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1.
The pressure, temperature and composition of ore fluids that resulted in gold deposition in the Archean, greenstone-hosted Hutti deposit have been studied using fluid inclusions and the compositions of arsenopyrite and chlorite. Five types of fluids have been identified in fluid inclusions in quartz veins associated with mineralization. They are (1) monophase CO 2-rich fluid; (2) low-salinity (0 to 14 wt% NaCl equivalent) and high-salinity (16 to 23 wt% NaCl equiv.) aqueous fluids; (3) high-salinity (28 to 40 wt% NaCl equiv.), polyphase aqueous fluids; (4) CO 2–H 2O–NaCl fluids of low salinity (0–8 wt% NaCl equiv.); and (5) a few carbonic inclusions with halite±nahcolite. The diversity of entrapped fluid composition is explained in terms of changes in fluid pressure and temperature which affect a more or less uniform supply of primary low-salinity CO 2–H 2O–NaCl fluid to the shear zone. Geothermobarometric studies indicate that during mineralization temperature ranged between 360 and 240 °C, and fluid pressure between 3,600 and 1,600 bar. The data are interpreted in terms of the cyclic fault-valve mechanism for active shear zones. Deposition of gold and sulfides has been studied on the basis of constraints from the composition of wall-rock chlorite, ore-mineral assemblages, and textural features. Tubular channels, 20 to 100 µm wide and up to 500 µm long that arise from fractures and C-planes in sheared quartz veins are reported for the first time. The channels have pyrrhotite, arsenopyrite, pyrite and gold at their distal ends, with calcite filling up the remaining part. These channels form in response to increases in T and P, by dissolution of quartz grains, guided by dislocations in them. At the P– T conditions of interest, gold and sulfide deposition takes place in the shears and fractures of quartz veins from CO 2–H 2O–NaCl ore fluid of low salinity and pH due to changes in phase compositions that occur during the process of shear failure of the enclosing rocks. In the wall rock where pH is buffered, gold deposition takes place from the predominant Au(HS) 2 - species with progressive sulfide deposition and decrease in SS, from 0.01 to 0.001 mol/kg as T falls from 360 to 240 °C.  相似文献   

2.
Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H2O-CO2-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V1) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ18O across the batholith (9.5 ± 0.4‰n = 12) suggests V1 precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V1 indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ18O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V2) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O-NaCl-CaCl2-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ18O (−3 to + 1.2‰), δ13CCO2 (−19 to 0‰) and δ34Ssulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V3). Correlations of salinity, temperature, δD and δ18O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ18O (1.2–2.5‰), high-δ13CCO2 (> −4‰), low-δ34Ssulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ18O (−5.4 to −3‰), low-δ13C (<−10‰), high-δ34Ssulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V3 veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997  相似文献   

3.
A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of ~15–20 wt.% equiv. NaCl. The δ34S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The δ18O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.  相似文献   

