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Pyrite zonation and source of gold in the Pangjiahe orogenic gold deposit, West Qinling Orogen, central China
Abstract
The Pangjiahe deposit is an orogenic gold deposit located on the northwestern margin of the Fengtai Basin and has proven reserves of 38 t Au at an average grade of 6.3 g/t. Phyllite is widely distributed in the mining area with Triassic granite porphyry and diabase dykes intruded in faults crosscutting it. Gold mineralization is mainly hosted in phyllite rocks. The granite porphyry present in the deposit is locally mineralized when it shares the same space with ore-controlling faults. Pyrite always displays zoning textures: Py0 and Py1 are present in the core; rim Py2 replaces and/or overgrows Py0 and Py1; and Py3 comprises the outermost rim. Previously determined in situ δ³⁴S values indicate that Py0, Py1, and Py2 and Py3 are diagenetic, magmatic hydrothermal, and hydrothermal ore stage in origin, respectively. Electron microprobe analysis (EMPA) mapping of the zoned pyrite (especially Py2) shows a negative correlation between sulfur and arsenic, indicating the substitution of sulfur by As¹⁻. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping and spot analysis of Py2 and Py3 reveal two Au and As relationships. In the mineralized phyllite and granite porphyry, Au has strong positive correlations with As, Sb, Cu, and Ag, similar to other arsenian pyrites in hydrothermal ore deposits. However, in altered samples, Au and As display decoupled geochemical behavior, especially in Py2, which has the same levels of As, Sb, Cu, and Ag but 2–5 orders of magnitude lower Au. We think that the ore fluid reacted with altered samples located distal to the ore-controlled fault (distal to the ore fluid center), and the fluid may not have been sufficiently charged with those elements, especially Au. Because As, Sb, Cu, and Ag could maintain at the same level in the ore fluid after precipitation of pyrite enriched in those elements, but Au was sharply depleted. This was also noted in a published fluid inclusion study in Carlin-type gold deposits in Guizhou Province, which has similar zoned pyrite texture. Based on the similar δ³⁴S values (8 ‰ and 10 ‰) and trace element enrichments (Au, As, Sb, Cu, and Ag) between hydrothermal ore-stage pyrite and the Devonian seafloor exhalation sequence in the Fengtai Basin, the source of the gold in the Pangjiahe deposit should be attributed to metamorphism of the Devonian seafloor exhalation sequence during the Triassic orogeny.
... However, it is consistent with the mineralization age of the nearby orogenic gold deposits, including the Pangjiahe (231.2 Ma) and the Matigou (234 Ma) gold deposit [34,92]. Furthermore, the Qinling Orogen's tectonic setting changed from compression to extension at around 230 Ma, which would have provided a favorable environment for auriferous fluid migration and precipitation in the shear zones. ...
The Anwangshan gold deposit is located in the northwestern part of the Fengtai Basin, Western Qinling Orogen (WQO). The gold ore is hosted within quartz syenite and its contact zone. The U–Pb weighted mean age of the quartz syenite is 231 ± 1.8 Ma. It is characterized by high potassium (K2O = 10.13%, K2O/Na2O > 1) and high magnesium (Mg# = 55.31 to 72.78) content, enriched in large ion lithophile elements (Th, U, and Ba) and light rare earth elements (LREE), with a typical “TNT” (Ti, Nb, and Ta) deficiency. The geochemical features and Hf isotope compositions (εHf(t) = −6.68 to +2.25) suggest that the quartz syenite would form from partial melting of an enriched lithospheric mantle under an extensional setting. Three generations of gold mineralization have been identified, including the quartz–sericite–pyrite (Py1) stage I, the quartz–pyrite (Py2)–polymetallic sulfide–early calcite stage II, and the epidote–late calcite stage III. In situ sulfur isotope analysis of pyrite shows that Py1 (δ34S = −1.1 to +3.8‰) possesses mantle sulfur characteristics. However, Py2 has totally different δ34S (+5.1 to +6.7‰), which lies between the typical orogenic gold deposits in the WQO (δ34S = +8 to +12‰) and mantle sulfur. This suggests a mixed source of metamorphosed sediments and magmatic sulfur during stage II gold mineralization. The fluid inclusions in auriferous quartz have three different types, including the liquid-rich phase type, pure (gas or liquid)-phase type, and daughter-minerals-bearing phase type. Multiple-stage fluid inclusions indicate that the ore fluids are medium-temperature (concentrated at 220 to 270 °C), medium-salinity (7.85 to 13.80% NaCleq) CO2–H2O–NaCl systems. The salinity is quite different from typical orogenic gold deposits in WQO and worldwide, and this is more likely to be a mixture of magmatic and metamorphic fluids as well. In summary, the quartz syenite should have not only a spatio-temporal but also a genetical relationship with the Anwangshan gold deposit. It could provide most of the gold and ore fluids at the first stage, with metamorphic fluids and/or gold joining in during the later stages.
... Some research evidences indicate that the ore-forming fluids and metals may originate from post-collision metamorphism and magmatism in the WQOB. (Yu et al., 2020a;Hu et al., 2020;Wang et al., 2020;Ma et al., 2021). At present, it is generally considered that the WQOB has undergone multi-stage merging processes from the Proterozoic to late Mesozoic in the South China Block and North China Block. ...
The West Qinling Orogenic Belt (WQOB) in central China records the tectonic evolution and deep geodynamics process associated with plate collision on the northeastern margin of the Qinghai–Tibet Plateau. The study of the deep–seated West Qinling structure is beneficial for revealing the evolution of the West Qinling crust during the continental collision orogeny, and also has significance for mineral exploration. In this study, the crustal structure and the distribution of major faults in the West Qinling Orogen are calculated by processing geophysical aeromagnetic and ground gravity data. The density and magnetic susceptibility differences between the West Qinling crust and the upper mantle are calculated by fitting, and the depth and trend of the faults related are inferred from the gravity and magnetic inversion results. Due to the overall subduction of the lower crust of the Ruoergai to the West Qinling Orogenic Belt, the structural deformation within the West Qinling is strong, and the geophysical magnetic field and gravity field are suddenly changed. It is indicated that the formation of the Diebu–Sanhe fault may have a strong correlation with the Mianlue suture zone, and their intersection may penetrate into the lower crust at a depth of about 50 km or more. The West Qinling structure represents important conduits for migrating magmatic–derived hydrothermal fluids. During the continental collision, the West Qinling major faults provided hydrothermal migration channels for ore–forming materials in different tectonic periods. Meanwhile, multiple the geological activities led to the formation of mineral deposits related to magmatic hydrothermal fluids.
Pyrite (FeS2), arsenopyrite (FeAsS) and löllingite (FeAs2) are exceptional gold concentrators on Earth; yet the exact redox and structural state of this “invisible” gold and the forces driving its intake and release by these minerals remain highly controversial. Here we applied high resolution X-ray absorption spectroscopy to Au-bearing pyrite and iron sulfarsenides from hydrothermal deposits and their synthetic analogues. We show that Au preferentially enters octahedral Fe structural sites [Au(As,S)6] enriched in As, by forming respectively [AuAs1–3S5–3], [AuAs3S3⋯AuAs6] and [AuAs6] atomic units in arsenian pyrite (>0.1–1.0 wt. % As), arsenopyrite and löllingite, implying a formal oxidation state of AuII in the minerals. In contrast, in As-poor pyrite, Au is dominantly chemisorbed as [AuIS2] moieties in much lower concentrations. Combined with experimental data on Au mineral-fluid partitioning, our findings imply a universal control exerted by arsenic on gold incorporation in iron sulfides and sulfarsenides via coupled Au-As redox reactions. These reactions account for the observed variations in invisible gold contents in the minerals from different hydrothermal deposit types and enable quantitative prediction of iron sulfarsenide ability in controlling gold concentration and distribution in hydrothermal systems.
Open access article:
https://www.geochemicalperspectivesletters.org/article2112/
In recent years, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping is a new key analytical application to obtain chemical compositions in analyzed sample. LA-ICP-MS mapping has great advantages, e. g., easy sample preparing, low cost of equipment, fast, low detection limit, multi elemental analysis, comparing to other modern mapping techniques. Here we present a new LA-ICP-MS mapping method based on the PhotonMachines laser ablation system with Agilent ICP-MS system. This mapping method is based on two-volume cell laser system and Matlab coding data reduction software named LaIcpMsSoftware (LIMS). This new method can generate multi-elements (more than 30) pictures of 3mm × 3mm mineral in less than 2 hours. Compared with other mapping software, LIMS has many advantages, e. g., easy to operation, variety data display mode and cutting section display, element ratio mapping, to facilitate data analysis and interpretation for geologist. We demonstrate mapping results of magmatic plagioclase, hydrothermal scheelite and skarn garnet and constrain the origin of such minerals in terms of mapping results. Our datasets reveal that LA-ICP-MS mapping can reveal geochemical details that cannot be appreciated with single spot analysis, and it is easier to get elements correlations to better understand the geochemical process in mineral. © 2017 Yanshi Xuebao/Acta Petrologica Sinica. All rights reserved.
