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1.
In the W Hoggar (Algeria), the major transcurrent N–S East Ouzzal shear zone (EOSZ) hosts several world-class gold deposits over a 100-km length. The late Pan-African EOSZ separates two contrasting Precambrian domains: the Archaean In Ouzzal block to the west (orthogneisses with subordinate metasediments, reworked and granulitized in the c. 2 Ga Eburnean event) and a Middle Proterozoic block to the east (again orthogneisses and metasediments, involved in the c. 600 Ma Pan-African event). The EOSZ is a mylonite belt, 1–3 km wide, with a 50-m-wide ultramylonite belt hosting numerous quartz veins and lenses (giant hydrothermal quartz system) associated with a quartz-sericite-pyrite-carbonate (beresite) alteration. These hydrothermal events occurred under ductile (evolving towards brittle) conditions, between 500 and 300 MPa, at 500–300°C, with aqueous-carbonic fluids derived both from underlying devolatilized metamorphic rocks and a mantle source, as recorded by stable (C, O) isotope data. No gold mineralization was associated with these typical mesothermal events. Following a pressure drop (to 130 MPa), related to the inception of extensional tectonics, the EOSZ was later percolated by a new set of hydrothermal fluids, evolved from basinal waters that deeply penetrated into the In Ouzzal basement. These fluids were Ca-bearing brines (up to 25% wt. eq. NaCl), characterized by high δD (-9 to + 18‰ range), mobilized by the thermal energy released by the late Pan-African granite magmatism (Taourirt granites). As demonstrated by Pb isotope data, the brines leached Au from the In Ouzzal granulites (which contain 3 ppb Au). Fluid inclusion studies indicate that gold was deposited from these brines in the EOSZ at a depth of c. 5 km, due to mixing and cooling with descending diluted fluids.  相似文献   

2.
The Dungash historic gold mine is located in the South Eastern Desert of Egypt. The gold-bearing quartz veins are hosted by the metavolcanic and metavolcaniclastic rocks along an ENE–WSW trending shear zone. Alteration types recorded in the wall rocks are sericitization, silicification, carbonatization, chloritization, sulfidization, ferruginization, and listwanitization. The ore mineral assemblage comprises arsenopyrite, pyrite, native gold, pyrrhotite, sphalerite, chalcopyrite, and galena. The primary sulfide mineral assemblage formed during a hypogene hydrothermal stage whereas anglesite and goethite occur as secondary supergene phases. Microthermometric fluid inclusion analysis revealed that the auriferous quartz precipitated from a moderately saline (5 to 11.22 wt% NaClequiv) solution at temperatures above the recorded homogenization temperatures (T h), which range from 380 to 177 °C. The minimum pressures of trapping are between 350 and 400 bars. The fluid evolution during mineralization is explained by mixing of a magmatic fluid with meteoric waters. Initially, the high temperature and moderately saline magmatic fluid dominated and progressively became diluted with meteoric waters. Highest gold content is recorded in the carbonatized zone and the quartz veins. However, gold content in the carbonatized zone of the footwall exceeds several times its content in the quartz veins and the carbonatized zone of the hanging wall.  相似文献   

