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1.
The Camagüey district, Cuba, is known for its epithermal precious metal deposits in a Cretaceous volcanic arc setting. Recently, the La Unión prospect was discovered in the southern part of the district, containing gold and minor copper mineralization interpreted as porphyry type. Mineralization is hosted in a 73.0 ± 1.5 Ma calc–alkaline I-type oxidized porphyry quartz diorite intrusive within volcanic and volcaniclastic rocks of the early Cretaceous Guáimaro Formation. The porphyry is affected by propylitic alteration and crosscut by a network of quartz and carbonate veinlets and veins. Chlorite, epidote, sericite, quartz, and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins. Late barite pseudomorphously replaces pyrite. Gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins, occurring as inclusions or filling fractures in pyrite with 4 g/t Au in bulk samples, and up to 900 ppm Au in in pyrite. Fluid inclusion and oxygen isotope data are consistent with a H2O–NaCl–(KCl) mineralizing fluid, derived from the quartz diorite magma, and trapped at least at 425°C and 1.2 kbar. This primary fluid unmixed into two fluid phases, a hypersaline aqueous fluid and a low-salinity vapor-rich fluid. Boiling during cooling may have played an important role in metal precipitation. Pyrite δ34S values for the La Unión prospect range between 0.71‰ and 1.31‰, consistent with a homogeneous magmatic sulfur source. The fluids in equilibrium with the mineralized rocks have estimated δ18O values from 8‰ to 11.8‰, calculated for a temperature range of 480–505°C. The tectonic environment of the La Unión prospect, its high gold and low copper contents, the physical–chemical characteristics of the mineralizing fluids and the isotopic signature of the alteration minerals and fluids indicate that the La Unión gold mineralization is similar to the porphyry gold type, even though the ore-related epidote–chlorite alteration can be classified as propylitic and not the classic potassic and/or phyllic alteration. The low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper, which is indicated by trapped chalcopyrite crystals in high-temperature fluid inclusions. The low-temperature paragenesis, represented by carbonate, zeolite and barite, indicates epithermal overprint. The study shows the potential for other gold porphyry-type deposits in the Cretaceous volcanoplutonic arc of Cuba.  相似文献   

2.
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   

3.
The Géant Dormant gold mine is a sulfide-rich quartz vein gold deposit hosted by a volcano-sedimentary sequence and an associated felsic endogenous dome and dikes. The auriferous quartz-sulfide veins were preceded by two synvolcanic gold-bearing mineralizing events: early sulfidic seafloor-related and later disseminated pyrite in the felsic dome. This deposit differs from classical Archean auriferous quartz vein deposits by the low carbonate and high sulfide contents of the veins and by their formation prior to ductile penetrative deformation. The δ18O values of quartz associated with seafloor-related auriferous sulfides average 11.9 ± 0.6‰ (n = 3). The seafloor hydrothermal fluids had a δ18O value of 3.2‰ calculated at 250 °C. The oxygen isotope composition of quartz and chlorite from veins average 12.5 ± 0.3‰ (n = 20) and 5.9 ± 1.1‰ (n = 4) respectively. Assuming oxygen isotope equilibrium between quartz and chlorite, the veins formed at a temperature of ∼275 °C, which is consistent with the calculated temperature of 269 ± 10 °C from chlorite chemistry. The gold-bearing fluids had a δ18O value of 4.7‰ calculated at 275 °C. The δ34S values of sulfides from the three gold events range from 0.6 to 2.8‰ (n = 32) and are close to magmatic values. Sulfur isotope geothermometry constrains the sulfide precipitation in the gold-bearing veins at a temperature of ∼350 °C. The similarity of the isotope data, the calculated δ18O of the mineralizing fluids and the likely seawater fluid source suggest that the three mineralizing events are genetically related to a volcanogenic hydrothermal system. The high value of the auriferous fluids (δ18O = 4.7‰) is attributed to a significant magmatic fluid contribution to the evolved seawater-dominated convective hydrothermal system. The two-stage filling of veins at increasing temperature from quartz-chlorite (275 °C) to sulfides (350 °C) may reflect the progressive maturation of volcanogenic hydrothermal systems. These results, together with field and geochemical data, suggest that formation of gold-rich volcanogenic systems require specific conditions that comprise a magmatic fluid contribution and gold from arc-related felsic rocks, coeval with the mineralizing events. This study shows that some auriferous quartz-vein orebodies in Archean terranes are formed in volcanogenic rather than mesothermal systems. Received: 12 December 1998 / Accepted: 5 July 1999  相似文献   

