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1.
Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses.The mineralized quartz veins are characterized by abundant carbonic (CO2 ± CH4 ± H2O) and aqueous-carbonic (H2O–NaCl–CO2 ± CH4) inclusions along intragranular trails, whereas aqueous inclusions (H2O–NaCl ± CO2) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ34S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ34SΣs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO2 via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites. 相似文献
2.
The Inata gold deposit is hosted in the Bouroum greenstone belt of northern Burkina Faso and contains ca. 5 Moz of gold resource. The greenstone belt is divided into 4 distinct domains: The Pali West, Pali-Minfo and Fété Kolé domains comprised of variable proportions of mafic to intermediated volcanic, volcaniclastic and sedimentary rocks, and the Sona Basin comprised of feldspathic sandstones and turbidites. Potential Tarkwaian-like conglomerates are rarely observed on the eastern margin of the basin. The stratigraphy is crosscut by a series of intrusions between 2172 ± 15 Ma and 2122 ± 4 Ma. A complex deformation sequence is recorded in the rocks and has been interpreted in a five stage scheme: early syn-depositional basin margin faults reactivated through time and partitioning all subsequent regional deformation (DeB); N–S compression (D1B > 2172 Ma); E-W compression (D2B, < ca 2122 Ma); NW–SE compression (D3B), and a late N–S compression (D4B). D2B-D4B overprint all rocks, including those of the Sona Basin and Tarkwaian-like conglomerates. Peak metamorphism is mid- to upper-greenschist facies.Mineralisation at Inata is hosted in black shales and volcaniclastic rocks of the Pali-Minfo domain and comprises shear-zone hosted quartz-tourmaline-ankerite veins with associated sulphides dominated by pyrite and arsenopyrite. Three generations of pyrite (py1, py2, py3) and one generation of arsenopyrite (apy2) have been identified. Py1 is parallel to bedding and early D1B foliation and not associated with gold. Py2 and apy2 are coeval, contain up to 1 ppm gold and are spatially associated with auriferous quartz veins. Py3 locally overprints previous assemblages and is also associated with Au. Fluid inclusions in quartz indicate H2O to H2O–CO2–NaCl fluids in auriferous quartz veins.Microscopic to macroscopic observation of fabric-mineral-vein crosscutting relationships indicate that mineralisation is syn-D2B, disrupted and remobilised during D3B. All observations and data are consistent with Inata representing an orogenic style of gold mineralisation formed relatively late in the evolution of the host terrane. 相似文献
3.
Located along the southern part of the Yarlung Zangbo suture zone in southern Tibet, Bangbu is one of the largest gold deposits in Tibet. Auriferous sulfide-bearing quartz veins are controlled by second- or third-order brittle fractures associated with the regional Qusong–Cuogu–Zhemulang brittle-ductile shear zone. Fluid inclusion studies show that the auriferous quartz contains aqueous inclusions, two-phase and three-phase CO2-bearing inclusions, and pure gaseous hydrocarbon inclusions. The CO2-bearing inclusions have salinities of 2.2–9.5% NaCleq, and homogenization temperatures (Th) of 167–336 °C. The δD, δ18O, and δ13C compositions of the Bangbu ore-forming fluids are − 105.5 to − 44.4‰, 4.7 to 9.0‰ and − 5.1 to − 2.2‰, respectively, indicating that the ore-forming fluid is mainly of metamorphic origin, with also a mantle-derived contribution. The 3He/4He ratio of the ore-forming fluids is 0.174 to 1.010 Ra, and 40Ar/36Ar ranges from 311.9 to 1724.9. Calculations indicate that the percentage of mantle-derived He in fluid inclusions from Bangbu is 2.7–16.7%. These geochemical features are similar to those of most orogenic gold deposits. Dating by 40Ar/39Ar of hydrothermal sericite collected from auriferous quartz veins at Bangbu yielded a plateau age of 44.8 ± 1.0 Ma, with normal and inverse isochronal ages of 43.6 ± 3.2 Ma and 44 ± 3 Ma, respectively. This indicates that the gold mineralization was contemporaneous with the main collisional stage between India and Eurasia along the Yarlung Zangbo suture, which resulted in the development of near-vertical lithospheric shear zones. A deep metamorphic fluid was channeled upward along the shear zone, mixing with a mantle fluid. The mixed fluids migrated into the brittle structures along the shear zone and precipitated gold, sulfides, and quartz because of declining temperature and pressure or fluid immiscibility. The Bangbu is a large-scale Cenozoic syn-collisional orogenic gold deposit 相似文献
4.