The El Callao district, with a total endowment of more than 2000 t Au, is considered to be the most prolific gold resource in Venezuela. Mineralization is hosted by a vein system that is genetically associated with the El Callao transpressional shear zone. This vein system consists of a network of interconnected quartz–albite–ankerite veins enveloping a large number of metabasaltic fragments that host gold-bearing pyrites. Based on detailed mineralogical, microstructural, and fluid inclusion studies, a pressure-temperature pathway was established for the evolution of the mineralizing fluid during shear-zone development and exhumation. This path is characterized by repeated episodes of fluid pressure fluctuation from lithostatic (higher than 1.6 kbar) to near-hydrostatic values (<0.4 kbar), recorded throughout the transition from the quasi-plastic to frictional deformation cortical domains. Each successive pressure drop induced boiling of the hydrothermal fluid, with the resulting fluid phase separation controlling: (i) pyrite and invisible gold crystallization, which occurred during ductile and ductile-brittle transition strain conditions, and (ii) primary gold remobilization with consequent native-refined gold precipitation, occurring mainly under brittle conditions. The metallogenic framework that was proposed for the El Callao shear zone can be used as a vector to explore and characterize other mineralized shear zones in the Guiana Shield and analogous orogenic systems worldwide.
Lots of Paleozoic chert outcrops were found in Mianlue tectonic zone which can reveal the sedimentary environment and tectonic background of western Qinling orogenic belt. In this paper, researches on the Devonian cherts from Wenxian which is located in the west of Mianlue tectonic zone have been done, including field work of the cherts outcrops, petrology and mineralogy of the cherts, major and trace elements of the cherts, and the sedimentary environment of cherts have also been analyzed. Moreover, combined with the studies related to cherts in the east of Mianlue tectonic zone, we conclude the sedimentary environment of Mianlue tectonic belt and the tectonic background of western Qinling orogenic belt in Paleozoic. Cherts from Wenxian can be divided into grey-white blocky and grey-dark brecciated cherts, and both of them are hot water sedimentary cherts with bedding or banded structure and conformity to upper and underlying strata in the field. And they are mainly made up of fine crystalline to microcrystalline quartz grain (84. 85% ∼99. 07%, average value is 91. 99%) with few calcite, sericite and mud material, while the grey-white cherts are purer than the other one with more SiO2. According to the Fe2O3/TiO2-Al2O3/(Al2O3+ Fe2O3), V/Y-TVV and LaN/CeN-Al2O3/(Al2O3+ Fe2O3) discrimination diagrams, cherts are mainly formed in or near the continental margin. It is noticeable that the grey-white cherts are affected by materials from continental margin less than the grey-dark one based on the fewer Σ REE contents (11. 96 × 10-6∼51. 06 × 10-6compare to 171. 3 × 10-6∼ 236.1 × 10-6). The rare earth patterns of the two kinds of cherts are similar with flat model after North American Shale Standardization (NASS). And the feature of cherts which shows no Ce anomaly (grey-dark is 0. 95 ∼ 1. 05) or weak Ce positive anomaly (grey-white is 1. 07 ∼ 1. 12), Eu weak negative anomaly (grey-dark cherts is 0. 66 ∼0. 87, the other is 0. 70 ∼ 0. 90), and low (La/Yb) SNvalue (grey-dark is 0. 95 ∼ 1. 10, the other is 1. 64 ∼ 4. 07) is similar to the classical cherts formed near the continent margin in the world. All the geology and geochemistry features suggest cherts from Wenxian are formed in or near the continental margin while the grey-dark cherts may contain more continent materials than the grey-white one. Compared with the studies of the cherts in the east of Mianlue tectonic zone in Paleozoic, it suggests that it is sea basin sedimentary environment in the north of South China Block continent margin from Early Paleozoic to early Late Paleozoic, where there are a lot of continental margin materials added into the SiO2-rich hot water, and then a series of cherts are formed from the west to the east of Mianlue tectonic zone. Meanwhile, western Qinling is under the continent rifting tectonic background, and a lot of rifting basins and deep fractures are formed in different stages of rifting activities which have a contribution to the forming of cherts.
High spatial resolution textural (scanning electron microscope (SEM)), chemical (electron microprobe (EMP)) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)), and sulfur isotopic (secondary ion mass spectrometry (SIMS)) analyses of pyrite from the Qiuling sediment-hosted gold deposit (232 ± 4 Ma) in the West Qinling orogen, central China were conducted to distinguish pyrite types and gain insights into the source and evolution of sulfur in hydrothermal fluids. The results reveal an enormous variation (−27.1 to +69.6 ‰) in sulfur isotopic composition of pyrite deposited during three paragenetic stages. Pre-ore framboidal pyrite, which is characterized by low concentrations of As, Au, Cu, Co, and Ni, has negative δ34S values of −27.1 to −7.6 ‰ that are interpreted in terms of bacterial reduction of marine sulfate during sedimentation and diagenesis of the Paleozoic carbonate and clastic sequences, the predominant lithologies in the deposit area, and the most important hosts of many sediment-hosted gold deposits throughout the West Qinling orogen. The ore-stage hydrothermal pyrite contains high concentrations of Au, As, Cu, Sb, Tl, and Bi and has a relatively narrow range of positive δ34S values ranging from +8.1 to +15.2 ‰. The sulfur isotope data are comparable to those of ore pyrite from many Triassic orogenic gold deposits and Paleozoic sedimentary exhalative (SEDEX) Pb-Zn deposits in the West Qinling orogen, both being hosted mainly in the Devonian sequence. This similarity indicates that sulfur, responsible for the auriferous pyrite at Qiuling, was largely derived from the metamorphic devolatization of Paleozoic marine sedimentary rocks. Post-ore-stage pyrite, which is significantly enriched in Co and Ni but depleted in Au and As, has unusually high δ34S values ranging from +37.4 to +69.6 ‰, that are interpreted to result from thermochemical reduction of evaporite sulfates in underlying Cambrian sedimentary rocks with very high δ34S values. The variations in Au content and sulfur isotopic compositions across a single ore-stage pyrite grain may reflect displacement of indigenous groundwater with low δ34S values by auriferous metamorphic fluids with high δ34S values. The very low-grade metamorphism of the host rocks and the metamorphic derivation of sulfur for the ore pyrite indicate that the Qiuling sediment-hosted gold deposit is an epizonal manifestation of an orogenic gold system in the West Qinling orogen.
A method is proposed for determining gold solubility in the common gold-bearing minerals (sulfides, etc.) using so-called 'gold-assisting elements' (GAE). These elements increase gold solubility in the fluid phase, enabling minerals formed from hydrothermal fluids to be saturated with gold. Two experimental approaches are discussed: (1) identifying the solid-solution limit of gold in a mineral structure by determining the maximum content of uniformly distributed gold constituent which remains unchanged with increasing gold content in the coexisting fluid phase, and (2) determining the gold distribution between the mineral under study and a reference mineral with a sufficiently high and well-defined solid-solution limit of gold and utilizing the phase composition correlation principle. Statistical treatment of analytical data for single crystals permits inference of the structurally bound gold constituent. Greenockite (α-CdS) incorporates a maximum of 50 ± 7 ppm Au in solid solution at 500°C and 1 kbar and this is used as a reference mineral to determine gold solubility in pyrite under the same conditions in the presence of As and Se as gold-assisting elements. The value obtained for gold solubility in pyrite (3 ± 1 ppm Au) is in reasonable agreement with the results of ion-probe microanalysis of natural pyrites. The data suggest that monovalent gold substitutes for divalent iron, giving rise to an acceptor center compensated by a donor defect, either a sulphur vacancy or a hydrosulphide ion replacing S22-.
This paper reports geochemical and Pb-Sr-Nd isotopic compositions from four Indosinian granitoid intrusions (the Heimahe, Wenqian, Daheba and Tongren intrusions), together with U-Pb zircon LA-ICPMS ages from two instrusions (the Heimahe and Wenqian intrusions) around the Gonghe Basin in Qinghai Province. The results show that the U-Pb zircon ages for the Heimahe and Wenqian intrusions are 235 2Ma and 218 2Ma, respectively. The granitoids from the four intrusions display SiO2 contents of 63.34 ∼ 68.06% and K2O/Na2 O ratios of 0.82 ∼ 1.36. They are metaluminous with A/CNK = 0.89 ∼ 1.0 and belong to medium-high potassium calc-alkaline series. Generally, they have similar trace element geochemical characteristics and REE patterns with (La/Yb)N, = 10 ∼ 15 for most samples and Eu/Eu * = 0.5 ∼ 0.8. Whole-rock initial Sr isotopic ratios (ISr) ranging from 0.70701 to 0.70952, and εNd (t) values from - 3.8 to - 8.4, indicating that their magma sources were dominantly derived from partial melting of continental crustal materials. They are characterized by high radiogenic Pb isotopic compositions with (206Pb/204Pb)t = 18.068 ∼ 18.748, (207Pb/204Pb)t = 15.591 ∼ 15.649 and (208Pb/204Pb)t = 38.167 ∼ 38.554. The geochemistry of the granitoids reveals that their magma sources were high-potassium meta-basalts in lower crust, which could be derived from an enriched mantle source in Proterozoic. However, these meta-basalts have small extent chemical heterogeneity. The geochemical probing to deep crustal compositions, combined with regional comparisons, indicates that the West Qinling, the Qaidam (including east Kunlan) and the Oulongbuluke blocks around the Gonghe basin have a common crustal basement, which have an affinity with the Yangtze block. According to analyses of regional geological setting, the early Indosinian (eg. the Heimahe intrusion) and the late Indosinian granitoids (eg. the Wenqian intrusion) have distinct geodynamic processes for their magma generation. The former was suggested to be related to break-off of a subducted slab; the latter resulted from lithospheric delemination after crustal thickening due to collision between continental plates in Triassic.