4.
Post-metamorphic quartz veins which occur over hundreds of square kilometres in the biotite zone of the Dalradian metamorphic belt consist of three principal types: anhedral quartz with pyrite, anhedral quartz with hematite, and prismatic quartz with hematite or rutile. The oxide minerals in anhedral veins have formed by oxidation of pre-existing sulphides, and gold was mobilized during this oxidation. Anhedral quartz veins formed from an aqueous fluid with up to 5 wt% dissolved salts and 16 wt% CO2 at about 300 °C. Texturally later prismatic quartz crystals formed from a compositionally similar fluid which was undergoing phase separation at the H2O-CO2 solvus at 160–200 °C and 500 to 1200 bars fluid pressure. Oxygen isotope ratios for quartz from the veins range from 12.0 to 15.3‰, with hematite-bearing veins generally isotopically heavier than pyrite-bearing veins. Calculated fluid oxygen isotope ratios range from + 8‰ for pyrite-bearing veins to -2‰ for late prismatic crystals. The mineralizing fluid contained a substantial component of meteoric water whose isotopic and chemical composition evolved with progressive water-rock interaction. Evolution of meteoric fluid composition involved migration of oxidation and oxygen isotope fronts in the down-flow direction as head-driven water passed through structurally controlled fractures in the schist pile. A gold solubility trough occurs for the observed fluid in the oxidation frontal zone. Gold remobilization and reprecipitation occurred progressively as the oxidation front migrated through the schist pile.  相似文献   

5.
Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones. The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly foliated (S1) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite, tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite, chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase alteration assemblages are developed in microlithons of the S2–S3 crenulation cleavage and are replaced along S3 by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D2 shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D3 dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C), which formed the auriferous laminated quartz veins. Later D4 crosscutting veins and D5 faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than in the western Dharwar craton. Editorial handling: G. Beaudoin  相似文献   

6.
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   

7.
The Mesozoic Yangzhaiyu lode gold deposit is situated in the southern edge of the North China craton. Gold mineralization is hosted in Archean amphibolite facies metamorphic rocks, and consists mainly of auriferous quartz veins. Pyrite is the predominant sulfide mineral, with minor amounts of chalcopyrite, sphalerite, and galena. Based on morphology and paragenesis, there are three generations of pyrite, termed as first generation (G1), second generation (G2), and third generation (G3). They have distinct contents, occurrences, and distribution patterns of gold. The coarse-grained, euhedral G1 pyrite contains negligible to low levels of gold, whereas both invisible and visible gold are present in the fine- to medium-grained G2 pyrite that is characterized by abundance of microfractures and porosities, forming a foam-like texture. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) depth profiles indicate that invisible gold occurs either as solid solution or as nanoparticles of gold-bearing tellurides in the G2 pyrite. Visible gold is widespread and present as irregular grains and stringers of native gold mostly along grain boundaries or filling microfractures of pyrite, likely resulting from remobilization of invisible gold once locked in the G2 pyrite. The G3 pyrite, invariably intergrown with chalcopyrite, sphalerite, and galena, contains the highest levels of invisible gold. There is a positive correlation between Au, Ag, and Te, indicating that gold occurs as submicroscopic Au-bearing telluride inclusions in the host minerals. Whenever gold, either invisible or visible, is present, As is always below or only marginally higher than the detection limit of LA-ICP-MS. This indicates that As played an insignificant role in gold mineralization. Tellurides are widespread in the auriferous quartz veins, consisting mainly of petzite, calaverite, hessite, altaite, and tellurobismuthite. Native gold commonly occurs as intergrowths with tellurides. Textural evidence indicates a precipitation sequence, in a temporal order, of calcaverite, petzite, altaite, tellurobismuthite, and hessite. Little amount of sulfide phases has been found in association with the tellurides, indicating that tellurides were deposited under low S fugacity (fS 2 ) and/or high Te fugacity (fTe 2 ) conditions. The textural relationships, when combined with fluid inclusion microthermometric data of auriferous quartz veins and tellurides thermodynamic data, permit estimation for logfTe 2 during telluride formation, which are −6.8 to −10.8 at 300°C and −9.6 to −17.6 at 250°C. Available geochronological and geochemical data suggest that Te was most likely derived from the late Mesozoic magmatic rocks widespread in the Xiaoqinling district and other parts of the southern North China craton, which were emplaced broadly contemporaneous with gold mineralization at Yangzhaiyu. This study highlights the role of Te and tellurides as important gold scavengers in As-deficient ore fluids.  相似文献   