3.
Gold mineralization in the West Hoggar shear zone,Algeria   总被引：1，自引：0，他引：1  
The Amesmessa gold prospect is located along a vertical N-S-trending crustal-scale ductile shear zone; stretching lineations are subhorizontal. This major shear zone is a Late Pan African dextral strike-slip fault of the Pharusian Belt of the Tuareg Shield (Algeria). The Amesmessa shear zone is asymmetric: strong thermal and deformational gradients are present along its western border where biotitic ultramylonites are in contact with a rigid Archean complex (In Ouzzal block), whereas there is a progressive gradation, through mylonite then protomylonite, to the Proterozoic gneiss of the Eastern block which displays co-axial Pan African structures. The Amesmessa shear zone is characterized by the presence of a felsic dike complex emplaced during shearing, and forming the most important parent material for ultramylonites. Basic magmas and carbonatites also intruded within the shear zone. The gold-rich quartz veins are located within the ultramylonitic western part of the shear zone. These N-S-trending laminated quartz veins formed during the late increments of shearing (plastic/brittle transition), by repeated syntectonic hydraulic fracturing along zones of rheological contrast parallel to foliation. The ore mineral association (pyrite, galena, native gold, sphalerite) crystallized in the deformed quartz matrix along late shear planes. Undeformed E-W trending banded quartz veins are present in the mylonitic eastern part of the shear zone; their gold content is low and no native gold has been observed. A strong hydrothermal alteration resulted in the development (along the walls of the N-S gold-bearing quartz veins) of a 5-m-wide carbonate-sericite-albite-pyrite secondary mineral association which implies an important CO2 supply and moderate temperature conditions. There is no alteration halo around the E-W quartz veins. Ultramylonites, hydrothermally altered rocks and quartz veins display similar REE patterns characterized by strong LREE enrichments. Shear-related fluids could be likely parental fluids for the Amesmessa gold mineralization and the associated hydrothermal alteration. Hydrothermal fluids were drawn into dilation zones and filled opening fractures along the main planar discontinuity of the most deformed rocks. The supply of CO2 may come from a deep-seated source as suggested by the presence of carbonatite dikes in the shear zones and the existence of CO2-H2O-rich fluid inclusions in quartz. The location of the gold-bearing quartz veins in the western part of the shear zone can be explained by the presence of strong thermal and rheological gradients.  相似文献   

4.
The Dongping gold deposit, situated on the northern margin of the North China Platform, is a composite deposit composed of auriferous quartz vein-type and altered rock-type ore bodies. It is hosted in the inner contact zone of an alkaline intrusion which was intruded into Archean metamorphic rocks and was formed not later than the Hercynian period. Auriferous quartz veins of the deposit are dated with the fluid inclusion Rb-Sr isochron method at 103 ± 4 Ma, indicating that the gold deposit was formed in the Yenshanian period.87Sr/86Sr sourcetracing shows the ore forming materials came dominantly from alkaline intrusions. These results, combined with other isotope and REE data, suggest that the Dongping gold deposit is not a traditional magmatic hydrothermal deposit, but a reworked hydrothermal deposit related to heated and evolved meteoric water. This project (49372105) is financially supported by the National Natural Science Foundation of China.  相似文献   

5.
The Huai Kham On gold deposit is located in the central part of the Sukhothai Fold Belt, northern Thailand. The Sukhothai Fold Belt represents an accretionary complex formed by subduction and collision between the Indochina and Sibumasu Terranes. There are many small gold deposits in the Sukhothai Fold Belt; however, the styles and formation environments of those gold deposits are not clear. The geology of the Huai Kham On deposit consists of volcanic and volcanosedimentary rocks, limestone, and low‐grade metamorphic rocks of Carboniferous to Triassic age. Gold‐bearing quartz veins are hosted by volcanic and volcanosedimentary rocks. The quartz veins can be divided into four stages. The mineral assemblage of the gold‐bearing quartz veins of Stages I and II comprises quartz, calcite, illite, pyrite, native gold, galena, chalcopyrite, and sphalerite. Quartz veins of Stage III consist of microcrystalline quartz, dolomite, calcite, pyrite, native gold, and chalcopyrite. Veins of Stage IV consist of calcite, dolomite, chlorite, and quartz. Fluid inclusions in quartz veins are classified into liquid‐rich two‐phase (Types IA and IB), carbonic‐aqueous (Type II), and carbonic (Type III) fluid inclusions. The homogenization temperatures of Types IA and II fluid inclusions that are related to the gold‐bearing quartz veins from Stages I to III ranged from 240° to 280°C. The δ18O values of quartz veins of Stages I to III range from +12.9 to +13.4‰, suggesting the presence of a homogeneous hydrothermal solution without temperature variation such as a decrease of temperature during the formation of gold‐bearing quartz veins from Stages I to III in the Huai Kham On gold deposit. Based on the calculated formation temperature of 280°C, the δ18O values of the hydrothermal solution that formed the gold‐bearing quartz veins range from +3.2 to +3.7‰, which falls into the range of metamorphic waters. The gold‐bearing quartz veins of the Huai Kham On deposit are interpreted to be the products of metamorphic water.  相似文献   