Using recent research results and previously published data of chronology and petrogenesis, the authors have divided the Indosinian granitoid magmatism into three stages in the middle segment of the Qinling orogen. The early stage of the magmatism is attributed to the northward subduction of oceanic slab coupled with the closure of the Mianxian-Lueyang ocean during 248-216 Ma. The middle stage of the magmatism occurred in the period from the collision between the northern margin of the Yangtze Craton and the North Qiling arc complex zone to the collapse of the orogen during 215-200 Ma. The late stage of the magmatism is considered to have resulted from the delamination of the orogenic root in the post-collision period during 200-195 Ma. Therefore, the Indosinian granitoid magmatism perfectly registered the evolutionary processes of the Qinling orogenic belt.
The El Callao mining district is the most important gold-producing region in Venezuela. It is hosted in the
Paleoproterozoic Guasipati-El Callao greenstone belt, which forms part of the Guayana craton, the Venezuelan extension of the Guiana Shield of South America. It consists of volcanic and volcanosedimentary sequences that were affected by several deformation events, particularly localized shear zones. The Colombia mine, the largest active mine in the district, produces 4 tonnes (t) (128,600 oz) of gold annually with reserves estimated at 740 t (24 Moz) and grades of up to 60 g/t. Gold mineralization is concentrated within a vein network in the Colombia corridor, a shear fracture-hosted mesh of interconnected quartz-ankerite-albite veins enclosing fragments of altered metabasaltic host rocks. Gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite, in which it occurs as invisible gold, micron-sized native gold inclusions, and filling fractures.
Based on optical and scanning electron microscopy-backscattered electron observations, two types of pyrite
are recognized: a simple-zoned pyrite and a less common oscillatory-zoned pyrite. Both types consist of a mineral inclusion-rich core and a clearer rim; however, in oscillatory-zoned pyrite, the latter is composed of complex rhythmic overgrowths of alternating As-rich and As-poor bands. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite. The highest concentrations (5–23 ppm Au) are found in oscillatory-zoned pyrite rims, which correlate with the highest As concentrations (16,000–23,000 ppm). In As-poor bands, Au (up to 1.5 ppm) and As (300–6,000 ppm) concentrations decrease by about an order of magnitude. Copper, Bi, Te, Sb, Pb, and Ag always occur with invisible gold, particularly in pyrite cores, suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids. Visible native gold grains occur as small inclusions throughout core and rim of both pyrite types, as well as in fractures within it. In both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and chlorite accompany native gold, and gold fineness ranges between 900 and 930.
At an early stage of vein mesh formation, pyrite formed in the metabasaltic fragments at the expense of
ankerite, which, in turn, resulted from alteration of Fe-Ti oxides. Gold, together with other chalcophile elements, was incorporated within the structure of pyrite, most likely by destabilization of metal sulfide complexes during ankerite replacement. Subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite, generating local and repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid, as evidenced by the presence of primary fluid inclusions containing immiscible liquid rich and vapor-rich aqueous-carbonic fluids. This process was responsible for a number of physical-chemical changes in the liquid, all of which contributed to the formation of the As- and Au-rich overgrowths in pyrite:
(1) removal of H2O into the vapor phase, inducing saturation of dissolved metals in the remaining liquid; (2) an increase in pH due to partition of H2S and CO2 into the vapor, thus decreasing the solubility of sulfide minerals;
and (3) an adiabatic decrease in temperature, lowering the solubility of As and Au in the liquid. Waning of
this process restored precipitation of As-poor pyrite, until the onset of a new cycle. Because pressure drops are more significant adjacent to open spaces, oscillatory-zoned pyrite probably crystallized near newly formed veins whereas simple-zoned pyrite formed away from them. Previously formed pyrite underwent fracturing during reactivation of the deformation, especially through the brittle deformation events that postdated shearing, resulting in local pulverization of pyrite. This newly created porosity facilitated fluid circulation and remobilization of structurally bound gold, as well as of other chalcophile elements (Ag, Cu, Bi, Te, Pb, and Sb), which reprecipitated together with pyrite in the form of native gold, sulfides, and tellurides, either as small inclusions or as larger grains within fractures. This remobilization process facilitates the exceptionally high gold tenor found in the deposit, where the Colombia corridor is intersected by the Santa Maria fault.
Massive sulfide deposits of Zn-Pb-Cu-Ag type in the Bathurst Mining Camp, New Brunswick, are hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence that has undergone complex polyphase deformation and associated regional metamorphism to the lower- to upper-greenschist grade. These factors are partly responsible for the present geometry and textural modification of these hydrothermal deposits, originally formed on the seafloor. Despite the importance of heteroge- neous ductile deformation, some primary features are evident, in particular fine-grained colloform pyrite and base- and precious- metal zonation within many of these deposits. The average Au content (bulk instrumental neutron-activation analyses; n = 215) of massive sulfides from 43 deposits in the Bathurst Mining Camp is 0.85 ppm, with values as high as 6.86 ppm. Positive correlations of Au content with Ag (r' = 0.53), As (r' = 0.57) and Sb = (r' = 0.65) suggest that Au is mainly associated with arsenian pyrite and a distal suite of elements (Au + Sb + As ± Ag). The submicroscopic Au contents of pyrite and arsenopyrite in eight deposits with elevated bulk Au contents were investigated using a Cameca IMS-4f secondary-ion mass spectrometer. Results suggest that arsenian pyrite is the most important host for Au in massive sulfides of the Bathurst Mining Camp, with an average Au content of 9.1 ppm, and values reaching 42.9 ppm. Arsenopyrite was found to contain much less Au, with an average of 2.7 ppm. Recrystallization of sulfides during greenschist-facies metamorphism has resulted in pyrite morphologies with vari- able Au contents. Invisible Au was in part released and adsorbed as submicroscopic inclusions on As-rich surfaces of pyrite and on arsenopyrite.
The Qinling orogen was formed by the joining of the North and South China blocks, but the timing of their integration has been debated for more than a decade. The controversies obviously stem from different approaches to reconstruction of the integration history. Two contrasting lines of evidence yield two different ages for collision of the North and South China blocks---middle Paleozoic and Late Triassic. The Shangdan suture within the Qinling was regarded in previous studies as the trace along which the North and South China blocks collided. Our studies, however, demonstrate that there are two sutures within the Qinling: the well-documented Shangdan suture and the newly discovered Mianlue suture. We show in this paper that the Late Proterozoic to early Mesozoic evolution of the Qinling involved interactions between the North China block, the North and South Qinling orogens, and the South China block. The middle Paleozoic collision along the Shangdan suture, as constrained by some evidence, accreted only the South Qinling orogen to the southern part (i.e., the North Qinling) of the North China block. Contemporaneous rifting of the South China block and subsequent drifting separated the South Qinling from the South China block during the middle to late Paleozoic. The separation of the South from the North China blocks is supported by other evidence, in particular, geomagnetic data. Evidently it was the Late Triassic collision of the South China block with the South Qinling orogen along the Mianlue suture that led to final integration of the North and South China blocks.
The stability and solubility of natural arsenopyrite (FeAsS) in pure water and moderately acid to slightly basic aqueous solutions buffered or not with H2 and/or H2S were studied at temperatures from 300 to 450°C and pressures from 100 to 1000 bar. The solubilities of FeAsS in pure water and dilute HCl/NaOH solutions without buffering are consistent with the formation of the As(OH)3⁰(aq) species and precipitation of magnetite. At more acid pH (pH ≤2), arsenopyrite dissolves either stoichiometrically or with formation of the As-FeAsS assemblage. In H2S-rich and H2-rich aqueous solutions, arsenopyrite dissolution results in the formation of pyrrhotite (±pyrite) and iron arsenide(s), respectively, which form stable assemblages with arsenopyrite.
Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of As along the Melones fault zone in the southern Mother Lode Gold District of California. Mine tailings and associated weathering products from partially submerged inactive gold mines at Don Pedro Reservoir, on the Tuolumne River, contain ∼20–1300 ppm As. The highest concentrations are in weathering crusts from the Clio mine and nearby outcrops which contain goethite or jarosite. As is concentrated up to 2150 ppm in the fine-grained (<63 μm) fraction of these Fe-rich weathering products.
