic regime transition from intraplate collision to lithospheric extension in the Qinling orogenic belt in late Mesizoic, thus resulting in the emplacement of large scale granite and meso-basic dikes in the Xiaoqinling and Xiongershan mountains area.
One geomagnetic pole and two virtual geomagnetic poles (VGPs) are now available from U-Pb dated dyke swarms/individual dykes from the Eastern Block, Central Zone and Western Block of the North China craton. The precise baddeleyite U-Pb age of Hengshan GU dyke in the Central Zone is 1769 ± 3 Ma and is part of the Taihang dyke swarm. This swarm has yielded a paleomagnetic pole position at Plat =36°N, Plong =247°E(dp =2°, dm =4°). A dyke at location DY in the Eastern Block of the North China craton yields a baddeleyite U-Pb age of a baddeleyite 1620. 8 ± 6. 9Ma, and a VGP at Plat =6. 9°N, Plong = 81. 9°E (dp =4. 31°, dm = 6.09°). A nearby dyke at site LW4 gives a zircon SHRIMP U-Pb age of 1157 ± 18Ma, and a VGP at Plat =27. 1°N, Plong = 162°E(dp = 15. 8°, dm = 18. 7°). Although secular variation has not been removed from the Shandong data, comparison of these three paleomagnetic pole positions of the North China craton with the Mesoproterozoic APWP of the Canadian Shield allows a continental reconstruction whereby the North China craton joined the supercontinent Columbia with the Canadian Shield from 1800 to 1600Ma, and the North China craton separated from the Canadian Shield sometime between 1600 to 1200Ma in response to the final break-up of the supercontinent Columbia.
Lying in the area of Lingbao county and Tongguan county, the Xiaoqinling goldfield is of gold-bearing quartz vein type occurring in the Archaean Taihua group; >1000 gold-bearing quartz veins exist which can be classified into the pyrite type and the polymetallic type. The gold mineralization during the hydrothermal activities may be divided into four stages: pyrite-quartz (I), quartz-pyrite (II), polymetallic sulphides (III) and carbonate-quartz (IV), stages II and III playing the major role in mineralization. The formation T of minerals ranges from 303 to 155oC. Gold occurs mainly as native gold having relative purity 900-960. It is suggested that the Xiaoqinling gold deposit was formed during the late Yanshanian period. An isotopic study of the gold ore vein yields an average delta D of 45.54per mille and delta 18O values from 8per mille to 11per mille, showing the characteristics of relatively normal magmatic water. The narrow variation of delta 34S values together with the behaviour of sulphur similar to that of meteoritic sulphur, implies that the sulphur was largely derived from the magma. An examination of minor elements in pyrite and quartz also demonstrates the dominant role of magmatic hydrothermal fluids. It is therefore considered that the Xiaoqinling gold deposit is genetically of the medium- low-T magmatic hydrothermal type.-J.M.H. Bureau of Geology & Mineral Resources of Henan Province, Zhengzhou, Henan, China.
The Baofeng basin is one of the most important volcanic basins in the northern part of the East Qinling, where there mainly occur volcanic rocks of the Early Cretaceous Daying Formation. The lavas consist predominantly of trachyandesite and basaltic trachyandesite, which is geochemically equivalent to shonshointic rocks. They are characterized by higher Na2 O + K2 O (8.04%-9.37%) and enrichment in large ion lithopbile elements (LILE) and light rare earth elements (LREE) and depletion in high field strength elements (HSFE). Initial Nd-Sr isotopic compositions are (143Nd /144Nd)i =0.5117-0.5118 and (87Sr/ 86Sr)i = 0.7067-0.7079. On the basis of summarizing the geological and geochemical characteristics of volcanic rocks, precise dating was performed of the trachyandesite and basaltic trachyandesite of the Daying Formation by using the SHRIMP U-Pb zircon dating technique. The results indicate that: zircon grains in basaltic trachyandesite are fine in size and scarce; the cathodoluminescence images show no distinct zoning structure; the SHRIMP concordia ages at two analyzed spots of the zircon grains are 148.5-132.4Ma, which probably represent the ages of relict zircons. However, there exist a large number of zircons in trachyandesite and the cathodoluminescence images of most zircons show distinct zoning structure, implying that they are magmatic zircons. The weighted average value of the SHRIMP concordia ages at 14 analyzed spots of the zircon grains is 117 ± 2Ma (MSWD = 1.3). These age data combined with the regional data indicate that volcanic rocks in the Baofeng basin in the East Qinling formed during the Early Cretaceous (117 ± 2Ma), synchronously with the formation of the A-type Donggou granite porphyry and Taishanmiao granite batholith, as well as the newly found Donggou and Tangjiaping porphyry Mo deposits. There occurred subducted components in the deep process of intracontinental orogeny in the East Qinling orogenic belt and its adjacent regions. The orogenic process terminated in the late Early Cretaceous, and lithospheric extension contributed to lower crustal delamination. All these provide important information for the understanding of the tectonic evolution of the Qiling-Dabie orogenic belt.