6.
The Shihu gold deposit, located in the middle-south section of the core of the Fuping mantle branch structure, is hosted in the Archean Fuping Group and adjacent to the quartz diorite porphyrite. The gold deposit is the only large gold deposit with reserves of more than 30 tons gold discovered in western Hebei Province so far. In order to constrain the timing of mineralization of this ore deposit, this paper focuses on the isotopic dating of zircon and pyrite. Zircons in gold-bearing quartz veins are magmatic in origin and no hydrothermal zircon has been found in such quartz veins, indicating that zircons were derived from the wall rocks. U–Pb ages of zircons fall mainly in the two domains: 2492 ± 82 and 136 ± 4 Ma, respectively, indicative of the contribution of the Fuping-Group TTG gneiss and Yanshanian igneous rocks, respectively. The Re–Os isotopic compositions of pyrites in the gold-bearing quartz veins yield an isochron age of 127 ± 31 Ma. Combined with other dating results, we suggest that the main metallogenic age of the Shihu gold deposit is 120–127 Ma.  相似文献   

7.
The Blue Dot gold deposit, located in the Archean Amalia greenstone belt of South Africa, is hosted in an oxide (± carbonate) facies banded iron formation (BIF). It consists of three stratabound orebodies; Goudplaats, Abelskop, and Bothmasrust. The orebodies are flanked by quartz‐chlorite‐ferroan dolomite‐albite schist in the hanging wall and mafic (volcanic) schists in the footwall. Alteration minerals associated with the main hydrothermal stage in the BIF are dominated by quartz, ankerite‐dolomite series, siderite, chlorite, muscovite, sericite, hematite, pyrite, and minor amounts of chalcopyrite and arsenopyrite. This study investigates the characteristics of gold mineralization in the Amalia BIF based on ore textures, mineral‐chemical data and sulfur isotope analysis. Gold mineralization of the Blue Dot deposit is associated with quartz‐carbonate veins that crosscut the BIF layering. In contrast to previous works, petrographic evidence suggests that the gold mineralization is not solely attributed to replacement reactions between ore fluid and the magnetite or hematite in the host BIF because coarse hydrothermal pyrite grains do not show mutual replacement textures of the oxide minerals. Rather, the parallel‐bedded and generally chert‐hosted pyrites are in sharp contact with re‐crystallized euhedral to subhedral magnetite ± hematite grains, and the nature of their coexistence suggests that pyrite (and gold) precipitation was contemporaneous with magnetite–hematite re‐crystallization. The Fe/(Fe+Mg) ratio of the dolomite–ankerite series and chlorite decreased from veins through mineralized BIF and non‐mineralized BIF, in contrast to most Archean BIF‐hosted gold deposits. This is interpreted to be due to the effect of a high sulfur activity and increase in fO2 in a H2S‐dominant fluid during progressive fluid‐rock interaction. High sulfur activity of the hydrothermal fluid fixed pyrite in the BIF by consuming Fe2+ released into the chert layers and leaving the co‐precipitating carbonates and chlorites with less available ferrous iron content. Alternatively, the occurrence of hematite in the alteration assemblage of the host BIF caused a structural limitation in the assignment of Fe3+ in chlorite which favored the incorporation of magnesium (rather than ferric iron) in chlorite under increasing fO2 conditions, and is consistent with deposits hosted in hematite‐bearing rocks. The combined effects of reduction in sulfur contents due to sulfide precipitation and increasing fO2 during progressive fluid‐rock interactions are likely to be the principal factors to have caused gold deposition. Arsenopyrite–pyrite geothermometry indicated a temperature range of 300–350°C for the associated gold mineralization. The estimated δ34SΣS (= +1.8 to +2.5‰) and low base metal contents of the sulfide ore mineralogy are consistent with sulfides that have been sourced from magma or derived by the dissolution of magmatic sulfides from volcanic rocks during fluid migration.  相似文献   

