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1.
The Osborne iron oxide–copper–gold (IOCG) deposit is hosted by amphibolite facies metasedimentary rocks and associated with pegmatite sheets formed by anatexis during peak metamorphism. Eleven samples of ore-related hydrothermal quartz and two pegmatitic quartz–feldspar samples contain similarly complex fluid inclusion assemblages that include variably saline (<12–65 wt% salts) aqueous and liquid carbon dioxide varieties that are typical of IOCG mineralisation. The diverse fluid inclusion types present in each of these different samples have been investigated by neutron-activated noble gas analysis using a combination of semi-selective thermal and mechanical decrepitation techniques. Ore-related quartz contains aqueous and carbonic fluid inclusions that have similar 40Ar/36Ar values of between 300 and 2,200. The highest-salinity fluid inclusions (47–65 wt% salts) have calculated 36Ar concentrations of approximately 1–5 ppb, which are more variable than air-saturated water (ASW = 1.3–2.7 ppb). These fluid inclusions have extremely variable Br/Cl values of between 3.8 × 10−3 and 0.3 × 10−3, and I/Cl values of between 27 × 10−6 and 2.4 × 10−6 (all ratios are molar). Fluid inclusions in the two pegmatite samples have similar 40Ar/36Ar values of ≤1,700 and an overlapping range of Br/Cl and I/Cl values. High-salinity fluid inclusions in the pegmatite samples have 2.5–21 ppb 36Ar, that overlap the range determined for ore-related samples in only one case. The fluid inclusions in both sample groups have 84Kr/36Ar and 129Xe/36Ar ratios that are mainly in the range of air and air-saturated water and are similar to mid-crustal rocks and fluids from other settings. The uniformly low 40Ar/36Ar values (<2,200) and extremely variable Br/Cl and I/Cl values do not favour a singular or dominant fluid origin from basement- or mantle-derived magmatic fluids related to A-type magmatism. Instead, the data are compatible with the involvement of metamorphic fluids that have interacted with anatectic melts to variable extents. The ‘metamorphic’ fluids probably represent a mixture of (1) inherited sedimentary pore fluids and (2) locally derived metamorphic volatilisation products. The lowest Br/Cl and I/Cl values and the ultra-high salinities are most easily explained by the dissolution of evaporites. The data demonstrate that externally derived magmatic fluids are not a ubiquitous component of IOCG ore-forming systems, but are compatible with models in which IOCG mineralisation is localised at sites of mixing between fluids of different origin. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorised users.  相似文献   

2.
The timing of Cu–Mo–U mineralisation at the Nori/RA prospect in the Paleoproterozoic Great Bear magmatic zone has been investigated using Re–Os molybdenite and 40Ar–39Ar biotite geochronology. The Re–Os molybdenite ages presented are the first robust sulphide mineralisation ages derived from the Great Bear magmatic zone. Cu–Mo–U mineralisation is hosted in early to syn-deformational hydrothermal veins consisting of quartz and K-feldspar or more commonly tourmaline-biotite-quartz-K-feldspar, with associated wall-rock alteration assemblages being predominantly biotite. Sulphide and oxide minerals consist of chalcopyrite, molybdenite and uraninite with lesser pyrite and magnetite. Elevated light rare earth elements and tungsten concentrations associated with the Cu–Mo–U mineralisation have also been reported at the prospect by previous workers. Molybdenite and uraninite occur intimately in dravitic tourmaline growth zones and at grain margins, attesting to their syngenetic nature (with respect to hydrothermal veining). Two molybdenite separates yield Re–Os model ages of 1,874.4 ± 8.7 (2σ) and 1,872.4 ± 8.8 Ma (2σ) with a weighted average model age of 1,873.4 ± 6.1 Ma (2σ). Laser step heating of biotite from the marginal alteration of the wall-rock adjacent to the veins yields a 40Ar–39Ar maximum cooling age of 1,875 ± 8 Ma (MSWD = 3.8; 2σ), indistinguishable from the Re–Os molybdenite model age and a previously dated ‘syn-tectonic’ aplitic dyke in the region. Dravitic tourmaline hosts abundant primary liquid–vapour–solid-bearing fluid inclusions. Analytical results indicate liquid–vapour homogenisation at >260°C constraining the minimum temperature of mineralisation. The solids, which are possibly trapped, did not homogenise with the liquid–vapour by 400°C. Salinities in the inclusions are variable. Raman spectra identify that at least some of the solids are calcite and anhydrite. Raman spectra also confirm the vapour phases contain some CO2; whereas clathrates or CH4 was not observed or detected. Quartz grains only host secondary fluid inclusions, which fluoresce under ultraviolet light, indicating trapped hydrocarbons. We speculate that these resulted from Phanerozoic fluid circulation through the Proterozoic basement. The collective interpretation of the age, hydrothermal character and associated metals, high temperature and variable salinity suggests that the Nori/RA Cu–Mo–U mineralisation can be linked with the earliest stages of plutonism in the Great Bear magmatic zone. From a regional perspective, the mineralisation may pre-date the extensive multi-element mineralisation now recognised as part of the iron oxide copper–gold (IOCG) spectrum of deposits. As IOCG provinces generally contain a variety of mineralisation styles, we interpret this as the earliest phase of the extensive mineralising system.  相似文献   