The Wangu gold deposit in northeastern Hunan, South China, is one of many structurally controlled gold deposits in the Jiangnan Orogen. The host rocks (slates of the Lengjiaxi Group) are of Neoproterozoic age, but the area is characterized by a number of Late Jurassic–Cretaceous granites and NE-trending faults. The timing of mineralization, tectonic setting and ore genesis of this deposit and many similar deposits in the Jiangnan Orogen are not well understood. The orebodies in the Wangu deposit include quartz veins and altered slates and breccias, and are controlled by WNW-trending faults. The principal ore minerals are arsenopyrite and pyrite, and the major gangue minerals are quartz and calcite. Alteration is developed around the auriferous veins, including silicification, pyritic, arsenopyritic and carbonate alterations. Field work and thin section observations indicate that the hydrothermal processes related to the Wangu gold mineralization can be divided into five stages: 1) quartz, 2) scheelite–quartz, 3) arsenopyrite–pyrite–quartz, 4) poly-sulfides–quartz, and, 5) quartz–calcite. The Lianyunshan S-type granite, which is in an emplacement contact with the NE-trending Changsha-Pingjiang fracture zone, has a zircon LA-ICPMS U–Pb age of 142 ± 2 Ma. The Dayan gold occurrence in the Changsha-Pingjiang fracture zone, which shares similar mineral assemblages with the Wangu deposit, is crosscut by a silicified rock that contains muscovite with a ca. 130 Ma 40Ar–39Ar age. The gold mineralization age of the Wangu deposit is thus confined between 142 Ma and 130 Ma. This age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of NE-trending extensional faults, the emplacement of Late Jurassic–Cretaceous granites and the formation of Cretaceous basins filled with red-bed clastic rocks in northeastern Hunan, which forms part of the Basin and Range-like province in South China. EMPA analysis shows that the average As content in arsenopyrite is 28.7 atom %, and the mineralization temperature of the arsenopyrite–pyrite–quartz stage is estimated to be 245 ± 20 °C from arsenopyrite thermometry. The high but variable Au/As molar ratios (>0.02) of pyrite suggest that there are nanoparticles of native Au in the sulfides. An integration of S–Pb–H–O–He–Ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks, possibly driven by hydraulic potential gradient created by reactivation of the WNW-trending faults initially formed in Paleozoic, with possible involvement of magmatic and mantle components channeled through regional fault networks. The Wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic, epithermal and Carlin-type gold deposits, and may be better classified as an “intracontinental reactivation” type as proposed for many other gold deposits in the Jiangnan Orogen. 相似文献
5.
The Dongping gold deposit hosted in syenites is one of the largest hydrothermal gold deposits in China and composed of ore veins in the upper parts and altered zones in the lower parts of the ore bodies. Pervasive potassic alteration and silicification overprint the wall rocks of the ore deposit. The alteration minerals include orthoclase, microcline, perthite, quartz, sericite, epidote, calcite, hematite and pyrite, with the quartz, pyrite and hematite assemblages closely associated with gold mineralization. The phases of hydrothermal alteration include: (i) potassic alteration, (ii) potassic alteration - silicification, (iii) silicification - epidotization - hematitization, (iv) silicification - sericitization - pyritization and (v) carbonation. Mass-balance calculations in potassic altered and silicified rocks reveal the gain of K2O, Na2O, SiO2, HFSEs and transition elements (TEs) and the loss of REEs. Most major elements were affected by intense mineral reactions, and the REE patterns of the ore are consistent with those of the syenites. Gold, silver and tellurium show positive correlation and close association with silicification. Fluid inclusion homogenization temperatures in quartz veins range from 154 °C to 382 °C (peak at 275 °C–325 °C), with salinities of 4–9 wt.% NaCl equiv. At temperatures of 325 °C the fluid is estimated to have pH = 3.70–5.86, log fO2 = − 32.4 to − 28.1, with Au and Te transported as Au (HS)2− and Te22 − complexes. The ore forming fluids evolved from high pH and fO2 at moderate temperatures into moderate-low pH, low fO2 and low temperature conditions. The fineness of the precipitated native gold and the contents of the oxide minerals (e.g., magnetite and hematite) decreased, followed by precipitation of Au- and Ag-bearing tellurides. The hydrothermal system was derived from an alkaline magma and the deposit is defined as an alkaline rock-hosted hydrothermal gold deposit. 相似文献
6.