4.
The Youjiang basin, which flanks the southwest edge of the Yangtze craton in South China, contains many Carlin-type gold deposits and abundant paleo-oil reservoirs. The gold deposits and paleo-oil reservoirs are restricted to the same tectonic units, commonly at the basinal margins and within the intrabasinal isolated platforms and/or bioherms. The gold deposits are hosted by Permian to Triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon. Paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz ± pyrite (stage I), a main stage of auriferous quartz + arsenian pyrite + arsenopyrite + marcasite (stage II), and a late stage of quartz + calcite + realgar ± orpiment ± native arsenic ± stibnite ± cinnabar ± dolomite (stage III). Bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks, particularly close to high grade ores, but is absent in barren sedimentary rocks. Bitumen dispersed in the mineralized rocks is closely associated and/or intergrown with the main stage jasperoidal quartz, arsenian pyrite, and arsenopyrite. Bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages II and III mineral assemblages. These observations suggest an intimate relationship between bitumen precipitation and gold mineralization. In the paleo-petroleum reservoirs that typically occur in Permian reef limestones, bitumen is most commonly observed in open spaces, either alone or associated with calcite. Where bitumen occurs with calcite, it is typically concentrated along pore/vein centers as well as along the wall of pores and fractures, indicating approximately coeval precipitation. In the gold deposits, aqueous fluid inclusions are dominant in the early stage barren quartz veins (stage I), with a homogenization temperature range typically of 230°C to 270°C and a salinity range of 2.6 to 7.2 wt% NaCl eq. Fluid inclusions in the main and late-stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon- and CO2-rich inclusions. The presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization, the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase. Aqueous inclusions in the main stage quartz associated with gold mineralization (stage II) typically have a homogenization temperature range of 200–230°C and a modal salinity around 5.3 wt% NaCl eq. Homogenization temperatures and salinities of aqueous inclusions in the late-stage drusy quartz and calcite (stage III) typically range from 120°C to 160°C and from 2.0 to 5.6 wt% NaCl eq., respectively. In the paleo-oil reservoirs, aqueous fluid inclusions with an average homogenization temperature of 80°C are dominant in early diagenetic calcite. Fluid inclusions in late diagenetic pore- and fissure-filling calcite associated with bitumen are dominated by liquid C2H6, vapor CH4, CH4–H2O, and aqueous inclusions, with a typical homogenization temperature range of 90°C to 180°C and a salinity range of 2–8 wt% NaCl eq. It is suggested that the hydrocarbons may have been trapped at relatively low temperatures, while the formation of gold deposits could have occurred under a wider and higher range of temperatures. The timing of gold mineralization in the Youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits. Among the limited isotopic data, the Rb–Sr date of 206 ± 12 Ma for Au-bearing hydrothermal sericite at Jinya as well as the Re–Os date of 193 ± 13 Ma on auriferous arsenian pyrite and 40Ar/39Ar date of 194.6 ± 2 Ma on vein-filling sericite at Lannigou may provide the most reliable age constraints on gold mineralization. This age range is comparable with the estimated petroleum charging age range of 238–185 Ma and the Sm–Nd date of 182 ± 21 Ma for the pore- and fissure-filling calcite associated with bitumen at the Shitouzhai paleo-oil reservoir, corresponding to the late Indosinian to early Yanshanian orogenies in South China. The close association of Carlin-type gold deposits and paleo-oil reservoirs, the paragenetic coexistence of bitumens with ore-stage minerals, the presence of abundant hydrocarbons in the ore fluids, and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated, migrated, and precipitated along with the hydrocarbons in an immiscible, gold- and hydrocarbon-bearing, basinal fluid system.  相似文献   

5.
Gold ore-forming fluids of the Tanami region, Northern Australia   总被引：1，自引：0，他引：1  
Fluid inclusion studies have been carried out on major gold deposits and prospects in the Tanami region to determine the compositions of the associated fluids and the processes responsible for gold mineralization. Pre-ore, milky quartz veins contain only two-phase aqueous inclusions with salinities ≤19 wt% NaCl eq. and homogenization temperatures that range from 110 to 410°C. In contrast, the ore-bearing veins typically contain low to moderate salinity (<14 wt% NaCl eq.), H2O + CO2 ± CH4 ± N2-bearing fluids. The CO2-bearing inclusions coexist with two-phase aqueous inclusions that exhibit a wider range of salinities (≤21 wt% NaCl eq.). Post-ore quartz and carbonate veins contain mainly two-phase aqueous inclusions, with a last generation of aqueous inclusions being very CaCl2-rich. Salinities range from 7 to 33 wt% NaCl eq. and homogenization temperatures vary from 62 to 312°C. Gold deposits in the Tanami region are hosted by carbonaceous or iron-rich sedimentary rocks and/or mafic rocks. They formed over a range of depths at temperatures from 200 to 430°C. The Groundrush deposit formed at the greatest temperatures and depths (260–430°C and ≤11 km), whereas deposits in the Tanami goldfield formed at the lowest temperatures (≥200°C) and at the shallowest depths (1.5–5.6 km). There is also evidence in the Tanami goldfield for late-stage isothermal mixing with higher salinity (≤21 wt% NaCl eq.) fluids at temperatures between 100 and 200°C. Other deposits (e.g., The Granites, Callie, and Coyote) formed at intermediate depths and at temperatures ranging from 240 to 360°C. All ore fluids contained CO2 ± N2 ± CH4, with the more deeply formed deposits being enriched in CH4 and higher level deposits being enriched in CO2. Fluids from deposits hosted mainly by sedimentary rocks generally contained appreciable quantities of N2. The one exception is the Tanami goldfield, where the quartz veins were dominated by aqueous inclusions with rare CO2-bearing inclusions. Calculated δ 18O values for the ore fluids range from 3.8 to 8.5‰ and the corresponding δD values range from −89 to −37‰. Measured δ 13C values from CO2 extracted from fluid inclusions ranged from −5.1 to −8.4‰. These data indicate a magmatic or mixed magmatic/metamorphic source for the ore fluids in the Tanami region. Interpretation of the fluid inclusion, alteration, and structural data suggests that mineralization may have occurred via a number of processes. Gold occurs in veins associated with brittle fracturing and other dilational structures, but in the larger deposits, there is also an association with iron-rich rocks or carbonaceous sediments, suggesting that both structural and chemical controls are important. The major mineralization process appears to be boiling/effervescence of a gas-rich fluid, which leads to partitioning of H2S into the vapor phase resulting in gold precipitation. However, some deposits also show evidence of desulfidation by fluid–rock interaction and/or reduction of the ore-fluid by fluid mixing. These latter processes are generally more prevalent in the higher crustal-level deposits.  相似文献   

