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Fluid Inclusions and CHOSPb Isotopes: Implications for the Genesis of the Zhuanshanzi Gold Deposit on the Northern Margin of the North China Craton: Fluid inclusions and isotope geochemistry
Abstract
The Zhuanshanzi gold deposit lies in the eastern section of the Xingmeng orogenic belt and the northern section of the Chifeng‐Chaoyang gold belt. The gold veins are strictly controlled by a NW‐oriented shear fault zone. Quartz veins and altered tectonic rock‐type gold veins are the main vein types. The deposits can be divided into four mineralization stages, and the second and third metallogenic stages are the main metallogenic stages. In this paper, based on the detailed field geological surveys, an analysis of the orebody and ore characteristics, microtemperature measurement of fluid inclusions, the Laser Raman spectrum of the inclusions, determination of CHOSPb isotopic geochemical characteristics, and so on were carried out to explore the origin of the ore‐forming fluids, ore‐forming materials, and the genesis of the deposits. The results show that the fluid inclusions can be divided into four types: type I – gas–liquid two‐phase inclusions; type II – gas‐rich inclusions; type III– liquid inclusions; and type IV – CO2‐containing three‐phase inclusions. However, they are dominated by type Ib – gas liquid inclusions and type IV – three‐phase inclusions containing CO2. The gas compositions are mainly H2O and CO2, indicating that the metallogenic system is a CO2H2ONaCl system. The homogenization temperature of the ore‐forming fluid evolved from a middle temperature to a low temperature, and the temperature of the fluid was further reduced due to meteoric water mixing during the late stage, as well as a lack of CO2 components, and eventually evolved into a simple NaClH2O hydrothermal system. CHOSPb isotope research proved that the ore‐forming fluids are mainly magmatic water during the early stage, with abundant meteoric water mixed in during the late stage. Ore‐forming materials originated mostly from hypomagma and were possibly influenced by the surrounding rocks, suggesting that the ore‐forming materials were mainly magmatic hydrothermal deposits, with a small amount of crustal component. The fluid immiscibility and the CO2 and CH4 gases in the fluids played an active and important role in the precipitation and enrichment of Au during different metallogenic stages. The deposit is considered a magmatic hydrothermal deposit of middle–low temperature.
... Ore Geology Reviews Sketch showing the location of the regional tectonics (a) (Sun et al, 2018), the geological map of the Chifeng-Chaoyang region (b) Sun et al, 2018), and the geological map of the Daxiyingzi deposit (c porphyry, and further inform about the mineralization process of the Daxiyingzi gold deposit. ...
... Ore Geology Reviews Sketch showing the location of the regional tectonics (a) (Sun et al, 2018), the geological map of the Chifeng-Chaoyang region (b) Sun et al, 2018), and the geological map of the Daxiyingzi deposit (c porphyry, and further inform about the mineralization process of the Daxiyingzi gold deposit. ...
... Therefore, the average observed δ 34 S values of the sulfide minerals can approximate the total δ 34 S values of the ore-forming fluids. The δ 34 S values of the Daxingyizi gold deposits are similar to those of the typical gold deposits in the Chifeng-Chaoyang gold ore concentration area, such as the Zhuanshanzi and Anjiayingzi gold deposits (Fu et al., 2016;Sun et al., 2018), indicating that the sulfur in the ore-forming fluids was derived from a magmatic source with negligible contamination/assimilation of country rock sulfur (Hoefs, 1997). In Fig. 15, the δ 34 S values of the ores are similar to the S isotopic compositions of arc basalt and andesite (Hoefs, 1997). ...
The Daxiyingzi gold deposit is located in the Chifeng-Chaoyang gold ore concentration area, toward the northern margin of the North China Craton. The ore bodies of the Daxiyingzi gold deposit are mainly hosted in the Manitu volcanic rocks, which are intruded on by the Mesozoic granite porphyry. The ore formation consists of four main stages: magnetite-hematite (Ⅰ), quartz-metal sulfide (II), gold-bearing polymetallic sulfide-quartz (III), and pyrite-carbonate (IV). The homogenization temperatures of the fluid inclusion range from 189.5 to 310.0 °C, while the salinity ranges from 0.53 to 13.55 wt.% NaCl equiv., corresponding to entrapment depths of 0.1 to 1.0 km below the paleowater table. The quartz from the stage II and III veins from Daxiyingzi have δDV-SMOW values range from −62.1 to −82.8 ‰, while the δ¹⁸OH2O values range from 0.46 to 1.26 ‰. The samples for galena, sphalerite, and pyrite from the Daxiyingzi veins have approximate S isotopic compositions, with δ³⁴S values ranging from 3.5 to 3.9‰, 3.9 to 4.2‰ and 3.5 to 5.0‰, respectively. The S-H-O isotopic data suggest that the ore-forming fluid had originated from magmatic sources. The zircon U-Pb ages of the volcanic rocks and granite porphyry are ca. 157–158 and 149 Ma, respectively, which suggests that the gold mineralization took place from 149 to 158 Ma. The volcanic rocks are belong to the high-K calc-alkaline to alkaline series, while the granite porphyry is belongs to A-type granite, which suggests an extensional tectonic setting. Our new data indicates that the Daxiyingzi gold deposit is a typical epithermal-mesothermal deposit formed by a Late Jurassic magmatic-hydrothermal system.
... 62,63 The region is rich in mineral resources and features various types of mineralization, such as epithermal gold, skarn copper, and porphyry copper−molybdenum deposits. Mainly include the Zhuanshanzi gold, Dahuanghutong and Xujiashuiquan copper, 62,63 Baituyingzi molybdenum, 64 and Yajishan and Baimashigou copper−molybdenum deposits. 65,66 Gold deposits represented by Zhanshanzi gold deposit primarily occur in the Permian Yujiabeigou Formation rhyolites and Indosinian granites, exhibiting strong geochemical anomalies of Au and Ag. ...
The focus of exploration geochemistry is an accurate interpretation of geochemical data and the precise extraction of anomaly information related to mineralization from complex geological information. However, geochemical data are component data and exhibit a closure effect. Thus, traditional statistical methods cannot adequately reveal and identify the distribution of deep-seated anomaly information. This paper focuses on the Sidaowanzi area in Inner Mongolia and uses multivariate component data analysis methods to process 1:50 000 soil geochemical data. Using the Exploratory Data Analysis (EDA) method, the spatial distribution and internal structure characteristics of raw, logarithmic, and isometric logarithmic ratio (ILR) transformed data were compared and, coupled with robust principal component analysis (RPCA) and elemental component biplots, the association between element combinations and mineralization indicated by these three types of data was revealed. The S-A method was used to decompose composite anomalies of the ILR transformed RPCA score data to extract the characteristics of elemental combination anomalies and background distribution, and the Fry analysis method was utilized to analyze the dominant mineralization direction within the area. The results show that (1) data transformed using the ILR eliminated the influence of the closure effect, making the data more uniform on a spatial scale and exhibiting characteristics of an approximately normal distribution. (2) The S-A method was further used to decompose the composite anomaly of the PC1 and PC2 principal component combinations. The screened-out anomaly and background fields can essentially reflect the ore-causing anomalies dominated by Au and Cu–Mo mineralization. Moreover, the extracted anomalies and background information closely align with known mineral deposits (prospects) and can effectively identify weakly retarded geochemical anomaly information. (3) Fry analysis based on geochemical anomalies indicates that the dominant mineralization directions, by an assemblage dominated by Au and Cu–Mo, predominantly occur in the NE, NW, and proximate EW orientations. The combined application of the aforementioned three methods for the quantitative analysis of geochemical data aims to explore a transferable methodological system, providing new insights and approaches for further prediction of mineralization potential.