3.
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   

4.
Fluid inclusions were studied in samples from the Ashanti, Konongo-Southern Cross, Prestea, Abosso/Damang and Ayanfuri gold deposits in the Ashanti Belt, Ghana. Primary fluid inclusions in quartz from mineralised veins of the Ashanti, Prestea, Konongo-Southern Cross, and Abosso/Damang deposits contain almost exclusively volatile species. The primary setting of the gaseous (i.e. the fluid components CO2, CH4 and N2) fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids. Microthermometric and Raman spectroscopic analyses of the inclusions revealed a CO2 dominated fluid with variable contents of N2 and traces of CH4. Water content of most inclusions is below the detection limits of the respective methods used. Aqueous inclusions are rare in all samples with the exception of those from the granite-hosted Ayanfuri mineralisation. Here inclusions associated with the gold mineralisation contain a low salinity (<6 eq.wt.% NaCl) aqueous solution with variable quantities of CO2. Microthermometric investigations revealed densities of the gaseous inclusions of 0.65 to 1.06 g/cm3 at Ashanti, 0.85 to 0.98 g/cm3 at Prestea, up to 1.02 g/cm3 at Konongo-Southern Cross, and 0.8 to 1.0 g/cm3 at Abosso/Damang. The fluid inclusion data are used to outline the PT ranges of gold mineralisation of the respective gold deposits. The high density gaseous inclusions found in the auriferous quartz at Ashanti and Prestea imply rather high pressure trapping conditions of up to 5.4 kbar. In contrast, mineralisation at Ayanfuri and Abosso/Damang is inferred to have occurred at lower pressures of only up to 2.2 kbar. Mesothermal gold mineralisation is generally regarded to have formed from fluids characterized by H2O > CO2 and low salinity ( ±  6 eq.wt.%NaCl). However, fluid inclusions in quartz from the gold mineralisations in the Ashanti belt point to distinctly different fluid compositions. Specifically, the predominance of CO2 and CO2 >> H2O have to be emphasized. Fluid systems with this unique bulk composition were apparently active over more than 200␣km along strike of the Ashanti belt. Fluids rich in CO2 may present a hitherto unrecognised new category of ore-forming fluids. Received: 30 May 1996 / Accepted: 8 October 1996  相似文献   