Tarmoola is a structurally controlled Archean orogenic gold deposit hosted in greenschist facies metamorphosed komatiite and trondhjemite in the Leonora district of the Eastern Goldfields province, Yilgarn craton. High-grade (>1 g/t Au) orebodies are located in komatiite wall rock adjacent to the eastern and northeastern margins of the asymmetrical, north-south-striking, Tarmoola trondhjemite intrusion. Gold-bearing veins postdate trondhjemite emplacement (ca. 2700 Ma), quartz diorite dikes (ca. 2667 Ma), and regional greenschist facies metamorphism. Textures and crosscutting relationships in gold-bearing veins indicate two stages of hydrothermal fluid infiltration associated with a single gold-related hydrothermal event: a volumetrically dominant, but gold-poor, stage I fluid and a gold-rich stage H fluid. Gold-bearing veins contain stage I milky quartz and pyrite that are overprinted by stage II quartz-ankerite-muscovite-chalcopyrite-sphalerite-galena-gold-tellurides +/- albite +/- chlorite +/- fuchsite +/- epidote +/- scheelite. Stage I hydrothermal alteration assemblages are different in trondhjemite and komatiite due to contrasting reactions between a common ore fluid and disparate wall-rock Chemistry. Stage II fluid-wall rock interaction was minor compared to stage I and is indicated by the overprinting of stage I mineral assemblages by stage II microveins. Wall-rock alteration proximal to veins in trondhjemite is characterized by replacement of igneous plagioclase, amphibole, biotite, and metamorphic chlorite by hydrothermal quartz, muscovite, ankerite, calcite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold, whereas in proximal alteration in komatiite, metamorphic chlorite and talc are replaced by ankerite, quartz, muscovite, albite, chlorite, fuchsite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold. The stage II fluid was enriched in H2O, CO2, Si, Ca, K, Na, S, Au, Ag, Cu, Pb, W, Bi, As, Mo, Zn, and Te. Based on fluid inclusion studies and stage II mineral equilibria, gold deposited from a homogeneous, neutral to slightly alkaline (pH 5.1-5.5), reduced, low-salinity (<5.5 wt % NaCl equiv) fluid that had a bulk composition of 78 mole percent H2O and 21 mole percent CO2, and trace amounts of CH4, C2H6, H-2, Ar, H2S, and He. Gold deposition occurred at 300degrees +/- 50degreesC and 0.5 to 3.0 kbars. Assuming lithostatic-fluid pressures, gold precipitated at a 2- to 10-km depth. Stage II gray quartz delta(18)O(fluid) values range from 5.9 to 7.5 per mil, whereas deltaD(fluid) values calculated from the dehydration of muscovite grains and measured directly from bulk fluid inclusion analyses of stage II gray quartz have ranges of -9 to -35 and -27 to -28 per mil, respectively. Hydrothermal ore fluids were transported from greater crustal depths to the site of gold deposition during the district-scale D, event by shallowly W dipping, reverse brittle-ductile shear zones in supracrustal rock and along the steeply east dipping trondhjemite contact. Associated subhorizontal east-west shortening caused the reactivation of the eastern trondhjemite margin and subparallel foliation, which facilitated the transport of hydrothermal fluids and the generation of gold-bearing veins and hydrothermal alteration zones in komatiite. East-west-striking fractures in trondhjemite aided the lateral migration of ore fluids away from trondhjemite margins and the formation of east-west-striking gold-bearing veins and broad alteration zones. Gold was most likely transported in the stage II fluid as bisulfide complexes. The sulfidation of trondhjemite and komatiite wall rock by the stage II fluid caused the destabilization of An bisulfide complexes and gold deposition. Potassium, Ca, and CO2 metasomatism of komatiite wall rock may have enhanced gold deposition via the acidification of the stage II fluid. The physicochemical characteristics of the Tarmoola ore fluid and relative timing of gold mineralization are consistent with the Yilgarn-wide, 2650 to 2630 Ma gold event. The host-rock chemistry, vein, and proximal wall-rock alteration mineral assemblages, and fluid temperature and pressures during gold deposition are also similar to that of earlier (ca. 2755 Ma) gold deposits in the Leonora district.
