Genesis Of Gold Mineralization Research Articles

Petrogenesis of the Laoshan Suite in the Jiaodong Peninsula (Eastern China): An Oxidized Low Ba–Sr A1-Type Granite

The Jiaodong Peninsula is closely related to Mesozoic granites in terms of spatial and temporal aspects. However, the specific association between the genesis of gold mineralization and these granites remains unclear. It is also ambiguous why Laoshan-type granites, which are similar to Mesozoic granites, are not gold mineralized. In this study, we analyzed the Laoshan granites and compiled Mesozoic magmatic rock data (Linglong, Guojialing, and Weideshan suites) of the Jiaodong Peninsula. We performed whole-rock major and trace elements, LA–ICP–MS zircon U–Pb geochronology and geochemistry analyses. Our zircon U–Pb data denote that the Laoshan granite was emplaced during the 118 ± 1 Ma. The Laoshan granite is characterized by high SiO2 content (76.03–80.28 wt.%), high TFe2O3/MgO (11.1–27.1) and Ga/Al (3.0–3.5) ratios, high zircon saturation temperature (809–850 °C), and negative Eu (Eu/Eu* = 0.05–0.08) anomalies, showing A-type granite characteristics. Furthermore, the Laoshan granite is identified as an A1-type granite with low Ba (33.1–42.0 ppm) and Sr (14.1–21.0 ppm) contents. It was formed in an extensional tectonic environment induced by the subducting slab roll-back of the Paleo-Pacific Plate, mainly from the partial melting of lower crustal materials, mixed with a small amount of mantle components. Zircon trace elements indicate that the Laoshan suite had relatively high oxygen fugacity and temperature (ΔFMQ = + 2.43 to + 4.22, T-Ti in zircon (mean) = 796 °C) compared to the pre-mineralization and contemporaneous mineralization magma. We propose that oxidized adakitic magma (Weideshan suite) may contribute to the enrichment and mineralization of gold. Although Laoshan-type granite also has a high oxygen fugacity, its location is distant from the ore-controlling faults, which ultimately hinders the formation of gold mineralization. From the Jurassic to the Cretaceous, the tectonic and geochemical properties of magmatic rocks in the Jiaodong Peninsula have changed, and gold deposits are formed in such a transitional process (compression to extension, reduction to oxidation).

Geology, geochemistry, and genesis of gold mineralization in the Chifumbazi deposit of the Tete Province, Irumide Belt, Mozambique

AbstractThe Chifumbazi gold deposit is located in the Chifunde District, Tete Province, northwestern Mozambique. The geology of the study area is composed of the Proterozoic Mualadzi Group comprising ultramafic metavolcanic rocks, quartzite, biotite‐schist, Macanda Mafic Metavolcanics, conglomerate, and granitoid of the Furancungo Suite. The geological characteristics and metallogenesis of gold mineralization in the Tete Province are poorly understood. This study describes the gold mineralization and associated hydrothermal alteration in the Chifumbazi deposit on the basis of petrography, whole‐rock geochemistry, mineral chemistry, fluid inclusion and sulfur isotope analyses. The mineralization in the Chifumbazi deposit includes auriferous quartz, calcite, dolomite and ankerite quartz‐chlorite, and chlorite‐pyrite veins and veinlets as well as sulfide dissemination hosted by altered meta‐granodiorite along shear zones and fracture zones. On a larger scale, the mineralized veins and veinlets occur along a discordant fracture system within a Pan‐African shear zone. The meta‐granodiorite host‐rock of the mineralization consists mainly of plagioclase, biotite, actinolite, and quartz, and alteration minerals calcite, dolomite, sericite, and chlorite. The meta‐granodiorite is enriched in LREE compared to HREE and shows negative Eu anomalies. The mineralization in the Chifumbazi deposit formed through three hypogene mineralization stages (Stages I–III) and one supergene stage (Stage IV). Stage I comprises quartz, calcite, dolomite, ankerite, pyrite, chalcopyrite, pyrrhotite and galena. Stage II comprises quartz, pyrrhotite, pyrite, chalcopyrite and native gold. Stage III comprises chlorite and pyrite; and Stage IV comprises secondary minerals goethite, bornite and covellite. Bulk chemical analysis of ores detected gold contents up to 5 ppm for a quartz vein of Stage II, where visible native gold is present in pyrite. The δ34SCDT values of pyrite and chalcopyrite of the Stages I–III range from +0.9 to +2.4 ‰. On the basis of the geological setting, mineralogy, and sulfur isotope signatures, the Chifumbazi deposit can be classified as orogenic.