Vein-type gold deposits in the Atud area are related to the metagabbro–diorite complex that occurred in Gabal Atud in the Central Eastern Desert of Egypt. This gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the NW–SE brittle–ductile shear zone, as well as along the contacts between them. By using the mass balance calculations, this work is to determine the mass/volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits. In addition, we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers. Two generations of quartz veins include the mineralized grayish-to-white old vein (trending NW–SE), and the younger, non-mineralized milky white vein (trending NE–SW). The ore minerals associated with gold are essentially arsenopyrite and pyrite, with chalcopyrite, sphalerite, enargite, and goethite forming during three phases of mineralization; first, second (main ore), and third (supergene) phases. Three main hydrothermal alteration zones of mineral assemblages were identified (zones 1–3), placed around mineralized and non-mineralized quartz veins in the underground levels. The concentrations of Au, Ag, and Cu are different from zone to zone having 25–790 ppb, 0.7–69.6 ppm, and 6–93.8 ppm; 48.6–176.1 ppb, 0.9–12.3 ppm, and 39.6–118.2 ppm; and 53.9–155.4 ppb, 0.7–3.4 ppm, and 0.2–79 ppm for zones 1, 2, and 3, respectively.The mass balance calculations and isocon diagrams (calculated using the GEOISO-Windows program) revealed the gold to be highly associated with the main mineralized zone as well as sericitization/kaolinitization and muscovitization in zone 1 more than in zones 2 and 3. The sericite had a higher muscovite component in all analyzed flakes (average XMs = 0.89), with 0.10%–0.55% phengite content in wall rocks and 0.13%–0.29% phengite content in mineralized quartz veins. Wall rocks had higher calcite (CaCO3) contents and lower MgCO3 and FeCO3 contents than the quartz veins. The chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite, with estimated formation temperatures of 289–295 °C and 301–312 °C, respectively. Albite has higher albite content (95.08%–99.20%) which occurs with chlorite in zone 3. 相似文献
7.
In the Eastern Dharwar craton, among the many shear zone-hosted lode gold deposits, those at Ramagiri and Penakacherla are located near the western margin of the craton. Mineralized quartz (± sulfide ± carbonate) veins are hosted by the schistose (metavolcanic and carbonaceous metasedimentary) rocks, in close spatial association with granitoids having quartz and quartzofeldspathic veins representing hydrothermal activities associated with them. Mineralized quartz veins from the ore zones (in Ramagiri and Penakacherla regions) and quartz (or pegmatitic) veins from the surrounding granitic terrane were chosen for δ18O analysis. Samples from the schistose and granitic domains show δ18Oquartz values in the range of 10.4–14.9 and 9.3–10.9‰ respectively. The ore-zone fluids from the Ramagiri and Penakacherla regions give δ18O values of 7.9 ± 1.5 and 5.1 ± 0.8‰, calculated at pressure-corrected temperatures obtained from fluid inclusion microthermometry. The late-magmatic fluid is relatively 18O-poor with δ18O values estimated at 4.5 ± 0.7‰ and the value is closer to what is obtained for the ore zones. Based on the δ18O values reported and a possible magmatic contribution to ore fluid deciphered from fluid inclusion characteristics, a genetic relationship between granitic magmatism and gold mineralization is surmised. The observed increase in the 18O/16O ratio from the magmatic fluid to ore fluid in the shear zone is attributed to interaction of the magmatic fluid with host metasediments, that agrees well with the variation in the CO2/CH4 ratio of carbonic component in such fluids. 相似文献
8.
Carbon (δ13CPDB) and oxygen (δ18OSMOW) isotopic compositions of auriferous quartz-carbonate veins (QCVs) of gold deposits from Sangli, Kabuliyatkatti, Nagavi, Nabapur and Mysore mining areas developed on the Central Lode system of the Gadag Gold Field (GGF) in the Neoarchaean Gadag schist belt of the Dharwar Craton, southern India have been examined for the first time to understand the origin of the mineralising fluids. In majority of the samples (46 out of 49), δ13Cpdb of carbonates of the QCVs fall in the range from − 2.2‰ to − 9.7‰ and the δ18O values range from 12.0‰ to 30.5‰ SMOW. The calculated fluid δ13C∑ C compositions for these deposits range from − 2.1‰ to − 9.6‰ and δ18OH2O from 6.8‰ to 25.9‰, respectively. Carbonate δ13C and fluid δ13C∑ C compositions of the carbonates of the QCVs of the GGF are not only distinct from the carbon isotope range of marine carbonates or meta-sedimentary carbonates of the Chitradurga schist belt, but are consistent with C-isotope values of magmatic (− 5 ± 3‰, Burrows et al., 1986) and/or mantle (− 6 ± 2‰, Ohmoto, 1986) carbonates. As dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO2 with δ13C values similar to or more enriched than parent rock, the carbonate or fluid δ13C ratios of the QCVs (which fall in the compositional range of mantle/magmatic derived CO2 or carbonates) obtained in this work cannot be the result of metamorphism. The present study corroborates our previous reports from Ajjanahalli and G.R. Halli gold deposits (Sarangi et al., 2012) occurring in the vicinity of the southern extension of the same crustal scale shear zone on which all the GGF deposits are located.The age of gold mineralisation in this area has been reported to be 2522 ± 6 Ma by Sarma et al., 2011. Chardon et al. (2011) have proposed large-scale remobilization of the older gneissic basement, as well as, emplacement of juvenile granites between 2559 Ma and 2507 Ma, close to the crustal scale shear zone along the eastern margin of the Chitradurga schist belt. Based on these observations and our isotope studies, it is proposed that gold mineralising fluids were derived from mantle/juvenile magmatic melts and were channelled through crustal scale shear zones to give rise to the gold deposits in the GGF. 相似文献
9.