8.
The Duolanasayi gold deposit, 60 km NW of Habahe County, Xinjiang Uygur Autonomous Region, is a mid-large-scale gold deposit controlled by brittle-ductile shearing, and superimposed by albitite veins and late-stage magma hydrothermal solutions. There are four types of pyrite, which are contained in the light metamorphosed rocks (limestone, siltstone), altered-mineralized rocks (chlorite-schist, altered albite-granite, mineralized phyllite), quartz veins and carbonatite veinlets. The pyrite is the most common ore mineral. The Au-barren pyrite is present mainly in a simple form and gold-bearing pyrite is present mainly in a composite form. From the top downwards, the pyrite varies in crystal form from {100} and {210} {100} to {210} {100} {111} to {100} {111}. Geochemical studies indicate that the molecular contents of pyrite range from Fe1.057S2 to Fe0.941S2. Gold positively correlates with Mn, Sr, Zn, Te, Pb, Ba and Ag. There are four groups of trace elements: Fe-Cu-Sr-Ag, Au-Te-Co, As-Pb-Zn and Mn-V-Ti-Ba-Ni-Cr in pyrite. The REE characteristics show that the total amount of REE (ΣREE) ranges from 32.35×10 -6 to 132.18×10 -6; LREE/HREE, 4.466-9.142; (La/Yb)N, 3.719-11.133; (Eu/Sm)N, 0.553-1.656; (Sm/Nd)N, 0.602-0.717; La/Yb, 6.26-18.75; δEu, 0.628-2.309; δCe, 0.308-0.816. Sulfur isotopic compositions (δ 34S=-2.46‰--7.02‰) suggest that the sulfur associated with gold mineralization was derived from the upper mantle or lower crust.  相似文献   

9.
Significant gold deposits in the western Tanami region of Western Australia include deposits in the Bald Hill and Coyote areas. The ca. 1,864 Ma Bald Hill sequence of turbiditic and mafic volcanic rocks hosts the Kookaburra and Sandpiper deposits and a number of smaller prospects. The ca. 1,835 Ma turbiditic Killi Killi Formation hosts the Coyote deposit and several nearby prospects. The Kookaburra deposit forms as a saddle reef within a syncline, and the Sandpiper deposit is localized within graphitic metasedimentary rocks along a limb of an anticline. Gold in these deposits is hosted by anastomosing quartz–(–pyrite–arsenopyrite) veins within quartz–sericite schist with disseminated arsenopyrite, pyrite, and marcasite (after pyrrhotite). Based on relative timing relationships with structural elements, the auriferous veins are interpreted to have been emplaced before or during the ca. 1,835–1,825 Ma Tanami Orogeny (regional D1). Gold deposition is thought to have been caused by pressure drops associated with saddle reef formation (Kookaburra) and chemical reactions with graphitic rocks (Sandpiper). The Coyote deposit, the largest in the western Tanami region, consists of a number of ore lenses localized along the limbs of the Coyote Anticline, which formed during the Tanami Orogeny. The largest lenses are associated with the Gonzalez Fault, which is located along the steeply dipping southern limb of this fold. Gold was introduced at ca. 1,790 Ma into dilatant zones that formed in local perturbations along this fault during later reactivation (regional D5) towards the end of a period of granite emplacement. Gold is associated with quartz–chlorite–pyrite–(arsenopyrite–galena–sphalerite) veins with narrow (<?5 mm) chloritic selvages. A quartz–muscovite–biotite–K–feldspar–(tourmaline–actinolite–arsenopyrite) assemblage, which is interpreted to relate to granite emplacement, overprints the regional greenschist facies metamorphic assemblage. The mineralogical similarity between this overprinting assemblage and the vein assemblage suggests that the auriferous veins at the Coyote deposit are associated with the granite-related metamorphic–metasomatic assemblage. Gold deposition is thought to have been caused by pressure drops within dilatant zones.  相似文献   