Composition and Geochemical Characteristics of Pyrite and Quartz: Constraints on the Origin of the Xinjiazui Gold Deposit, Northwestern Margin of the Yangtze Block, China

The Xinjiazui gold deposit, a newly discovered deposit, is situated in the northwestern margin of the Yangtze Block, China. The source and genesis of gold mineralization are poorly understood. It is urgent to use the H–O isotopic composition of quartz and geochemistry of pyrite to evaluate the origins of the Au and ore-forming fluids of this deposit. Three types of pyrite were identified, including synsedimentary framboidal pyrites (Py0), the directional arrangement of pyrites in pre-mineralization stage (Py1), and euhedral coarse-grain pyrites in the quartz–sulfide veins of the mineralization stage (Py2). The As content in Py2 is relatively higher than Py0 and Py1, indicating that the ore-forming fluids are strongly enriched in As. The δ34S values of Py2 (+5.50–+13.34‰) overlap with the S1–2M phyllite (+7.25‰–+8.70‰). This result is consistent with the Pb isotopic composition of Py2, showing that the source of ore-forming materials was derived from the S1–2M phyllite. Meanwhile, the variations in quartz’s H and O isotopic composition suggest that the ore-forming fluids were derived originally from metamorphic fluid. Additionally, the Au mineralization is strictly controlled by the shear zone. Above all, we would like to classify the Xinjiazui deposit as an orogenic gold deposit.

Petrogenesis of Early Cretaceous granitoids and mafic enclaves from the Jiaodong Peninsula, eastern China: Implications for crust-mantle interaction, tectonic evolution and gold mineralization

Early Cretaceous granitic plutons in the Jiaobei Terrane, Eastern China, contain abundant mafic enclaves (MMEs), which are important to understand the tectonic settings and genesis of gold mineralization in the Jiaodong Peninsula. Here, we combine petrology, geochronology, and isotopic geochemistry for MMEs and host granitoids to constrain their tectonic evolution and understand the genesis of coeval gold mineralization in the Jiaodong Peninsula during the Early Cretaceous. Zircon U-Pb dating reveals two magmatic events: (1) the earlier magmatic event (130–126 Ma), represented by Congjia MME (CJ-MMEs) and host granodiorite, and (2) the later magmatic event (121–116 Ma), including Gushan MME (GS-MMEs) and host granite. Field relationship, disequilibrium textures, and elemental and Sr-Nd-Hf isotopic compositions indicate that MMEs and host granitoids formed through mixing/mingling of mafic and felsic magmas. However, significant differences between CJ-MMEs and GS-MMEs indicate that the former has adakitic affinity and was produced by mixing of magmas from lithospheric mantle and mafic lower crust in deep hot zone, whereas the latter has arc-magmatic affinity, suggesting enriched lithospheric mantle source. Higher SiO2 and lower MgO contents in the host granitoids indicate that they were originated from reworking ancient NCC lower crust. Our results reveal crustal growth during the earlier magmatic event, followed by rapid crustal/lithospheric thinning in the later magmatic event, all triggered by ongoing rollback of subducting Paleo-pacific plate. These results combined with complex assemblages of sulfides and volatiles in GS-MMEs indicate that the lithospheric thinning, the rise of enriched lithospheric mantle-derived melts to shallow crustal levels, and crustal-mantle interactions around ∼ 120 ± 3 Ma, are the main factors for massive gold precipitation in the Jiaodong Peninsula.

Geochemistry of gold-bearing metamorphic rocks of the Natitingou area, Atacora structural unit, Northwestern Bénin (West Africa): Implications for Au genesis