6.
Several F, Pb, Zn and Ba deposits are located in the province of Zaghouan in north-eastern Tunisia. They are hosted in Lower Liassic or Upper Jurassic reef limestones, and the overlying condensed Carixian phosphatic limestones and Campanian marls, respectively. The mineralization occurs in three types of orebodies: stratiform replacement heaps and lenses (Jebel Stah and Hammam Zriba), breccia fillings and dissolution void fillings (Sidi Taya) and lodes (Jebel Oust). More than one generation of fluorite is observed in the stratiform deposits. Microthermometric analyses of the inclusion fluids observed in fluorite and quartz show that the economic concentrations of fluorite have deposited from moderate to highly saline (12–22.5 wt% NaCl equivalents) hydrothermal (110–160 °C) mineralizing fluids at the center (Jebel Stah, Sidi Taya) and to the east of the province (Hammam Zriba). Late remobilizations, observed in the stratiform deposits, are related to the circulation of a warmer (up to 185 °C) but less saline (10 wt% NaCl equivalents) fluid (Jebel Stah) and more saline (12–22 wt% NaCl equivalents) fluid (Hammam Zriba). The highest temperature (up to 250 °C) and salinity (32–34 wt% NaCl equivalents) are observed to the west of the province of Zaghouan (Jebel Oust). Less saline (3–6 wt% NaCl equivalents) and moderately hot to hot fluids (up to 220 ± 20 °C) and rich in gaseous CO2 invade most of the ore deposits in later stages and give rise to the massive quartz within fractures at Jebel Stah. Chemical analyses of the fluids extracted from the inclusions occuring in fluorite show compositions dominated by the presence of Na+, Ca2+ and Cl− ionic species and allow the mean temperature of the fluids in the source reservoir to be estimated as 275 ± 25 °C. The circulation of the ore-forming fluids is triggered by a regional tectonic extensional phase which occurs within the post-Jurassic to ante-Miocene time interval. The deposition of the economic concentrations of fluorite resulted from the decrease in pressure and temperature of the hydrothermal brines (Jebel Oust), along with the increase in the dissolved calcium activity (Jebel Stah and Sidi Taya), or a decrease in salinity due to the mixing with a hot, less saline and Na-poor, Ca-rich fluid (Hammam Zriba). The mineralogical associations (CaF2, PbS, ZnS, BaSO4) hosted within carbonate rocks, the temperatures and the salinities of the fluids that gave rise to the more important ore deposits (110–160 °C and 12–22.5 wt% NaCl equivalents), their composition (Na, Ca, Cl) and the molar ratios between the major ionic species, as well as the presence of liquid hydrocarbons in the mineralizing fluids, show that the ore deposits of the province of Zaghouan belong to the carbonate-hosted F, Pb, Zn, Ba Mississippi Valley-type deposits. Received: 23 June 1995 / Accepted: 18 November 1996  相似文献   