The Macraes deposit (> 10 Moz resource) is a Cretaceous orogenic system hosted in the Hyde-Macraes Shear Zone (HMSZ) which was mineralised under lower greenschist facies during later stages of lower greenschist facies metamorphism of host metasedimentary schists. Gold is encapsulated primarily in sulphides that have replaced silicates in ductile shears that are focussed in micaceous rocks. The shears anastomose around structurally competent lenses, and were enhanced by hydrothermal graphite deposition and alteration of albite to muscovite. In contrast, scheelite with minor auriferous sulphides occurs in multigenerational quartz veins that filled fractures in competent lithologies. Hence, scheelite was deposited coevally with gold, from the same hydrothermal fluid, but in different structural settings from most gold at all scales from millimetres to hundreds of metres. Consequentially, there is weak correlation between Au and W at all scales in the deposit. Multigenerational gold and scheelite mineralisation occurred during progressive deformation in the shear zone in two contrasting structural and mineralogical styles in syn-deformationally weakening gold-bearing micaceous shears, and in syn-deformationally hardened competent rocks that became silicified and veined with quartz and scheelite. Hydrothermal fluid flow in the gold-bearing shears occurred at the grain boundary, microshear, and microfracture scales, and was slow (< 1 m/year), continuous, and pervasive. In contrast, vein formation in more competent lithologies was episodic, locally rapid (> hundreds of m/year), and was controlled by fracture permeability. The Au and W enrichment in the Macraes deposit resulted from regional scale metal mobility, driven by coeval recrystallisation in higher-grade (upper greenschist to amphibolite facies) metamorphism that persisted structurally below the Macraes deposit for at least 10 Ma after mineralisation ceased. 相似文献
10.
The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO2 inclusions, CO2–H2O inclusions, hydrocarbon inclusions, and hydrocarbon–H2O inclusions. The ore-forming fluids are characterized by a H2O + CO2 + CH4 ± N2 system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H2O + NaCl → H2O + NaCl + CO2 + CH4 ± N2 → H2O + NaCl ± CH4 ± CO2 with a slight decrease in homogenization temperature and salinity. The δ18O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δDH2O and calculated δ18OH2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ13CPDB and δ18OSMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H2O–NaCl–CO2 fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold. 相似文献
11.
The Tasiast gold deposits are hosted within Mesoarchean rocks of the Aouéouat greenstone belt, Mauritania. The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite-quartz-pyrrhotite ± actinolite ± garnet ± biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units. The West Branch deposit is hosted within meta-igneous rocks, mainly diorites and quartz diorites that lie stratigraphically below host rocks of the Piment deposits. Most of the gold mineralisation at West Branch is hosted by quartz–carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. At Tasiast, gold mineralisation has been defined over a strike length > 10 km and to vertical depths of 740 m. All of the significant mineralised bodies defined to date dip moderately to steeply (45° to 70°) to the east and have a south–southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the Tasiast thrust. An age of 2839 ± 36 Ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation. 相似文献
12.
The Qianfanling Mo deposit, located in Songxian County, western Henan province, China, is one of the newly discovered quartz-vein type Mo deposits in the East Qinling–Dabie orogenic belt. The deposit consists of molybdenite in quartz veins and disseminated molybdenite in the wall rocks. The alteration types of the wall rocks include silicification, K-feldspar alteration, pyritization, carbonatization, sericitization, epidotization and chloritization. On the basis of field evidence and petrographic analysis, three stages of hydrothermal mineralization could be distinguished: (1) pyrite–barite–quartz stage; (2) molybdenite–quartz stage; (3) quartz–calcite stage.Two types of fluid inclusions, including CO2-bearing fluid inclusions and water-rich fluid inclusions, have been recognized in quartz. Homogenization temperatures of fluid inclusions vary from 133 °C to 397 °C. Salinity ranges from 1.57 to 31.61 wt.% NaCl eq. There are a large number of daughter mineral-CO2-bearing inclusions, which is the result of fluid immiscibility. The ore-forming fluids are medium–high temperature, low to moderate salinity H2O–NaCl–CO2 system. The δ34S values of pyrite, molybdenite, and barite range from − 9.3‰ to − 7.3‰, − 9.7‰ to − 7.3‰ and 5.9‰ to 6.8‰, respectively. The δ18O values of quartz range from 9.8‰ to 11.1‰, with corresponding δ18Ofluid values of 1.3‰ to 4.3‰, and δ18D values of fluid inclusions of between − 81‰ and − 64‰. The δ13CV-PDB values of fluid inclusions in quartz and calcite have ranges of − 6.7‰ to − 2.9‰ and − 5.7‰ to − 1.8‰, respectively. Sulfur, hydrogen, oxygen and carbon isotope compositions show that the sulfur and ore-forming fluids derived from a deep-seated igneous source. During the peak collisional period between the North China Craton and the Yangtze Craton, the ore-forming fluids that derived from a deep igneous source extracted base and precious metals and flowed upwards through the channels that formed during tectonism. Fluid immiscibility and volatile exsolution led to the crystallization of molybdenite and other minerals, and the formation of economic orebodies in the Qianfanling Mo deposit. 相似文献
13.