The Natitingou area within the Atacora Structural Unit (ASU) of the Pan-African Dahomeyide orogenic belt, northwestern Bénin, is characterized by the monocyclic metasedimentary formations, amphibolites and orthogneisses which belong to the Internal zone of the belt. Representative samples of these different gold-bearing rocks have been analyzed geochemically to determine their petrogenesis, mode of emplacement, tectonic setting and role in the genesis of gold mineralization in the area. The orthogneisses are subalkaline, peraluminous and emplaced in a syn-collisional arc setting while the amphibolites are metaluminous, tholeiitic with an E-MORB affinity and emplaced in a rifted lithospheric setting. The metasedimentary rocks, comprising quartzites and mica schists are cross-cut by auriferous quartz veins. The metasedimentary rocks are recycled sediments varying from wackes to sublitharenite in composition and deposited in a passive margin setting. The quartz veins similar to the metasedimentary rocks are enriched in LREE, depleted in HREE and showed a negative Eu-anomaly (0.31–0.72). Their chondrite-normalised REE patterns deciphered them as Upper Continental Crustal materials which were probably derived from the reworking of their host metasedimentary rocks. Gold concentration in quartzites, amphibolites, mica schists, quartz veins and orthogneiss were 0.004–9.587, 0.26–1.76, 0.0005–0.464, 0.023–0.147 and 0.0−0.0005 ppm, respectively. Devolatilization process of the country-rocks, is the most probable mechanism for the ore-forming fluid, which could have been generated during the Pan-African Dahomeyide event. Remobilisation of gold and associated metal (As, W, Cu, Mo, Sb, and Sn) after the Pan-African Dahomeyide orogeny is suggested for enrichment of gold along shear and fault zones of the Natitingou area.

Rb–Sr geochronology and geochemistry of pyrite from the Shihu gold deposit, central North China Craton: Implication for the timing and genesis of gold mineralization

The Shihu quartz vein‐type gold deposit in the central North China Craton occurs within Neoarchean TTG (tonalite‐trondhjemite‐granodiorite) gneisses and shows spatial relationship with quartz diorite porphyry. The occurrence of the ore body is controlled by the N‐S fracture. Primary mineral assemblage of the ore body is composed of quartz, calcite, and base‐metal sulphides, including pyrite, galena, sphalerite, chalcopyrite, and pyrrhotite. Eight pyrite samples from veins of the main mineralization stage yield an isochron age of 129 ± 3 Ma (MSWD = 1.6), coinciding with the major Early Cretaceous magmatic event, and associated lithospheric thinning recognized in the NCC. The (87Sr/86Sr)i values of the pyrite (0.7102 ± 0.0006) are lower than the continental crust mean value (0.719) and higher than that of the mantle initial ratio (0.707), suggesting mixed crust–mantle source for the metallogenic components. Three types of hydrothermal pyrite from the early to major ore‐forming stages were identified in the quartz vein, including Py(I) as disseminated euhedral grains, Py (II) as dense bulks, and Py (III) as intergrowth with base‐metal sulphides. Electron microprobe analysis of the pyrite grains indicates a trend from S‐deficient to S‐enriched and from Fe‐enriched to Fe‐deficient stages, suggesting an increase in sulphur fugacity and reducing conditions. The arsenic in pyrite shows decrease against increasing Au during Stages II and III, implying that As is not the only trigger of gold precipitation, but electrochemical‐geochemical barrier played a major role in the formation of the ore. The Co and Ni concentrations, as well as Co/Ni ratios in pyrite, are relatively high, which provide an additional clue to the link with the regional magmatic event. Combined with the textural features, we propose a wider potential for gold exploration in this region, linking the mineralization with a major transitional tectonic regime.

Alteration mineralogy, fluid inclusions and stable isotope studies from Chigargunta and Bisanatham gold deposits, South Kolar Greenstone Belt, Dharwar Craton, India: Implications on genesis of gold mineralization