8.
The Bepkong gold deposit is located in the Wa–Lawra belt of the Paleoproterozoic Baoulé-Mossi domain of the West African Craton, in NW Ghana. It occurs in pelitic and volcano-sedimentary rocks, metamorphosed to greenschist facies, in genetic association with zones of shear interpreted to form during the regional D3 deformational event, denominated DB1 at the deposit scale. The ore zone forms a corridor-like body composed of multiple quartz ± carbonate veins surrounded by an alteration envelope, characterized by the presence of chlorite, calcite, sericite, quartz and disseminated pyrite, arsenopyrite plus subordinate pyrrhotite and chalcopyrite. The veins contain only small proportions of pyrite, whereas most of the sulphides, particularly arsenopyrite, occur in the altered host rock, next to the veins. Pyrite is also common outside of the ore zone. Gold is found in arsenopyrite, where it occurs as invisible gold and as visible – albeit micron-size – grains in its rims, and as free gold within fractures cross-cutting this sulphide. More rarely, free gold also occurs in the veins, in fractured quartz. In the ore zone, pyrite forms euhedral crystals surrounding arsenopyrite, but does not contain gold, suggesting that it formed at a late stage, from a gold-free hydrothermal fluid.  相似文献   

9.
Orogenic Gold Mineralization in the Qolqoleh Deposit, Northwestern Iran   总被引：1，自引：1，他引：1  
The Qolqoleh gold deposit is located in the northwestern part of the Sanandai‐Sirjan Zone, northwest of Iran. Gold mineralization in the Qolqoleh deposit is almost entirely confined to a series of steeply dipping ductile–brittle shear zones generated during Late Cretaceous–Tertiary continental collision between the Afro‐Arabian and the Iranian microcontinent. The host rocks are Mesozoic volcano‐sedimentary sequences consisting of felsic to mafic metavolcanics, which are metamorphosed to greenschist facies, sericite and chlorite schists. The gold orebodies were found within strong ductile deformation to late brittle deformation. Ore‐controlling structure is NE–SW‐trending oblique thrust with vergence toward south ductile–brittle shear zone. The highly strained host rocks show a combination of mylonitic and cataclastic microstructures, including crystal–plastic deformation and grain size reduction by recrystalization of quartz and mica. The gold orebodies are composed of Au‐bearing highly deformed and altered mylonitic host rocks and cross‐cutting Au‐ and sulfide‐bearing quartz veins. Approximately half of the mineralization is in the form of dissemination in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in quartz–sulfide microfractures, microveins and veins. Only low volumes of gold concentration was introduced during ductile deformation, whereas, during the evident brittle deformation phase, competence contrasts allowed fracturing to focus on the quartz–sericite domain boundaries of the mylonitic foliation, thus permitting the introduction of auriferous fluid to create disseminated and cross‐cutting Au‐quartz veins. According to mineral assemblages and alteration intensity, hydrothermal alteration could be divided into three zones: silicification and sulfidation zone (major ore body); sericite and carbonate alteration zone; and sericite–chlorite alteration zone that may be taken to imply wall‐rock interaction with near neutral fluids (pH 5–6). Silicified and sulfide alteration zone is observed in the inner parts of alteration zones. High gold grades belong to silicified highly deformed mylonitic and ultramylonitic domains and silicified sulfide‐bearing microveins. Based on paragenetic relationships, three main stages of mineralization are recognized in the Qolqoleh gold deposit. Stage I encompasses deposition of large volumes of milky quartz and pyrite. Stage II includes gray and buck quartz, pyrite and minor calcite, sphalerite, subordinate chalcopyrite and gold ores. Stage III consists of comb quartz and calcite, magnetite, sphalerite, chalcopyrite, arsenopyrite, pyrrhotite and gold ores. Studies on regional geology, ore geology and ore‐forming stages have proved that the Qolqoleh deposit was formed in the compression–extension stage during the Late Cretaceous–Tertiary continental collision in a ductile–brittle shear zone, and is characterized by orogenic gold deposits.  相似文献   