We report sulfur isotopic compositions of sulfi des of various paragenetic stages in the giant Sukhoi Log sediment-hosted orogenic Au deposit in Russia. The overall mean value and the signifi cant variability in early pyrite indicate that the sulfur was from the reduction of seawater sulfate. The later generations of sulfi de have δ 34 S values in successively smaller ranges, coincident with the mode that is around the median value of the whole data set. Together with textural evidence, sulfi de trace element data, and gold occurrence, the data demonstrate that metamorphism has gradually homogenized the early sulfur, accompanied by the segregation of quartz and the release of Au from the lattice of early pyrite and its reprecipitation as inclusions in later pyrite. The S isotopic compositions of sulfi des in Sukhoi Log, and many other major orogenic Au deposits hosted in sedimentary rocks of various ages, show a pattern generally parallel to the seawater sulfate curve through geologic time, indicating that the sulfur in most sediment-hosted orogenic Au deposits was probably also originally from the reduction of seawater sulfate. We conclude that sulfi dation and gold mineralization in many sediment-hosted orogenic gold deposits was early during basin evo- lution when seawater was the principal active fl uid, rather than later, during or after basin inversion, as proposed in current models.
AssrRAcr Gold contained in the structure of the common sulfide. minerAls and that present as discrete inclusions smaller than 1000 A are collectively termed "invisible gold", not detec- table by optical and scanning electron microscopy. Con- centrations of invisible gold range from less than 0.5 ppm to greater than 1000 ppm in sulfide grains from 12 ore deposits, as determined by ion-probe microanalysis (Secon- dary Ion Mass Spectrometry). Concentrations vary con- siderably from deposit to deposit, and between discrete ore facies within a single deposit. Minerals that most readily act as hosts are arsenopyrite and arsenic-rich varieties of pyrite. There is a positive correlation between the concen- trations of arsenic and gold in pyrite' suggesting that sub- stitution of gold into that structure is facilirated by the presence of arsenic. Where two or more distinct popula- tions of arsenopyrite coexist, abundances of "invisible gold" are greater in the finer-grained type. Pyrrhotite, chal- copyrite, bornite, galena and tetrahedrite generally contain minor concentrations of "invisible gold"'
A new mode of arsenic incorporation into arsenian pyrite has been discovered. Electron microprobe analyses and elemental maps of arsenian pyrite from Pueblo Viejo, Dominican Republic, show that its chemical composition varies from (Fe(0.998)As(0.003))(1.001)S(2) to Fe(0.963)As(0.050)Cu(0.003)Ag(0.001))(1.017)S(2) and that arsenic is inversely correlated with Fe. High-resolution transmission electron microscopy (HRTEM) images show that some arsenic in this pyrite is present as nanoscale inclusions of amorphous As-Fe-S in a matrix of arsenian pyrite. The amorphous inclusions display negative facets with a Cubic or rectangular morphology, typical of negative inclusions of pyrite and arsenopyrite or marcarsite, respectively, and they are oriented parallel to the lattice fringes (100) of the arsenian pyrite matrix. Elemental maps collected by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) show that the inclusions have a higher content of arsenic than the surrounding pyrite, and TEM-EDX analyses indicate approximate atomic proportions of 62 at% S, 28 at% Fe, and 10 at% As for the inclusions, which are near the minimum melting temperature in the As-Fe-S ternary. These observations suggest that the inclusions were trapped as liquids during growth of the surrounding arsenian pyrite. Although not a new mineral, this constitutes a third form for arsenian pyrite, which has previously been shown to contain arsenic as either As(1) or As(3+).
The Matigou gold deposit, located in the Fengtai Basin, is newly discovered and contains more than 30 t Au. The gold ore bodies are mainly hosted in shear structures as auriferous quartz veins and in the upper Devonian Luohansi Formation as disseminated ores. Granite porphyry and dacite porphyry dykes have been recognized in the ore zone. The granite porphyry dykes normally share the same structure as gold ore bodies and are locally mineralized. Dacite porphyry dykes likely intruded later, as they occasionally cut gold ore bodies. Pyrite is common in all kinds of rocks in the deposit. Multiple isotope (H-O-S) analyses of different minerals (quartz and pyrite), together with the geological characteristics, suggest that the Matigou deposit should be classified as an orogenic gold deposit like others in the Fengtai Basin. Zircon U-Pb dating of magmatic dykes and ⁴⁰Ar/³⁹Ar dating of hydrothermal ore-stage sericite both indicate that the Matigou deposit formed at ∼234 Ma. It is coeval with the Pangjiahe deposit (231.2 Ma) 20 km to the west but much older than previously published orogenic gold ages (216 to 200 Ma). The ca. 230 Ma gold mineralization occurred between two episodes of magmatism (245-235 Ma and 227-205 Ma) in the Qinling orogen. The later orogenic gold mineralization is consistent with the later episode of magmatism, which is widely believed to have formed in a postcollisional extensional regime. The early episode of magmatism is still debated as related to either subduction or an early postcollisional stage. Because the ca. 230 Ma gold mineralization displays no strong deformation or metamorphism after formation, and the orogenic gold deposits imply a relatively extensional background. Hence, the early episode of magmatism may have intruded in a compressional regime after continental collision. The ca. 230 Ma gold mineralization occurred at the beginning of the transition in a tectonic setting from compression to transpression. Otherwise, the gold ore bodies would have been destroyed to some extent during the late collision. Due to the continuous extension background, more orogenic gold deposits with ages of 216 to 230 Ma are expected to be discovered in the Qinling orogen.
Gold (Au) is largely hosted by pyrite in a variety of hydrothermal systems, but the incorporation of Au into pyrite under disequilibrium conditions remains poorly understood. We integrate synchrotron X-ray fluorescence microscopy, electron backscatter diffraction, nanoscale secondary ion mass spectrometry, and laser ablation–multicollector–inductively coupled plasma–mass spectrometry to constrain the processes that sequester Au into zoned pyrite in the hydrothermal cement of breccia ores from the world-class Daqiao orogenic Au deposit, central China. Euhedral pyrite cores with oscillatory and sector zoning, variable δ34S values, and lower Au-As contents than the mantles are attributed to crystallization during oxidation of metal-depleted ore fluids with local variation in fluid conditions. The isotopically uniform colloform mantles are formed by pyrite crystallites separated by low-angle boundaries and are characterized by unusual decoupling of Au and As. Mantle formation is attributed to rapid disequilibrium precipitation from a metal-rich FeS2-supersaturated fluid. Incorporation of Au into the pyrite mantles was facilitated by abundant lattice defects produced by rapid nucleation. Gold-As–poor pyrite rims were deposited from an evolved ore fluid or other metal-depleted fluids. These results show that chemical variations recorded by fine layering within minerals can provide valuable insights into disequilibrium mass transfer and ore formation. The decoupling between Au and As in pyrite mantles indicates that As is not always a reliable proxy for Au enrichment in rapidly crystallized porous pyrite.
