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1.
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
2.
Gold mineralization at Jonnagiri, Dharwar Craton, southern India, is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units, typical of Archean greenstone–granite ensembles. The proximal alteration assemblage comprises of muscovite, plagioclase, and chlorite with minor biotite (and carbonate), which is distinctive of low- to mid-greenschist facies. The laminated quartz veins that constitute the inner alteration zone, contain muscovite, chlorite, albite and calcite. Using various calibrations, chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 °C and 268 to 324 °C, respectively. Gold occurs in the laminated quartz veins both as free-milling native metal and enclosed within sulfides. Fluid inclusion microthermometry and Raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous–gaseous (H2O–CO2–CH4 + salt) fluid that underwent phase separation and gave rise to gaseous (CO2–CH4), low saline (~ 5 wt.% NaCl equiv.) aqueous fluids. Quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and P–T regime. Although the estimated P–T range (1.39 to 2.57 kbar at 263 to 323 °C) compare well with the published P–T values of other orogenic gold deposits in general, considerable pressure fluctuation characterize gold mineralization at Jonnagiri. Factors such as fluid phase separation and fluid–rock interaction, along with a decrease in f(O2), were collectively responsible for gold precipitation, from an initial low-saline metamorphogenic fluid. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits in Australia and Canada confirms that fluids of low saline aqueous–carbonic composition with metamorphic parentage played the most dominant role in the formation of the Archean lode gold systems. 相似文献
3.
The Larafella Au-prospect (Burkina Faso) lies within dacitic rocks of the Palaeoproterozoic Birimian greenstone belts. Gold mineralization is intimately associated with zones of cataclastic deformation. Whilst the lode-vein mineralization is closely associated with CO2-rich fluid inclusions, the barren quartz veins are characterized by H2O ± salt-bearing inclusions. Geochemical studies on the immediate wall-rock of the quartz veins have shown an increase of As in zones of gold enrichment, while alteration overprints such as carbonatization and chloritization cannot be correlated unequivocally with Au-mineralization. Consequently, fluid inclusion studies of quartz veins and As-anomalies constitute important exploration tools for mesothermal gold mineralization, since Au-rich zones can be distinguished from Au-depleted zones. 相似文献
4.
The gold mineralization of the Hutti Mine is hosted by nine parallel, N–S trending, steeply dipping, 2–10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D2 shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle–ductile D3 shearing and intense quartz veining. The development of a S2–S3 crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D2 shearing is associated with a pervasively developed distal chlorite–sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite–plagioclase alteration in the center of the shear zones. D3 is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S3. The average size of the laminated vein systems is 30–50 m along strike as well as down-dip and 2–6 m in width.Mass balance calculations suggest strong metasomatic changes for the proximal biotite–plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite–sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ18O values of the whole rock from around 7.5‰ for the host rocks to 6–7‰ for the distal chlorite–sericite and the proximal biotite–plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow.The ductile D2 deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold–sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D3 deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization. 相似文献
5.
Mineralized veins at Major's Creek consist of preponderant quartz and carbonate gangue with gold, Au-Ag tellurides and base metal sulphides within silicified and sericitized dykes or granodiorite of the Braidwood Granite. Fluid inclusion studies indicate deposition throughout the range 350–80°C by low salinity fluids. Significant Au-Ag telluride mineralization took place at a temperature of about 155°C. Mineral deposition was due to the separation of a liquid CO2 phase from an originally CO2-rich aqueous fluid. Observed argillic alteration is a consequence of acid leaching above the boiling zone. Mineralization is epithermal in character and probably formed during the existence of a hydrothermal convective system. A relationship with similar epithermal gold deposits in the adjacent Eden-Yalwal Rift zone is inferred. 相似文献
6.
U Khin Zaw 《Mineralium Deposita》1984,19(2):138-144
The Pennaichaung and Yetkanzintaung W-Sn Prospects are located in Tavoy Township, Tennasserim Division, southern Burma. The W-Sn mineralization at the Pennaichaung is closely related with a small, satellitic granitoid pluton of presumably Late Mesozoic age, which intruded the metaclastic rocks of Mergui Group (mostly Carboniferous). The mineralized quartz veins at the Pennaichaung penetrated the granitoid-metasedimentary rocks contact. In contrary to the Pennaichaung deposit, the W-Sn veins at the Yetkanzintaung are exclusively in the metasedimentary rocks of slates and quartzites of Margui Group. Mineralized quartz veins in the Pennaichaung area trend NNE-SSW, NW-SE and NE-SW with a maximum thickness of 30 cm, but only quartz veins trending NNE-SSW are found to be productive and contained economically workable wolframite and cassiterite. Majority of the mineralized quartz veins in the Yetkanzintaung area trend approximately N-S with easterly dip of 50°–70°. The thickness of the ore veins in the Yetkanzintaung area are thinner than those of the Pennaichaung and range from 1 cm to 20 cm with an average width of 5 cm. Fluid inclusion studies of the quartz from the ore veins cutting the granitoid in the Pennaichaung area have yielded a filling temperature range of 170°–270°C with a maximum mode of 220°C, while quartz crystals from the ore veins in the nearby metasedimentary rocks gave a filling temperature range of 140°–220°C with a maximum mode of 160°C. Hence, the Pennaichaung deposit was thought to have emplaced under a filling temperature range of 140°–270°C. A similar low filling temperature range was recorded for the Yetkanzintaung deposit. Quartz from the Yetkanzintaung ore veins have yielded filling temperatures of 200°–240°C, whereas the fluorites associated with the mineralized quartz veins gave a temperature range of 140°–160°C. Limited freezing runs indicate a salinity of less than 5 NaCl equivalent weight percent for inclusions in quartz from both orebodies. No fluid inclusion evidence of boiling of ore fluids nor presence of liquid CO2 was observed in this study. Thus, the ore fluids responsible for the W-Sn mineralization at the Pennaichaung and Yetkanzintaung areas were of low temperature, diluted, CO2-deficient, NaCl brines. 相似文献
7.