5.
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   

6.
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   

7.
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   

8.
The Profitis Ilias gold deposit, located on the western part of Milos Island, Greece, is the first epithermal gold deposit discovered in the Pliocene–Pleistocene Aegean volcanic arc. Estimated ore reserves are 5 million tonnes grading 4.4 g/tonne Au and 43 g/tonne Ag. The deposit is closely associated with a horst and graben structure, and occurs in a series of steep interconnected crustiform-banded quartz veins up to 3 m wide, extending to depths of at least 300 m. The mineralisation occurs in three stages and is hosted by 3.5–2.5 Ma old silicified and sericitised rhyolitic lapilli-tuffs and ignimbrites. It consists of pyrite, galena, chalcopyrite, electrum and native gold. Additionally, adularia occurs with quartz mainly in veins. Homogenisation temperatures of primary liquid-rich inclusions vary from 145 to 399 °C for the ore stage, and 112 to 263 °C for the post-ore stage. Salinities range between 0.1 and 11.4 wt% NaCl equiv. and 0.93 to 8.5 wt% NaCl equiv. for the ore stage and the post-ore stage, respectively. Rare vapour-rich inclusions in ore stage quartz homogenise between 368 and 399 °C and estimates of eutectic melting (−25 to −38 °C) indicate the presence of Ca and Mg in the ore fluids. Sample elevation versus fluid inclusion Th–salinity relationships show (1) a high-salinity trend, where moderate-temperature (300–250 °C) and moderate-salinity brines (∼3 wt% NaCl equiv.) trend to high-salinity (up to 15 wt% NaCl equiv.) fluids with lower (∼25–50 °C) homogenisation temperatures, and (2) a high-Th trend where moderate-salinity and moderate-temperature brines (200–250 °C; 3 wt% NaCl equiv.) develop into low-salinity (<1 wt% NaCl equiv.), high-temperature (>350 °C) fluids. These trends are best explained by extreme boiling and vapourisation phenomena between 200 and 250 °C. The 430–450 m asl (metres above sea level) level marks the transition between a lower liquid-dominated segment of the system where only the steep high-salinity trend is seen, and an upper vapour-dominated segment where the high-Th trend or a combination of both are seen. There is a close spatial association between mineable gold grades and the upper segment of the system. Depth-to-boiling curves suggest that the paleo-surface was ∼200 m above the present summit of Profitis Ilias. Comparison of the mineralisation and fluid geochemistry at Profitis Ilias with that of the nearby modern geothermal system indicates that the processes of metal mineralisation have probably been continuous since the Late Pliocene. Received: 24 February 2000 / Accepted: 15 July 2000  相似文献   

9.
Mo-Bi mineralization occurs in subvertical and subhorizontal quartz-muscovite-± K-feldspar veins surrounded by early albitic and later K-feldspathic alteration halos in monzogranite of the Archean Preissac pluton, Abitibi region, Québec, Canada. Molybdenite is intergrown with muscovite in the veins or associated with K-feldspar in the alteration halos. Mineralized veins contain five main types of fluid inclusions: aqueous liquid and liquid-vapor inclusions, aqueous carbonic liquid-liquid-vapor inclusions, carbonic liquid and vapor inclusions, halite-bearing aqueous liquid and liquid-vapor inclusions, trapped mineral-bearing aqueous liquid and liquid-vapor inclusions. The carbonic solid in frozen carbonic and aqueous-carbonic inclusions melts in most cases at −56.7 ± 0.1 °C indicating that the carbonic fluid consists largely of CO2. All aqueous inclusion types and the aqueous phase in carbonic inclusions have low initial melting temperatures (≥70 °C), requiring the presence of salts other than NaCl. Leachate analyses show that the bulk fluid contains variable proportions of Na, K, Ca, Cl, and traces of Mg and Li. The following solids were identified in the fluid inclusions by SEM-EDS analysis: halite, calcite, muscovite, millerite (NiS), barite and antarcticite (CaCl2 · 6H2O). All are interpreted to be trapped phases except halite which is a daughter mineral, and antarcticite which formed during sample preparation (freezing). Aqueous inclusions homogenize to liquid at temperatures between 75 °C and 400 °C; the mode is 375 °C. Aqueous-carbonic inclusions homogenize to liquid or vapor between 210 °C and 400 °C. Halite-bearing aqueous inclusions homogenize by halite dissolution at approximately 170 °C. Aqueous inclusions containing trapped solids exhibit liquid-vapor homogenization at temperatures similar to those of halite-bearing aqueous inclusions. Temperatures of vein formation, based on oxygen isotopic fractionation between quartz and muscovite, range from 342 °C to 584 °C. The corresponding oxygen isotope composition of the aqueous fluid in equilibrium with these minerals ranges from 1.2 to 5.5 per mil with a mean of 3.9 per mil, suggesting that the liquid had a significant meteoric component. Isochores for aqueous fluid inclusions intersect the modal isotopic isotherm of 425 °C at pressures between 590 and 1900 bar. A model is proposed in which molybdenite was deposited owing to decreasing temperature and/or pressure from CO2-bearing, moderate to high salinity fluids of mixed magmatic-meteoric origin that were in equilibrium with K-feldspar and muscovite. These fluids resulted from the degassing of a monzogranitic magma and evolved through interaction with volcanic (komatiitic) and sedimentary country rocks. Received: 6 February 1997 / Accepted: 28 January 1998  相似文献   