7.
Summary ?The NW–SE-trending Yulong porphyry Cu–Mo ore belt, situated in the Sanjiang0 area of eastern Tibet, is approximately 400 km long and 35 to 70 km wide. Complex tectonic and magmatic processes during the Himalayan epoch have given rise to favorable conditions for porphyry-type Cu–Mo mineralization. Porphyry masses of the Himalayan epoch in the Yulong ore belt are distributed in groups along regional NW–SE striking tectonic lineaments. They were emplaced mainly into Triassic and Lower Permian sedimentary-volcanic rocks. K–Ar und U–Pb isotopic datings give an intrusion age range of 57–26 Ma. The porphyries are mainly of biotite monzogranitic and biotite syenogranitic compositions. Geological and geochemical data indicate that the various porphyritic intrusions in the belt had a common or similar magma source, are metaluminous to peraluminous, Nb–Y–Ba-depleted, I-type granitoids, and belong to the high-K calc-alkaline series. Within the Yulong subvolcanic belt a number of porphyry stocks bear typical porphyry type Cu–Mo alteration and mineralization. The most prominent porphyry Co–Mo deposits include Yulong, Malasongduo, Duoxiasongduo, Mangzong and Zhanaga, of which Yulong is one of the largest porphyry Cu (Mo) deposits in China with approximately 8 × 106 tons of contained Cu metal. Hydrothermal alteration at Yulong developed around a biotite–monzogranitic porphyry stock that was emplaced within Upper Triassic limestone, siltstone and mudstone. The earliest alteration was due to the effects of contact metamorphism of the country rocks and alkali metasomatism (potassic alteration) within and around the porphyry body. The alteration of this stage was accompanied by a small amount of disseminated and veinlet Cu–Mo sulfide mineralization. Later alteration–mineralization zones form more or less concentric shells around the potassic zone, around which are distributed a phyllic or quartz–sericite–pyrite zone, a silicification and argillic zone, and a propylitic zone. Fluid inclusion data indicate that three types of fluids were involved in the alteration–mineralization processes: (1) early high temperature (660–420 °C) and high salinity (30–51 wt% NaCl equiv) fluids responsible for the potassic alteration and the earliest disseminated and/or veinlet Cu–Mo sulfide mineralization; (2) intermediate unmixed fluids corresponding to phyllic alteration and most Cu–Mo sulfide mineralization, with salinities of 30–50 wt% NaCl equiv and homogenization temperatures of 460–280 °C; and (3) late low to moderate temperature (300–160 °C) and low salinity (6–13 wt% NaCl equiv) fluids responsible for argillic and propylitic alteration. Hydrogen and oxygen isotopic studies show that the early hydrothermal fluids are of magmatic origin and were succeeded by increasing amounts of meteoric-derived convective waters. Sulfur isotopes also indicate a magmatic source for the sulfur in the early sulfide mineralization, with the increasing addition of sedimentary sulfur outward from the porphyry stock. Received August 29, 2001; revised version accepted May 1, 2002 Published online: November 29, 2002  相似文献   

8.
Numerous gold deposits and occurrences were recognized in the regions of tectonomagmatic activation in the southeastern Siberian Platform. They are located in four metallogenic zones: the Ket-Kap (skarns, quartz veins, and stockworks; gold-bearing lodes in silicitolites; and argillisite-sericite metasomatites), Ulkan (clayey-micaceous metasomatites, quartz veins), Preddzhugdzhur (quartz veins, skarns, and sericite-hydromicaceous metasomatites), and Uda (sericite-hydromicaceous metasomatites). The skarn mineralization is of Meosozoic age, while the mineralization in the quartz veins, quartz-hydromicaceous metasomatites, and quartz-sulfide veins may have a Meosozoic, Paleozoic, or Late Paleozoic age. The highest temperatures were determined for the ore formation in the Preddzhugdzhur skarns (500–715 °C) and the hydrothermal-metasomatic rocks of the Ket-Kap zone (510–530 °C). The composition of gas-liquid inclusions in the minerals of these rocks is dominated by aqueous Na, K, and Ca chloride solutions with salinity up to 40 wt % NaCl equiv; fluid contains CO2. Quartz veins and stockworks of the Ket-Kap zone were formed under high (up to 465°C) and moderate temperatures and salinity up to 32 wt % NaCl equiv. Sometimes, the minerals in these rocks contain inclusions of low-density CO2. The gold-bearing veins of the Preddzhugdzhur zone formed at 225–230°C and salinity of 1–2 wt % NaCl equiv. The ore-bearing solutions in the gold-bearing veins of the Ulkan zone are characterized by a potassium-sodium-chlorine composition and salinity of 2–10 wt % NaCl equiv., and the temperature of their formation was 220–280 °C.  相似文献   