There has been an effort in the recent years to reverse the damage in stream ecosystems produced by the transformation of the natural landscapes into agricultural and urban land uses. Although many studies address hydrologic, hydraulic, and water quality changes in streams associated with land use change, there is still a need to relate land use change to shifts in species to identify specific causes of ecosystem changes observed. Starting in the late 1800's the landscape in Champaign County, east central Illinois, has been modified from a natural wet prairie to productive agricultural land. And modification of the streams has included channelization and dredging. This paper will assess the ecological consequence of land use and channel change by analyzing change in fish communities. The interpretation of community change will use an autecological approach, which assists in identifying relationships between habitat change and fish habitat requirements, and therefore support watershed management and restoration design.
The Rb-Sr isotopic dating of pyrite mineral from ore deposits can directly provide mineralization age. However, many geological
factors may affect the Rb-Sr isotopic system, which baffles application of this method. Employing ultra-low procedural blank
Rb-Sr method, we have dated pyrites separated from the No. 4 breccia pipe of the Qiyugou gold deposit, western Henan Province.
Single grains of euhedral pyrite crystal with few micro-cracks yield an isochron age of 126 ± 11 Ma, which represents time
of the main mineralization stage of the deposit. Pyrite grains of cataclastic type show nevertheless scattered Rb-Sr isotopic
composition and no reasonable isochron can be defined. Crystal morphology and mineral inclusion studies reveal that Rb and
Sr of pyrite mineral probably are preserved mainly in biotite, K-feldspar, and sericite mineral inclusions. The dating results
likely suggest that cataclastic pyrite is not suitable for the Rb-Sr dating due to modification of the Rb-Sr isotopic system
by later hydrothermal activity of fluid.
The Muruntau gold deposit in the Kyzylkum desert of Uzbekistan is the largest single deposit (⪢ 1100 tonnes of gold) of the class of low-sulfide syndeformation/synigenous gold deposits formed in the brittle/ductile transition zone of the crust within transpressional shear zones. Hosted by the Cambrian to Ordovician Besopan Suite, the ores were deposited in pre-existing thrust-fault- and metamorphism-related permeabilities and in synmineralization dilational zones created in a large fault-related fold. The Besopan Suite is a 5,000-m-thick sequence of turbiditic siltstones, shales and sandstones. The ore is primarily localized at the base of the Besopan-3 unit, which is a 2,000-m-thick series of carbonaceous shales, siltstones, sandstones and cherts. Initial gold deposition took place within the Sangruntau-Tamdytau shear zone, which was developed along the stratigraphic contact between the Besopan-3 and Besopan-4 units. During the mineralization process, folding of the Besopan Suite and a left-step adjustment in the Sangruntau-Tamdytau shear zone were caused by two concurrent events: (1) the activation of the left-lateral Muruntau-Daugyztau shear zone that developed at nearly a 90° angle to the preceding shear zone and (2) the intrusion of granitoid plutons. These structural events also resulted in the refocusing of hydrothermal fluid flow into new zones of permeability.
The largest gabbroic intrusion, Jiaoziyan, in the Northern Orthogneiss
of the Dabie Shan was investigated to test whether such intrusions
represent arc-related or collision-related magma, as postulated in
recent regional tectonic models. The Jiaoziyan intrusions comprise a
gabbro-monzogabbro suite and are locally characterized by cumulus
textured anhydrous silicates. Petrologic indications against arc
magmatism include (1) coexisting olivine-plagioclase compositions beyond
the documented range of arc gabbros, and (2) a compositional trend of
Al/Ti ratios in the gabbroic clinopyroxenes that is significantly
different from subduction-related gabbros worldwide. In contrast, the
Jiaoziyan rocks are mineralogically similar to rift gabbros.
Geochronologic and geometric arguments against collision-related
magmatism (i.e., as a consequence of slab breakoff) include the
following: (1) crystallization of the Jiaoziyan mafic intrusion
postdated the Triassic (245 240 Ma) subduction and peak orogenic
metamorphism by ˜100 m.y. and (2) the rocks of the Dabie
Cretaceous magmatic suite, including Jiaoziyan, are on the footwall
plate, rather than on the hanging wall, of the Triassic subduction-zone
framework and do not show a linear distribution. These observations
argue that the voluminous Early Cretaceous magmatism in the Northern
Orthogneiss is geodynamically unrelated to the Qinling-Dabie orogeny,
but was caused by extensional tectonism related to regional magmatic
underplating beneath the thinned Dabie crust.