《Ore Geology Reviews》2010,37(4):265-281
Axi is a low-sulfidation type epithermal gold deposit hosted in Paleozoic subaerial volcanic rocks in the western Tianshan orogenic belt, Xinjiang, China. The resource is more than 50 t gold at an average grade of > 4.4 ppm. The deposit occurs in the Tulasu volcanic fault-basin in the Paleozoic active continental margin on the northern side of the Yili-Central Tianshan plate. The host rocks are andesitic volcaniclastic rocks of the Paleozoic Dahalajunshan Formation, and the orebodies occur as veins in annular faults of a paleocaldera. Mineralization at Axi can be subdivided into five stages: quartz and/or chalcedony vein, quartz vein, quartz-carbonate vein, sulfide vein and carbonate vein. There are two types of ore host: quartz vein and altered rocks. Ore minerals are native gold, electrum, pyrite, marcasite, arsenopyrite, hematite, limonite, and trace amounts of pyrargyrite, polybasite, naumannite, cerargyrite, sphalerite, chalcopyrite, tetrahedrite, galena, pyrrhotite and clausthalite; gangue minerals are mainly quartz, chalcedony, illite, calcite, siderite, dolomite, adularia and laumontite. The main wall-rock alteration is silicification and phyllic alteration, carbonatization and propylitization. The deposit is characterized by an enrichment, relative to crustal abundance, of Au, Ag, As, Sb, Bi, Hg, Se, Te and Mo, depletion in base metals (Cu, Pb, and Zn), and a low Ag/Au ratio (0.5–3.7).Three types of fluid inclusions were recognized in quartz from the major mineralization stages: liquid aqueous inclusions, liquid-rich two-phase inclusions and small amounts of vapor-rich two-phase inclusions. Microthermometric measurements indicate that the final ice melting temperatures are − 0.3 to − 4.4 °C, corresponding to salinities of 0.5–6.9 wt.% NaCl equiv. (2.2 wt.% NaCl equiv. in average). The peak temperatures of ice melting varies from − 0.4 to − 1.9 °C, corresponding to salinities of 0.7–3.1 wt.% NaCl equiv. Homogenization temperatures range mainly between 120 and 240 °C, with an average of 190 °C and a maximum of 335 °C. The fluid density is 0.73 to 0.95 g/cm3 and thus the estimated maximum mineralization depth is about 700 m.Hydrogen and oxygen isotopic compositions of the ore fluids lie within a narrow range: δDH2O is − 98 to − 116‰ and δ18OH2O − 1.8 to 0.4‰. 3He/4He ranges from 0.0218 to 0.138 Ra, with an average of 0.044 Ra, indicating that He derived predominantly from crust with negligible mantle He in the ore fluids. By contrast, the 40Ar/36Ar ranges from 317.7 to 866.0, suggesting that crust-derived radioactive 40Ar⁎ accounts for 7.0 to 66%, and atmospheric 40Ar about 43 to 93% in the ore fluids. Hydrogen, oxygen, carbon, sulfur and noble gas isotopes indicate that the ore-forming fluids of the Axi gold deposit consisted predominantly of circulating meteoric water. Ore-forming metals may have derived mainly from the host volcaniclastic rocks of the Dahalajunshan Formation and basement rocks. The occurrence of adularia, platy calcite, and quartz or sulfide aggregates as pseudomorphs after bladed calcite in ore veins, and occurrence of aqueous liquid, and liquid-rich and vapor-rich two-phase inclusions, indicates that boiling of the ore-forming fluid have occurred, leading to supersaturation of the hydrothermal solution and deposition of ore metals. This is the main mineralization mechanism for quartz-vein type ores in Axi. The ore-forming fluid was buffered to a near-neutral pH in a reduced environment during mineralization. The preservation of this Paleozoic Axi deposit and its discovery required a rapid accumulation of sediments in the basin after formation of the deposit, and minimal amount of erosion after Late Cenozoic uplift. 相似文献
14.