The Lianhuashan Au deposit, located in the northern margin of North China Craton, is hosted in the metamorphic rocks of Neoarchean Wulashanyan Group along NNW- to N-trending faults, with pyrite as the predominant ore mineral. The mineralization process can be divided into four stages, involving stage I quartz-pyrite, stage II quartz-magnetite-chalcopyrite, stage III quartz-marcasite-pyrite, and stage IV quartz-calcite. Three types of fluid inclusions (FIs) have been identified, namely, pure carbonic inclusions (PC-type), aqueous-carbonic inclusions (AC-type), and aqueous inclusions (A-type). Stage I auriferous quartz veins host all three types of FIs, inclusions from stage II quartz are AC- and A-type, and only A-type FIs are recognized in stage III quartz and stage IV calcite veins. The FIs of stages I to IV were homogenized at temperatures of 300–360 °C, 275–326 °C, 252–289 °C, and 220–264 °C, with salinities of 4.0–11.7 wt%, 3.4–9.3 wt%, 2.1–7.0 wt%, and 1.4–3.1 wt% NaCl eqv., respectively. The ore-forming fluids belong to a medium-temperature, low-salinity H2O–NaCl–CO2 system. Gold deposition was likely caused by fluid immiscibility. The δDH2O and δ18OH2O values of ore fluids vary from −121.1 to −112.4‰ and from 2.8 to 6.2‰, respectively, within the range of enriched mantle-derived fluids in the North China Craton. The carbon isotope compositions of calcite (δ13CPDB = −2.87 to −2.38‰) show the features of mantle carbon. S-Pb isotope compositions of pyrite (δ34S = 2.58-3.96, 206Pb/204Pb = 16.077-16.119, 207Pb/204Pb = 15.329-15.332, and 208Pb/204Pb = 37.305-37.338) reveal that the ore-forming materials may have originated from crustal rocks. Based on the geological and geochemical observations, we suggest that the Lianhuashan deposit can be classified as an orogenic gold deposit.
The Honghuagou Au deposit is located in the Chifeng-Chaoyang region within the northern margin of the North China Craton. The auriferous quartz veins are mainly hosted in the mafic gneiss and migmatite of the Neoarchean Xiaotazigou Formation along NNW- and NE-striking faults, with pyrite as the predominant ore mineral. The gold mineralization process can be divided into two stages, involving stage I quartz-pyrite and stage II quartz-calcite-polymetallic sulfide. Three types of fluid inclusions (FIs) have been identified in the Honghuagou deposit, namely, carbonic inclusions, aqueous-carbonic inclusions, and aqueous inclusions. Quartz of stage I contains all types of FIs, whereas only aqueous inclusions are evident in stage II veins. The FIs of stages I and II yield homogenization temperatures of 275–340 °C and 240–290 °C with salinities of 3.4–10.7 wt% and 1.4–9.7 wt% NaCl eqv., respectively. The ore-forming fluids are characterized by medium temperature and low salinity, belonging to the H2O–NaCl–CO2 system. The δ¹⁸OH2O values of the ore fluids are between 2.1‰ and 5.9‰, within the range of enriched mantle-derived fluids in the North China Craton. The carbon isotope compositions of calcite (δ¹³CPDB = -4.4‰ to -4‰) are also similar to mantle carbon. He–Ar isotope data (³He/⁴He = 0.38-0.44 Ra; ⁴⁰Ar/³⁶Ar = 330-477) of fluid inclusions in pyrite indicate a mixed crustal and mantle source for the ore-forming fluids. Whereas, S-Pb isotope compositions of sulfides reveal that ore metals are principally derived from crustal rocks. On the basis of available geological and geochemical evidence, we suggest that the Honghuagou deposit is an orogenic gold deposit.
The leaf morphology and epidermal anatomy of a species of Pterophyllum, a morphological genus of Bennettitales, were studied in detail based on leaf fossil collected from the Middle Jurassic Yaojie Formation in the Baojishan Basin, Gansu Province, northwestern China. The fossil lamia is large; the segments are broadly linear to rectangular, opposite, each segment containing ca. 20 thin veins. The leaf cuticle is thin, hypostomatic, possessing of syndetocheilic stomatal complex. According to a detailed comparison on the basis of leaf morphology and cuticle features to other similar fossil plants, the fossil material was attributed to Pterophyllum propinquum Goeppert, which add to the details of the rachis, leaf cuticles of the species. Combined with the characteristics of paleoclimate change during the Mesozoic in China, the responses of the palaeogeographical distribution of Pterophyllum to paleoclimate change in different periods of Mesozoic in China are discussed. Moreover, based on the fossil floral assemblage and the cuticle features of P. propinquum, we presumed that it is a warm and humid climate in the Middle Jurassic in Baojishan basin, where the P. propinquum is a typical plant member.
Keywords: LA-ICP-MS Trace elements Pyrite Chang'an gold deposit Sanjiang Tethyan metallogenic domain The Chang'an gold deposit, Yunnan province is one of five large gold deposits in the Ailaoshan gold belt that is the most important ore belt of the Sanjiang Tethyan metallogenic domain. Geochemical study of pyrite in the deposit was conducted using laser ablation inductively coupled plasma mass spectroscopy. Three types of hydrothermal pyrite were identified in the ores and wall rocks, i.e., the coarse euhedral crystals in syenite (Py1), the coarse grains disseminated in altered sandstones or sandstone ores (Py2), and the fine-grained euhedral pyrite in sandstone ores (Py3). Their trace elements exhibit different concentrations, associations and rim-core zoning, implying different geneses and crystallization processes. The cores of Py1 were formed by magmatic fluids and have the lowest concentrations of Au, As, Cu and Zn. The cores of Py2 were formed by metamorphic fluids and have relatively high Au, Ag, Ni, Pb and Cu concentrations. The Py3 grains and the growth rims of Py1 and Py2 show consistently high contents of Au, As, Pb and Co, suggesting that all of them were rapidly deposited from a mixing fluid system that was probably composed of fluids of metamorphic and magmatic origins. This interpretation is supported by the observation that the high-grade ores generally contain lots of Py2 and Py3. Hence we consider that the deposit was formed during India-Asia collision, slightly postdate the 33-35 Ma tectono-magmatism.
Alteration mineral assemblages are important to the understanding of and exploration for hydrothermal ore deposits. Conventional mapping tools may not identify fine-grained minerals or define important compositional variations. Field portable short-wave infrared (SWIR) spectrometers solve some of these problems and provide a valuable tool for evaluating the distribution of alteration assemblages. Spectrometers such as the PIMA-II allow rapid identification of minerals and mineral-specific variations at a field base. Mineral assemblages, integrated with other exploration data, are then used to target drill holes and guide regional exploration programs. Data collection must be systematically organized and carried out by a trained operator. Analysis of data sets requires the use of spectral reference libraries from different geological environments and may be aided in some cases by computer data processing packages. Integration of results with field observations, petrography, and X-ray diffraction analysis is necessary for complete evaluation. The PIMA (portable infrared mineral analyzer) has been used successfully in the high-sulfidation epithermal, low-sulfidation epithermal, volcanogenic massive sulfide (VMS) and intrusion-related environments. Case studies from these systems demonstrate the ability to rapidly acquire and process SWIR data and produce drill logs and maps. The resulting information is critical for targeting.
Plumbotectonics is an attempt to model the geochemical behaviour of U, Th and Pb, among major terrestrial reservoirs in agreement with observational data. By recycling rock through the orogenic environment, a dynamically communicating upper crust, lower crust, and mantle can produce the required patterns of lead-isotope evolution.
Thermogenic hydrocarbons entirely deriving from the thermal degradation of organic matter usually exhibit methane to ethane plus propane ratios smaller than 100. We present hydrocarbon distribution data of continental hydrothermal gases, whose methane has been inde-pendently identifi ed to derive from the abiogenic reduction of CO 2 . We fi nd that excess amounts of methane with respect to thermogenic hydrocarbon distributions are characteristic for the investigated gases. A similar pattern is observed for well discharges whose temperatures are too high to support any microbially mediated methanogenesis. These fi ndings strongly suggest that abiogenic methane production in continental-hydrothermal systems is a more widespread process than previously assumed. The maximum contribution of such emissions to the modern atmospheric CH 4 budget is estimated at ~1%. INTRODUCTION The identifi cation of natural pathways leading to the formation of methane (CH 4) and higher hydrocarbons is of ultimate importance for reasons of energy exploitation, understanding the origin of life on Earth, modeling global atmospheric levels of CH 4 through time, and looking for indicators of extraterrestrial life (e.g., Zahnle, 1986; Catling et al., 2001; Etiope and Klusman, 2002; Formisano et al., 2004). The natural occurrence of biogenic (microbial and thermogenic) hydrocarbon production is well documented (e.g., Whiticar, 1999). Abiogenic formation of hydrocarbons from gaseous or dissolved CO 2 and CO is known to proceed under labora-tory conditions according to the Fischer-Tropsch-type (FTT) Reactions 1 and 2 (e.g., Foustoukos and Seyfried, 2004; McCollom and Seewald, 2007): CO + [2 + (m/2n)] H = (1/n) C H + 2H O
The St Ives Goldfield of Western Australia hosts high grade gold mineralization in a variety of lithologies within ∼2.7 Ga greenstone belts. The low salinity CO2-bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones, or from external magmatic sources, and independently sourced abiogenic-CH4 has been suggested as an important control on gold mineralization. In order to better constrain possible fluid sources, we applied a combination of He–Ne–Ar–Kr–Xe isotope and Cl–Br–I analyses to quartz and carbonate veins containing H2O–CO2 fluid inclusions, pyrite ± magnetite/hematite; and quartz veins containing CH4-rich fluid inclusions, pyrrhotite ± pyrite.