9.
The Shasta gold-silver deposit, British Columbia, Canada, is an adularia-sericite-type epithermal deposit in which deposition of precious metals coincided with the transition of quartz- to calcite-dominant gangue. Mineralization is associated with stockwork-breccia zones in potassically altered dacitic lapilli tuffs and flows, and consists of pyrite, sphalerite, chalcopyrite, galena, acanthite, electrum and native silver. Pre- and post-ore veins consist solely of quartz and calcite, respectively. Fluid inclusion microthermometry indicates that ore minerals were deposited between 280 ° and 225 °C, from a relatively dilute hydrothermal fluid (˜1.5 wt.% NaCl equivalent). Abundant vapor-rich inclusions in ore-stage calcite are consistent with boiling. Oxygen and hydrogen isotopic data (δ18Ofluid = −1.5 to −4.1‰; δDfluid = −148 to −171‰) suggest that the fluid had a meteoric origin, but was 18O-enriched by interaction with volcanic wallrocks. Initial (˜280 °C) fluid pH and log f O2 conditions are estimated at 5.3 to 6.0, and −32.5 to −33 bar, respectively; during ore deposition, the fluid became more alkaline and oxidizing. Ore deposition at Shasta is attributed to localization of meteoric hydrothermal fluids by extensional faults; mineralization was controlled by boiling in response to hydraulic brecciation. Calcite and base metal sulfides precipitated due to the increase in pH that accompanied boiling, and the associated decrease in H2S concentration led to precipitation of gold and silver. Received: 23 February 1995 / Accepted: 16 April 1996  相似文献   

10.
The Samgwang mine is located in the Cheongyang gold district (Cheonan Metallogenic Province) of the Republic of Korea. It consists of eight massive, gold-bearing quartz veins that filled NE- and NW-striking fractures along fault zones in Precambrian granitic gneiss of the Gyeonggi massif. Their mineralogy and paragenesis allow two separate vein-forming episodes to be recognized, temporally separated by a major faulting event. The ore minerals occur in quartz and calcite of stage I, associated with fracturing and healing of veins. Hydrothermal wall-rock alteration minerals of stage I include Fe-rich chlorite (Fe/(Fe+Mg) ratios 0.74-0.81), muscovite, illite, K-feldspar, and minor arsenopyrite, pyrite, and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, argentite, pyrargyrite, and argentian tetrahedrite. Only calcite was deposited during stage II. Fluid inclusions in quartz contain three main types of C–O–H fluids: CO2-rich, CO2–H2O, and aqueous inclusions. Quartz veins related to early sulfides in stage I were deposited from H2O–NaCl–CO2 fluids (1,500–5,000 bar, average 3,200) with T htotal values of 200°C to 383°C and salinities less than about 7 wt.% NaCl equiv. Late sulfide deposition was related to H2O–NaCl fluids (140–1,300 bar, average 700) with T htotal values of 110°C to 385°C and salinities less than about 11 wt.% NaCl equiv. These fluids either evolved through immiscibility of H2O–NaCl–CO2 fluids as a result of a decrease in fluid pressure, or through mixing with deeply circulated meteoric waters as a result of uplift or unloading during mineralization, or both. Measured and calculated sulfur isotope compositions (δ34SH2S = 1.5 to 4.8‰) of hydrothermal fluids from the stage I quartz veins indicate that ore sulfur was derived mainly from a magmatic source. The calculated and measured oxygen and hydrogen isotope compositions (δ18OH2O = −5.9‰ to 10.9‰, δD = −102‰ to −87‰) of the ore-forming fluids indicate that the fluids were derived from magmatic sources and evolved by mixing with local meteoric water by limited water–rock exchange and by partly degassing in uplift zones during mineralization. While most features of the Samgwang mine are consistent with classification as an orogenic gold deposit, isotopic and fluid chemistry indicate that the veins were genetically related to intrusions emplaced during the Jurassic to Cretaceous Daebo orogeny.  相似文献   

11.
The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17–23 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370–405 °C) with early sulphide remobilisation in Tsumeb at 275 °C, whereas they are lower at Berg Aukas (up to 255 °C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct. Received: 5 May 1999 / Accepted: 10 November 1999  相似文献   

12.
The Hokko prospect is located in the Minamikayabe area southwestern Hokkaido, Japan, where gold-bearing quartz veins of Pliocene age are exposed at the surface. The alteration mineral assemblage is typical of low-sulfidation epithermal systems, with the quartz veins associated with adularia alteration overprinted on Late Miocene propylitic alteration. Fluid inclusion studies of the vein quartz reveal mean homogenization temperatures of approximately 220 °C, and the co-existence of low-salinity (<2 wt.% NaCl equivalent) and moderate salinity (2 to 12 wt.% NaCl equivalent) fluid inclusions within the same veins. The moderate salinity fluid inclusions (2–12 wt.% NaCl equivalent) typically have relatively low homogenization temperatures between 150° to 200 °C. The results obtained from stable isotope analysis of  δ18O in quartz vein material showed a gradual decrease in  δ18O signatures with increasing depth. The majority of the samples have calculated fluid source signatures (δ18OH2O) between −8.0 and −10.0‰, but there is a significant change in the composition above 185 m drill depth. The shallower samples in particular show a wide range of oxygen isotope signatures that are associated with the moderate salinity fluid inclusions. We interpret that low-salinity inclusions within the Hokko system represent the composition of the liquid phase of the fluid, before boiling, and that the moderate-salinity inclusions are representative of the residual liquid phase, after extensive non-adiabatic boiling and vapor loss in an open system. This mechanism resulted in the entrapment of fluids with variable salinities at the same time, and in close proximity to each other. This is also reflected in the  δ18OH2O values which become more variable and heavier where the moderate-salinity inclusions occur. Deposition of ore minerals within the Hokko vein system also occurred at this time as a result of boiling and gas loss. Received: 30 May 1997 / Accepted: 6 January 1998  相似文献   