10.
A bulk geochemical study has been carried out on fluid inclusion leachates extracted from quartz veins from porphyry Cu deposits in Butte, Montana, USA and Bingham Canyon, Utah, USA. The leachates mostly represent low-salinity magmatic–hydrothermal fluid inclusions. Their halogen ratios (Br/Cl) of fluid inclusion leachates were determined by ion chromatography, and δ37Cl values of the leachates were measured by continuous-flow isotope ratio mass spectrometry. Br/Cl ratios from early pre-Main stage and later Main stage veins at Butte range from 0.60 to 1.88 × 10−3 M. Ratios are similar in pre-Main stage veins with sericite bearing selvages and Main stage samples ranging from 0.81 to 1.08 × 10−3 and from 0.92 to 1.88 × 10−3 M, respectively, clustering below seawater (1.54 × 10−3 M) and overlapping mantle values (~1–2 × 10−3 M). Two samples associated with early pre-Main stage potassic alteration yield distinctly lower Br/Cl ratios of 0.60 and 0.64 × 10−3 M. Butte δ37Cl values range from −0.8‰ to −2.3‰ with no significant difference between pre-Main stage and Main stage samples. Br/Cl ratios for quartz veins from Bingham Canyon range from 0.18 to 3.68 × 10−3 M. Br/Cl ratios from Bingham range above and below previously reported for porphyry copper deposits. In contrast to Butte, δ37Cl values for Bingham are lower, ranging from −0.9‰ to −4.1‰. In the absence of any processes which can significantly fractionate chlorine isotopes at high temperatures, we suggest that the porphyry system at Bingham, and to a lesser extent at Butte, have inherited negative chlorine isotopic signatures from the subducting slab generated at low temperatures.  相似文献   

11.
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   

12.
Halogen ratios (Br/Cl and I/Cl) and concentrations provide important information about how sedimentary formation waters acquire their salinity, but the possible influence of organic Br derived from sedimentary wall-rocks is rarely quantified. Here, it is demonstrated that Br/Cl versus I/Cl mixing diagrams can be used to deconvolve organic Br contributions; that organic matter has a limited range of Br/I ratios; and that organic Br is a more significant component in Zn–Pb deposit ore fluids than previously recognised. The significance of these findings is illustrated for the Lennard Shelf Zn–Pb deposits of Western Australia.Fluid inclusions related to Lennard Shelf Zn–Pb mineralisation have variable salinity and hydrocarbon contents. The halogen data from these fluid inclusions require mixing of three fluid end-members: (1) an evaporated seawater bittern brine (30 wt.% NaCl equiv.) with greater than seawater Br/Cl ratio; (2) a lower salinity pore fluid (?5 wt.% NaCl equiv.) with moderately elevated Br/Cl and I/Cl; and (3) fluids with Br/Cl ratios of ～5 times seawater and extremely elevated I/Cl ratios of ～11,500 times seawater. The first two fluids have 40Ar/36Ar of 300–400 and greater than air saturated water 36Ar concentrations that are typical of fluid inclusions related to Zn–Pb mineralisation. The third ‘organic-rich’ fluid has the highest 40Ar/36Ar ratio of up to 1500 and a depleted 36Ar concentration.Mineralisation is interpreted to have resulted from mixing of Zn-rich evaporitic brines and H2S present in hydrocarbons. It is suggested that aqueous fluids acquired organic Br and I from hydrocarbons, and that hydrocarbons exsolving from the aqueous fluid removed noble gases from solution. Interaction of variably saline brines and hydrocarbons could account for the variable Br/Cl and I/Cl composition, and 36Ar concentrations, recorded by Lennard Shelf fluid inclusions. The distinct 40Ar/36Ar signature of the fluid with the highest I/Cl ratio suggests the hydrocarbons and brines were sourced independently from different parts of the sedimentary basin. These data indicate the complementary nature of halogen and noble gas analysis and provide new constraints on important mixing processes during sediment-hosted Zn–Pb mineralisation.  相似文献   