Salinities of H[sub 2]O-salt inclusions are most often determined by measuring the melting temperature of ice in the inclusion and then referring this value to an equation or table describing the relationship between salinity and freezing-point depression. Generally, data for the system H[sub 2]O-NaCl are used to determine an NaCl-equivalent salinity, owing to lack of information concerning the salts (or other electrolytes) actually contributing to the freezing-point depression. The equation most often used to determine the salinity of H[sub 2]O-salt inclusions from freezing measurements is that of Potter et al (1978), which is based on a regression of data available in the literature at that time. More recently, Hall et al (1988) experimentally redetermined the ice-melting temperatures of H[sub 2]O-NaCl-KCl solutions having compositions ranging from pure water to the ternary eutectic and to each of the two binary (H[sub 2]O-NaCl and H[sub 2]O-KCl) eutectics.
Basement rocks of Paleogene sedimentary basins form the present physiographic highs on the West Coast of South Island, New Zealand. Apatite and zircon fission‐track ages of these basement rocks allow interpretations to be made on the depths of Paleogene burial and the Miocene uplift history of the Victoria Range. Apparent apatite fission‐track ages increase generally with elevation from 10 to 37 Ma. The ages of the high‐altitude samples represent uplift of rocks that have been only partially annealed during burial, while the young samples (i.e., those between 10 and 16 Ma) have been buried sufficiently deeply to allow annealing of all pre‐existing tracks. A rapid uplift pulse at this time reflects a major phase of compressional tectonics. Zircon ages also exhibit a general altitudinal correlation, but are not fully reset, and hence indicate that burial was to depths of less than 8 km.
The northern margin of the North China craton is well-endowed with lode gold deposits hosting a resource of approximately 900 tonnes (t) of gold. The ~1,500-km-long region is characterized by east-trending blocks of metamorphosed Archean and Proterozoic strata that were episodically uplifted during Variscan, Indosinian, and Yanshanian deformational and magmatic events. At least 12 gold deposits from the Daqinshan, Yan-Liao (includes the Zhangjiakou, Yanshan, and Chifeng gold districts), and Changbaishan gold provinces contain resources of 20-100 t Au each. Most deposits are hosted in uplifted blocks of Precambrian metamorphic rocks, although felsic Paleozoic and Mesozoic plutons are typically proximal and host ~30% of the deposits. The lodes are characterized by sulfide-poor quartz veins in brittle structures with low base metal values and high Au:Ag ratios. Although phyllic alteration is most common, intensive alkali feldspar metasomatism characterizes the Wulashan, Dongping, and Zhongshangou deposits, but is apparently coeval with Variscan alkalic magmatism only at Wulashan. Stepwise 40Ar-39Ar geochronology on 16 samples from gangue and alteration phases, combined with unpublished SHRIMP U-Pb dates on associated granitoids, suggest that gold mineralizing events occurred during Variscan, Indosinian, and Yanshanian orogenies at circa 350, 250, 200, 180, 150, and 129 Ma. However, widespread Permo-Triassic (~250 Ma) and Early Jurassic (~180 Ma) thermal events caused variable resetting of most of the white mica and K-feldspar argon spectra, as well as previously reported K-Ar determinations. Compiled and new stable isotope and fluid inclusion data show that most ཎO values for ore-stage veins range from 8 to 14, indicating a fluid in equilibrium with the Precambrian metamorphic basement rocks; 'D values from fluid inclusions range widely from -64 to -154, which is indicative of a local meteoric component in some veins; and highly variable ཞS data (+7 to -17), even within individual deposits, indicate various local country-rock sources for sulfur. Fluid inclusions from all districts show variable homogenization temperatures between 240 and 400 C, and are consistently characterized by low salinity, H2O-CO2 - CH4, N2 solutions. Although the data are largely consistent with that from orogenic gold veins, intrusion-related veins and epithermal veins are also recognized. The multiple episodes of mineralization are coincident with episodic tectonic reactivations and associated magmatism along the northern margin of the North China craton. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00126-001-0239-2.
The Zhuanshanzi hornblende monzogranite is located in the the Mesozoic tectonic magmatic active belt which between the North China craton and the Xingmeng orogeny belt. Testing of major elements, rare-earth elements (REEs), trace elements and LA-ICP-MS zircon U-Pb dating of the hornblende monzogranite have be carried out to accurately determined the forming age and study the dynamics background of the granite,rock. Results show that the LA-ICP-MS zircon U-Pb age of the hornblende monzogranite is (245.8±3.1) Ma. (MSWD=3.7,n=17), and belongs to the Early Triassic. Geochemically, the granite is characterized by high concentrations of silica (w(SiO2)= 71.68%-72.85%), alkali (w(Na2O+K2O)= 8.98%-9.20%), and alumina (w (Al2O3)=13.45%-13.77%); low concentrations of calcium (w(CaO)=0.81%-0.99%); and with low values of TFeO/MgO (average is 6.55), indicating the hornblende monzogranite belongs to high-K, calc-alkaline high differentiation I type, weak aluminum granite. The granite is enriched in LREE and relatively depleted in HREE(LaN/YbN=21.94-24.33), and obvious Eu anomaly (δEu=0.66-0.68), which indicates that a large number of the plagioclase fractionation occured during the magmatic processes. In addition, the granite is enriched in Th, U, La and P, and depleted in Ba, Nb, Ti, Sr and Ta, and so on. In summary, the Zhuanshanzi granite is mainly formed by partial melting of crust material, with a few new mantle source composition. It is formed under the extensional tectonic setting in post-collision or post-orogenic stage after the North China plate and Siberia plate's eventual collision.
The SHRIMP U-Pb dating results on zircons from granitoid intrusions in Jinchanggouliang- Erdaogou gold orefield suggest that this region experienced at least three phases of magmatism during Mesozoic, which are represented by the emplacements of the Xitaizi monogranite, the Loushan pyroxene-bearing quartz diorite, and the Xiduomiangou granodiorite as well as dioritic dike at 218 +/- 4 Ma ( Indosinian), 161 +/- 1 Ma ( early Yanshanian) and 126 +/- 1 Ma ( late Yanshanian), respectively. The geochemical characteristics of these intrusions indicate that they were related to orogenesis, formed at intracontinental extensional periods of late- or post-orogenesis. The age of 126 +/- 1 Ma of the pre- or syn-gold mineralization dioritic dike from Erdaogou mine district constrains the maximum age of the gold mineralization in the area, which, in combination with previous geochronology data, indicates that gold mineralization in the area took place at between 126similar to118 Ma. This shows that the gold deposits in the district are essentially coeval with those of other gold-concentrating provinces in North China Craton, such as Jiaodong, Xiaoqinling and Liaodong, and further confirms that the late Yanshanian is the most important and extensive period of gold mineralization in north China. In addition, the similarities of the late timing of gold mineralization with respect to the latest felsic magmatism in these major gold-concentrating areas are probably indicative of that gold mineralization is likely a latest stage product of orogeneses or regional tectono-magmatism.
Most fluid inclusion trapped from a homogeneous fluid but some may trapped from a heterogeneous fluids (immiscibility process). In the nature, there are a lot of immiscible processes and systems, including immiscibility between basic magma and felsic magma, magma and hydrothermal fluids; magma and CO(2) fluid; saline fluids and CO(2) fluid etc. The fluid inclusions trapped from homogeneous and heterogeneous processes are with different characteristics and in somewhat is not easy to be distinguished. The immiscibility process is an important process of mineralization. Especially in the gold deposition, pegmatite formation and porphyry Cu-Mo system.
Pyrite is an ordinary mineral which appears in different types of deposits and it is an important gold-bearing mineral in gold deposits. Through studying the compositional typomorphic characteristics of pyrite, we have achieved much genesis information. This paper studied major and trace elements of pyrite of Jiaodong gold deposits by using δFe-δS, δFe/δS-As and (Fe+S)-As systems and achieved major elements characteristics of Jiaodong gold deposits. The results show that quartz-vein type, altered-rock type and interlayer slippage type of gold deposits in Jiaodong all have its typical characteristics for each distinct ore-forming environment. The result achieved in this paper proves that the compositional typomorphic method of pyrite is well applicable to each sub-type of gold deposits in Jiaodong, and is helpful for studying pyrite compositional typomorphism and for prospecting gold deposits.