10.
The Picacho gold deposit, located in southeasternmost California, is a low-grade gold deposit in a nearly flat-lying denudational fault of regional extent and probable Oligocene age. The deposit is hosted by intensely fractured and faulted Mesozoic leucogranite and by chloritic augen gneiss and schist, and is overlain unconformably and in fault contact by unmineralized late Oligocene Quechan volcanic rocks. The deposit is structurally characterized by normal and normal-oblique faults of low to high dip at shallow depths in the mine, merging downward with a synchronous, low-dipping ore-stage extensional fault system (the Chocolate Mountains/Gatuna Fault) of probable Oligocene age in deeper portions of the deposit. The fault system was infiltrated during much of its active life by hot, dilute, highly exchanged meteoric water having temperatures of 170°–210° C, salinity <2 wt% NaCl equivalent and calculated δ18Ofluid between −2.6‰ and 5.2‰. This main-stage fluid precipitated quartz, pyrite, and specular hematite, accompanied by silicification and sericitization. Auriferous ore-stage pyrite was precipitated late in the fault evolution probably by mixing of reducing ore fluid with relatively oxidized main-stage fluid during regional Oligocene extension on the Chocolate Mountains/Gatuna Fault. The Picacho deposit is characterized by a gold–arsenic–antimony geochemical signature consistent with bisulfide complexing of gold in reducing fluid, in contrast with typical denudation fault-hosted base-metal-rich deposits associated with high-salinity fluids elsewhere in the southwestern United States. The deposit is overprinted by Miocene normal faults having a wide range of dips. These postore faults are associated with red earthy hematite precipitation, pyrite oxidation, and supergene enrichment of gold. Editorial handling: L. Meinert  相似文献   

11.
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   

12.
The Jurassic (approximately 145 Ma) Nambija oxidized gold skarns are hosted by the Triassic volcanosedimentary Piuntza unit in the sub-Andean zone of southeastern Ecuador. The skarns consist dominantly of granditic garnet (Ad20–98) with subordinate pyroxene (Di46–92Hd17–42Jo0–19) and epidote and are spatially associated with porphyritic quartz-diorite to granodiorite intrusions. Endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone. Exoskarn has an outer K- and Na-enriched zone in the volcanosedimentary unit. Gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide-poor vugs and milky quartz veins and veinlets in association with hematite. Fluid inclusion data for the main part of the prograde stage indicate the coexistence of high-temperature (500°C to >600°C), high-salinity (up to 65 wt.% eq. NaCl), and moderate- to low-salinity aqueous-carbonic fluids interpreted to have been trapped at pressures around 100–120 MPa, corresponding to about 4-km depth. Lower-temperature (510–300°C) and moderate- to low-salinity (23–2 wt.% eq. NaCl) aqueous fluids are recorded in garnet and epidote of the end of the prograde stage. The microthermometric data (Th from 513°C to 318°C and salinity from 1.0 to 23 wt.% eq. NaCl) and δ18O values between 6.2‰ and 11.5‰ for gold-bearing milky quartz from the retrograde stage suggest that the ore-forming fluid was dominantly magmatic. Pressures during the early retrograde stage were in the range of 50–100 MPa, in line with the evidence for CO2 effervescence and probable local boiling. The dominance of magmatic low-saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system, which could have delayed the ingression of external fluids until relatively low temperatures were reached. The resulting low water-to-rock ratios explain the weak retrograde alteration and the compositional variability of chlorite, essentially controlled by host rock compositions. Gold was precipitated at this stage as a result of cooling and pH increase related to CO2 effervescence, which both result in destabilization of gold-bearing chloride complexes. Significant ingression of external fluids took place after gold deposition only, as recorded by δ18O values of 0.4‰ to 6.2‰ for fluids depositing quartz (below 350°C) in sulfide-rich barren veins. Low-temperature (<300°C) meteoric fluids (δ18Owater between −10.0‰ and −2.0‰) are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities (about 100°C and 25 wt.% eq. NaCl). The latter are similar to low-temperature fluids (202–74.5°C) with δ18O values of −0.5‰ to 3.1‰ and salinities in the range of 21.1 to 17.3 wt.% eq. CaCl2, trapped in calcite of late veins and interpreted as basinal brines. Nambija represents a deep equivalent of the oxidized gold skarn class, the presence of CO2 in the fluids being partly a consequence of the relatively deep setting at about 4-km depth. As in other Au-bearing skarn deposits, not only the prograde stage but also the gold-precipitating retrograde stage is dominated by fluids of magmatic origin.  相似文献   