13.
Fluid inclusion and structural studies were carried out at the Guarim gold deposit in the Palaeoproterozoic Tapajós province of the Amazonian craton. Guarim is a fault-hosted gold deposit cutting basement granitoids. It consists of a quartz vein, which is massive in its inner portions, grading laterally either to a massive or to cavity-bearing quartz vein associated with hydrothermal breccias. The wallrock alteration comprises chlorite, carbonate, white mica and sulphide minerals, with free gold occurring within quartz grains and spatially associated with sulphide mineral grains. Petrographic, microthermometric and Laser Raman investigations recognised CO2-rich, mixed H2O–CO2, and H2O fluid inclusions. The coexisting CO2 and H2O–CO2 inclusions were interpreted as primary immiscible fluids that formed the gold-bearing vein. The H2O inclusions were considered a product of later infiltration of fluids unrelated to the mineralising episode. The mineralising fluid has CO2 ranging typically from 5–10 mol%, contains traces of N2, has salinities of ∼5 wt% NaCl equiv., and densities varying between 0.85 and 0.95 g/cm3. The P–T estimations bracket gold deposition between 270–320 °C and 0.86–2.9 kb; ƒO2–ƒS2–pH estimates suggest a reduced, near-neutral character for the fluid. Variations in the physico-chemical properties, as demonstrated by the fluid inclusion study, resulted from a combination of fluid immiscibility and pressure fluctuation. This interpretation, combined with textural and structural evidence, suggests the emplacement of the mineralised vein in an active fault and at a rather shallow level (4–7 km). The geological and structural setting, deposit-scale textures and structures, wallrock alteration and physico-chemical fluid properties are compatible with those of epizonal to mesozonal orogenic lode gold deposits. Received: 3 March 2000 / Accepted: 21 October 2000  相似文献   

14.
The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold, and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO2 content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate that ore fluid pH ranged between 4.2 and 5.3, fO 2 between 10−38.8 and 10−39.6 bars, and mΣs between 10−3.2 and 10−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean lode-gold mineralisation.  相似文献   

15.
Sediment-hosted disseminated gold mineralisation at Zarshuran, NW Iran   总被引：1，自引：0，他引：1  
Mineralisation at the Zarshuran, NW Iran, occurs on the flank of an inlier of Precambrian rocks hosted in black silty calcareous and carbonaceous shale with interbedded dolomite and limestone varying in thickness from 5 to 60 m and extending along strike for approximately 5–6 km. Two major, steeply dipping sets of faults with distinct trends occur in the Zarshuran: (1) northwest (310–325) and (2) southwest (255–265). The main arsenic mineralisation occurs at the intersection of these faults. The mineral assemblage includes micron to angstrom-size gold, orpiment, realgar, stibnite, getchellite, cinnabar, thallium minerals, barite, Au-As-bearing pyrite, base metal sulphides and sulphosalts. Hydrothermal alteration features are developed in black shale and limestone around the mineralisation Types of alteration include: (1) decalcification, (2) silicification, (3) argillisation, (4) dolomitisation, (5) oxidation and acid leaching and (6) supergene alteration. The early stage of mineralisation involved removal of carbonates from the host rocks, followed by quartz precipitation. The main stage includes massive silicification associated with argillic alteration. In the late stage veining became more dominant and the main arsenic ore was deposited along fault cross cuts and gouge. These characteristics are typical of Carlin-type sediment-hosted disseminated gold deposits. The early stage of mineralisation contains only two-phase aqueous fluid inclusions. The main stage has two groups of three-phase CO2-bearing inclusions with minor CH4 ± N2, associated with high temperature, two-phase aqueous inclusions. During the late stage, fluids exhibit a wide range in composition, salinity and temperature, and CH4 becomes the dominant carbonic fluid with minor CO2 associated with a variety of two-phase aqueous fluid inclusions. The characteristics of fluids at the Zarshuran imply the presence of at least two separate fluids during mineralisation. The intersections of coexisting carbonic and aqueous inclusion isochores, together with stratigraphic and mineral stability evidence, indicate that mineralisation occurred at 945 ± 445 bar and 243 ± 59 °C, implying a depth for mineralisation of at least 3.8 ± 1.8 km (assuming a lithostatic pressure gradient). Fluid density fluctuations and the inferred depth of formation suggest that the mineralisation occurred at the transition between overpressured and normally pressured regimes. Geochronologic studies utilising K/Ar and Ar/Ar techniques on hydrothermal argillic alteration (whole rock and separated clay size fractions) and on volcanic rocks, indicates that mineralisation at Zarshuran formed at 14.2 ± 0.4 Ma, and was contemporaneous with nearby Miocene volcanic activity, 13.7 ± 2.9 Ma. It is proposed that mineralisation was the result of the infiltration of hydrothermal fluids containing a magmatic gas component, and that it was localised in the Zarshuran Unit because of the redox boundary that it provided and/or because it lay between an overpressured region at depth and a zone of circulating, hydrostatically pressured fluids above. Received: 10 December 1997 / Accepted: 5 March 1999  相似文献   