Arsenic (As) and gold (Au) are closely associated in many gold deposits, both being hosted in Fe-sulfide minerals (pyrite, marcasite, and arsenopyrite), partly because As geochemistry controls Au accumulation. Yet, the partitioning behavior of As between pyrite, arsenopyrite, and hydrothermal fluids remains poorly understood. Here, we introduce solid-solution models for As in pyrite and As in arsenopyrite into a thermochemical model of fluid-rock interaction, and use it to evaluate the effects of temperature, redox state, and fluid-flow dynamics on As—and Au by association—partitioning. We find that As concentrations in pyrite decrease with increasing temperature, despite the widening of the solid-solution composition range. This is related to the preferential partitioning of As into fluids at higher temperatures. Simulations of infiltration of rock-buffered H2O-CO2-As fluids into low-As pyrite (As:S = 0.01) ores reveal a continuous enrichment of As in pyrite with increasing fluid:rock ratio. The modeling suggests that upgrading of early-formed low-grade ores by multistage hydrothermal events can generate large gold deposits. In this scenario, an anomalously Au-rich fluid is not needed, but instead, p
The Daqiao gold deposit is hosted in organic-rich Triassic pumpellyite-actinolite facies metamorphosed turbidites in the West Qinling orogen, central China. Gold mineralization is characterized by high-grade hydraulic breccias (B and C ores) that overprint an earlier tectonic breccia (A ore). A complex paragenesis is defined by four sulfide stages: S1 diagenetic preore pyrite (py), S2 hydrothermal early ore disseminated pyrite and marcasite (mc), S3 main ore pyrite and marcasite aggregates, and S4 late ore coarse-grained marcasite with minor pyrite and stibnite. However, multiple generations of pyrite and marcasite may develop within one individual stage. Ore-related hydrothermal alteration is dominated by intensive silicification, sulfidation, sericitization, and generally distal minor carbonatization. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analyses show that the stage S1 py1 from the shale interlayers within turbidites contains low gold contents (mean of 0.05 ppm) and other trace elements (Mn, Co, Ni, Cu, Mo, Bi, and Pb), indicating an anoxic to euxinic sedimentary environment. Stage S2 contributed only minimally to the gold endowment with relatively low gold in various sulfides including py2 (mean of 0.09 ppm), py3 (0.84 ppm) to py4 (0.70 ppm), along with mc1 (0.02 ppm) and mc2 (0.14 ppm). Most of the gold was deposited in stage S3, which formed rapidly crystallized, irregular (e.g., framboids, colloform and cyclic zonation) cement-hosted py5a (mean of 27.35 ppm), py5b (9.71 ppm), and mc3 (5.94 ppm) during repeated hydraulic fracturing. Other trace elements (e.g., Ag, As, Sb, Hg, Tl, and W) are also significantly enriched in the main ore-stage pyrite and marcasite. Little or no gold is detected in the S4 py6 and mc4. Sulfur isotopes determined from in situ LA-multicollector (MC)-ICP-MS analyses of hydrothermal pyrite and marcasite from the Daqiao deposit vary significantly from –31.3 to 22.0 (δ34S values) but fall mostly between –10 to 10 and provide important information on the source and evolution of sulfur and of the ore-forming fluids. The results show that S2 ore fluids (mean δ34Ssulfide = –0.8 to 5.2) were most likely derived from deep-seated Paleozoic carbonaceous sediments during regional metamorphism associated with orogenesis of the West Qinling orogen. Main ore S3 fluids (mean δ34Ssulfide = –9.7 to –6.0) are relatively depleted in ³⁴S relative to those of S2, presumably due to fluid oxidation associated with hydraulic fracturing caused by the overpressurized fluids. The textural, chemical, and isotopic data indicate two distinct gold-introducing episodes at Daqiao, forming sulfide disseminations during early ore S2 and cement-hosted sulfide aggregates during main ore S3. The S2 mineralization took place in a tectonic breccia beneath low-permeability shale seals that capped the flow of deep-seated metamorphic fluids, facilitating reaction with preexisting carbonaceous material and the host turbidites to form sulfide disseminations and pervasive silicification. Raman spectroscopy analysis suggests that carbonaceous material in the ores is poorly crystallized, with low maturity, giving estimated temperatures of 283° to 355°C that are much higher than those of the ore fluids (100°–240°C). This temperature difference indicates an in situ sedimentary origin modified by the regional pumpellyite-actinolite facies metamorphism for the carbonaceous material in the host rocks, rather than a hydrothermal origin. In S3, continuous flux of hydrothermal fluids caused fluid overpressure and consequent hydraulic fracturing of the competent silicified rocks. Subsequent rapid fluid pressure fluctuations led to phase separation and thus massive oxidation of ore fluids, which triggered fast precipitation of gold and other trace elements within the fine-grained irregular sulfides. Results presented here, in combination with geologic evidences, suggest that the Daqiao gold deposit can be best classified as the shallow-crustal epizonal orogenic type, genetically associated with orogenic deformation and regional metamorphism of the West Qinling orogen.
The sedimentary- and granite porphyry-hosted Pangjiahe gold deposit is located in the north belt of the South Qinling Orogen, with a proven reserve of 38 t Au at 6.3 g/t. A total of 26 samples, from various rock types and locations from the Panjiahe deposit, were taken in order to determine the paragenesis, sulfur source and absolute age of mineralization. Magmatic activities of ca. 240 Ma occurred around the whole study area, which is common in the northwest part of the West Qinling orogen. Field observations confirm that the emplacement of granite porphyry dykes is earlier than the gold mineralization, because they were both altered and mineralized. Moreover, post ore stage diabase dykes, which cut cross the ore bodies, display an age of ca. 220 Ma. Zircons from strongly mineralized granite porphyry samples show typical hydrothermal trace element characteristics. Theses zircons return a concordia age at ca. 230 Ma. Therefore, we infer that the ore-forming age of the Pangjiahe deposit is at ca. 230 Ma. Optical microscopy, EMPA and LA-(MC)-ICPMS have been used to determine the physical and chemical features of pyrite. Five stages of pyrite have been identified in mineralized phyllite and granite porphyry rocks. Diagenetic pyrite in phyllite, Py1-P, displays a wide range of δ³⁴S (−1.5% to +9.4%). The magmatic pyrite in the granite porphyry, Py1-G, display relatively narrow range of δ³⁴S (+2% to +5%) consistent with magmatic sulfur. The main ore stage pyrite in both rocks, Py2 (Py2-P and Py2-G), is the most common sulfide in the deposit. Py2 grains predominantly show oscillatory zoning and either replace or overgrown the Py1-P and Py1-G, or occur as individual fine grains. Arsenopyrite is typically associated with Py2. Pyrite grains in late stage quartz (Qz) veins that cross cut phyllite and granite, Py-Qz, display weak oscillatory zoning. Native gold is commonly observed as inclusions in the Py-Qz grains or within/proximal to quartz veins. Py3 pyrite (Py3-P and Py3-G), which also belong to the later ore stage and can be distinguished by the native gold inclusions in them, all occur as the outer rims that overgrown or replace on Py2. The ore stage Py2, Apy, Py-Qz and Py3 have a similar narrow range in sulfur isotopic composition, from +8% to +10%. This range is slightly lower than the age equivalent SEDEX Pb-Zn deposits situated in graben basins to the East, but totally different from the earlier Py1-P and Py1-G. EMPA element mapping and spot analyses, and in-situ sulfur isotope analyses indicate that the ore stage pyrites in both phyllite and granite rocks have similar major element chemistry and sulfur isotopic composition, indicating that they are likely sourced from the same ore fluids. Therefore, we propose that prograde metamorphism of the underlying Devonian sedimentary sequences during the seafloor exhalation, which are thought to be enriched in Au, As, Sb, B, F and S, may have released H2S that forming these auriferous sulfides. Gold mineralization in the south belt of the South Qinling orogen shows the same mineralization style as observed in the North belt. However, mineralization is younger and the δ³⁴S values of the hydrothermal ore stage sulfide are consistent with magmatic sulfur, which is consistent with intense magmatic activities within the South Qinling orogen. The δ³⁴S values in the South Belt are distinctly different from those of gold deposits in the north belt. We propose that the source of the sulfur and gold in north belt came from the Devonian graben basins, and in the south belt came from a deep magmatic source. More focused studies of the gold and Devonian graben basin hosted SEDEX Pb-Zn deposits are required to further prove or develop this hypothesis.
The Lihir (also known as Ladolam) Au deposit in Papua New Guinea is a telescoped ore deposit, in which volcanic sector collapse(s) led to superimposition of shallow-level Au-rich epithermal mineralization upon preexisting but genetically related, porphyry-style alteration. This superimposition created a giant 56 Moz Au resource but also created complications for ore processing, specifically with regard to the difficulties in processing the refractory Au-rich pyritic ore. We have analyzed trace element zonation and composition of pyrite grains using laser ablation-inductively coupled plasma-mass spectrometry imaging coupled with NaOCl etching from a subset of spatially and paragenetically constrained pyrite-bearing samples from the Lienetz orebody. Pyrite grains belong to either porphyry or epithermal stages or are composite pyrite grains with a multistage history. Trace element zonation and metal contents of pyrite are unique for each paragenetic event, providing insights into the nature of the mineralizing fluids. Early generations of coarse-grained pyrites that formed under higher-temperature porphyry-style conditions have low trace element contents compared with epithermal-stage pyrites, except for Co, Ni, and Se. Later generations of oscillatory zoned pyrites that formed under lower-temperature epithermal conditions are enriched in trace elements such as As, Mo, Ag, Sb, Au, and Tl. The composite pyrites are relatively coarse grained and display textural and geochemical evidence of modification (i.e., dissolution and reprecipitation). They are interpreted to be porphyry-stage pyrite grains that have been overgrown by rims of delicate banded epithermal-style pyrite enriched in Au, As, and other trace elements. The composite pyrite grains are volumetrically dominant in the deep-seated anhydrite zone at Lienetz. Because Au is concentrated only along the rims of these pyrite grains, the pyrite can be subjected to a shorter period of oxidation and leaching to liberate most of the Au. This is in contrast to areas dominated by high-grade epithermal-stage mineralization where pyrite grains are As- and Au-rich throughout and thus require longer oxidation and processing time. Understanding Au deportment in telescoped deposits is therefore essential for optimizing mineral processing and can significantly impact the economics of mining complex hybrid ore deposits.
On the base of comprehensively researching regional geological background, geochemical and geophysical characteristics in Feng-Tai multi-metal ore cluster region and combined with systematicly analyzing discovered mineral deposits here, there could be divided into three metallogenic series, including Devonian carbonate rock-hosted lead and zinc deposit metallogenic series, Devonian sedimentary reformation typed copper deposit metallogenic series, and Mesozoic orogenesis-related gold deposit metallogenic series. It is concluded that Changgou-Donggou, Qiliping in Fengzhou and Weiziping-Donggou-Jianzigou region are very important for exploration in the near future, and point clear prospecting direction for Changgou-Donggou, Tiziya-Sanjiaoya region. It is considered that discovered lead and zinc zone in Changgou-Donggou located in the same secondary hydrothermal sedimentary basin with Baguamiao gold deposit, which range coincide with that of He-2 anomaly, where hase good metallogenic condition. South limb (overturned limb) and plunging end of Changgou anticline and Changgou-Baguamiao segment of Donggou anticline are animportant exploration target for lead, zinc and gold. These have important significance to guide exploration work in Feng-Tai region.