Geochemical Features and Formation of the Auriferous Cherts in the Mojiang Gold Deposit,Yunnan 总被引：1，自引：0，他引：1
Auriferous cherts in the Middle Carboniferous Jinchang Formation are the dominant host rocks of auriferous quartz veins and mixed orebodies comprised of gold-bearing quartz veins and cherts in the Mojiang gold deposit.The rocks exhibit sedimentary texture and structure and are composed of hot-water deposited minerals.The FeO,Fe2O3,Au and Ag contents of the auriferous cherts are high;the Cr,Ni and Co contents are also high but significantly variable;MnO/TiO2 and TFe/TiO2 ratios are relatively higy.As viewed from a few diagrams that distinguish different chert formations,the auriferous cherts are in or near the range of hot-water deposited cherts.Because the correlation coefficients between Au contents and those of Cr, Ni of the rocks are negative,a great Au amount in the cherts might not be brought about by later hydrothermal alterations.The rare-earth elements,O and Si isotopic compositions of the auriferous cherts demonstrate that the cherts belong to hot-water deposited rocks.The later hydrothermal alterations made the petrochemical compositions of the cherts deviate from the characteristics of hot-water deposition.In general,the geological and geochemical features of the auriferous cherts demonstrate that the rocks were formed by hot water deposition. 相似文献
15.
The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73 Ga volcanic and sedimentary sequences, as well as 2.75–2.72 Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70 Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5 kbar pressures and at 500–600 °C temperatures affected the belt at around 2.70 Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe3 +–Al3 +-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ11B data for this kind of tourmaline is from − 17.2‰ to − 12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe3 +–Al3 + substitution) and δ11B values between − 19.0‰ and − 9.6‰. The uvite–foitite compositional trend and δ11B ‰ values between − 24.1% and − 13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ11B values between − 14.5‰ and − 10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ34SVCDT values measured in pyrite, chalcopyrite and pyrrhotite is from − 9.1 to + 8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ34SVCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ34SVCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ34SVCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ65CuNBS978 values from chalcopyrite is from − 1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ65CuNBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ34SVCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes. 相似文献
16.
The Laowan metallogenic belt in China is an important metallogenic belt within the Tongbai orogenic belt, and contains the medium-sized Laowan and Shangshanghe gold deposits, the small Huangzhuyuan lead–zinc–silver–gold deposit and some gold and Cu–Pb occurrences. These deposits are hosted in Mesoproterozoic plagioclase amphibolite (or schist) and mica-quartz schist. The gold ores are mainly quartz veins and veinlets and disseminated altered ores. Subordinate ore types include massive sulfides and breccias. The Laowan gold deposit is characterized by three right-stepping en-echelon fracture-controlled alteration zones that dip gently to the south and includes disseminated, sheeted and stockwork ores. These lodes were formed by the interaction of ore-forming fluid with foliated-to laminated cataclasite within the transpressional faults. The Shangshanghe gold deposit is characterized by parallel ore lodes that dip steeply to the north, and includes quartz veins and breccias in addition to ores in altered wallrocks. These lodes were formed by focusing of fluids into transtensional faults. These ore controlling faults displaced early barren quartz veins 10 m horizontally with a dextral sense of motion. The ore-hosting structures at the Laowan and Shangshanghe deposits correspond to the P and R-type shears of a brittle dextral strike-slip fault system, respectively, which make angles of about 15° and − 15° to the Laowan and Songpa boundary faults. The ore-controlling fault system post-dated formation of a ductile shear zone, and peak regional metamorphism. This precludes a genetic relationship between hydrothermal mineralization and regional metamorphism and ductile shear deformation. These gold deposits are not typical orogenic gold deposits. The metallogenic belt displays district-scale-zoning of Mo → Cu–Pb–Zn–Ag → Au relative to Songpa granite porphyry dike zone, suggesting the mineralization may be closely related to the granite porphyry. Measured δ34S of sulfides and δ18O and δD of fluid inclusion waters in auriferous quartz also are consistent with a magmatic source for sulfur and ore fluids. The similarity of Pb isotope ratios between the ores and Yanshanian granitoids suggests a similar source. As the age (139 ± 3 Ma) of granite porphyry obtained by zircon U–Pb isotope overlaps the mineralization age (138 ± 1 Ma: Zhang et al., 2008a), the gold and polymetallic metallogenesis of the Laowan gold belt has close spatial, temporal and possibly genetic relationships with Yanshanian high level magmatism. 相似文献
17.