There are many Neoarchaean orogenic gold hosting greenstone belts in the Dharwar Craton (DC), India; among them the southern extension of the Kolar greenstone belt (SKGB) contain promising gold deposits such as Chigargunta and Bisanatham. Auriferous laminated quartz-calcite veins are hosted mainly within the sheared and altered Champion gneiss and amphibolite in Chigargunta and Bisanatham respectively. The hydrothermal alteration mineral assemblages show three main alteration zones at Chigargunta. However, only two alteration zones were identified at Bisanatham. The hydrothermal mineral assemblage at Chigargunta and Bisanatham indicate a similar mineralogy that consists of tourmaline, muscovite, biotite, chlorite, epidote, sericite, calcite and quartz along with sulfide and gold. Sulfides associated with ore are predominantly pyrrhotite, pyrite, arsenopyrite, chalcopyrite, ±molybdenite in both the places. Native gold is mainly associated with pyrrhotite in the inner and proximal zone of both the deposits. Fluid inclusion petrography and microthermometry from quartz-calcite veins within the mineralized zone at Chigargunta reveal a low to medium salinity (0.5 to 13.3 wt% NaCl equiv.) H2O-NaCl-CO2-CH4 ± N2 bearing fluid. However, at Bisanatham only H2O-NaCl fluid with low salinity (1.56 to 6.44 wt% NaCl equiv.) was observed. P-T conditions of the gold mineralization at the Chigargunta and Bisanatham deposits range between 1.7 and 3.5 kbar pressure/285 °C to 378 °C temperature and 0.8 to 1.2 kbar pressure/365 °C to 405 °C temperature respectively. Alteration mineralogy, ore mineral assemblages, fluid compositions and P-T conditions from the SKGB confirm that the near neutral to slightly alkaline mineralizing fluid transported gold as Au(HS)2− complexes with low ƒO2 conditions. The observed drastic pressure and temperature fluctuation suggests that fault-valve mechanism/pressure cycling occurred at the time of gold precipitation.Carbon (δ13CCO2) isotopic composition of ore fluid deduced from calcite varies from −2.4 to +3.3‰ (average: −0.3‰) and −2.1 to +1.4‰ (average: −0. 8‰) for Chigargunta and Bisanatham respectively. Oxygen isotopic (δ18OH2O) values for both the deposits vary from +7.2 to +14.6‰ (average: +9.3) and +7.3 to +8.9‰ (average: +8.3‰) respectively. Ore-forming fluid could have possibly been derived from the decarbonation of marine carbonates during the metamorphic devolatilization of greenstones. The slight intra-deposit variation of the carbon isotopic value implies the Rayleigh fractionation with progressive consumption of the CO2 as a result of interaction between hydrothermal fluid and wall rock as well as immiscibility of CH4 and CO2 fluid. Ore fluid sulfur (δ34SH2S) isotopic composition, derived from δ34S values of sulfides, varies from −0.42 to +2.43‰ and +0.25 to +2.34‰ for Chigargunta and Bisanatham respectively. Both the sulfur and carbon isotopic values, from these deposits are comparable with other orogenic gold deposits in the DC and elsewhere in the world. Although carbon isotopic values possibly indicate the metamorphic fluid source, the sulfur isotopes signify average crustal sulfur composition that is from crustally derived/modified fluid. Based on the field-geological, mineralogical, fluid, and isotopic compositions, the possible fluid source could be from the metamorphic devolatilization process of the thick greenstone sequence in the SKGB. However, a magmatic source could be further tested with geochronological studies on intrusives in the area.

The 40Ar/39Ar dating of quartz: new insights into the metallogenic chronology of the Jinchang gold deposit and its geological significance

The Jinchang gold deposit has been extensively studied, but precise dates for its formation are debated. Native gold mainly occurs as inclusions within pyrite and quartz. In this study, we analysed quartz crystals coeval with gold precipitation from two different types of mineralization using the ArgusVI multi-collector noble gas mass spectrometer by the stepwise crushing technique to resolve the timing and genesis of gold mineralization. 40Ar/39Ar dating of quartz samples (J12Q) from breccia ore yields a plateau age of 109.87 ± 0.86 Ma, and an inverse isochron age of 109.87 ± 0.88 Ma. Quartz samples (J18Q) from vein ore yields a slightly younger plateau age of 107.76 ± 0.85 Ma, with an inverse isochron age of 107.76 ± 0.92 Ma. These dates place the ore-forming age of the Jinchang gold deposit at 107~110 Ma, much younger than previously published radiometric ages, suggesting the gold mineralization is spatio-temporally associated with the granite porphyry. The formation of the Jinchang gold deposit is consistent with the regional late Mesozoic porphyry-epithermal gold mineralization event in the Yanbian-Dongning area. Finally, our study shows that 40Ar/39Ar of quartz can be used as a powerful tool to date the formation ages of hydrothermal ore deposits.

Zircon indicators of fluid sources and ore genesis in a multi-stage hydrothermal system: The Dongping Au deposit in North China