13.
Summary ?Data from secondary fluid inclusions in barite and quartz occurring in strongly silicified lithologies (“Quarzite”) along the inter-Ordovician unconformity in SW Sardinia suggest that two distinct fluids are recorded in the mineral phases. (A), a high temperature (250–360 °C) – low salinity (0–6% NaCl equiv.) fluid, measured in quartz, (B), a high salinity (6–20% NaCl equiv.) – low temperature (<80–200 °C) fluid, measured in barite. Fluid (B), though less abundant than A, has also been recorded in quartz, whereas traces of fluid (A) were found in barite. The characteristics of fluid (B) make it akin to the well known saline formation waters responsible for the deposition of the post-Variscan barite and galena ores in SW Sardinia. It is inferred that at least two distinct post-depositional hydrothermal events took place, with fluids circulating along the inter-Ordovician unconformity. Fluid (A) is related to the last phases of Variscan tectonics, whereas the timing of the second hydrothermal event (fluid B) spans the interval from Permian to Mesozoic. Due to the lack of measurable primary inclusions and to the ubiquitous evidence for the stratigraphic control of the silicification, as well as to the marked deformation of the “Quarzite” horizon, it cannot be excluded that at least part of this hydrothermal event could have taken place in pre-Variscan (Silurian?) times. An alternative model is to consider the deposition of the “Quarzite” to be unrelated to pre-Variscan hydrothermal events, but to fluid circulation along Late-Variscan structures. Received July 12, 2001; revised version accepted March 11, 2002  相似文献   

14.
Fluid inclusion microthermometry, Raman spectroscopy and noble gas plus halogen geochemistry, complemented by published stable isotope data, have been used to assess the origin of gold-rich fluids in the Lachlan Fold Belt of central Victoria, south-eastern Australia. Victorian gold deposits vary from large turbidite-hosted ‘orogenic’ lode and disseminated-stockwork gold-only deposits, formed close to the metamorphic peak, to smaller polymetallic gold deposits, temporally associated with later post-orogenic granite intrusions. Despite the differences in relative timing, metal association and the size of these deposits, fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low-salinity aqueous, CO2-bearing fluids. The majority of these fluid inclusions also have similar 40Ar/36Ar values of less than 1500 and 36Ar concentrations of 2.6–58 ppb (by mass) that are equal to or much greater than air-saturation levels (1.3–2.7 ppb). Limited amounts of nitrogen-rich fluids are present at a local scale and have the highest measured 40Ar/36Ar values of up to 5,700, suggesting an external or distinct source compared to the aqueous fluids. The predominance of low-salinity aqueous–carbonic fluids with low 40Ar/36Ar values, in both ‘orogenic’ and ‘intrusion-related’ gold deposits, is attributed to fluid production from common basement volcano-sedimentary sequences and fluid interaction with sedimentary cover rocks (turbidites). Aqueous fluid inclusions in the Stawell–Magdala deposit of western Victoria (including those associated with N2) preserve mantle-like Br/Cl and I/Cl values. In contrast, fluid inclusions in deposits in the eastern structural zones, which contain more abundant shales, have elevated molar I/Cl ratios with maximum values of 5,170 × 10−6 in the Melbourne Zone. Br/I ratios in this zone range from 0.5 to 3.0 that are characteristic of fluid interaction with organic-rich sediments. The maximum I/Cl and characteristic Br/I ratios provide evidence for organic Br and I released during metamorphism of the shales. Therefore, the regional data provide strong evidence for the involvement of sedimentary components in gold mineralisation, but are consistent with deeper metamorphic fluid sources from basement volcano-sedimentary rocks. The overlying sediments are probably involved in gold mineralisation via fluid–rock interaction.  相似文献   