Longwu Gorge ophiolite belt located in Tongren Area, Qinghai Province, is at the jointing part of West Qinling and South Qilian orogenic belts. The ophiolite geochronology is very important for the study of tectonic evolution of orogenic belt and the reconstruction of the ancient ocean-land and plate tectonic framework. LA-ICP-MS zircon U-Pb dating of gabbro from the Longwu Gorge ophiolite suggests an isotopic age of (250.1±2.2) Ma (MSWD=0.7), which indicates that the Longwu Gorge ophiolite was the product of magmatic activity during the process of extension of paleo-ocean basin along the jointing part of West Qinling and South Qilian in the late Permian to early Triassic. Zircons from the granodiotite intruding the ophiolite yielded an age of 244±1.4 Ma, which was younger than the formation of ophiolite, but it is significantly older than the Indosinain coHisional granitoids in the region. Therefore, it may reflect the emplacement age of the Longwu Gorge ophiolite.
The main purpose of this paper is two-fold. The first is to summarize the specific types of information on ore-forming processes (e. g. , conditions and mechanisms of ore formation, sources of ore constituents) that can be obtained from a stable isotope study of an ore deposit, and the principles behind the methodologies. The uncertainties, limitations, and problems that are associated with the methodologies are discussed. The second purpose is to summarize some of the important concepts and models of ore-forming processes that have been developed from stable isotope investigations of ore deposits.
Invisible gold in natural and synthetic arsenian pyrite and marcasite correlates with anomalous As content and Fe deficiency, and high contents of invisible gold in most natural and all synthetic arsenopyrite correlate with excess As and Fe deficiency. As-rich, Fe-deficient arsenopyrite synthesized hydrothermally contains up to 3.0 wt% Au uniformly distributed in growth zones of light backscattered electron contrast. At the Deep Star gold deposit. Carlin Trend, Nevada, the sulfide compositions apparently span the full range of metastability from FeS 2 to near FeAsS (40 at% S); arsenian pyrite contains up to 0.37 wt% Au, but arsenopyrite has excess S and is relatively Au poor. Observed minimum Fe contents are 29.1 at% in arsenian pyrite and marcasite from the Deep Star deposit and 31.3 at% in synthetic arsenopyrite. We suggest that invisible gold in arsenian pyrite and marcasite and arsenopyrite from sediment-hosted gold deposits represents Au removed from ore fluids by chemisorption at As-rich, Fe-deficient surface sites and incorporated into the solids in metastable solid solution. However, the oxidation state of invisible gold (Au 0, Au 1+) remains uncertain because the chemisorption process is intrinsically nonsystematic in terms of crystal-chemical parameters and does not result in definitive atomic substitution trends.
There are a lot of granite from Xiahe to Lixian along the west Qinling which is of the adakite geochemical feature. The paper reports the SHRIMP dating of Yeliguan and Xiahe granite plutons. The SHRIMP ages are 245 ± 6 Ma and 238 ± 4 Ma respectively, belong to early Indosinian period. The adakite is rich of potassium, is classified by high potassium calc-alkalic series. Geology and geochemical data show that the adakite in the western Qinling was formed in the active plate margin, and was associated with the melting of thicken lower crust in the active crust margin. The dating implicated that the convergent and consuming of palaeotethys was processing at early Indosinian.
Zoned arsenopyrite crystals from the Chatelet and Villeranges deposits (Creuse, France), rich in Au(1.5 wt.%) and Sb (1.6 wt.%) show a strong deficiency in Fe. Gold and Sb concentrations are independent of the As/S ratio. The presence of As on the Fe-site (As[Fe]), varies from one growth zone to another. The number of As atoms on the Fe-site is negatively correlated with (Au + Sb)at, the slope of regression lines being -0.5. This indicates a substitution mechanism 2 As[Fe] ⇄(Au,Sb) + Fe. The existence of Fe(III) in the non-polar state in the FeAsS crystal structure implies the presence of Au(III) and/or Sb(III). There is an abridged English version. -English summary
Ion-probe microanalysis and ion mapping by SIMS have established that submicroscopic ppm-levels of gold in arsenian pyrite from stratabound and stratiform gold deposits are associated exclusively with As-rich growth bands; the gold is most probably incorporated in solid solution in arsenian pyrite via As-rich growth surfaces. The surface layers of stained sections of arsenian pyrite and arsenopyrite contain three species of sulfur (disulfide, elemental S, and sulfate), and the proportion of elemental S in surface layers of pyrite increases with As content of the substrate. Surface oxidation of pyrite by potassium permanganate solution is promoted by As-for-S substitution; in this way, color development can be correlated with the distribution of As and "invisible' Au. -from Authors
This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural, or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements (particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se, Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic- and sulfide-rich metalliferous black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian pyrite and nanoparticles of free gold. During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence. LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite.
The Meikle mine exploits one of the world's highest grade Carlin-type gold deposits with reserves of ca. 220 t gold at an average grade of 24.7 g/t. Locally, gold grades exceed 400 g/t. Several geologic events converged at Meikle to create these spectacular gold grades. Prior to mineralization, a Devonian hydrothermal system altered the Bootstrap limestone to Fe-rich dolomite. Subsequently the rocks were brecciated by faulting and Late Jurassic intrusive activity. The resulting permeability focused flow of late Eocene Carlin-type ore fluids and allowed them to react with the Fe-rich dolomite. Fluid inclusion data and mineral assemblages indicate that these fluids were hot (ca. 220degreesC),of moderate salinity (<6 wt % NaCl equiv), acidic, and H2S rich. Goldrich pyrite formed by dissolution of dolomite and sulfidation of its contained Fe. Where dissolution and replacement were complete, ore-stage pyrite and other insoluble minerals were all that remained. Locally, these minerals accumulated as internal sediments in dissolution cavities to form ore with gold grades >400 g/t. Petrographic observations, geochemical data, and stable isotope results from the Meikle mine and other deposits at the Goldstrike mine place important constraints on genetic models for Meikle and other Carlin-type gold deposits on the northern Carlin trend. The ore fluids were meteoric water (deltaD = -135parts per thousand, delta(18)O = -5parts per thousand) that interacted with sedimentary rocks at a water/rock ratio of ca. 1. and temperatures of ca. 220degreesC. The absence of significant silicification suggests that there was little cooling of the ore fluids during mineralization. These two observations strongly suggest that ore fluids were not derived from deep sources but instead flowed parallel to isotherms. The gold was transported by H2S (delta(34)S = 9parts per thousand), which was derived from Paleozoic sedimentary rocks. The presence of auriferous sedimentary exhalative mineralization in the local stratigraphic sequence raises the possibility that preexisting concentrations of gold contributed to the Carlin-type deposits. Taken together our observations suggest that meteoric water evolved to become an ore fluid by shallow circulation through previously gold- and sulfur-enriched rocks. Carlin-type gold deposits formed where these fluids encountered permeable, reactive Fe-rich rocks.
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.
Sulfide nodules composed of pyrite occur within black shales in the Lucky Bay area of the Bulong Domain, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia that correlate with the Neoarchean Black Flag Group. Detailed petrography reveals a variety of shapes and textures, from spherical to ovoid and zoned to uniform. Some nodules have recrystallized, resulting in a later generation of pyrite. All have been affected by post-depositional metamorphism, as evidenced by pressure shadows containing quartz ± mica ± carbonate assemblages. LA-ICP-MS analyses show that these nodules are enriched in a range of trace elements, including Co, Ni, Cu, Ag, Sb, Te, Au, Tl, Pb, and Bi. Gold and Te concentrations range from 0.3 to 1 ppm and 10 to 50 ppm, respectively. The Black Flag Group equivalents stratigraphically and structurally underlie a younger basin east of Lucky Bay that contains turbidites, sandy debris flows and BIF, the latter of which hosts several gold deposits at Randalls, 60 km SE of Kalgoorlie. It is likely that the lateral equivalents of the nodular sulfide-bearing black shale originally occurred stratigraphically beneath what is now the ore horizon at Randalls. Given its anomalous Au and Te content, this lithology is a potential contributor of gold to the BIF-hosted gold deposits of the Randalls Goldfield, as well as other similar gold resources elsewhere in the Eastern Goldfields Superterrane.