16.
The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (LCO2 + Laq ± VCO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (LCO2 ± VCO2) and aqueous H2O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H2O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm3, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f S2 to the range of 10−10 to 10−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f O2 through pressure fluctuation and CO2 effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.  相似文献   

17.
The Tuwaishan, Baoban, Erjia, Bumo and other gold deposits in western Hainan occur in Precambrian metamorphic clastic rocks and are structurally controlled by the Gezhen shear zone. Fluid inclusion studies have been carried out of the gold deposits mentioned above. The homogenization temperatures of the whole fluid inclusion population range from 140°C to 370°C, indicating that gold was precipitated mainly at 240–250°C. The salinities are within the range of 2.0–9.2 wt% NaCl equiv. and the pressure of formation of the deposits was estimated at about 270×105−500×105Pa, corresponding to a depth of about 1.1–2.0 km under lithostatic confinement. Chemical studies show that the ore fluid is of the Na+(K+)-Ca2+-Cl−(F−) type. Theδ 18O andδD values of the fluid vary from −2.7‰- +4.4‰ and −50‰–−87‰ Evidence developed from fluid inclusions and geological setting indicates that the ore fluid was a mixture of magmatic and meteoric-hydrothermal waters. Changes in chemical composition andδ 18O andδD of fluid inclusions from one ore field to another seem to be related with regional tectonism, metamorphism and magmatism.  相似文献   