The syenite-hosted Dongping gold deposit represents the largest gold deposit on the northern margin of the North China Craton. Here, we present new UPb age, trace-element concentration, and Hf-isotope data from magmatic and hydrothermal zircons to better constrain the timing and genesis of gold mineralization. Geochronological data, combined with field and petrologic results, document two mineralization events. The earlier event, in the mid-Devonian (~380 Ma), was associated with the intrusion of the Shuiquangou syenite, whereas the later event, in the early Cretaceous (~140 Ma), was related to the emplacement of the Shangshuiquan granite. Devonian hydrothermal zircons extracted from gray auriferous quartz veins were formed via metasomatic alteration of magmatic zircons derived from the syenite, by an oxidizing F-rich fluid. In contrast, Cretaceous hydrothermal zircons hosted in the altered Shangshuiquan granite and milky gold-rich quartz ores were precipitated directly from a high-temperature Zr-saturated aqueous fluid that was contaminated by basement materials, possessing a substantially different Hf-isotopic composition from the contemporaneous magmatic zircons. We propose that the Devonian Au mineralization event at Dongping involved a magmatic fluid and auto-metasomatism during crystallization of the Shuiquangou syenite complex, whereas the Cretaceous Au mineralization event involved a fluid exsolved from the Shangshuiquan alkaline pluton. Results from this study highlight the potential utility of zircons as indicators of fluid sources and ore genesis in a multi-stage hydrothermal system.

Gold in Devono-Carboniferous red beds of northern Britain

Authigenic gold occurs in Devono-Carboniferous red beds in northern Britain. Red beds exhibit concentrations of gold and pathfinder elements for gold mineralization including tellurium and mercury, at redox boundaries. Detailed studies of samples from Millport, Isle of Cumbrae, Firth of Clyde, show particles of native gold up to 10 µm size, typically with less than 15 wt% silver. Their context indicates that the gold was concentrated during diagenesis, in rocks that had not experienced regional temperatures above 100°C. These occurrences add to other evidence of a role for red beds in the genesis of gold mineralization.

The role of the granite emplacement and structural setting on the genesis of gold mineralization in Egypt

The Eastern Desert of Egypt is well known as a gold-mining district since ancient times. Gold mineralization is closely associated with the granitic rocks in such way that the mineralization is either hosted by or occurs immediately adjacent to the granite intrusions. Granitic rocks accompanying gold mineralization in the Eastern Desert can be grouped into three categories i.e. syn-late tectonic calc-alkaline granites, calc-alkaline to mildly alkaline granites of the transitional stage and post-tectonic alkaline granites.Tectonically, gold mineralization is linked with the tectonothermal stages that were operative during the evolution of the Arabian–Nubian Shield (ANS). During the primitive stages of the island-arc formation, pre-orogenic gold mineralization (auriferous exhalites) was formed by hot brines accompanying submarine volcanic activity. No role for the granite is observed in this stage. Syn-orogenic gold mineralization (i.e. gold hosted in altered ophiolitic serpentinites along thrust faults and in sutures, quartz veins hosted in the metavolcano-sedimentary assemblage and/or the I-type granitic rocks surrounding them) connected with the collision and accretion stage is characterized by emplacement of calc-alkaline (I-type) older granite batholiths. Shear fractures reflected in brittle–ductile shear zones and amphibolite-green schist facies regional metamorphism were broadly contemporaneous with this intense compressional tectonic regime. Available fluid inclusion microthermometry and isotopic studies reveal that both metamorphic and magmatic fluids related to the syn-late tectonic calc-alkaline granites were operative. A further indication for the role of the granites is indicated by the presence of some concentrations of Antimony, Bismuth, Molybdenum, Tungsten, Rubidium, Beryllium, Tin, Yttrium, Ytterbium, Tantalum and Niobium in some auriferous quartz veins in the Egyptian gold mines.In the cratonal development of the (ANS), the land underwent a transitional stage between the major subduction-related calc-alkaline magmatic activity and the subsequent post-tectonic plutonism represented by the alkaline granites. This transitional stage is dominated by the eruption of Dokhan volcanics and deposition of molass-type Hammamat sediments. At ~590–530Ma, the Arabian–Nubian Shield was deformed by post-accretionary structures, in the form of N-trending shortening zones such as the Hamisana shear zone and NW-trending strike-slip faults such as the Najd fault system. The regional NNW–SSE directed extension opened spaces that were progressively sealed with different magmatic phases including among them a considerable proportion of rocks referred to as “younger granites” in the Egyptian literature. Late-orogenic gold mineralization connected with the transitional stage is represented principally by the gold-bearing quartz veins traversing Hammamat molasse sediments, quartz veins traversing syn-extensional younger granites and generally quartz veins in ductile to brittle shears related to the Najd fault system and within Hamisana shear zone and its splays.By the end of Pan African orogeny until the Tertiary, the basement was intermittently intruded by a number of sub-alkaline to per alkaline granite bodies that host Mo, Sn, W, Nb–Ta and U mineralization in the Eastern Desert of Egypt. Anorogenic gold mineralization connected with post-orogenic granites is represented by small amounts of the element in disseminations, stockworks and quartz veins of Sn–W–Ta–U mineralization.The present review shows that gold mineralization in Egypt is an expression of two major cycles with distinct magmatic and tectonic characteristics, and the two cycles were separated by a transitional stage. The emplacement of granites in the compressional cycle played an important role in metamorphosing the country rocks by producing the heat energy required for the regional metamorphism and the providing of the magmatic fluids. The H2O–CO2 fluids enriched in volatiles were released at the greenschist–amphibolite facies transition at 450°–500°C and mixed with the I-type calc-alkaline granite related fluids and both moved down a temperature gradient away from the amphibolite-green schist transition at depth to a lower temperature regime in the upper levels where it is deposited in brittle–ductile shear zones. With the extensional cycle, the syn-extensional granite intrusions acted as heat engine in such way that the heat of the granite drove the convective cells to circulate through the auriferous host-granite contacts, leaching gold and other elements and depositing it in structurally favorable sites. In addition, the contrasts in competency between the granites with brittle deformational characteristics and the surrounding country rocks with a ductile response to stress, led to a generation of extensive fracture pattern within the more competent unit.