13.
Both stratiform/stratabound and granite-related models have been used to explain the genesis of W(Mo) deposits in the Okiep copper district in western Namaqualand, South Africa. Apparently, stratabound mineralization (Fe-rich wolframite with accessory molybdenite) occurs in foliation-parallel quartz veins in high-grade (∼750 °C, 5–6 kbar) metapelites of the Wolfram Formation, and less commonly in small bodies of silicified leucogranites and pegmatites. Six Re–Os ages for molybdenites from four deposits (Nababeep Tungsten Far West, Kliphoog, Narrap, Tweedam) range between 1000 ± 4 and 1026 ± 5 Ma. These molybdenites define a well-constrained 187Re–187Os isochron with an age of 1019 ± 6 Ma, which is interpreted as the age of W(Mo) mineralization. This age is significantly younger than Proterozoic protolith ages for supracrustal rocks and the emplacement ages for the main intrusive suites, but geologic evidence requires overlap with a period of high-grade metamorphism. We suggest that W(Mo) mineralization is genetically linked to intra-crustal magmatic processes at ∼1020 Ma, thereby precluding the ∼1060 Ma Concordia granite as the source for mineralizing fluids. A narrow range of positive δ34S compositions (+3.6 to +4.5‰) for eight molybdenites from five W(Mo) mines is consistent with a SO2-rich fluid and a granite-related genetic model. Post-peak metamorphic deformation and metamorphism of W(Mo) ores is most likely related to the retrograde stage of the Namaquan orogeny, which overlaps emplacement of late-orogenic, evolved granites and pegmatites, and the formation of W(Mo) deposits in western Namaqualand. Therefore, the effects of retrograde Namaquan metamorphism extend at least to ∼1020 Ma or, alternatively, these W(Mo) veins were affected by a poorly constrained later event (e.g. early Pan-African). Received: 12 September 1999 / Accepted: 20 April 2000  相似文献   

14.
Summary The Kassiteres-Sappes district represents a multi-centered, porphyry-epithermal system developed during the Oligocene to Miocene at a composite calc-alkaline to high-K calc-alkaline volcanic edifice. Precious and base metal mineralization postdates the emplacement of dacite and rhyolite porphyries and is partly superimposed on earlier microdiorite-related porphyry-style mineralization exposed at the Koryfes Hill prospect. A second mineralized porphyry-type system genetically related to a dacite porphyry body developed near the St Demetrios deposit. Tellurides occur mainly at the St Barbara prospect and the St Demetrios deposit. Based on petrographic, electron microprobe, and scanning electron microscope analyses, hessite, petzite, sylvanite, altaite, stützite and native tellurium occur in the St Barbara prospect. These tellurium-bearing minerals are hosted in intermediate-sulfidation type veins and accompanied by pyrite, chalcopyrite, tetrahedrite-group minerals, galena and native gold/electrum. The St Demetrios mineralization includes hessite, altaite, stützite, and tetradymite in close spatial relation to a high-sulfidation assemblage composed of enargite, chalcopyrite, goldfieldite, and native gold. Tellurides were deposited at logfTe2 values of −8.5 to −7.1 and logfS2 values of −10.7 to −7.9 (275 °C). The ore systems are characterized by Au, Ag, Te, Bi, and Mo, which suggests a magmatic contribution to the mineralizing fluids. Ore-forming components were likely derived from both the dacite and rhyolite porphyries.  相似文献   