《Chemie der Erde / Geochemistry》2016,76(4):659-676
The Hercynian late-orogenic granites from the Gerês massif, northern Portugal, underwent intense hydrothermal activity along tectonic structures striking N-S and NE-SW. The first hydrothermal stage is characterized by the albitization of feldspars (primary K-feldspar and plagioclase) followed by the dissolution of magmatic quartz, the chloritization of biotite, and the muscovitization of magmatic biotite and feldspars and occasionally of authigenic albite. Whole-rock geochemistry shows a decrease of SiO2, K2O and Rb and an increase of Na2O, Al2O3 and Sr amounts during the albitization. The second hydrothermal stage is characterized by a mineral assemblage consisting of secondary quartz, albite, chlorite, hematite, apatite, muscovite, epidote, sphene and carbonates, which infilled the cavities produced by the early quartz leaching.Mass balance calculations demonstrate a noticeable element mobilization during the granite alteration. The average ∑REE of the Gerês granite is nearly constant at 154 ppm, but ranges from 91 to 163 ppm in the altered rocks. A relative LREE depletion and a slight HREE enrichment associated with a negative Eu anomaly characterize the feldspathization process of Gerês granite. The average ∑REE of the Carris granite is at 159 ppm but in the altered epidote + chlorite + hematite assemblage it reaches about 201 ppm with a slight HREE increase.Two different fluids were involved in the hydrothermal alteration. A first aqueous fluid (Lw1), with a low to intermediate salinity (<10 wt.% eq. NaCl) circulated along the main structures (N-S, NE-SW and NW-SE), characterized by entrapment temperature lower than 350 °C and maximum pressure of 115 MPa followed by a later colder and more saline fluid (Lw2) under a temperature of 220 °C and a pressure of about 27 MPa.Oxygen isotope data obtained on magmatic quartz indicate δ18Oquartz of + 9.3 and + 11.0‰, pointing to an interaction of magmatic fluids with others of meteoric origin. For the secondary quartz, δ18Owater of +0.5 and +0.7‰ were calculated for a temperature of 250 °C, suggesting a meteoric fluid with a possible seawater signature.The K-Ar data of K-feldspar from the altered rocks confirm an age of 273.6 ± 11.7 Ma, attributed to the first alteration process. Younger K-Ar ages between 155.8 ± 6.7 Ma and 124 ± 5.3 Ma were also obtained in the feldspathized rocks, confirming the late hydrothermal activity.Albitization and quartz dissolution of granitic rocks from the Gerês massif occurred at depths shallower than 5 km, induced by the circulation of fluids along brittle structures, during the orogenic uplift and extensional tectonics which affected the Iberian Massif in the Early Permian. A second major event is attributed to late hydrothermal circulations of aqueous brines until a depth of about 3 km, presumably derived from interaction with sub-surface evaporites throughout Late Jurassic to Early Cretaceous. These late hydrothermal events probably reflect the rifting episodes and the rising of geothermal gradient, associated with the opening of the Atlantic Ocean and Gulf of Biscay, respectively. 相似文献
18.
《Ore Geology Reviews》2010,37(4):333-349
Gold mineralization at Jonnagiri, Dharwar Craton, southern India, is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units, typical of Archean greenstone–granite ensembles. The proximal alteration assemblage comprises of muscovite, plagioclase, and chlorite with minor biotite (and carbonate), which is distinctive of low- to mid-greenschist facies. The laminated quartz veins that constitute the inner alteration zone, contain muscovite, chlorite, albite and calcite. Using various calibrations, chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 °C and 268 to 324 °C, respectively. Gold occurs in the laminated quartz veins both as free-milling native metal and enclosed within sulfides. Fluid inclusion microthermometry and Raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous–gaseous (H2O–CO2–CH4 + salt) fluid that underwent phase separation and gave rise to gaseous (CO2–CH4), low saline (~ 5 wt.% NaCl equiv.) aqueous fluids. Quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and P–T regime. Although the estimated P–T range (1.39 to 2.57 kbar at 263 to 323 °C) compare well with the published P–T values of other orogenic gold deposits in general, considerable pressure fluctuation characterize gold mineralization at Jonnagiri. Factors such as fluid phase separation and fluid–rock interaction, along with a decrease in f(O2), were collectively responsible for gold precipitation, from an initial low-saline metamorphogenic fluid. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits in Australia and Canada confirms that fluids of low saline aqueous–carbonic composition with metamorphic parentage played the most dominant role in the formation of the Archean lode gold systems. 相似文献
19.