New tectono-geochronological constraints on timing of shearing along the Ailao Shan-Red River shear zone: Implications for genesis of Ailao Shan gold mineralization

Several world class gold deposits are located along the Ailao Shan (ALS) belt in eastern Tibet, China. The genesis of gold mineralization along the belt, however, has been the subject of debates in the last decades, which highlights the importance of dating shearing, magmatism and mineralization along the Ailao Shan-Red River shear zone (ASRR). Through detailed field observations and microscopic analysis, a group of leucocratic intrusions from within and outside the shear zone along the ALS belt are investigated in the present paper. Pre-, syn- and post-shearing intrusions are grouped based on structural and microstructural analysis. LA-ICP-MS and SIMS dating of the intrusions revealed the existence of two age populations, a group of ages older than 30Ma and the other younger than 28Ma. The former are distributed both within and outside the shear zone, and the latter, in contrast, occur only within the shear zone. Our new results show that the ductile shearing along the ASRR shear zone initiated since ca. 30Ma ago.The dating results place major constraints on timing of shearing along the ASRR shear zone and have profound implications on the genesis of gold mineralization along the ALS belt. The present study reveals that ductile shearing along the ASRR shear zone was resulted from extrusion of the Indochina block late during the Indian–Eurasian plate collision. Meanwhile, we conclude that the gold mineralization took place prior to the shearing, but in close relation to an early magmatism (>30Ma) ascribed to post-collisional extension collapse involving mantle processes, early during the plate collision.

Timing, tectonic implications and genesis of gold mineralization in the Xincheng gold deposit, China: C–H–O isotopes, pyrite Rb–Sr and zircon fission track thermochronometry

The Xincheng gold deposit located at the northwestern edge of the Jiaodong Peninsula in the North China Craton has a reserve of ~96t Au and is among the largest in the Jiaodong gold province. Here, disseminated- and stockwork-style ores are hosted in Mesozoic granitoids. The mineralization and alteration are largely controlled by the regional Jiaojia fault. The C–H–O isotope values in the ores indicate that the ore-forming fluids were derived mainly from magmatic sources. New Rb–Sr geochronological data on hydrothermal pyrite from the Xincheng deposit presented in this study indicate that the main gold mineralization occurred between 126Ma and 122Ma. The zircon fission track of convergent ages and decomposed divergent ages from the ores and altered rocks show four groups of ages: ~135Ma, ~124Ma, 96–75Ma, and 66–52Ma. The ~135Ma is related to the exhumation and cooling of the Linglong granitoids. The ~124Ma is considered to represent the principal stage of gold mineralization, and is consistent with the pyrite Rb–Sr ages. The younger ages of 96–75Ma and 66–52Ma resulted from subsequent annealing caused by later tectonic activity. The timing of gold mineralization in the Xincheng gold deposit broadly coincides with the switch in stress field in eastern China from extension to transpression, resulting from the change in subduction direction of the Pacific Plate from NE–SW to NW–SE, and marked by extension-related voluminous magmatism associated with the Cretaceous giant igneous event in eastern China.