The Dayingezhuang gold deposit, located in the central section of the Zhaoping Fault Zone, northwestern Jiaodong Peninsula, within the phyllic zone in the footwall of main fault. Wall-rock alteration is intense and has a variety of types that include potash feldspathization, silication, sericitization, pyritization, chloritization and carbonatation. Amongst these alterations, silication, sericitization and pyritization are considered more closely related with the gold mineralization. Based on the cutting relations of veins and paragenesis of ore minerals, we determined three major mineralization stages: gold-quartz-pyrite, gold (silver) -quartz-polymetallic sulfides, quartz-calcite-pyrite. Employing the decrepitation thermometry to the ore quartz, we identified three concentrated ranges: 330-510°C, 240-330°C and 240-330°C, which are corresponding with each of the mineralization stage above. The ore-forming fluids in the Dayingezhuang gold deposit are characterized by medium temperature and rich in CO2content, contain a small amount of volatile gases, such as CH4, C2H6and H2S. The scheme of NaCl-H2O-CO2and the high level content of C2H6existed in all of mineralization stages indicate that the ore-forming fluids mostly are the metamorphic water. Even though the gas-liquid content from different mineralization stages are similar with each other, we still observed certain rules along the ore-forming evolution. The trend of increasing N2content indicates that the ore-forming fluids system switch to an open system in the late stage, and atmospheric water began to take part in the ore-forming fluids. The high quantity of H2S in the early gold mineralization indicates that the gold possibly was migrated as Au-S complex. The climbing ratios of C1-/SO2-4and Na+/K+show that the ore-forming system convert from the CO2-H2O-K2SO4into CO2-H2O-NaCl system with the evolving process. The concentration of Na+, C1-, K+and SO2-4decreased from early stage to later stage indicates that the salinity of ore-fluid declined. The higher ratio of H2O/CO2in the later stage represents that the fluid boiling may occurred in the middle stage. Overall, we think that a variety of fluid processes, including change of fluid inclusion types and fluid boiling, have been suggested for gold precipitation.
Sanjia gold deposit is a typical lode gold deposit in the Muping-Rushan gold belt of Jiaodong Peninsula, eastern China. Gold occurs mainly in pyrite- and polymetallic sulfide-quartz vein/veinlet stockworks. Fluid inclusions in the altered wallrocks and gold ores show C02 -H2O-NaCl fluids of three main types, namely H2 0-C02, C02 -rich and aqueous inclusion. Early milk white quartz contains original H 20-C02 inclusions. In the pyrite quartz vein and polymetallic-sulphide quartz vein, C02-rich inclusions occur in isolation or cluster with large range of H2O/CO2, whereas the aqueous inclusions occur as secondary trails in microfractures cutting earlier quartz grains. On occasion C02-rich inclusions with variable aqueous phase were co-occurrence within the same clusters, and mostly homogenize to liquid or vapor at the same temperatures, indicating immiscibility did occur at the deposit. Quartz and calcite in the late ore-forming stage developed original aqueous inclusion. Microthermometric data show the homogenization temperatures of the C02-H20 inclusions in the early ore-stage fall in the range between 280 and 416°C. Homogenization temperatures, obtained mainly from C02-rich inclusions with relatively small C02 bubbles in the main ore-stages, range from 210 to 330°C Homogenization temperatures of H20 aqueous inclusions at same stage with C02-rich inclusions are between 253 and 377 °C , Homogenization temperatures (to liquid) of the aqueous inclusions in the late stage range from 176 to 207°C Fluid immiscibility caused by change of mineralizing stress possibly led to the gold precipitation at Sanjia.
The spatial distribution map of 65 mid-large gold-deposits hosted in the granite-greenstone terrains of the North China Craton is first drawn. These gold deposits mainly concentrate in the Mesozoic remobilized Yinshan-Yan-shan-Liaoning-Jilin intracontinental collisional orogenic belt, the northern Qinling and the Jiaodong Mesozoic collisional orogenic belts, and the Mesozoic intracontinental fault-magmatic belts developed along the Taihangshan and the Tan-Lu faults; their mineralizing time is predominantly Jurassic-Cretaceous, i. e. the Yanshanian. The metallogenic geody-namic background is exactly the compression-to-extension transition regime during continental collision.
Orogenic gold deposits of all ages, from Paleoarchean to Tertiary, show consistency in chemical composition. They are the products of aqueous-carbonic fluids, with typically 5–20 mol% CO2, although unmixing during extreme pressure fluctuation can lead to entrapment of much more CO2-rich fluid inclusions in some cases. Ore fluids are typically characterized by significant concentrations of CH4 and/or N2, common estimates of 0.01–0.36 mol% H2S, a near-neutral pH of 5.5, and salinities of 3–7 wt.% NaCl equiv., with Na > K > > Ca,Mg. This fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation. Nevertheless, there are broad ranges in more robust fluid-inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths.
The study of metallogenic systems includes two branches: the geological background and the system's interior structure. The geological background analysis is based on the dissection of the macroscopic environments of representative metallogenic belts or typical ore deposit concentration areas. It emphasizes the contributions of both the layered structure of the earth and the coupling processes of the transformation of tectonic regimes, magmatic activity and fluid influx to the activation of regional ore-forming fluid and the initiation of large-scale mineralization. The study of interior structure aims to clarify the temporal-spatial structure of the mineralized web, the physical mechanism of ore-forming fluid transportation and the chemical process of the enrichment-migration-precipitation of metallogenic elements. It considers the relationship between these elements by analyzing the tectonics-fluids-mineralization process in multi-scales, including orefield, ore deposit and ore body scale. Taking the Northwestern Jiaodong ore deposit concentration area as an example, and using a method of interior system analysis, the authors try to reveal the influence of the exterior geological environment of the ore-forming system on the metallogenesis of the interior ore deposit concentration area, and to establish the intrinsic relations of ore-forming factors (ore-forming products, ore-forming processes etc.). Results show that the transition of stress-strain field properties, created by the transformation of regional tectonic regimes, leads to regional ore-forming activity, which occurs in the temporal-spatial interfaces where the shearing stress alters from compression to tensile. The transformation also forms the complex and multivariate ore-controlled structural features. The differences of spatial structural features and mechanical properties result in the variety of regional metallogeny. Variations of stress-strain field properties in the ore deposit concentration area range cause the mutation of physicochemical parameters of ore-forming fluid, initiating the deposition of the metallogenic materials in the fluid. Over geological time, the fluid's Eh declined, and changed from an oxidation state to a deoxidation state. The fluid transport mode, which was under the control of different structural features, is the internal cause of metallogenic variety. The complexity of the ore-forming process mainly shows in the multi-fractal features of the spatial distribution of the gold ore bodies' grade and thickness, the self-similarity of alteration zoning, the inflation of ore-forming activity, and the variety of ore-forming products. This research is a new method of study in metallogeny and also an important approach to deepen the understanding of metallogenesis.
This essentially practical manual is designed to fill the gap between the theory and practice of studying fluid inclusions in minerals. All the major methods of examination and analysis are examined in detail and illustrated with practical applications. The correct techniques for the collection and preparation of material for optical examination are described.-R.A.H.
It is concluded that thiocomplexes of gold in which Au(I) is complexed by a sulphur donor ligand such as HS- are probably the dominant mechanism of gold transport in hydrothermal systems. In higher-T systems, >400oC, of high salinity and low activity of reduced sulphur simple chloro- and hydroxochloro-complexes may be important. Gold deposition in hydrothermal systems will be in response to changes in T, P, Eh, pH and decreasing activities of complexing ligands. In addition, amorphous As and Sb sulphide soils are very efficient in extracting gold from aqueous solutions, both at ambient and elevated T.-C.N.
Most fluid inclusion trapped from a homogeneous fluid but some may trapped from a heterogeneous fluids (immiscibility process). In the nature, there are a lot of immiscible processes and systems, including immiscibility between basic magma and felsic magma, magma and hydrothermal fluids; magma and CO2 fluid; saline fluids and CO2 fluid etc. The fluid inclusions trapped from homogeneous and heterogeneous processes are with different characteristics and in somewhat is not easy to be distinguished. The immiscibility process is an important process of mineralization. Especially in the gold deposition, pegmatite formation and porphyry Cu-Mo system.