15.
Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO2-H2O inclusions, which have a wide range in CO2 content and coexist with lesser primary CO2-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO2 in stage I inclusions and 10–20 mol% CO2 in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.% NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th total than, their coexistent CO2-H2O inclusions. The coexisting CO2-rich, CO2-H2O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO2-H2O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for stage II fluids. The CO2-H2O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids. The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage II veins. Received: 15 May 1997 / Accepted: 10 June 1998  相似文献   

16.
《Applied Geochemistry》2001,16(1):35-55
Formation waters within Upper Carboniferous sandstones in the sub-sea Prince and Phalen coal mines, Nova Scotia, originated as residual evaporative fluids, probably during the precipitation of Windsor Group (Lower Carboniferous) salts which underlie the coal measures. Salinity varies from 7800 to 176,000 mg/l, and the waters are Na–Ca–Cl brines enriched in Ca, Sr and Br and depleted in Na, K, Mg and SO4 relative to the seawater evaporation curve. Br:Cl and Na:Cl ratios suggest that the brine composition corresponds to an evaporation ratio of as much as 30. The brines lie close to the meteoric line on H/O isotopic plots but with a compositional range of δ18O from −4.18 to −6.99 and of δD from −42.4 to −23.5, distant from modern meteoric or ocean water. Mine water composition contrasts with that of nearby salt-spring brines, which are inferred to have originated through dissolution of Windsor Group evaporites by modern meteoric waters. However, a contribution to the mine waters from halite dissolution and from Br in organic matter cannot be ruled out. Present concentrations of several elements in the brines can be explained by water–rock interaction. The original Windsor brines probably moved up into the overlying coal-measure sandstones along faults, prior to the Late Triassic. The high salinity and irregular salinity distribution in the Phalen sandstones suggests that the brines have undergone only modest dilution and are virtually immobile. In contrast, Prince waters show a progressive increase in salinity with depth and are inferred to have mixed with surface waters. Basinal brines from which these modern formation fluids were derived may have been important agents in base-metal and Ba mineralisation from the mid-Carboniferous onwards, as saline fluid inclusions are common in Zn–Pb sulphide deposits in the region.  相似文献   