15.
Young orogenic gold mineralisation in active collisional mountains, Taiwan   总被引：1，自引：0，他引：1  
Gold-bearing vein systems in the high mountains of Taiwan are part of the youngest tectonic-hydrothermal system on Earth. Tectonic collision initiated in the Pliocene has stacked Eocene–Miocene marine sedimentary rocks to form steep mountains nearly 4 km high. Thinner portions of the sedimentary pile (∼5 km) are currently producing hydrocarbons in a fold and thrust belt, and orogenic gold occurs in quartz veins in thicker parts of the pile (∼10 km) in the Slate Belt that underlies the mountains. Metamorphic fluids (2–5 wt.% NaCl equivalent) are rising from the active greenschist facies metamorphic zone and transporting gold released during rock recrystallisation. Metamorphic fluid flow at the Pingfengshan historic gold mine was focussed in well-defined (4 km3) fracture zones with networks of quartz veins, whereas large surrounding volumes of rock are largely unveined. Gold and arsenopyrite occur in several superimposed vein generations, with ankeritic alteration of host rocks superimposed on chlorite–calcite alteration zones as fluids cooled and became out of equilibrium with the host rocks. Mineralising fluids had δ18O near +10‰, δ13C was between −1‰ and −6‰ and these fluids were in isotopic equilibrium with host rocks at ∼350°C. Ankeritic veins were emplaced in extensional sites in kink fold axial surfaces, formed as the rock mass was transported laterally from compressional to extensional regimes in the orogen. Rapid exhumation (>2 mm/year) of the Slate Belt is causing a widespread shallow conductive thermal anomaly without igneous intrusions. Meteoric water is penetrating into the conductive thermal anomaly to contribute to crustal fluid flow and generate shallow boiling fluids (∼250°C) with fluid temperature greater than rock temperature. The meteoric-hydrothermal system impinges on, but causes only minor dilution of, the gold mineralisation system at depth.  相似文献   

16.
Vein-type tin mineralization in the Dadoushan deposit, Laochang ore field, Gejiu district, SW China, is predominantly hosted in Triassic carbonate rocks (Gejiu Formation) over cupolas of the unexposed Laochang equigranular granite intrusion. The most common vein mineral is tourmaline, accompanied by skarn minerals (garnet, diopside, epidote, phlogopite) and beryl. The main ore mineral is cassiterite, accompanied by minor chalcopyrite, pyrrhotite, and pyrite, as well as scheelite. The tin ore grade varies with depth, with the highest grades (~1.2 % Sn) prevalent in the lower part of the vein zone. Muscovite 40Ar–39Ar dating yielded a plateau age of 82.7 ± 0.7 Ma which defines the age of the vein-type mineralization. Measured sulfur isotope compositions (δ 34S = −4.1 to 3.9 ‰) of the sulfides (arsenopyrite, chalcopyrite, pyrite, and pyrrhotite) indicate that the sulfur in veins is mainly derived from a magmatic source. The sulfur isotope values of the ores are consistent with those from the underlying granite (Laochang equigranular granite, −3.7 to 0.1 ‰) but are different from the carbonate wall rocks of the Gejiu Formation (7.1 to 11.1 ‰). The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (δ 18OH2O = −2.4 to 5.5 ‰, δD = −86 to −77 ‰) suggest an initially magmatic fluid which gradually evolved towards meteoric water during tin mineralization.  相似文献   

17.
In the Mazowe area some 40 km NW of Harare in Zimbabwe, gold mineralization is hosted in a variety of lithologies of the Archean Harare-Bindura-Shamva greenstone belt, in structures related to the late Archean regional D2/3 event. Conspicuous mineralzogical differences exist between the mines; the mainly granodiorite-hosted workings at Mazowe mine are on pyrite-rich reefs, mines of the Bernheim group have metabasalt host rocks and are characterized by arsenopyrite-rich ores, and Stori's Golden Shaft and Alice mine, both in metabasalts, work sulfide-poor quartz veins. In contrast to the mineralogical diversity, near-identical fluid inventories were found at the different mines. Both H2O-CO2-CH4 fluids of low salinity, and highly saline fluids are present and are regarded to indicate fluid mixing during the formation of the deposits. Notably, these fluid compositions in the Mazowe gold field markedly contrast to ore fluids “typical” of Archean mesothermal gold deposits on other cratons. Stable isotope compositions of quartz from the various deposits (δ18O=10.8 to 13.2‰ SMOW), calcite (δ18O=9.5 to 11.9‰ SMOW and δ13C=−3.2 to −8.0‰ PDB), inclusion water (δD=−28 to −40‰ SMOW) and sulfides (δ34S=1.3 to 3.2‰ CDT) are uniform within the range typical for Archean lode gold deposits worldwide. The fluid and stable isotope compositions support the statement that the mineralization in the Mazowe gold field formed from relatively reduced fluids with a “metamorphic” signature during a single event of gold mineralization. Microthermometric data further indicate that the deposits formed in the PT range of 1.65–2.3 kbar and 250–380 °C. Ages obtained by using the Sm/Nd and Rb/Sr isotope systems on scheelites are 2604 ± 84 Ma for the mineralization at Stori's Golden Shaft mine, and 2.40 ± 0.20 Ga for Mazowe mine. The Archean age at Stori's is regarded as close to the true age of gold mineralization in the area, whereas the Proterozoic age at Mazowe mine probably reflects later resetting. Received: 30 September 1998 / Accepted: 17 August 1999  相似文献   