The Qinling–Dabie–Sulu orogenic belt is the junction between the North and South China blocks, which resulted from the final amalgamation of China continents during the Indosinian. Indosinian granitoids are widespread in the Qinling orogen, and their geneses can thus constrain the evolution of China continent. We carried out a combined U–Pb zircon dating and geochemical study for the Shuangpengxi granodiorite pluton and the Xiekeng diorite–granodiorite pluton in the middle part of the West Qinling orogen. U–Pb zircon dating shows that the magma crystallization ages of 242 ± 3 Ma for the Shuangpengxi pluton and ~244–242 Ma for the Xiekeng pluton. Geochemical and Sr–Nd–Hf isotopic compositions reveal that the magma of the Shuangpengxi granodiorite was derived from partial melting of crustal materials. The Xiekeng diorites can be divided into high-Al diorite and high-Mg diorite. Both of them resulted from partial melting of enriched lithospheric mantle, but their mantle source had been modified by previous slab-derived melt. The high-Al diorite was formed by fractional crystallization of olivine, pyroxene and/or preferential accumulation of plagioclase, and the high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. The Xiekeng granodioritic porphyry was formed by mixing of crust-derived and mantle-derived melts. We propose that the Early Indosinian magmatism resulted from break-off of subducted oceanic slab after collision. The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the West Qinling.
A ‘new’ type of arsenian pyrite was formed during experimental replacement of magnetite under hydrothermal conditions (T = 125 and 220 °C; Psat) and in the presence of S(-II) and various As-containing species. The amount of As in pyrite depended on the As-source, with sources containing cationic As (As(II), As(III) and As(V)) resulting in considerably higher amounts of As in the product arsenian pyrite than anionic sources. The highest As content was 23.83 ± 0.20 wt%, corresponding to a S:Fe:As molar ratio of 2:0.58:0.42. Electron probe micro-analyses revealed an inverse correlation between the Fe and As contents in the arsenian pyrite, indicating that As is substituting for Fe. Arsenic concentrations were highly inhomogeneous within the pyrite rim; in general, higher As contents were found within solid pyrite growing on the outer rim, compared to the highly porous and texturally complex pyrite found close to the reaction boundary. This likely reflects different uptake mechanisms for As during the pyrite nucleation and growth stages. X-ray Absorption Near Edge Structure (XANES) analyses showed that the As in the arsenian pyrite was predominantly in the form of As(II). Cross-sectional X-ray photoelectron spectroscopy (XPS) analysis of the arsenian pyrite confirmed the presence of As(II), but also showed evidence for more oxidized species (As(III) and As(V) oxides), as well as small amounts of polymeric As–As bonding. This indicates a large difference between As in the bulk (XANES measurements) and at the pyrite surface (XPS). Ab initio XANES calculations are consistent with As replacing Fe in pyrite in the form of As(II). Our experimental study suggests that the formal oxidation state of As in this type of arsenian pyrite is close to +2, and that in addition to fluid composition and oxidation state, the reaction path leading to pyrite formation plays a significant role in controlling the chemistry of arsenian pyrite.
Porphyry copper deposits are currently the world's largest source of copper and molybdenum, and are also among the largest reservoirs of gold in the upper crust. Despite the fact that pyrite is a ubiquitous mineral phase in these deposits and secondary Cu enrichment processes are commonly controlled by the abundance of this sulfide, the major and trace ele-ment chemistry of pyrite from porphyry systems remains unconstrained. In this study, we report the first comprehensive trace element database of pyrite from the Dexing deposit, China's largest porphyry Cu deposit. By combining high-spatial resolu-tion and X-ray mapping capabilities of electron microprobe analysis (EMPA) with low detection limits and depth-profiling capabilities of secondary-ion mass spectrometry (SIMS) in a suite of samples from the Dexing deposit, we show that the con-centrations of precious metals (e.g., Au, Ag), metalloids (e.g., As, Sb, Se, Te) and heavy metals (e.g., Cu, Co, Ni, Zn, Hg) in pyrite from porphyry systems are more significant than previously thought. Among the elements analyzed, Cu, As, Au and Ni are the most abundant with concentrations that vary from sub-ppm levels to a few wt.% (i.e., 3 wt.% As, 0.2 wt.% Ni). Detailed wavelength-dispersive spectrometry (WDS) X-ray maps and SIMS depth vs. isotope concentration profiles reveal that pyrite from the Dexing deposit is characterized by complex chemical zoning where the studied elements occur in different mineralogical forms. While As occurs as a structurally bound element in pyrite, Cu and Au can occur as both solid solution and micro-to nano-sized particles of chalcopyrite and native Au (or Au tellurides), respectively, indicating that pyrite can control metal speciation and partitioning during porphyry Cu mineralization. The well-developed oscillatory zoning detected in pyrite, where Cu-rich, As-depleted growth zones alternate with Cu-depleted, As-rich layers, indicates that Cu is geochemically decoupled from As, suggesting that this selective partitioning of metals into pyrite is most likely the result of changes in hydrothermal fluid composition. Ó 2012 Published by Elsevier Ltd.
Granitoids varying in composition from biotite–amphibole granodiorite to monzogranite with rapakivi textures have recently been recognized in the Qinling orogen (central China). The granitoids are magnesian, calc-alkalic and metaluminous (A/CNK=0.71–0.95). They contain many mafic to intermediate microgranular enclaves. The enclaves are quenched mafic melts in granitic host magma as evidenced by fine-grained sinuous margins against or transitional contacts with the host granitoids as well as disequilibrium textures and mineral assemblages. U–Pb zircon dating of the granitoids yields: 209±2Ma and 212±2Ma and the enclaves 210±2Ma and 211±2Ma, respectively. The host granitoids and enclaves show obvious silica gap but similar REE and trace element patterns. The whole-rock 87Sr/86Sr(t), εNd(t) and zircon εHf(t) values for the granitoids vary from 0.70516 to 0.70634, −2.3 to −4.4 and 0.7 to −7.6, and for the enclaves from 0.70467 to 0.70600, −1.1 to −4.0, and 1.2 to −8.8, respectively, showing overlapped εNd(t) and zircon εHf(t) values. All these suggest that the two magmatic systems represented by the host rock and enclave are probably derived from distinct sources with sufficient interaction, or common origin but underwent different degrees of crustal contamination. Mineralogical and geochemical data point to enclave dissolution in silicic magma as a major process accounting for the origin of the rapakivi texture. The possible bimodal magmatism suggested by the coeval granitoids and lamprophyre dykes, combined with the structural pattern, geochemical features and regional tectonics, suggest a post-collision setting for the Qinling rapakivi-textured granitoid plutons.
Many hydrothermal Cu–Mo–Au deposits related to granitoid intrusives were recently discovered in the West Qinling Orogenic Belt (WQOB). These deposits were mainly formed during the late Indosinian epoch (ca. 214Ma), and the regional geological setting of Cu–Mo ore formation in WQOB during this epoch is poorly understood until now. This paper describes the geochronology and geochemistry of the Wenquan ore-bearing pluton, a composite granite body, to study the geologic background of magmatic emplacement and ore formation. The Mo mineralisation occurs at the contact between a fine-grained biotite monzogranite and a medium- to fine-grained porphyritic monzogranite. Zircon 206Pb/238U ages of 223±3Ma (biotite monzogranite) and 225±3Ma (porphyritic monzogranite) were obtained. Geochemical analyses show that the Wenquan pluton is a high-K calc-alkaline to shoshonite series rock with relatively high LREE and low HREE and a moderate to weak negative Eu anomaly. Relatively negative anomalies of Ba, Ti, P, Nb, Ta also exist. These results imply that the Wenquan pluton was emplaced during a transitional process (from collision to extension) between the Yangtze Craton and North China Craton. During the later Indosinian epoch, the East Qinling Orogenic Belt (EQOB) and WQOB had similar tectonic settings, and intensive magmatic activity and Mo mineralisation occurred. The EQOB was then involved in the Mesozoic subduction of the Pacific plate, and its subsequent tectonic evolution was different from that of the WQOB.
Very well preserved fossil vent chimneys from the Silurian Yaman-Kasy volcanogenic-hosted massive sulfid deposit in the Southern Urals range in decreasing temperatures from chalcopyrite-pyrite black smoker to sphalerite-chalcopyrite-marcasite-pyrite gray smoker to sphalerite-quartz-barite white smoker assemblages. Laserablation ICPMS analyses show systematic trace element distribution patterns across chimneys. Coarse-grained layers of chalcopyrite in the central conduits are relatively high in Se and Sn but are low in other elements. Chalcopyrite at the margins of such layers is enriched in Bi, Co, Au, Ag, Pb, Mo, Te, and As, which reside ind\ltrpar\nowidctlpar microinclusionsof tellurides and/or sulfoarsenides. Sphalerite in the conduits and the outer chimney wall contains elevated Sb, As, Pb, Co, Mn, U, and VAntimony, As, and Pb reside in microinclusions of a galena-fahlore assemblage, whereas the Co and Mn likely substitute for Zn2+ in the sphalerite structure. The highest concentrations of most traceelements are found in colloform pyrite within the outer wall of the chimneys and likely result from rapid precipitation under high-temperature-gradient conditions. The trace element concentrations in the outer-wall colloform pyrite decrease in the following order, from the outer wall inward: Tl > Ag > Ni > Mn > Co > As > Mo > Pb > Ba > V > Te > Sb > U > Au > Se > Sn > Bi, governed by the strong temperature gradient. In contrast, pyrite in the high- to mid-temperature central conduits exhibit concentration of Se, Sn, Bi, Te, and Au. The zone between the inner conduit and outer wall is characterized by recrystallization of colloform pyrite to euhedral pyrite, which becomes depleted in all trace elements except Co, As and Se. The mineralogical and trace element variations between chimneys are likely due to increasing fO2 and decreasing temperature caused by mixing of hydrothermal fluids with cold oxygenated seawater. Average values of Se (a high-temperature element) decrease in the order from black to gray to white smoker chimneys. The medium-temperature association (Te, Bi, Co, Mo, and Au) is typically present in the gray smoker chimneys. The white smoker chimneys are depleted in most elements except for Ag, Tl, Te, Sb, and As, probably due to the dilution of the vent fluid by seawater which penetrates deeper parts of the hydrothermal system. U and V are concentrated in the outer wall of most chimneys due to their extraction from seawater associated with the more reduced fluids of black and gray smokers.