At the Canadian Arrow deposit, Matheson, separate base- and precious-metal-bearing fracture systems are present in a trondhjemite stock transected by deformed lamprophyre dykes. Peripheral to the stock, high-Fe tholeiitic pillow basalts, with interbedded felsic units, experienced amphibolite-facies hydrothermal or regional metamorphism and intense ductile deformation focussed within mylonite zones at the interflow horizons. The trondhjemite stock experienced pervasive reaction with sea-water, accompanied by conversion of plagioclase to albite, concomitant enrichment of Na, and shifts of albite up to delta 18O = 14, with attendant, variably negative, quartz-albite fractionations. Galena-dominated veins were precipitated from hypersaline CaCl2-NaCl hydrothermal fluids of evolved marine origin. Later, Au-bearing quartz veins formed during hydraulic fracturing. Alteration in vein selvages involved the reaction of albite to K-feldspar, with gains of Si, K, Rb, Ba, CO2 and S, and loss of Fe, Mg and Na. The hydrothermal fluids implicated in this vein array are described. Lamprophyre dykes containing trondhjemitic xenoliths were injected along two subparallel fracture systems, showing the same geometry and orientation as the Au-bearing veins. These dykes are thought to be mantle derivatives. The conjunction of trondhjemitic and lamprophyric rocks with major structures and Au deposits is interpreted in terms of transcrustal fractures utilized as conduits for high-Na magmas from the base of the crust, for alkalic magmas from the mantle, and for discharge of hydrothermal fluids from a metamorphic or magmatic reservoir. Major- and trace-element analyses (11) of the Canadian Arrow Au deposit, abundances of major- and selected trace-elements in trondhjemites and lamprophyres, and isotopic compositions of whole-rocks and mineral separates, are presented.-P.Br.
We have measured the carbon and helium abundances and isotopic compositions of high-pressure carbon dioxide fluid inclusions in ultramafic xenoliths from oceanic hotspot volcanos to examine the extent to which He and C are separated by igneous processes, and to determine whether or not the ‘undegassed’ isotopic character of hotspot helium extends to carbon. These measurements place limits on upper mantle C contents and the on the fate of carbon recycled at subduction zones.Xenolith fluid inclusions from the Loihi-Hawaii, Re´union and Kerguelen hotspots exhibit carbon isotopic compositions (δ13C= −1.6to−10.8‰) similar to, although somewhat more variable, than MORB. The greater larger range can be ascribed to physicochemical processes associated with the volcanic systems and there is no evidence that hotspots and ridges tap isotopically distinct carbon sources. CO2 and He abundances vary by more than three orders of magnitude (1–260 ppm C and 10−9–10−6 cc STP He/g) and are strongly correlated. The samples' C/3He ratios (2–20 × 109) largely overlap with mid-ocean ridge basalt values (1–7 × 109). The small overall scatter of C/3He ratios argues against residual carbon phases during melting and against large-scale diffusive He transport in the mantle. In contrast to these similarities, most hotspot xenoliths and basalts with high3He/4He ratios have higher C/4He ratios (4–40 × 104, uncorrelated with C or He abundance) than mid-ocean ridge basalts (0.5–7 × 104). Given the similarity in C/3He ratios, the lower C/4He ratios of MORB with respect to hotspot sources must be produced by radiogenic4He production.The measured C/4He ratios suggest that the upper mantle carbon content must be less than 500 ppm C and is probably in the range 50–250 ppm C. The upper limit relies only on the observed MORB C/4He ratios, a maximum mantle U concentration of 26 ppb, and the conclusion from helium isotopic compositions that most upper mantle4He must be radiogenic rather than trapped primordial4He. The lower C range relies on the dynamics of the two-reservoir mantle model proposed by Kellog and Wasserburg [1] to explain ridge-hotspot He isotopic differences, and on the hypothesis that the upper (ridge source) and lower (hotspot source) mantle reservoirs had similar initial C/4He and3He/4He ratios. Within the framework of an initially homogeneous mantle which has differentiated into two reservoirs, significant carbon recycling to the upper mantle is inconsistent with the similarity of hotspot and ridge C/3He ratios. However, recycling of carbon to the lower mantle is consistent with C-He systematics and can account for both the similarity of ridge and hotspot carbon isotopic compositions and the low exospheric C inventory.