The Yangshan gold deposit, Wenxian county, Gansu province, containing 308 t Au with average grade of 4.74g/t, is now ranked as the China’s largest gold deposit. Locating in western Qinling Orogen, central China, it is a syn-collisionally formed Carlin-like gold deposit. Its orebodies are controlled by an east-trending shear-zone and hosted in the Devonian carbonaceous carbonate-phyllite-slate sequence or the granite-porphyry dikes intruding into the Devonian strata. The 13CCO2 (PDB) and 13CCH4( PDB) ratios of fluid inclusions within quartz separates range -2.5 ~ -5.6‰and -23.1 ~ -32.6‰, respectively, suggesting that the ore-fluids have been mainly sourced, through metamorphism and/or reworking, from the Devonian strata or/and similar lithologies which comprise carbonaceous phyllite, slate, chert and carbonate. This conclusion is strongly supported by the 18O values of the early- and main-stage ore fluids, which range from 9.5‰ to 15.3‰, with corresponding D values between -86 ~ -73‰. Two 18O values of late ore-forming stage ore-fluids are 2.7‰ and 6.8‰, implying a significant input of meteoric water. In general, the ore-forming fluid-system varies from early, deep, metamorphic fluid to late, shallow, meteoric water. It is worthy of stating that the above results and understandings are consistent with conclusions drawn from our studies of fluid inclusions and ore geology.
Here we present the first study of pyrite textures observed in particulate matter from an active shallow submarine vent site, which is characterized by large spatial and temporal variations in the physico-chemical conditions caused by a combination of geochemical processes and microbial activity. Morphologic characterization of pyrite crystals from suspended particulate matter in the discharged fluid of a shallow submarine vent system near Punta Mita, Nayarit, Mexico, showed diverse crystal morphologies and aggregates; well-defined framboids are only observed within the deposits of fine-layered calcite (calcareous tuff) formed around the vents, whereas particulate matter contains diverse pyrite crystal forms as globular, cubic, octahedral, and pyritohedral but no framboids. Available sulfur isotopic data indicate that pyrite is formed as a consequence of microbial sulfate reduction in a hydrothermal reducing environment. The results of our study in a natural system provide evidence of the effect of variations in key parameters, such as redox state, on the pyrite morphologies and framboid size distribution, and support the conclusions of numerous studies that have attempted to explain pyrite crystallization processes that generate different morphologies as a result of large variations in the physico-chemical conditions.
There are several gold deposits in the eastern section of the regional Jiang−Shao Fault between the Yangtze and Cathaysia blocks in South China. Auriferous quartz veins in these deposits are strictly hosted in second-order NE–trending ductile shear zones. The ores generally contain low amounts of sulfide minerals (<5%), with pyrite as the most common sulfide mineral hosting native gold. Detailed fluid inclusion work and Rb–Sr dating were conducted on the auriferous quartz veins from the Pingshui and Huangshan deposits. H2O–CO2 inclusions (type I) and aqueous inclusions (type II) ubiquitously coexist in the main mineralization stage veins in the Huangshan and Pingshui deposits. Type I and II inclusions in the Huangshan deposit have similar homogenization temperatures at 214 to 282°C, but different salinities with 1.2 to 6.0 % and 2.7 to 8.7 % wt.% NaCl equivalent, respectively. In the gold orebodies from the Pingshui deposit, type I and II inclusions also have similar homogenization temperatures ranging from 236 to 304°C, but different salinities ranging from 1.2 to 6.4 % and from 3.2 to 9.8 % wt.% NaCl equivalent, respectively. Fluid inclusion observations and microthermometric results show that the ore fluids are low salinity and CO2-rich. Petrography and microthermometric results of fluid inclusions suggest that extensive fluid immiscibility occurred during the gold mineralization stage. Rb–Sr dating of quartz-hosted fluid inclusions (ca. 450 Ma) for the gold mineralization at Pingshui, combined with previous radiometric age data (ca. 397 Ma) of gold mineralization at Huangshan, suggest that the regional gold mineralization was formed in the Early Paleozoic. This study suggests that there is an Early Paleozoic orogenic gold belt in the eastern section of the Jiang−Shao Fault, formed in response to the coeval northward underthrusting of the Cathaysia Block beneath the Yangtze Block during the Caledonian Orogeny in South China.
Methane is widely developed in hydrothermal fluids from reduced porphyry copper deposits, but its origin remains enigmatic. The occurrence of methane in fluid inclusions at the Late Carboniferous Baogutu reduced porphyry copper deposit in western Junggar, Xinjiang, NW-China, presents an excellent opportunity to address this problem. A systematic study including fluid inclusion Laser-Raman and CO2–CH4 carbon isotope analyses, igneous and hydrothermal mineral H–O isotope analyses, and in situ major, trace element and Sr isotopic analyses of hydrothermal epidote was conducted to constrain the origin of CH4 and CH4-rich fluids. The δ2H and δ18O of water in equilibrium with igneous biotite ranges from −65.0‰ to −66.0‰ and +7.2‰ to +7.4‰, respectively, indicating notable degassing of probably supercritical fluids in the magma chamber. The wide range of δ2H (−58.0‰ to −107.0‰, n = 23) for water within quartz suggests the existence of significant hydrothermal fluid boiling. Water–rock interaction is the most likely mechanism leading to the wide range of δ18O values for water in vein quartz with water/rock ratios (wt.% in O) of 0.15 to 0.75 and 0.13 to 0.46 for a closed and open system, respectively. Detailed Laser-Raman analyses indicate CO2 in apatite included in granodiorite porphyry phenocrystic biotite that records the carbon species of the early stage magmatic stage, whereas later hydrothermal fluids containing CH4 with trace or without CO2 are found in inclusions of vein quartz. We propose that CH4 is probably transformed from CO2 by Fischer–Tropsch type reactions at 500 °C, assumed from CO2–CH4 C isotope equilibrium. The (87Sr/86Sr)i of hydrothermal epidote yields values of 0.70369–0.70404, consistent with that reported for the whole rocks. The δ13CCH4CCH4 (−28.6‰ to −22.6‰) and δ2HCH4HCH4 (−108.0‰ to −59.5‰) are characteristic of abiogenic methane. The measured δ13CCO2δ13CCO2 shows a slightly depleted 13C (−13.5‰ to −7.2‰) relative to upper mantle (−6‰), probably due to the combined effects of minor (less than 0.5%) sedimentary organic matter contamination in the mantle and carbon isotope fractionation occurring during late degassing.
Combining the results indicates that CO2 likely originated from the upper mantle with trace addition of sedimentary organic matter. During the uplift or emplacement of the granitoids, significant degassing caused the depletion of 13C and 2H. As the granitoids cooled, notable hydrothermal fluid boiling and water–rock interaction produced the depletion of 2H and 18O, respectively, and the magmatic CO2 was reduced to CH4 by Fischer–Tropsch type reactions that probably occurred during Ca–Na and potassic hydrothermal alteration.
The ubiquity of Au-bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of Au and As in this sulfide occurs under a wide range of physico-chemical conditions. Despite significant advances in the last 20 years, fundamental factors controlling Au and As ratios in pyrite from ore deposits remain poorly known. Here we explore these constraints using new and previously published EMPA, LA-ICP-MS, SIMS, and μ-PIXE analyses of As and Au in pyrite from Carlin-type Au, epithermal Au, porphyry Cu, Cu–Au, and orogenic Au deposits, volcanogenic massive sulfide (VHMS), Witwatersrand Au, iron oxide copper gold (IOCG), and coal deposits. Pyrite included in the data compilation formed under temperatures from ∼30 to ∼600 °C and in a wide variety of geological environments. The pyrite Au-As data form a wedge-shaped zone in compositional space, and the fact that most data points plot below the solid solubility limit defined by Reich et al. (2005) indicate that Au1+ is the dominant form of Au in arsenian pyrite and that Au-bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native Au. The analytical data also show that the solid solubility limit of Au in arsenian pyrite defined by an Au/As ratio of 0.02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal-chemical properties of pyrite and post-depositional alteration. Compilation of Au–As concentrations and formation temperatures for pyrite indicates that Au and As solubility in pyrite is retrograde; Au and As contents decrease as a function of increasing temperature from ∼200 to ∼500 °C. Based on these results, two major Au–As trends for Au-bearing arsenian pyrite from ore deposits are defined. One trend is formed by pyrites from Carlin-type and orogenic Au deposits where compositions are largely controlled by fluid-rock interactions and/or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids. The second trend consists of pyrites from porphyry Cu and epithermal Au deposits, which are characterised by compositions that preserve the Au/As signature of mineralizing magmatic-hydrothermal fluids, confirming the role of this sulfide in controlling metal ratios in ore systems.