A model for the genesis of gold mineralization in carbonaceous schists of the Southern Urals

Based on the peculiarities of gold manifestations in carbonaceous schists of the Southern Urals, a model for the genesis of gold mineralization is proposed. The pattern takes a complex of related processes into consideration: sedimentation, transformation as a result of submergence, dynamometamorphism, and contact metamorphism. Hence, prognostic-prospecting factors for searching for the hydrothermal-metamorphogenic gold mineralization in carbonaceous deposits are proposed.

Constraints on the genesis of gold mineralization at the Homestake Gold Deposit, Black Hills, South Dakota from rhenium–osmium sulfide geochronology

The Homestake gold deposit, located in the Black Hills, South Dakota, USA, is one of the largest known hydrothermal gold deposits globally, with total mining production exceeding 40 Moz Au. Rhenium–osmium geochronology of ore-associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern Black Hills. Arsenopyrite yields a rhenium–osmium (Re–Os) age of 1,736 ± 8 Ma (mean squared weighted deviation = 1.6), consistent with existing age constraints for gold mineralization, whereas Re–Os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age. This is taken to indicate that the Re–Os arsenopyrite chronometer is robust to at least 400°C, whereas the Re–Os pyrrhotite chronometer is likely disturbed by temperatures of 300–350°C. The Re–Os arsenopyrite age and initial Os ratio (0.28 ± 0.15) are interpreted to indicate that gold was introduced at ca. 1,730 Ma, coincident with the onset of exhumation of crustal blocks and, possibly, the earliest intrusive phases of Harney Peak granite magmatism. New in situ U–Pb monazite analyses from an aplite dike in the east-central Black Hills indicate that granite magmatism was a protracted event, persisting until at least ca. 1,690 Ma.

Geologic, Geochemical, and Geochronological Constraints on the Genesis of Gold Mineralization at Poplar Mountain, Western New Brunswick, Canada

The Poplar Mountain gold occurrence in western New Brunswick is hosted in the Poplar Mountain volcanic complex (PMVC), which is located along the southern segment of the regional Woodstock fault zone. The PMVC consists of three principal units including, in ascending order, a porphyritic felsic volcanic unit, a volcaniclastic unit, and a mafic volcanic unit. U-Pb dating of zircon indicates that the age of the volcanic rocks is younger than 459 ± 3 Ma, and 40Ar/39Ar dating of mineralization-associated sericite indicates that the age of mineralization is 411 ± 3.7 Ma. Gold-mineralized zones occur in all the lithologic units but mainly in the porphyritic felsic volcanic unit. Gold zones are not controlled by individual faults but are characterized by high-density brittle fracturing, thin carbonate-quartz veining, and intensive ankerite-sericite alteration, superimposed on an earlier chlorite-cal- cite-quartz alteration. Gold is associated with arsenopyrite, which mostly occurs as disseminations in the host rocks, and to a lesser extent with quartz-carbonate-sericite veins and quartz-cemented breccias. Geochemical data indicate that mineralization-associated alteration is characterized by enrichment of K, Rb, Cs, Ca, Sr, Ba, As, Sb, W, C, and S. Fluid inclusion data indicate that the mineralizing fluids are composed of H 2O, salts, and CO2 with variable amounts of N2 and CH4; salinities range from 2.3 to 10.6 eq. wt.% NaCl, but mainly from 2.3 to 5.6 eq. wt.% NaCl, and homogenization temperatures lie mainly between 220° and 270°C. Fluid pressures at the site of mineralization are estimated to have been 770 to 1200 bars, corresponding to a depth of 2.9 to 4.6 km at lithostat- ic pressure. The δ18OSMOW and δ13CPDB values of mineralization-related ankerite range from +14.5‰ to +16.5‰, and -6.8‰ to -8.3‰, respectively. The δ18O values of the ore-forming fluids are estimated to be 6.4‰ to 8.3‰ at a temperature of 250°C. These carbon and oxygen isotope data fall in the field of magmatic fluids and in part the field of metamorphic fluids. Considering the geological setting of the region, abundant granitic intrusions, the similarity between the age of mineralization (411 ± 3.7 Ma) and the nearby Pokiok batholith (402-415 Ma), and the enrichment of granophile elements in mineralization-related alteration, the ore-forming fluids were probably derived from a granitic intrusion underneath the PMVC. These fluids were focused along structures related to the Woodstock fault. The Poplar Mountain gold occurrence might be compared to some granitic intrusion-related gold systems based on the geochemical data presented herein. © 2008 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.