The studied Mokrsko-West (90–100 t Au), Mokrsko-East (30 t Au) and Čelina (11 t Au) deposits represent three spatially and genetically interrelated deposits of supposed affiliation to the intrusion-related gold deposit type. The deposits differ in their dominant host rocks, which are represented by ca 354 Ma old biotite tonalite (Mokrsko-West) and Neoproterozoic volcanic and volcanosedimentary rocks (Mokrsko-East, Čelina). Another difference lies in the style of veining — densely spaced networks of 0.1–5 mm thin veins (Q2) within the tonalite, compared to thick (usually 5–20 cm; Q1–2) and widely spaced veins within the Neoproterozoic rocks.Five generations of quartz veins, referred to as Q0 through Q4 were distinguished: Q0 veins are the oldest and ore-barren, Q1 veins mark the onset of the Au-ore formation, Q2 veins its culmination and Q3 veins its fading. Late quartz gangue (Q4) is associated with uneconomic Ag–Pb–Zn vein-type ores hosted by calcite–barite–(quartz) veins.Quartz vein thickness (~ 0.3 to ~ 300 mm), spacing (~ 3 mm to ~ 500 mm), distribution, and related extensional strain (ca. 3–25%) evolve systematically across the studied ore district, reflecting both the major host rock and other tectonic factors. Detailed study of vein dimension parameters (thickness, length, width, aspect ratios) allowed estimation of the probable depth of the fluid source reservoir (~ 2 km or ~ 4 km) below the present surface. The depth to the fluid source seems to increase through time, being the shallowest for the Q0 veins and the deepest for the Q2 veins. Two independent methods of estimating fluid overpressure are discussed in the paper. Fluid overpressure during vein formation decreases from the Q0 through the Q2 veins, from 10 to 4 MPa or from 26 to 10 MPa, depending on the assumed tensile strength of the tonalite (5.5 and 15 MPa, respectively).The origin of joints and veins is discussed in terms of the stress orientation and crack-seal and crack-jump mechanisms. Field relationships unambiguously indicate that the veins hosted by Neoproterozoic rocks originated by reopening of the pre-existing extension joints (J1) due to fluid overpressure. The origin of the densely-spaced thin veins (Q2) hosted by the tonalite at the Mokrsko-West deposit is, however, less certain. It is probable that the tonalite was already affected by microfracturing analogous to the J1 joints prior to the formation of quartz veins.The formation of the Q1–2 veins at the Mokrsko-East deposit was constrained by the Re–Os dating of molybdenite to 342.9 ± 1.4 Ma. The ore-bearing hydrothermal system is thus ca 12 Ma younger than the tonalite that hosts the Mokrsko-West deposit. A similar ca 15–2 Ma difference between the age of the host-intrusion and the age of the hydrothermal event was encountered in several other gold deposits in the vicinity of the Central Bohemian Plutonic Complex. Two hypotheses to explain this are discussed in the paper. 相似文献
20.
The Sanshandao gold deposit (reserves of more than 200 t Au and average grade of 3.96 g/t), located at northwestern edge of the Jiaodong Peninsula, eastern North China Craton, is one of the largest gold deposits in the Jiaodong gold province. In this deposit, disseminated- and stockwork-style ores are hosted in Mesozoic granitoids; mineralization and alteration are largely controlled by the regional Sanshandao–Cangshang fault. Host granitic rocks for the deposit display a complex paragenetic sequence of alteration and mineralization. Activities of the Sanshandao–Cangshang fault created structurally controlled permeability allowing for infiltration of hydrothermal fluids, leading to diffusive K-feldspar alteration on the two fault planes. Later, large scale diffusive sericitization symmetrically developed across the main fault, and partially overprinted the earlier K-feldspar alteration. Following the sericitization, relatively small scale silicification occurred, but now it is only retained in the hanging wall of the main fault. Subsequently, the fault gouge formed as a “barrier layer”, which is impermeable for later fluids to move upward. After that, strong pyrite–sericite–quartz alteration occurred only in the footwall of the main fault, and was accompanied by gold precipitation. The last stage carbonation and quartz-carbonate veins marked the waning of gold-related hydrothermal activity. Mass-balance calculations indicate complex behaviors of different types of elements during fluid–rock interaction. Most major elements were affected by intensive mineral replacement reactions. As expected, the fluid-mobile elements, LILE and LREE, generally show moderate to high mobility. It is notable that even the commonly assumed fluid-immobile elements, such as HREE and HFSE, tend to be changed to various degrees. In addition, Y–Ho, Zr–Hf and Nb–Ta fractionations are observed in altered domains. Studies on alteration assemblages and fluid inclusions suggest that the ore-forming fluids were characterized by low salinity (≤ 8.4 wt.% NaCl equiv.), moderate temperature (300–400 °C), weakly acidic (pH: 3–5), and relatively reducing (log fO2: ~–28) characteristics. In this type of fluids, gold was most likely transported as Au(HS)2− complex. With alteration going on, log (aK+/aH+) of fluids generally decreased due to significant formation of secondary K-bearing minerals. In addition, there might be a decrease of fO2 from pre-gold alteration stage to the main gold mineralization stage, and decrease of fO2 was probably one of the factors controlling gold precipitation. The Sr and Nd isotopic compositions of hydrothermal minerals, combined with previous H–O and He–Ar isotopic studies, indicate that the hydrothermal fluids were mainly derived from crustal sources (e.g., degassing of felsic magmas and meteoric water), but with involvement of mantle derived components. The gold mineralization event just coincided with reactivation of the North China Craton, as marked by asthenosphere upwelling, voluminous igneous rocks, and high crustal heat flow, which may have provided sufficient heat energy and fluid input required for the formation of the gold deposits. 相似文献