VEIN-HOSTED gold deposits in low- to medium-grade metamorphic terrains are commonly associated with low-salinity hydrothermal fluids rich in CO2 and/or CH4. Fluid inclusion studies of gold mineralization indicate that the ore fluid comprised co-existing CO2/CH4-rich and H2O-rich phases, and that phase separation played an integral part in gold deposition. In hydrothermal solution gold is present as the Au(HS)2- complex. Precipitation of gold caused by decreasing ligand activity involving the formation of iron sulphides from wall-rock iron oxides and silicates is clearly relevant to gold deposits associated with iron formations and iron-rich igneous rock. It cannot, however, be used to explain the common association of gold deposits with black shales or schists where wall-rock iron is in the form of sulphides, and therefore generally in equilibrium with the hydrothermal fluid, or with granitoids or felsic volcanics, where the amount of iron is low. This latter association may be explained by the partitioning of H2S into the non-aqueous phase during fluid immiscibility, but the general applicability of this mechanism is not known. Here we present a new synthesis of experimental data from a variety of sources which puts this mechanism on a semi-quantitative basis, and suggest that it may be applicable to a wide variety of hydrothermal gold environments.
Oxygen isotopic compositions of quartz and feldspar in greenschist-grade mylonites from the Blue Ridge thrust and the Brevard zone in the southern Appalachians were analyzed by laser microprobe to examine the effect of deformation on isotopic behavior. In mylonites, texturally homogeneous polycrystalline quartz ribbons have a constant isotopic composition (δ18O = 12.9 ± 0.0‰, n = 3), whereas monocrystalline quartz ribbons, which display heterogeneous intercrystalline strain and only minor recrystallization, have variable δ18O values (11.6 ± 0.5‰, n= 5). Alkali feldspars in samples that contain fluid inclusion-decorated microcracks, reflecting heterogeneous deformation, show a range in isotopic composition (8.8 to 10.2; mean = 9.4 ± 0.7‰, n= 3). In contrast, recrystallized myrmekite rims surrounding alkali feldspar augen in Brevard zone mylonites are isotopically heavier by about 1% (9.2 ± 0.1‰, n = 5) compared to the cores 8.3 ± 0.3‰, n = 4), reflecting isotopic homogenization during neocrystallization. Deformation mechanisms that result in heterogeneous strain on the grain scale (either crystal plastic or brittle) are associated with only partial isotopic homogenization, whereas deformation mechanisms that result in homogeneous strain (e.g., recrystallization, neocrystallization) are associated with isotopic homogenization on the grain scale. Agreement between measured quartz-feldspar isotopic temperatures and calculated temperatures using a finite difference model indicates diffusional exchange occurred between phases during closed system cooling, and that the measured temperatures in the mylonites are maximum temperatures for the deformation. The approximate agreement between measured temperatures in some mylonites and the calculated Dodson quartz closure temperatures indicates that isotopic exchange below Tc quartz was not substantial. The necessary conditions under which isotopic temperatures in mylonites correspond to the deformation temperature are outlined. On the basis of this study and reconsideration of older data, the onset of total dynamic recrystallization in quartz is estimated to be about 350°C in natural shear zones. Together with reaction weakening of feldspar observed in the mylonites, the temperature interval 350-400°C is likely to be important for weakening of both quartz and feldspar in the continental crust.
LIMNOLOGY January 1977 AND Volume 22 Number OCEANOGRAPHY Microbial methane consumption reactions and their effect on methane distributions in freshwater and marine environment? William Institute S. Reeburgh and David T. Heggk of Marine Science, University of Alaska, Fairbanks Abstract A survey of reported methane distributions in sediments and the adjacent overlying water shows distinct differences between freshwater and marine environments. These differences may be explained by the activities of sulfate-reducing bacteria and appear to be the re- sult of differences in sulfate concentration between freshwater and marine environments. Measurements of methane in freshwater an d marine environments have been re- ported by many workers over the past 20 years. Table 1 summarizes the studies that we have consulted for the water columns and sediments of both environments. Physical mixing processes are similar in lake and marine sediments, so a compari- son of methane distributions is possible and should yield information on similari- ties and differences in chemical reactions occurring in these environments. Such a comparison is more difficult for the water column, as there are few marine circulation analogs of stratified lakes. Anoxic basins are the nearest analogs, so studies on the Black Sea, the Cariaco Trench, and Lake Nitinat are included in Table 1. A great deal of emphasis has been placed on improving the precision and accuracy of gas measurements in sediments and ex- plaining the formation of methane. Much less emphasis has been placed on methane consumption. No summary of the reported methane distributions has been attempted. Although an mlusually wide variety of sampling and analytical techniques have been used in the sediment studies (e.g. Reeburgh 1968; Barnes 1973; Martens 1974), distributions from either freshwater or marine environments show reasonable internal consistency. Distinct differences in the methane distributions from freshwater and marine environments are also evident, suggesting that some general process must be responsible. We will use here the re- sults from recent laboratory and field stud- ies of the requirements, activities, and dis- tributions of aerobic methane-oxidizing bacteria, as well as bacteria capable of an- aerobic sulfate reduction, methane produc- tion, and methane oxidation, to point out the locations of the above bacterial reac- tions and to provide a general explanation of the differences in the freshwater and ma- rine methane distributions. Data The results of the studies cited in Ta- ble 1 are too numerous to plot conveniently in a single figure, so representative exam- ples of sediment methane distributions in 1 This work was supported in part by Na- tional Science Foundation grants GA-19380 and GA-41209. Contribution 277, Institute of Marine Science, University of Alaska. LIMNOLOGY AND OCEANOGRAPHY JANUARY 1977, V. 22( 1)
Phanerozoic intrusion-related gold deposits have many consistent characteristics such as a spatial, temporal, and geochemical association with moderately reduced (predominantly ilmenite-series) 1-type intrusions. The deposits exhibit a range of characteristics that vary over a wide range of emplacement depths (<1->7 km). Deposits in shallow crustal settings (~<5 km) are associated with stocks, sills, dikes, and volcanic domes and include systems with epithermal-style veins to breccia and stockwork similar to porphyry-type settings. Deeper systems (~>5 km) have characteristics of mesothermal environments, and are hosted by plutons containing sheeted veins, greissen, and disseminated gold. Fluid characteristics also vary with depth. Deposits in shallow environments contain high-temperature (>350°C), immiscible brine (>30 wt % NaCl equiv) and low-salinity (<5 wt % NaCl equiv) vapor that commonly contains carbon dioxide. Deposits in deeper environments contain abundant low-salinity, carbon dioxide-rich aqueous fluids (<10 wt % NaCl equiv) that, in some deposits, are postdated by moderate- to high-salinity brines (10-40 wt % NaCl equiv). These contrasting fluid types are interpreted to be magmatic in origin and are the result of the complex interplay between exsolution of different volatiles (carbon dioxide, water, and chlorine) from felsic magmas emplaced at different crustal levels. Experimental studies have shown that carbon dioxide will exsolve from felsic magmas at much higher pressures than water and chlorine, due to its lower solubility in the melt. Thus, deposits formed from felsic magmas in deeper environments will contain abundant, carbon dioxide-rich fluids. Subsequent fluids released from the magma are likely to be water rich and more saline. Felsic magmas in shallow environments will contain lower concentrations of carbon dioxide that will partition into the vapor phase during separation between brine and vapor.
Equilibrium constants for oxygen isotope exchange between quartz and
water have been measured from 195°C (1000 ln α = 12.0) to
750°C (1000 ln α = 0.4). Over limited temperature ranges the
behavior of fractionation with temperature can be approximated by 1000
ln α = 3.38 (106 T-2) - 3.40 for
200°-500°C and by 1000 ln α = 2.51 (106
T-2) - 1.96 for 500°-750°C. The results of
measurements in the quartz-water system can be combined with analogous
results from other mineral systems to make mineral-pair isotopic
thermometers for application to problems of petrogenesis.