17.
The Pering deposit is the prime example of Zn–Pb mineralisation hosted by stromatolitic dolostones of the Neoarchean to Paleoproterozoic Transvaal Supergroup. The hydrothermal deposit centers on subvertical breccia pipes that crosscut stromatolitic dolostones of the Reivilo Formation, the lowermost portion of the Campbellrand Subgroup. Four distinct stages of hydrothermal mineralisation are recognised. Early pyritic rock matrix brecciation is followed by collomorphous sphalerite mineralisation with replacive character, which, in turn, is succeeded by coarse grained open-space-infill of sphalerite, galena, sparry dolomite, and quartz. Together, the latter two stages account for ore-grade Zn–Pb mineralisation. The fourth and final paragenetic stage is characterised by open-space-infill by coarse sparry calcite. The present study documents the results of a detailed geochemical study of the Pering deposit, including fluid inclusion microthermometry, fluid chemistry and stable isotope geochemistry of sulphides (δ34S) and carbonate gangue (δ13C and δ18O). Microthermometric fluid inclusion studies carried out on a series of coarsely grained crystalline quartz and sphalerite samples of the latter, open-space-infill stage of the main mineralisation event reveal the presence of three major fluid types: (1) a halite–saturated aqueous fluid H2O–NaCl–CaCl2 (>33 wt% NaCl equivalent) brine, (2) low-salinity meteoric fluid (<7 wt% NaCl) and (3) a carbonic CH4–CO2–HS− fluid that may be derived from organic material present within the host dolostone. Mixing of these fluids have given rise to variable mixtures (H2O–CaCl2–NaCl ±(CH4–CO2–HS−), 2 to 25 wt% NaCl+CaCl2). Heterogeneous trapping of the aqueous and carbonic fluids occurred under conditions of immiscibility. Fluid temperature and pressure conditions during mineralisation are determined to be 200–210°C and 1.1–1.4 kbar, corresponding to a depth of mineralisation of 4.1–5.2 km. Chemical analyses of the brine inclusions show them to be dominated by Na and Cl with lesser amounts of Ca, K and SO4. Fluid ratios of Cl/Br indicate that they originated as halite saturated seawater brines that mixed with lower salinity fluids. Analyses of individual brine inclusions document high concentrations of Zn and Pb (∼1,500 and ∼200 ppm respectively) and identify the brine as responsible for the introduction of base metals. Stable isotope data were acquired for host rock and hydrothermal carbonates (dolomite, calcite) and sulphides (pyrite, sphalerite, galena and chalcopyrite). The ore-forming sulphides show a trend to 34S enrichment from pyrite nodules in the pyritic rock matrix breccia (δ34S = −9.9 to +3.7‰) to paragenetically late chalcopyrite of the main mineralisation event (δ34S = +30.0‰). The observed trend is attributed to Rayleigh fractionation during the complete reduction of sulphate in a restricted reservoir by thermochemical sulphate reduction, and incremental precipitation of the generated sulphide. The initial sulphate reservoir is expected to have had an isotopic signature around 0‰, and may well represent magmatic sulphur, oxidised and leached by the metal-bearing brine. The δ18O values of successive generations of dolomite, from host dolostone to paragenetically late saddle dolomite follow a consistent trend that yields convincing evidence for extensive water rock interaction at variable fluid–rock ratios. Values of δ13C remain virtually unchanged and similar to the host dolostone, thus suggesting insignificant influx of CO2 during the early and main stages of mineralisation. On the other hand, δ13C and δ18O of post-ore calcite define two distinct clusters that may be attributed to changes in the relative abundance in CH4 and CO2 during waning stages of hydrothermal fluid flow.  相似文献   

18.
Textural and geochemical studies of inclusions in topaz from greisens in the Hensbarrow topaz granite stock (St. Austell, Cornwall) are used to constrain the composition of fluids responsible for late stage greisening and mineralisation. The topaz contains an abundant and varied suite of inclusions including aqueous liquid + vapour (L + V), quartz, zinnwaldite, albite, K-feldspar, muscovite, ilmenorutile, apatite, columbite, zircon, varlamoffite [(Sn, Fe)(O, OH)2] and qitianlingite [(Fe+2,Mn+2)2(Nb,Ta)2W+6O10]. Primary L + V inclusions in topaz show relatively high T h (mainly 300 to >500 °C) and a narrow range of salinities (23–30 wt % NaCl equivalent) compared with those in greisen quartz (150–450 °C, 0–50 wt % NaCl equivalent). Textures indicate that topaz formed earlier than quartz and the fluid inclusion data are interpreted as indicating a cooling of the hydrothermal fluids during greisenisation, mixing with meteoric waters and a decrease in pressure causing intermittent boiling. The presence of early-formed albite and K-feldspar as inclusions in the topaz is likely to indicate that the greisen-forming fluid became progressively more acid during greisenisation. The most distinctive inclusions in the topaz are wisp- and bleb-shaped quartz, < 50 μm in size, which show textural characteristics indicating former high degrees of plasticity. They often have multiple shrinkage bubbles at their margins rich in Sn, Fe, Mn, S and Cl and, more rarely, contain euhedral albite, K-feldspar, stannite or pyrrhotite crystals up to 40 μm in size. The quartz inclusions show similar morphologies to inclusions in topaz from quartz-topaz rocks elsewhere which have been interpreted as trapped “silicate melt”. Their compositions are, however, very different to those expected for late stage topaz-normative granitic melts. From their textural and chemical characteristics they are interpreted as representing crystallised silica colloid, probably trapped as a hydro gel during greisenisation. There is also evidence for the colloidal origin of inclusions of varlamoffite in the topaz. These occurrences offer the first reported evidence in natural systems for the formation of colloids in high temperature hydrothermal fluids. Their high ore carrying potential is suggested by the presence of varlamoffite and the occurrence of stannite, pyrrhotite and SnCl within the quartz inclusions. Received: 9 April 1996 / Accepted: 12 November 1996  相似文献   