Laser ablation ICP-MS imaging of gold and other trace elements in pyrite from four different sediment-hosted gold-arsenic
deposits has revealed two distinct episodes of gold enrichment in each deposit: an early synsedimentary stage where invisible
gold is concentrated in arsenian diagenetic pyrite along with other trace elements, in particular, As, Ni, Pb, Zn, Ag, Mo,
Te, V, and Se; and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic
and/or hydrothermal pyrite or as narrow gold-arsenic rims on the outermost parts of the overgrowth hydrothermal pyrite. Compared
to the diagenetic pyrites, the hydrothermal pyrites are commonly depleted in Ni, V, Zn, Pb, and Ag with cyclic zones of Co,
Ni, and As concentration. The outermost hydrothermal pyrite rims are either As-Au rich, as in moderate- to high-grade deposits
such as Carlin and Bendigo, or Co-Ni rich and As-Au poor as in moderate- to low-grade deposits such as Sukhoi Log and Spanish
Mountain. The early enrichment of gold in arsenic-bearing syngenetic to diagenetic pyrite, within black shale facies of sedimentary
basins, is proposed as a critical requirement for the later development of Carlin-style and orogenic gold deposits in sedimentary
environments. The best grade sediment-hosted deposits appear to have the gold climax event, toward the final stages of deformation-related
hydrothermal pyrite growth and fluid flow.
Sediment-hosted gold deposits in Guizhou, China, are hosted in late Paleozoic and early Mesozoic sedimentary rocks along the southwest margin of the Precambrian Yangtze craton. They have characteristics similar to Carlin-type gold deposits in Nevada and are notably enriched in As, Sb, Hg, and Tl. The Shuiyindong and Yata deposits consist of disseminated, strata-bound sulfides in Permian bioclastic limestone and fault-controlled mineralization in Middle Triassic calcareous clastic rocks, respectively. Mineralization in both deposits consists of barren milky quartz veins, disseminated gold-bearing arsenian pyrite and arsenopyrite, stibnite, realgar, and orpiment. The barren milky quartz veins occur in the ore-controlling structures with an envelope of gold mineralization in the host rock consisting of disseminated gold-bearing arsenian pyrite and arsenopyrite and replacement-style quartz veinlets. Later drusy quartz, stibnite, realgar, and orpiment fill fractures and vugs on the periphery of gold mineralization. Petrography, microthermometry, laser Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) analyses of fluid inclusions are used to characterize the chemical evolution of ore fluids at Shuiyindong and Yata. Early barren milky quartz veins trapped aqueous fluid with moderate salinity (6.0 wt % NaCl equiv) and minor CO2 (<2.4 mol %) at a temperature of 230° ± 30°C. This fluid contains measurable B, Na, K, Cs, Sr, As, Sb, and Au (3.8 ± 0.5-5.7 ± 2.3 μg/g) determined by LA-ICP-MS and is interpreted to approximate the mineralizing fluid responsible for precipitation of early barren quartz veins. Low-salinity (0.9-2.3 wt % NaCl equiv), CO2-bearing (6.3-8.4 mol %), aqueous fluids are recorded in replacement-style quartz veinlets. They formed gold-bearing arsenian pyrite and arsenopyrite and quartz veinlets at a temperature of 210° ± 20°C and pressures of 450 to 1,150 bars, corresponding to depths between 1.7 and 4.3 km under lithostatic conditions. Fluid inclusions in late stibnite-orpiment-realgar-quartz veins contain high CO2 (58-64 mol %) and N2 (19.2-23.7 mol %) with trace CH4 (up to 1.6 mol %). The increasing content of CO2 and decrease in the concentrations of Au and As in aqueous-carbonic inclusions are interpreted to result from carbonate dissolution at the level of the deposits during gold mineralization. Covariance of Au, As, Sb, and Sr concentrations in the fluids is interpreted to reflect deposition of As and Sb sulfides and gold concurrent with carbonate crystallization. Iron is below detection limit (∼400 μg/g) in all fluid inclusion types, suggesting that the ore fluids were Fe poor but possibly sulfur rich, possibly explaining their exceptionally high gold contents. Iron in sulfide minerals was probably derived from dissolution of ferroan minerals in the host rocks, sulfidized by H 2S-rich fluids to precipitate arsenian pyrite, arsenopyrite, and gold. Pressure fluctuation induced by faulting resulted in local fluid immiscibility and led to late deposition of realgar, orpiment, stibnite, and calcite. Hydrogen and oxygen isotope compositions of ore fluids (δD(H 2O) = -35 to -68‰, δ18O(H2O) = 4-16.5‰) indicate a metamorphic origin, possibly related to crustal thickening and prograde metamorphism during the late Yanshanian orogeny.
Gold mineralization at Sukhoi Log in eastern Siberia is hosted in a deformed Neoproterozoic organic-bearing and pyritic black shale and siltstone sequence that is folded into a tight overturned anticline. The deposit contains about 30 million ounces of gold at air average grade of 2.0 g/t Au and is one of the largest known undeveloped gold resources. The high-grade gold zone forms a gently dipping tabular body in the core of the anticline. The best gold grades occur in narrow, bedding-parallel pyrite-quartz veinlets that have been folded during the main deformation event. Lower grade gold is associated with disseminated pyrite developed in and around the high-grade core of the deposit. Detailed paragenetic studies of the mineralization and host rocks have defined six stages of pyrite development in the carbonaceous sediments. The two earliest forms of pyrite, termed py(1) and py(2), are commonly developed in stratiform layers of micron-sized crystals, framboids and fine euhedra, which are interpreted as synsedimentary to early diagenetic in origin. Coarser grained, bedding-parallel aggregates of inclusion-rich pyrite, termed py(3), contain inclusions of arsenopyrite, native gold and gold tellurides and are interpreted to form during late diagenesis and earliest deformation. Coarse euhedral pyrite, py(4), overgrows the earlier pyrite (py(1), py(2), and py(3)), and the slaty cleavage developed in the host rocks, indicating a syndeformation timing. Late-stage, inclusion-free pyrite, py(5), overgrows and replaces earlier sulfides and is considered to be syn- to late deformation. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses of the various pyrite types indicate that the synsedimentary py(1) contains the highest levels of invisible gold, varying from 0.4 to 12.1 ppm, with a mean of 3.22 ppm An, and 1,900 ppm As. Py-1 is also enriched in a suite of trace elements (Mo, Sb, Ni, Co, Se, Te, Ag, Cu, Pb, Zn, Mn, Ba, Cr, U, V), which are similar to those concentrated by organic processes in euxinic sedimentary environments. Later generations of pyrite, from py2 to py5, including pyrite in bedding-parallel pyrite-quartz veinlets, contain progressively lower contents of invisible gold and most other trace elements. However, this metamorphic and postmetamorphic pyrite contains microinclusions of free gold, arsenopyrite, pyrrhotite, sphalerite, and chalcopyrite. The paragenetic, textural, and chemical relationships at Sukhoi Log suggest that gold was clearly initially introduced prior to cleavage development, accompanying sedimentation of the organic-rich shales and fixed during diagenesis within the structure of diagenetic arsenian pyrite. Subsequently, accompanying deformation, gold was liberated from recrystallized diagenetic pyrite to become concentrated as free gold and gold tellurides within metamorphic pyrite and folded bedding-parallel pyrite-quartz veinlets. Two key processes are considered vital to the formation of the Sukhoi Log deposit: original synsedimentary and early diagenetic concentration of gold, dissolved within arsenian pyrite in organic-rich black shales, and metamorphic processes that liberated gold from the early forms of arsenian pyrite, to be concentrated as free gold, and gold tellurides within late diagenetic and metamorphic pyrite and associated pyrite-quartz veinlets in the core of an overturned anticline. These ore-forming processes are unlikely to be unique to Sukhoi Log; other black-shale and turbidite-hosted deposits that occur in rifted continental margin environments, which have undergone collision and basin inversion, may form by similar processes.