18.
The Tanami region of northern Australia has emerged over the last two decades as the largest gold-producing region in the Northern Territory. Gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks, and by sulfide-rich replacement zones within iron formation. Although limited, geochronological data suggest that most mineralization occurred at about 1,805–1,790 Ma, during a period of extensive granite intrusion, although structural relationships suggest that some deposits predate this period. There are three main goldfields in the Tanami region: the Dead Bullock Soak goldfield, which hosts the world-class Callie deposit; The Granites goldfield; and the Tanami goldfield. In the Dead Bullock Soak goldfield, deposits are hosted by carbonaceous siltstone and iron formation where a late (D5) structural corridor intersects an early F1 anticlinorium. In The Granites goldfield, deposits are hosted by highly sheared iron formation and are interpreted to predate D5. The Tanami goldfield consists of a large number of small, mostly basalt-hosted deposits that probably formed at a high structural level during D5. The D5 structures that host most deposits formed in a convergent structural regime with σ 1 oriented between E–W and ENE–WSW. Structures active during D5 include NE-trending oblique thrust (dextral) faults and ESE-trending (sinistral) faults that curve into N- to NNW-trending reverse faults localized in supracrustal belts between and around granite complexes. Granite intrusions also locally perturbed the stress field, possibly localizing structures and deposits. Forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid-crust. In areas characterized by the N- to NW-trending faults, orebodies also tend to be N- to NW-trending, localized in dilational jogs or in fractured, competent rock units. In areas characterized by ESE-trending faults, the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during E–W- to ENE–WSW-directed transpression. Many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation. Ore deposition occurred at depths ranging from 1.5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430°C, similar to lode–gold fluids elsewhere in the world. These fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow, although it is also possible that the fluids were derived from coeval granites. Lead isotope data suggest that lead in the ore fluids had multiple sources. Hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids. Gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks. Fluid unmixing is caused by three different processes: (1) CO2 unmixing caused by interaction of ore fluids with carbonaceous siltstone; (2) depressurization caused by pressure cycling in shear zones; and (3) boiling as ore fluids move to shallow levels. Deposits in the Tanami region may illustrate the continuum model of lode–gold deposition suggested by Groves (Mineralium Deposita 28:366–374, 1993) for Archean districts.  相似文献   

19.
Ore deposits of the Charters Towers Goldfield (CTGF) are mainly hosted by fault-fill veins. Extensional (∼8% of all veins) and stockwork-like (∼3%) veins are less common and of little economic significance. Crosscutting relationships and published structural and geochronological data indicate a Late Silurian to Early Devonian timing of gold mineralization, coincident with regional shortening (D4) and I-type magmatism. Paragenetic relationships, which are uniform in veins everywhere within the CTGF, suggest that vein formation commenced with the deposition of large volumes of buck quartz (stage I), followed by buck and comb quartz, and significant pyrite and arsenopyrite precipitation (stage II). Gold was introduced during stage III, after earlier sphalerite and coincident with galena and chalcopyrite. Narrow, discontinuous calcite veins of stage IV mark the waning of gold-related hydrothermal activity or a later unrelated episode. Ore zones within the veins are everywhere composed of comb and/or gray quartz, calcite and/or ankerite and bands or clusters of fractured pyrite that are spatially associated with galena, sphalerite or chalcopyrite. Low-grade or barren vein sections, on the other hand, are mainly composed of milky buck quartz with little evidence for modification, overprinting or interaction with later fluids. Gold-related hydrothermal wall-rock alteration is symmetrically zoned, displaying proximal sericite–ankerite and distal epidote–chlorite–hematite assemblages that may be taken to imply wall-rock interaction with near neutral fluids (pH 5–6). Isocon plots assuming immobile Al, P, Ti, Y and Zr consistently indicate As, K, Pb, S and Zn enrichment and Na, Si and Sr depletion in altered wall-rock specimens relative to the least altered rocks. Alteration assemblages, quartz textures, fault rocks and published fluid inclusion and stable isotope data imply that the veins were formed under conditions of episodic fluid overpressuring (∼0.9–3.8 kbar), at a depth of ∼7 km and a temperature of ∼310°C. The published fluid inclusion data also imply that gold precipitation may have been brought about by fluid mixing. However, physi- and chemisorption of gold complexes onto sulfide surfaces may have been important depositional processes and controls on gold enrichment at the millimeter to centimeter scale, given that most gold particles are attached to the surfaces of pyrite crystals of stage II or to etch-pits and fracture surfaces within the earlier pyrite.  相似文献   