10.
The Penjom gold deposit lies on the eastern side of the Raub‐Bentong Suture line within the Central Belt of Permo‐Triassic rocks, near Kuala Lipis, Pahang, Malaysia. The geology of the deposit is dominated by a sequence of fine‐ to coarse‐grained rhyolitic to rhyodacitic tuff, tuff‐breccia and a minor rhyolitic–rhyodacitic volcanic series, associated with argillaceous marine sedimentary rocks consisting of shale with subordinate shalely limestone of Padang Tungku Formation and Pahang Volcanic Series. Fine‐ to coarse‐grained tonalite and quartz porphyry intruded this unit. The main structural features of the area are north–south‐trending left‐lateral strike‐slip faults and their subsidiaries, which generally strike north–south and dip moderately to the east (350°–360°/40°–60°). Mineralization at the Penjom gold deposit is structurally controlled and also erratic laterally and vertically. The gold mineralization can be categorized as (i) gold associated with carbonate‐rich zones hosted within dilated quartz veins carrying significant amount of sulfides; (ii) gold disseminated within stockwork of quartz–carbonate veins affiliated with tonalite; and (iii) gold often associated with arsenopyrite and pyrite in quartz–carbonate veins and stringers hosted within shear zones of brittle–ductile nature in all rock types and in brittle fractured rhyodacitic volcanic rocks. Sphalerite, chalcopyrite, tetrahedrite and pyrrhotite are the minerals accompanying the early stage of gold mineralization. These minerals also suffered from local brittle deformation. However, most of the gold mineralization took place after the deposition of these sulfides. Galena appears somewhat towards the end of gold mineralization, whereas tellurium and bismuth accompanied gold contemporaneously. The gold mineralization occurred most probably due to the metamorphogenic deformational origin concentrated mostly in the shear zone. The mineralization is strongly controlled by the wall rock (e.g. graphitic shale), the sulfide minerals and fluid–rock interaction.  相似文献   

11.
The Huilvshan gold deposit in the west Junggar (Xinjiang, China) is hosted in chloritized basalts, chlorite–siderite-altered basalts, and quartz–siderite rocks. Our study demonstrated that all these rocks were derived by alteration of basalts in a shear zone. The orebodies, consisting of quartz-sulfide veins and disseminated sulfides, formed in five stages: quartz–muscovite (I), disseminated sulfides (II), quartz–ankerite-sulfide (III), quartz–calcite-sulfide (IV), and quartz–calcite (V). Auriferous minerals are native gold in stage III and electrum in stage IV. During alteration of basalts in the shear zone, ore-forming elements were released from basalts to ore-forming fluid. Compared with fresh basalts, sulfide-bearing chloritized basalts contain much higher Sr, Ba, P, La, Ce, U, Mn, Ni, Zn, As, Ag, and Au contents. Phase analysis of the As–Cu–Fe–S–O system with the SUPCRT92 software package indicates that a decrease of the aH2S value, caused by the fluid–rock reactions and crystallization of sulfides, induced gold precipitation.  相似文献   

12.
Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the Syama-Bananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit, mineralisation hosted in altered basalt is associated with an intense ankerite–quartz–pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz–porphyry rocks.In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz–Fe–carbonate–sulphide lodes in mafic volcanics, and quartz–sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.  相似文献   

13.
The Junction gold deposit, in Western Australia, is an orogenic gold deposit hosted by a differentiated, iron‐rich, tholeiitic dolerite sill. Petrographic, microthermometric and laser Raman microprobe analyses of fluid inclusions from the Junction deposit indicate that three different vein systems formed at three distinct periods of geological time, and host four fluid‐inclusion populations with a wide range of compositions in the H2O–CO2–CH4–NaCl ± CaCl2 system. Pre‐shearing, pre‐gold, molybdenite‐bearing quartz veins host fluid inclusions that are characterised by relatively consistent phase ratios comprising H2O–CO2–CH4 ± halite. Microthermometry suggests that these veins precipitated when a highly saline, >340°C fluid mixed with a less saline ≥150°C fluid. The syn‐gold mineralisation event is hosted within the Junction shear zone and is associated with extensive quartz‐calcite ± albite ± chlorite ± pyrrhotite veining. Fluid‐inclusion analyses indicate that gold deposition occurred during the unmixing of a 400°C, moderately saline, H2O–CO2 ± CH4 fluid at pressures between 70 MPa and 440 MPa. Post‐gold quartz‐calcite‐biotite‐pyrrhotite veins occupy normal fault sets that slightly offset the Junction shear zone. Fluid inclusions in these veins are predominantly vapour rich, with CO2?CH4. Homogenisation temperatures indicate that the post‐gold quartz veins precipitated from a 310 ± 30°C fluid. Finally, late secondary fluid inclusions show that a <200°C, highly saline, H2O–CaCl2–NaCl–bearing fluid percolated along microfractures late in the deposit's history, but did not form any notable vein type. Raman spectroscopy supports the microthermometric data and reveals that CH4–bearing fluid inclusions occur in syn‐gold quartz grains found almost exclusively at the vein margin, whereas CO2–bearing fluid inclusions occur in quartz grains that are found toward the centre of the veins. The zonation of CO2:CH4 ratios, with respect to the location of fluid inclusions within the syn‐gold quartz veins, suggest that the CH4 did not travel as part of the auriferous fluid. Fluid unmixing and post‐entrapment alteration of the syn‐gold fluid inclusions are known to have occurred, but cannot adequately account for the relatively ordered zonation of CO2:CH4 ratios. Instead, the late introduction of a CH4–rich fluid into the Junction shear zone appears more likely. Alternatively, the process of CO2 reduction to CH4 is a viable and plausible explanation that fits the available data. The CH4–bearing fluid inclusions occur almost exclusively at the margin of the syn‐gold quartz veins within the zone of high‐grade gold mineralisation because this is where all the criteria needed to reduce CO2 to CH4 were satisfied in the Junction deposit.  相似文献   