19.
Analyses of fluid-inclusion leachates from ore deposits show that Na/Br ratios are within the range of 75 - 358 and Cl/Br 67 - 394, respectively, and this variation trend coincides with the seawater evaporation trajectory on the basis of the Na/Br and Cl/Br ratios. The average Cl/Br and Na/Br ratios of mineralizing fluids are 185 and 173 respectively, which are very close to the ratios ( 120 and 233 ) of the residual evaporated seawater past the point of halite precipitation. It is suggested that the original mineralizing brine was derived from highly evapo-rated seawater with a high salinity. However, the inclusion fluids have absolute Na values of 69.9—2606.2 mmol kg^-1 and Cl values of 106.7 — 1995.5 mmol kg^-1. Most of the values are much less than those of seawater: Na, 485 mmol kg^-1 and Cl, 566 mmol kg^-1 , respectively; the salinity measured from fluid inclusions of the deposits ranges from 2.47 wt% to 15.78 wt% NaCl equiv. The mineralizing brine has been diluted. The δ ^18O and δD values of ore-forming fluids vary from -8.21‰ to 9.51‰ and from -40.3‰ to -94.3‰, respectively. The δD values of meteoric water in this region varied from - 80‰ to - 100‰ during the Jurassic. This evidenced that the ore-forming fluids are the mixture of seawater and meteoric water. Highly evaporated seawater was responsible for leaching and extracting Pb, Zn and Fe, and mixed with and diluted by descending meteoric water, which resulted in the formation of ores.  相似文献   

20.
Gold mineralization in the Kolar schist belt of the Dharwar craton occurs dominantly in the form of a sulfide-poor Au-quartz lode (the Champion lode exposed in the Mysore and other mines) and sulfide-rich auriferous lodes (from the Nundydroog mine). Fluid inclusion microthermometric experiments were conducted on primary inclusions in quartz intimately associated with Au-mineralization. Homogenization studies on aqueous-biphase (L + V), aqueous polyphase (L + V+ halite) and aqueous-carbonic (LCO2± VCO2 + Laq) inclusions from the Champion lode furnish a temperature range of 120 to 420 °C. Freezing of aqueous biphase inclusions and dissolution of halite in the aqueous polyphase inclusions provide salinity of 5 to 50 wt.% NaCl equivalent. Fluid inclusion thermobarometry from the total homogenization of aqueous-carbonic inclusions and from intersecting isochores of coeval pure-carbonic (LCO2± VCO2) and pure-aqueous inclusions constrain the P-T path of evolution of the fluid in the Champion lode. Gold precipitation was likely to have been brought about in response to a sharp fall in pressure with attendant unmixing of liquid-CO2 from the parent H2O-CO2 fluid of possible metamorphic origin. This would imply transportation of gold by some pressure-sensitive complex such as the Au-carbonyl. Fluid characteristics are different in the sulfide-rich auriferous lodes, as indicated by the virtual absence of the CO2-bearing and the halite-bearing inclusions. The fluid evolution path, as evident from the crude positive colinearity of temperature and salinity, is due to mixing of a low (≤200 °C) temperature-low saline (≤7 wt.% NaCl equivalent) fluid with a high temperature (≥400 °C)-high saline (≥50 wt.% NaCl equivalent) fluid. The lack of CO2 and association of Au with sulfides indicate a different mode of gold transport, as chloride or bisulfide complexing, deposition of which was possibly brought about by fluid mixing. Received: 17 April 1997 / Accepted: 30 June 1998  相似文献   

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