The middle and lower Yangtze River Valley and adjacent regions are the most important metallogenic belt of gold (and copper)-bearing skarn deposits in China. The total gold reserves in this belt have been estimated at more than 600 t. The gold-bearing skarns are mainly distributed in the southeastern Hubei, Tongling and northern Anhui regions. Favorable tectonic settings are depressions and fold zones of the platforms, i.e., mobile belts. These skarns are hosted by platformal limestone, dolomitic limestone and dolomite of the Triassic, Carboniferous-Permian and Middle to Lower Cambrian formations. The related intrusions are Yenshanian (180 to 113 Ma) calc-alkaline quartz monzodiorite, granodiorite, quartz monzonite, monzogabbro, and their hybabyssal facies. The intrusions have high Fe2O3/FeO (>0.5) and intermediate initial 87Sr/86Sr ratios (0.7046 to 0.7087). Their REE distribution patterns are LREE-enriched and exhibit smooth, right-dipping curves. These suggest that the source materials mainly came from upper mantle, with contamination by sialic crustal components. The auriferous skarns are both calcic and magnesian, but calcic skarns are most common. The constituent minerals of the calcic skarns are diopside, garnet, wollastonite, vesuvianite and scapolite, whereas magnesian skarns are dominated by forsterite, spinel, diopside, phlogopite, chondrodite and clinohumite, with abundant superimposed serpentine, clinochlore and brucite. The compositions of coexisting pyroxenes and garnets are diopside and andradite, indicating the high oxygen fugacity and low acidity conditions. Gold is closely associated with Cu (Pb, Zn) sulfides and exists mainly in the form of native gold and electrum. Arsenides, tellurides, bismuthides and selenides are present in many ore deposits. Therefore, Cu, As, Bi, Te, Ag, Pb, Zn, Se and Co are the major metals present in the deposits and are important geochemical ore-searching indicators. In some Au (Fe, Cu) magnesian skarns, magnesiomagnetite, magnesioferrite and ludwigite are locally abundant. The metasomatic zoning in many gold skarn deposits is very distinct consisting of an outward sequence of: Fe (Cu)→Cu (Mo)→Cu (Au)→Au (Cu)→Au (Pb, Zn). The geologic characteristics of Au (Cu) skarn deposits that formed in the mobile platformal setting of China have distinct differences compared to Au skarns formed in orogenic belts at convergent plate margins in British Columbia and the western USA.
Abstract Located in Alxa Zuoqi (Left Banner) of Inner Mongolia, China, the Zhulazhaga gold deposit is the first large-scale gold deposit that was found in the middle-upper Proterozoic strata along the north margin of the North China craton in recent years. It was discovered by the No. 1 Geophysical and Geochemical Exploration Party of Inner Mongolia as a result of prospecting a geochemical anomaly. By now, over 50 tonnes of gold has been defined, with an average Au grade of 4 g/t. The ore bodies occur in the first lithological unit of the Mesoproterozoic Zhulazhagamaodao Formation (MZF), which is composed mainly of epimetamorphic sandstone and siltstone and partly of volcanic rocks. With high concentration of gold, the first lithological unit of the MZF became the source bed for the late-stage ore formation. Controlled by the interstratal fracture zones, the ore bodies mostly appear along the bedding with occurrence similar to that of the strata. The primitive ore types are predominantly the altered rock type with minor ore belonging to the quartz veins type. There are also some oxidized ore near the surface. The metallic minerals are composed mainly of pyrite, pyrrhotite and arsenopyrite with minor chalcopyrite, galena and limonite. Most gold minerals appear as native gold and electrum. Hydrothermal alterations associated with the ore formation are actinolitization, silicatization, sulfidation and carbonation.
A total of 100 two-phase H2O-rich and 7 three-phase daughter crystal-bearing inclusions were measured in seven gold-bearing quartz samples from the Zhulazhaga gold deposit. The homogenization temperatures of the two-phase H2O-rich inclusions range from 155 to 401°C, with an average temperature of 284°C and bimodal distributions from 240 to 260°C and 300 to 320°C respectively. The salinities of the two-phase H2O-rich inclusions vary from 9.22wt% to 24.30wt% NaCl equiv, with a mode between 23wt% and 24wt% NaCl equiv. Comparatively, the homogenization temperatures of the three-phase daughter crystal-bearing inclusions vary from 210 to 435°C and the salinities from 29.13wt% to 32.62wt% NaCl equiv. It indicates that the ore-forming fluid is meso-hypothermal and characterized by high salinity, which is apparently different from the metamorphic origin with low salinity. It suggests a magmatic origin of the gold-bearing fluid.
The δ18O values of quartz from auriferous veins range from 11.9 to 16.3 per mil, and the calculated δ18O values in equilibrium with quartz vary from 1.06 to 9.60 per mil, which fall between the values of meteoric water and magmatic water. It reflects that the ore-forming fluid may be the product of mixing of meteoric water and magmatic water. Based on geological and geochemical studies of the Zhulazhaga gold deposit, it is supposed that the volcanism in the Mesoproterozoic might make gold pre-concentrate in the strata. The extensive and intensive Hercynian tectono-magmatic activity not only brought along a large number of ore-forming materials, but also made the gold from the strata rework. It can be concluded that the ore bodies were mainly formed in late hydrothermal reworking stage. Compared with typical gold deposits associated with epimetamorphic clastic rocks, the Zhulazhaga deposit has similar features in occurrence of ore bodies, ore-controlling structure, wall-rock alterations and mineral assemblages. Therefore, the Zhulazhaga gold deposit belongs to the epimetamorphic clastic rock type.
The Heilongjiang Complex is a sequence of high-pressure metamorphic rocks, located along the suture zone that separates the Jiamusi–Khanka (–Bureya) and Songliao–Zhangguangcai blocks in NE China (and extending northward into Far East Russia). The complex consists of mafic–ultramafic rocks, various quartzo–feldspathic schists and radiolarian-bearing quartzite (formerly chert). The rocks were metamorphosed up to epidote–blueschist facies, with P–T conditions of approximately T = 320–450 °C and P = 0.9–1.1 GPa. The lithological association and major and trace element compositions indicate that the blueschists were metabasalts of OIB and E-MORB affinity, most likely generated in a rift setting at the western margin of the Jiamusi Block that later underwent subduction. Magmatic zircons extracted from two samples of epidote–blueschist facies metabasalts from Mudanjiang have SHRIMP U–Pb 206Pb/238U ages of 213 ± 2 Ma and 224 ±7 Ma, whereas similar rocks ∼ 200 km farther north at Yilan have ages of 258 ± 2 Ma and 259 ± 4 Ma. These data define the protolith ages of the metabasalts as Late Triassic and Late Permian, respectively. These ages limit the timing of high-pressure metamorphism in the Heilongjiang Complex to post-Late Triassic, consistent with argon data reported from previous studies. Inherited zircon components in all four epidote–blueschist facies samples show distinct populations at 290–330 Ma, 420–530 Ma, 670–910 Ma and > 1065 Ma. Such ages are also a feature of the Central Asia Orogenic Belt (CAOB) to the west, supporting the view that the Jiamusi Block was most likely the rifted easternmost segment of the CAOB and not an exotic block derived from Gondwana. Final closure between the Jiamusi–Khanka–Bureya and Songliao blocks took place in the latest Triassic to Early Jurassic, with the two blocks accreted as a result of Pacific Ocean subduction. This suggests that the Heilongjiang Complex records the time when northward movement of the combined Mongolia–North China Block toward Siberia was waning and becoming surpassed by the onset of Pacific accretion from the east, which has dominated the tectonics of NE China and Far East Russia since the Early Jurassic.
There are six distinct classes of gold deposits, each represented by metallogenic provinces, having 100's to >1000 tonne gold
production. The deposit classes are: (1) orogenic gold; (2) Carlin and Carlin-like gold deposits; (3) epithermal gold-silver
deposits; (4) copper-gold porphyry deposits; (5) iron-oxide copper-gold deposits; and (6) gold-rich volcanic hosted massive
sulfide (VMS) to sedimentary exhalative (SEDEX) deposits. This classification is based on ore and alteration mineral assemblages;
ore and alteration metal budgets; ore fluid pressure(s) and compositions; crustal depth or depth ranges of formation; relationship
to structures and/or magmatic intrusions at a variety of scales; and relationship to the P-T-t evolution of the host terrane.
These classes reflect distinct geodynamic settings. Orogenic gold deposits are generated at mid-crustal (4–16 km) levels proximal
to terrane boundaries, in transpressional subduction-accretion complexes of Cordilleran style orogenic belts; other orogenic
gold provinces form inboard, by delamination of mantle lithosphere, or plume impingement. Carlin and Carlin-like gold deposits
develop at shallow crustal levels (<4 km) in extensional convergent margin continental arcs or back arcs; some provinces may
involve asthenosphere plume impingement on the base of the lithosphere. Epithermal gold and copper-gold porphyry deposits
are sited at shallow crustal levels in continental margin or intraoceanic arcs. Iron oxide copper-gold deposits form at mid
to shallow crustal levels; they are associated with extensional intracratonic anorogenic magmatism. Proterozoic examples are
sited at the transition from thick refractory Archean mantle lithosphere to thinner Proterozoic mantle lithosphere. Gold-rich
VMS deposits are hydrothermal accumulations on or near the seafloor in continental or intraoceanic back arcs.