14.
Abstract: The Dongping deposit, located near the center of the northern margin of the north China craton, is one of the largest gold deposits in China. It is spatially, temporally, and genetically associated with the shallowly-emplaced Hercynian Shuiquan-gou alkaline intrusive complex. The complex intrudes high-grade metamorphic rocks of the Archean Sanggan Group along a deep-seated fault zone within the north China craton. Four major ore bodies (Nos. 1, 2, 22, and 70), consisting mainly of a set of en echelon lenses and veins, have been delineated at the Dongping deposit. Hypogene hydrothermal activities can be divided into four periods from early to late including: (1) gold-bearing K–feldspar–quartz stockworks and veins; (2) disseminated sulfide and gold zones; (3) gold-bearing quartz veins, and (4) barren calcite-quartz veins. Individual veins and stockwork systems can be traced along strike for 125 to 600 m and downdip for 100 to 600 m; they range from 0. 5 to 3 m in thickness. The mineralogical composition of the ore in the first three hypogene periods is relatively simple. It is composed of pyrite, galena, sphalerite, magnetite, specularite, chalcopyrite, native gold, electrum, calaverite, and altaite. Gangue minerals include K–feldspar, quartz, sericite, chlorite, epidote, albite, and calcite. Ore grade averages 6 g/t Au, but varies between 4. 14 and 22. 66 g/t Au. Gold is generally fine-grained and not visible in hand specimen. Fluid inclusions in ore-bearing quartz of periods 1, 2, and 3 are CO2–rich, variable salinity (2. 5–21 wt% equiv. NaCl), and have variable homogenization temperatures of 195° to 340°C. Quartz in the gold-bearing K–feldspar–quartz stockworks (period 1), disseminated sulfide and gold zones (period 2), and the gold-bearing quartz veins (period 3) has calculated δ18OH2O values between –1. 7 and 6. 9%, and δ values of fluid inclusion waters between –101 and –66%. All these isotope data of the ore-forming fluids plot between the magmatic fluid field and the meteoric water line. Sulfide minerals disseminated in host rocks show positive δ34S values of 1. 9 to 3. 5%. Pyrite separates from he gold-bearing K–feldspar–quartz stockworks and veins (period 1) have a δ34S range of –4. 3 to 0. 5%, whereas δ34S values of pyrite, chalcopyrite, galena, and sphalerite from the disseminated sul-fide and gold zones (period 2) and the gold-bearing quartz veins (period 3) vary from –5. 3 to –13. 4%. Gold ores are also characterized by relatively radiogenic lead isotope compositions compared to those of the alkaline syenite host rock. The data are interpreted as indicative of a mixing of lead from the alkaline intrusive complex with lead from Archean metamorphic rocks. The combined fluid inclusion measurements, sulfur, oxygen, hydrogen, and lead isotope data, and petrological observations indicate that the Dongping deposit was formed from the mixing of these magmatic fluids with meteoric waters. The deposit is, therefore, believed to be a product of Hercynian alkaline igneous processes within the north China craton.  相似文献   