20.
A set of sheeted quartz veins cutting 380 Ma monzogranite at Sandwich Point, Nova Scotia, Canada, provide an opportunity to address issues regarding fluid reservoirs and genesis of intrusion-related gold deposits. The quartz veins, locally with arsenopyrite (≤5%) and elevated Au–(Bi–Sb–Cu–Zn), occur within the reduced South Mountain Batholith, which also has other zones of anomalous gold enrichment. The host granite intruded (P = 3.5 kbars) Lower Paleozoic metaturbiditic rocks of the Meguma Supergroup, well known for orogenic vein gold mineralization. Relevant field observations include the following: (1) the granite contains pegmatite segregations and is cut by aplitic dykes and zones (≤1–2 m) of spaced fracture cleavage; (2) sheeted veins containing coarse, comb-textured quartz extend into a pegmatite zone; (3) arsenopyrite-bearing greisens dominated by F-rich muscovite occur adjacent the quartz veins; and (4) vein and greisen formation is consistent with Riedel shear geometry. Although these features suggest a magmatic origin for the vein-forming fluids, geochemical studies indicate a more complex origin. Vein quartz contains two types of aqueous fluid inclusion assemblages (FIA). Type 1 is a low-salinity (≤3 wt.% equivalent NaCl) with minor CO2 (≤2 mol%) and has T h = 280–340°C. In contrast, type 2 is a high-salinity (20–25 wt.% equivalent NaCl), Ca-rich fluid with T h = 160–200°C. Pressure-corrected fluid inclusion data reflect expulsion of a magmatic fluid near the granite solidus (650°C) that cooled and mixed with a lower temperature (400°C), wall rock equilibrated, Ca-rich fluid. Evidence for fluid unmixing, an important process in some intrusion-related gold deposit settings, is lacking. Stable isotopic (O, D, S) analyses for quartz, muscovite and arsenopyrite samples from vein and greisens indicate the following: (1) δ18Oqtz = +11.7‰ to 17.8‰ and δ18Omusc = +10.7‰ to +11.2‰; (2) δDmusc = −44‰ to−54‰; and (3) δ34Saspy = +7.8‰ to +10.3‰. These data are interpreted, in conjunction with fluid inclusion data, to reflect contamination of a magmatic-derived fluid (d18OH2O {\delta^{{{18}}}}{{\hbox{O}}_{{{{\rm{H}}_{{2}}}{\rm{O}}}}}  ≤ +10‰) by an external fluid (d18OH2O {\delta^{{{18}}}}{{\hbox{O}}_{{{{\rm{H}}_{{2}}}{\rm{O}}}}}  ≥ +15‰), the latter having equilibrated with the surrounding metasedimentary rocks. The δ34S data are inconsistent with a direct igneous source based on other studies for the host intrusion (d18OH2O {\delta^{{{18}}}}{{\hbox{O}}_{{{{\rm{H}}_{{2}}}{\rm{O}}}}}  = +5‰) and are, instead, consistent with an external reservoir for sulphur based on δ34SH2S data for the surrounding metasedimentary rocks. Divergent fluid reservoirs are also supported by analyses of Pb isotopes for pegmatitic K-feldspar and vein arsenopyrite. Collectively the data indicate that the vein- and greisen-forming fluids had a complex origin and reflect both magmatic and non-magmatic reservoirs. Thus, although the geological setting suggests a magmatic origin, the geochemical data indicate involvement of multiple reservoirs. These results suggest multiple reservoirs for this intrusion-related gold deposit setting and caution against interpreting the genesis of intrusion-related gold deposit mineralization in somewhat analogous settings based on a limited geochemical data set.  相似文献   

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