18.
Located in Alxa Zuoqi (Left Banner) of Inner Mongolia, China, the Zhulazhaga gold deposit is the first largescale gold deposit that was found in the middle-upper Proterozoic strata along the north margin of the North China craton in recent years. It was discovered by the No. l Geophysical and Geochemical Exploration Party of Inner Mongolia as a result of prospecting a geochemical anomaly. By now, over 50 tonnes of gold has been defined, with an average Au grade of 4 g/t. The ore bodies occur in the first lithological unit of the Mesoproterozoic Zhulazhagamaodao Formation (MZF), which is composed mainly of epimetamorphic sandstone and siltstone and partly of volcanic rocks. With high concentration of gold,the first lithological unit of the MZF became the source bed for the late-stage ore formation. Controlled by the interstratal fracture zones, the ore bodies mostly appear along the bedding with occurrence similar to that of the strata. The primitiveore types are predominantly the altered rock type with minor ore belonging to the quartz veins type. There are also some oxidized ore near the surface. The metallic minerals are composed mainly of pyrite, pyrrhotite and arsenopyrite with minor chalcopyrite, galena and limonite. Most gold minerals appear as native gold and electrum. Hydrothermal alterations associated with the ore formation are actinolitization, silicatization, sulfidation and carbonation. A total of 100 two-phase H2O-rich and 7 three-phase daughter crystal-beating inclusions were measured in seven goldbearing quartz samples from the Zhulazhaga gold deposit. The homogenization temperatures of the two-phase H2O-rich inclusions range from 155 to 401℃, with an average temperature of 284℃ and bimodal distributions from 240 to 260℃ and 300 to 320℃ respectively. The salinities of the two-phase H2O-rich inclusions vary from 9.22wt% to 24.30wt% NaCl eqniv, with a mode between 23 wt% and 24wt% NaC1 equiv. Comparatively, the homogenization temperatures of the threephase daughter crystal-beating inclusions vary from 210 to 435℃ and the salinities from 29.13wt% to 32.62wt% NaCl equiv. It indicates that the ore-forming fluid is meso-hypothermal and characterized by high salinity, which is apparently different from the metamorphic origin with low salinity. It suggests a magmatic origin of the gold-bearing fluid. The δ^18O values of quartz from auriferous veins range from 11.9 to 16.3 per mil, and the calculated δ^18OH2O values in equilibrium with quartz vary from 1.06 to 9.60 per mil, which fall between the values of meteoric water and magmatic water. It reflects that the ore-forming fluid may be the product of mixing of meteoric water and magmatic water.Based on geological and geochemical studies of the Zhulazhaga gold deposit, it is supposed that the volcanism in the Mesoproterozoic might make gold pre-concentrate in the strata. The extensive and intensive Hercynian tectono-magmatic activity not only brought along a large number of ore-forming materials, but also made the gold from the strata rework. It can be concluded that the ore bodies were mainly formed in late hydrothermal reworking stage. Compared with typical gold deposits associated with epimetamorphic clastic rocks, the Zhulazhaga deposit has similar features in occurrence of ore bodies, ore-controlling structure, wall-rock alterations and mineral assemblages. Therefore, the Zhulazhaga gold deposit belongs to the epimetamorphic clastic rock type.  相似文献   

19.
Tongkeng-Changpo is the largest tin deposit within the giant Dachang polymetallic tin ore field in Guangxi, southern China, which is part of a large skarn system associated with Cretaceous granitoids. The Tongkeng-Changpo mineralization consists of veins and stockworks in the upper levels and replacement stratiform orebodies (mantos) at lower levels. Based on textural relationships, three major mineralizing stages can be recognized: stage I with cassiterite, sulphides, stannite, tourmaline, and quartz; stage II with cassiterite, sulphides, sulphosalts, quartz, and calcite; and stage III with calcite as the main phase. The study of fluid inclusions has shown that there are two main fluid types: CO2 and NaCl-H2O. Homogenization temperatures are 270 to 365°C, 210 to 240°C, and 140 to 190°C for stages I, II, and III, respectively. Salinities range from 1 to 7 wt.% NaCl equiv. in the early ore stage and 3 to 10 wt.% NaCl equiv. in the late stages. Laser Raman Spectroscopy indicates that the inclusion fluids in stages I and II were of carbono-aqueous composition, with minor amounts of CH4 and H2S, whereas those in stage III were aqueous. Helium isotopic analyses of inclusion fluids indicate that the 3He/4He ratios in the ore veins are in between 1.2 to 2.9 Ra (Ra = 1.4 × 10−6, modern atmospheric ratio), and range from 1.6 to 2.5 Ra in the stratiform orebodies. This range of 3He/4He ratios is significantly higher than that of crustal fluids (0.01–0.05 Ra). The similar characteristics of fluid inclusions and their He isotopic composition, as well as age constraints, indicate that the ore veins and stratiform orebodies of the Tongkeng-Changpo deposit formed from the same hydrothermal system, likely related to granite intrusions of the Mesozoic Yanshanian tectono-thermal event. In addition, the high R/Ra ratios indicate a mantle contribution in the ore fluids.  相似文献   

20.
The Kuh-e-Surmeh carbonate-hosted zinc-lead deposit, located within the Simply Folded Belt of the Zagros Mountains in southwestern Iran, is an orogen-related Mississippi Valley type deposit originally formed in the foreland Thrust Belt of the Zagros Mountains. Structural and textural observations indicate that ore deposition took place as open-space fillings in brecciated carbonate rock and as internal sediments consisting of fine-grained ore minerals interlayered with carbonates. The preferred genetic model for the concentration of the ore metals is that of dewatering of the Zard-Kuh basin due to regional tectonic compaction tectonism and expulsion of basin-derived fluids into the highly porous and brecciated dolomitized rocks of the Dalan Formation. The metals precipitated from dense basinal brine (15 wt% equiv. NaCl) at low temperatures (less than 200 °C), typically within strata of a Late Paleozoic carbonate platform. Received: 21 July 1998 / Accepted: 20 August 1999  相似文献   

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