15.
Gold-bearing quartz veins of the Taihua Group consisting of Archean metavolcanic rocks are a main gold deposit type in the Xiao Qinling area,one of the three biggest gold production areas in China.The quartz veins experienced strong alteration characterized by a typical mesothermal hydrothermal altered mineral assemblage.The grade of gold is affected by the contents of sulphides,e.g.galena,pyrite and chalcopyrite.Results of minor elements analysis for the of gold-bearing quartz veins indicate higher contents of Au and high contents of Ag,Pb,Cu,Cd,W,and Mo.Abundant fluid inclusions were found in the gold-bearing quartz veins.Three types of fluid inclusions were identified:(1) aqueous inclusions;(2) CO 2-bearing inclusions;and(3) daughter crystal-bearing fluid inclusions.Homogenization temperatures ranged from 110 to 670℃ with low and high peaks appearing at 160 180℃ and 280 300℃,respectively.The salinity of aqueous inclusions varies between 1.8 wt% and 38.2 wt% NaCl.The homogenization temperature and salinity show a positive correlation.The H and O isotopes of fluid inclusions in the gold-bearing quartz veins indicate that magmatic solution and metamorphic hydrothermal solution,together with meteoric water,were involved in the formation of gold-bearing fluid.Mesozoic magma activities related to granite intrusions should be the main source of CO 2 fluid with higher temperature and salinity.  相似文献   

16.
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17.
The Chatree deposit is located in the Loei‐Phetchabun‐Nakhon Nayok volcanic belt that extends from Laos in the north through central and eastern Thailand into Cambodia. Gold‐bearing quartz veins at the Q prospect of the Chatree deposit are hosted within polymictic andesitic breccia and volcanic sedimentary breccia. The orebodies of the Chatree deposit consist of veins, veinlets and stockwork. Gold‐bearing quartz veins are composed mainly of quartz, calcite and illite with small amounts of adularia, chlorite and sulfide minerals. The gold‐bearing quartz veins were divided into five stages based on the cross‐cutting relationship and mineral assemblage. Intense gold mineralization occurred in Stages I and IV. The mineral assemblage of Stages I and IV is characterized by quartz–calcite–illite–laumontite–adularia–chlorite–sulfide minerals and electrum. Quartz textures of Stages I and IV are also characterized by microcrystalline and flamboyant textures, respectively. Coexistence of laumontite, illite and chlorite in the gold‐bearing quartz vein of Stage IV suggests that the gold‐bearing quartz veins were formed at approximately 200°C. The flamboyant and brecciated textures of the gold‐bearing quartz vein of Stage IV suggest that gold precipitated with silica minerals from a hydrothermal solution that was supersaturated by boiling. The δ18O values of quartz in Stages I to V range from +10.4 to +11.6‰ except for the δ18O value of quartz in Stage IV (+15.0‰). The increase in δ18O values of quartz at Stage IV is explained by boiling. PH2O is estimated to be 16 bars at 200°C. The fCO2 value is estimated to be 1 bar based on the presence of calcite in the mineral assemblage of Stage IV. The total pressure of the hydrothermal solution is approximately 20 bars at 200°C, suggesting that the gold‐bearing quartz veins of the Q prospect formed about 200 m below the paleosurface.  相似文献   