Geochemistry, Geochronology and Genesis of Gold Mineralization in Nurt of Northern Altay, Xinjiang: A Case Study on the Aketishikan Gold Deposit

Abstract Gold deposits such as the Aketishikan, Togetobie, Tasbig‐Kokeydlas, Kums and Hongshanzui gold deposits in the Nurt area in Altay of Xinjiang were found in Member 3 rhyolite tufflava, fragmental lava and ignimbrite of the Carboniferous Hongshanzui Group. Trace and rare earth elements, sulfur, lead, oxygen and hydrogen isotopes, and geochronological studies indicate that the ore‐forming material was mostly supplied by the Carboniferous volcanic rocks through water‐rock interaction under a low‐to‐moderate temperature, and the hydrothermal ore‐forming fluid came from meteoric water with some magmatic water input evolved from the granitic magmas. Gold deposits in the Nurt area as well as in the northern Altay might form in multiple stages, and the Yanshanian mineralization period should be paid more attention besides the Variscan mineralization period.

Geochemistry, alteration, and genesis of gold mineralization in the Okote area, southern Ethiopia

Ductile shear zone hosted mesothermal gold mineralization of the Okote area is located in southern Ethiopia. Three N-S striking ductile shear zones, with different intensity of shearing and hydrothermal alteration, cut the mafic rocks. The gold-mineralized parts of these shear zones reveal zonings: slightly altered but not sheared protolith at shear boundaries, transitional zone, and mylonite zones. Auriferous quartz-carbonate-tourmaline veins occur mainly in the mylonite zone. The ore minerals of the veins and their wall rocks are pyrite, chalcopyrite, pyrrhotite, gold, and accessory chalcocite, covellite, galena, and melonite (NiTe2). The textural relationships among minerals in these alteration zones indicate that epidote, ferroamphiboles, and magnetite were formed first, followed by chlorite, ankerite, pyrrhotite, chalcopyrite I, and K-feldspar, and, finally, calcite, chlorite, biotite, tourmaline, gold, and galena. Primary fluid inclusion data from auriferous type 2 quartz veins (qv2) indicate aqueous-carbonic inclusions with low salinity (<6.59 wt % NaCl equivalent), 0.38 to 0.90 g/cm3 in density that homogenized at 218°C to 345°C. Most of the inclusions decrepitate at 220°C to 388°C before or immediately after homogenization. Chlorite geothermometry gives temperatures of formation that range from 230°C to 410°C with modes at 250°C and 380°C, in good agreement with fluid inclusion data. Chemical mass balance studies, using samples from meta-gabbro and alteration products, reveal addition of K2O, P2O5, volatile, Ba, Sr, V, and Cu into wall rock and loss of MgO, CaO, and SiO2 from the wall rock to the veins accompanying gold mineralization. Chondrite-normalized REE patterns of samples show HREE enrichments in meta-gabbro, a flat pattern with a positive Eu anomaly in the epidote-amphibole-magnetite rich transitional zone, and HREE enrichment with a negative Cc anomaly in the mylonite zone. Stable isotope ratios of sulfur, carbon, and oxygen indicate a predominance of deep-seated fluids of metamorphic and magmatic signatures. Considering the combined structural and spatial association of gold with greenschist facies, the mineral and wall rock chemistry, fluid inclusion data, and isotopic data presented here, we conclude that the Okote gold mineralization formed by interaction of structurally focused hydrothermal fluids with mafic rocks.

A Study on the Occurrence of Gold in Unoxidized Carlin-Type Ores in China Using AEM, SEM-EDX and SXRF

The knowledge about the occurrence of gold is essential both to the explanation for the genesis of gold mineralization in its deposits and to the evaluation and exploration or even smelt process of its ores. It has been well known that the gold occurrence in the Carlin-type ores still remains a difficult question to be answered because of the tiny scale of its locality and its very low content.This paper reports the results of our analysis on some gold bearing minerals in the Carlin-type ores discovered during recent years in China with combined use of analytical electron microscopy (AEM), scanning electron microscopy-energy dispersive X ray spectrometry (SEM-EDX) and synchrotron X ray flourescence analysis (SXRF) techniques as following:(1) Some gold occurred as submicron size grains in the ores (see Photo 1-4 and Figure 1-3) with grain size generally less than 0.2 micron.(2) It has been found by AEM and SEM-EDX observation and SXRF analysis that gold occurred as micrograins embedded in the boundaries of clay or quartz minerals rather than, as said, entered the lattice or adhered as a covering film to the surface of clay minerals (see Figure 4).