The compressional tectonics of orogenic gold deposits is generated by terrane accretion; high heat flow stems from crustal
thickening, delamination of overthickened mantle lithosphere inducing advection of hot asthenosphere, or asthenosphere plume
impingement. Ore fluids advect at lithostatic pressures. The extensional settings of Carlin, epithermal, and copper-gold porphyry
deposits result from slab rollback driven by negative buoyancy of the subducting plate, and associated induced convection
in asthenosphere below the over-riding lithospheric plate. Extension thins the lithosphere, advecting asthenosphere heat,
promotes advection of mantle lithosphere and crustal magmas to shallow crustal levels, and enhances hydraulic conductivity.
Siting of some copper-gold porphyry deposits is controlled by arc parallel or orthogonal structures that in turn reflect deflections
or windows in the slab. Ore fluids in Carlin and epithermal deposits were at near hydrostatic pressures, with unconstrained
magmatic fluid input, whereas ore fluids generating porphyry copper-gold deposits were initially magmatic and lithostatic,
evolving to hydrostatic pressures. Fertilization of previously depleted sub-arc mantle lithosphere by fluids or melts from
the subducting plate, or incompatible element enriched asthenosphere plumes, is likely a factor in generation of these gold
deposits. Iron oxide copper-gold deposits involve prior fertilization of Archean mantle lithosphere by incompatible element
enriched asthenospheric plume liquids, and subsequent intracontinental anorogenic magmatism driven by decompressional extension
from far-field plate forces. Halogen rich mantle lithosphere and crustal magmas likely are the causative intrusions for the
deposits, with a deep crustal proximal to shallow crustal distal association. Gold-rich VMS deposits develop in extensional
geodynamic settings, where thinned lithosphere extension drives high heat flow and enhanced hydraulic conductivity, as for
epithermal deposits. Ore fluids induced hydrostatic convection of modified seawater, with unconstrained magmatic input. Some
gold-rich VMS deposits with an epithermal metal budget may be submarine counterparts of terrestrial epithermal gold deposits.
Real time analogs for all of these gold deposit classes are known in the geodynamic settings described, excepting iron oxide
copper-gold deposits.
The solubility of gold in aqueous sulphide solutions has been determined from pH20°C ≈ 4 to pH20°C ≈ 9.5 in the presence of a pyrite-pyrrhotite redox buffer at temperatures from 160 to 300°C and 1000 bar pressure. Maximum solubilities were obtained in the neutral region of pH as, for example, with mNaHS = 0.15 m, pH20°C = 5.96, T = 309°C, P = 1000 bar where a gold solubility of 225 mg/kg was obtained. It was concluded that three thio gold complexes contributed to the solubility. The complex Au2(HS)2S2− predominated in alkaline solution, the Au(HS)2− complex occurred in the neutral pH region, and in the acid pH region, it was concluded with less certainty that the Au(HS)° complex was present. Formation constants calculated forAu2(HS)2S2− and Au (HS)2− emphasize their high stability. In the temperature range from 175 to 250°C, values of pβ for Au2(HS)2S2− vary from −53.0 to 47.9 (±1.6) and from −23.1 to −19.5 ( ± 1.5) for Au(HS)2−. Equilibrium constante for the dissolution reactions, and 2Au° + H2S + 2H8− ⇋ Au(HS)2− + H2 vary from pKm = +2.4 to +2.55 (±0.10) for Au2(HS)2S2− and from pKn = + 1.29 to + 1.19 (±0.10) for Au(HS)2− over the temperature range 175 to 250°C. Enthalpies of these dissolution reactions were calculated to be ΔHm° = −5.2 ±2.0 kcal/mol and ΔHn° = +1.7 ±2.0 kcal/mol respectively. It was concluded that gold is probably transported in hydrothermal ore solutions as both thio and chloro complexes and may be deposited in response to changes in temperature, pressure, pH, oxidation potential of the system and total sulphur concentration.
The diagenetic cycling of carbon within recent unconsolidated sediments and soils generally can be followed more effectively by discerning changes in the dissolved constituents of the interstitial fluids, rather than by monitoring changes in the bulk or solid organic components. The major dissolved carbon species in diagenetic settings are represented by the two carbon redox end-members CH4 and CO2. Bacterial uptake by methanogens of either CO2 or “preformed” reduced carbon substrates such as acetate, methanol or methylated amines can be tracked with the aid of carbon () and hydrogen () isotopes. The bacterial reduction of CO2 to CH4 is associated with a kinetic isotope effect (KIE) for carbon which discriminates against . This leads to carbon isotope separation between CO2 and CH4 (εC) exceeding 95 and gives rise to values as negative as −110‰ vs. PDB. The carbon KIE associated with fermentation of methylated substrates is lower (εC is ca. 40 to 60, with values of −50‰ to −60‰). Hydrogen isotope effects during methanogenesis of methylated substrates can lead to deuterium depletions as large as vs. SMOW, whereas, bacterial D/H discrimination for the CO2-reduction pathway is significantly less (δDCH4 ca. −170‰ to −250‰). These field observations have been confirmed by culture experiments with labeled isotopes, although hydrogen isotope exchange and other factors may influence the hydrogen distributions. Bacterial consumption of CH4, both aerobic and anaerobic, is also associated with KIEs for C and H isotopes that enrich the residual CH4 in the heavier isotopes. Carbon fractionation factors related to CH4 oxidation are generally less than εC=10, although values >20 are known. The KIE for hydrogen (εH) during aerobic and anaerobic CH4 oxidation is between 95 and 285. The differences in C and H isotope ratios of CH4, in combination with the isotope ratios of the coexisting H2O and CO2 pairs, differentiate the various bacterial CH4 generation and consumption pathways, and elucidate the cycling of labile sedimentary carbon.
Although hydrocarbon-bearing fluids have been known from the alkaline igneous rocks of the Khibiny intrusion for many years, their origin remains enigmatic. A recently proposed model of post-magmatic hydrocarbon (HC) generation through Fischer-Tropsch (FT) type reactions suggests the hydration of Fe-bearing phases and release of H2 which reacts with magmatically derived CO2 to form CH4 and higher HCs. However, new petrographic, microthermometric, laser Raman, bulk gas and isotope data are presented and discussed in the context of previously published work in order to reassess models of HC generation. The gas phase is dominated by CH4 with only minor proportions of higher hydrocarbons. No remnants of the proposed primary CO2-rich fluid are found in the complex. The majority of the fluid inclusions are of secondary nature and trapped in healed microfractures. This indicates a high fluid flux after magma crystallisation. Entrapment conditions for fluid inclusions are 450–550 °C at 2.8–4.5 kbar. These temperatures are too high for hydrocarbon gas generation through the FT reaction. Chemical analyses of rims of Fe-rich phases suggest that they are not the result of alteration but instead represent changes in magma composition during crystallisation. Furthermore, there is no clear relationship between the presence of Fe-rich minerals and the abundance of fluid inclusion planes (FIPs) as reported elsewhere. δ13C values for methane range from − 22.4‰ to − 5.4‰, confirming a largely abiogenic origin for the gas. The presence of primary CH4-dominated fluid inclusions and melt inclusions, which contain a methane-rich gas phase, indicates a magmatic origin of the HCs. An increase in methane content, together with a decrease in δ13C isotope values towards the intrusion margin suggests that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks.
This study is a comprehensive, stable isotope survey of the marine carbonate-dominated, upper Paleo- to lower Neoproterozoic stratigraphy of Jixian County, China. Carbonate-associated sulfate (CAS) was extracted and measured for δ³⁴SCAS using the same samples analyzed for δ¹³Ccarbonate. This integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of Proterozoic seawater. We specifically sampled marine carbonate intervals from the lower section of the Chuanlinggou Formation, Changcheng Group (ca. 1700 Ma) to the top of the Jingeryu Formation, Qingbaikou Group (ca. 800 Ma). δ¹³Ccarbonate values are mostly negative in the upper Paleoproterozoic Changcheng Group, with an ascending trend from −3‰ to 0‰. We observed variation of approximately 0 ± 1‰ in the Mesoproterozoic Jixian Group, and positive values of +2 ± 2‰ characterize the lower Neoproterozoic Qingbaikou Group. Stratigraphic variations in δ³⁴SCAS are more remarkable in their ranges and magnitudes, including conspicuously high values exceeding +30‰ in the three intervals at ca. 1700 Ma, 1300–1100 Ma, and 1000–900 Ma. In the Changcheng Group, δ³⁴SCAS values are typically higher than +25‰, with only a few values of less than +15‰. In contrast, most of the data spanning from the Mesoproterozoic Tieling Formation of the Jixian Group to the lower Neoproterozoic Jingeryu Formation of the Qingbaikou Group are highly variable between +10‰ and +25‰, with some values exceeding +25‰.