18.
《Ore Geology Reviews》2008,33(3-4):511-542
The Morro Velho gold deposit, Quadrilátero Ferrífero region, Minas Gerais, Brazil, is hosted by rocks at the base of the Archean Rio das Velhas greenstone belt. The deposit occurs within a thick carbonaceous phyllite package, containing intercalations of felsic and intermediate volcaniclastic rocks and dolomites. Considering the temporal and spatial association of the deposit with the Rio das Velhas orogeny, and location in close proximity to a major NNW-trending fault zone, it can be classified as an orogenic gold deposit. Hydrothermal activity was characterized by intense enrichment in alteration zones of carbonates, sulfides, chlorite, white mica±biotite, albite and quartz, as described in other Archean lode-type gold ores. Two types of ore occur in the deposit: dark gray quartz veins and sulfide-rich gold orebodies. The sulfide-rich orebodies range from disseminated concentrations of sulfide minerals to massive sulfide bodies. The sulfide assemblage comprises (by volume), on average, 74% pyrrhotite, 17% arsenopyrite, 8% pyrite and 1% chalcopyrite. The orebodies have a long axis parallel to the local stretching lineation, with continuity down the plunge of fold axis for at least 4.8 km. The group of rocks hosting the Morro Velho gold mineralization is locally referred to as lapa seca. These were isoclinally folded and metamorphosed prior to gold mineralization. The lapa seca and the orebodies it hosts are distributed in five main tight folds related to F1 (the best examples are the X, Main and South orebodies, in level 25), which are disrupted by NE- to E-striking shear zones. Textural features indicate that the sulfide mineralization postdated regional peak metamorphism, and that the massive sulfide ore has subsequently been neither metamorphosed nor deformed. Lead isotope ratios indicate a model age of 2.82 ± 0.05 Ga for both sulfide and gold mineralization. The lapa seca are interpreted as the results of a pre-gold alteration process and may be divided into carbonatic, micaceous and quartzose types. The carbonatic lapa seca is subdivided into gray and brown subtypes. Non-mineralized, gray carbonatic lapa seca forms the hanging wall to the orebodies, and is interpreted as the product of extreme CO2 metasomatism during hydrothermal alteration. This dolomitic lapa seca ranges in composition from relatively pure limestone and dolomite to silty limestone and dolomite. The brown carbonatic and micaceous lapa secas are the host rocks to gold. These units are interpreted to correspond to the sheared and hydrothermal products of metamorphosed volcaniclastic and/or volcanic rocks of varying composition from dacitic to andesitic, forming various types of schists and phyllites. The high-grade, massive sulfide orebodies occur at the base of the gray carbonatic lapa seca. Both disseminated mineralization and quartz veins are hosted by micaceous lapa seca. The data are consistent with a model of epigenetic mineralization for the lapa seca, from a hydrothermal fluid derived in part from the Archean basement or older crust material.  相似文献   

19.
《Ore Geology Reviews》2008,33(3-4):571-595
The orogenic banded iron formation (BIF)-hosted Au mineralization at São Bento is a structurally-controlled, hydrothermal deposit hosted by Archean rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero region, Brazil. The deposit has reserves of 14.3 t Au and historical (underground) production of 44.6 t Au between 1987 and 2001. The oxide-facies São Bento BIF is mineralized at its lower portion, where in contact with carbonaceous, pelitic schists, particularly in the proximity of sulfide-bearing quartz veins. Shear-related Au deposition is associated with the pervasive, hydrothermal sulfidation (mainly arsenopyrite) of the Fe-rich bands of the São Bento BIF. Auriferous, sulfide- and quartz-rich zones represent proximal alteration zones. They are enveloped by ankerite-dominated haloes, which reflect progressive substitution of siderite and magnetite within the BIF by ankerite and pyrrhotite, respectively. The São Bento BIF was intensely and extensively deformed, first into open, upright folds that evolved into tight, asymmetric, isoclinal folds. The inverse limb of these folds attenuated and gave way to sheath folds and the establishment of ductile thrusts. Mineralized horizons at São Bento result from early structural modifications imposed by major transcurrent and thrusts faults, comprising the Conceição, Barão de Cocais and São Bento shear zones. Dextral movement on the SW–NE-directed Conceição shear zone may have generated splays at a compressional side-stepping zone, such as the São Bento shear zone, which is the structural locus for the São Bento gold mineralization. Relaxation of the Conceição shear zone under more brittle conditions resulted in the development of dilatational zones where gold–sulfide–quartz veins formed. These structures are considered to have been generated in the Archean. Geochronological data are scarce, with Pb–Pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single-stage growth curve at 2.65 Ga. A later compressional, ductile deformation of unknown age overprinted, rotated and flattened the original, N60E-directed structure of the whole rock succession, with development of planar and linear fabrics that appear similar to Proterozoic-aged structures. Fluid inclusion studies indicate low salinity, aqueous fluids, with or without CO2 and/or CH4, with extremely variable CO2/CH4 ratios, of probable metamorphic origin. Fluid evolution shows a paragenetic decrease in the carbonic phase from 10–15% to 5%, and increase in the H2O/(CO2 + CH4) and CO2/CH4 ratios, suggesting important interaction with carbonaceous sediment. Trapping conditions indicate a temperature of 300 °C at 3.2 kbar.  相似文献   

20.
《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   

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