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
8.
F. Masoudi B. Mehrabi M. Rezai Aghdam B. W. D. Yardley 《Journal of the Geological Society of India》2009,73(3):407-418
In the Sanandaj-Sirjan zone of metamorphic belt of Iran, the area south of Hamadan city comprises of metamorphic rocks, granitic
batholith with pegmatites and quartz veins. Alvand batholith is emplaced into metasediments of early Mesozoic age. Fluid inclusions
have been studied using microthermometry to evaluate the source of fluids from which quartz veins and pegmatites formed to
investigate the possible relation between host rocks of pegmatites and the fluid inclusion types. Host minerals of fluid inclusions
in pegmatites are quartz, andalusite and tourmaline. Fluid inclusions can be classified into four types. Type 1 inclusions
are high salinity aqueous fluids (NaCleq >12 wt%). Type 2 inclusions are low to moderate salinity (NaCleq <12 wt%) aqueous fluids. Type 3 and 4 inclusions are carbonic and mixed CO2-H2O fluid inclusions. The distribution of fluid inclusions indicate that type 1 and type 2 inclusions are present in the pegmatites
and quartz veins respectively in the Alvand batholith. This would imply that aqueous magmatic fluids with no detectable CO2 were present during the crystallization of these pegmatites and quartz veins. Types 3 and 4 inclusions are common in quartz
veins and pegmatites in metamorphic rocks and are more abundant in the hornfelses. The distribution of the different types
of fluid inclusions suggests that CO2 fluids generated during metamorphism and metamorphic fluids might also contribute to the formation of quartz veins and pegmatites
in metamorphic terrains. 相似文献
9.
The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO2-rich, H2O-rich and mixed H2O–CO2 fluid inclusions in the gold-bearing quartz veins. The H2O–CO2 inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ34S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO2. 相似文献
10.
The Xihuashan tungsten deposit is closely related to a small highly evolved granitic intrusion. The fluid phases associated with the wolframite-bearing quartz veins have been investigated using microthermometry and the Raman microprobe; they are highly variable in density and composition. The earlier fluids are low-density and low-salinity CO2-bearing aqueous solutions circulating at temperatures up to 420 °C, and low-salinity (2–3 equiv. wt% NaCl) aqueous solutions without traces of CO2 circulating at high temperatures 280°–400 °C) involved in a specific hydrothermal fracturing event; limited unmixing occurs at 380 °C and 200–100 bar in response to a sudden pressure drop. The second types of fluids related to deposition of idiomorphic drusy quartz are typical CO2-bearing aqueous solutions with low salinity (2.5 equiv. wt% NaCl) homogenizing at low to moderate temperatures (180°–340 °C). The late fluids characterize the sulfide deposition stage; they are aqueous fluids with variable salinities homogenizing in the liquid phase between 100° and 275 °C. The Xihuashan hydrothermal evolution resulted from a discontinuous sequence of specific events occurring between 420° and 150 °C and during a continuous hydrothermal evolution of the system during cooling. The role played by the CO2-rich fluids in the transport and deposition of tungsten in the hydrothermal environment is discussed. 相似文献
11.
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
12.
Fluid immiscibility in late-Alpine gold-bearing veins,Eastern and Northwestern European Alps 总被引：4，自引：0，他引：4
Gold-bearing quartz veins fill late-Alpine brittle structures in Pennine nappes of Austria (in the Tauern window) and in northern Italy. The veins formed in the latter stages of uplift of the Alps. Fluid inclusions in veins sampled from Böckstein, Austria, and Valle Anzasca, Italy have a wide variety of compositions, ranging from aqueous brine (about 5 wt% NaCl equiv.) to about 50 mol% CO2. At room temperature, the inclusions range with increasing CO2 content from two-phase aqueous, through three-phase in which the CO2 homogenizes to vapour, to three-phase with CO2 homogenizing to liquid. This wide range of inclusion compositions is interpreted as evidence for fluid immiscibility, with most inclusions being accidental mixtures of the two end-member immiscible fluids. The homogenization temperatures of the aqueous inclusions, 200–280°C, gives the best estimate of temperature of formation of the veins. Vein formation fluid pressure at Böckstein and Valle Anzasca was about 1 kbar, and Böckstein veins formed at lower pressure than Valle Anzasca veins. Fluid immiscibility may have contributed to deposition of gold at both Valle Anzasca and Böckstein, and possibly many other uplift-related Alpine gold localities. 相似文献
13.
Many of the zinc-lead deposits of NE Washington State are poorly known examples of Mississippi Valley Type (MVT) mineralization. This study compares inclusion fluids from the Josephine Breccia ores with the later cross-cutting sulfide-bearing quartz veins. The breccia ores are cemented mainly by open space fillings of dolomite, sphalerite, quartz, galena, jasperoid and calcite. Replacement is of minor importance. Ore and gangue deposition occurred over the range 150–250 °C with most of the temperatures less than 200 °C. The aqueous brines typically contain 17–23 equivalent weight percent NaCl with often substantial amounts of Ca and/or Mg chlorides. Homogenization temperatures do not delineate any cooling or paragenetic sequence. The cross-cutting vein quartz contains CO2-rich inclusions with overall densities usually less than 0.7 g/cc and homogenization temperatures from 250–325 °C. Sulfur isotope analyses yield two populations with the quartz vein ores being lighter (<13 permil CDT) than the average for the conformable ores. The later veins are not remobilized MVT sulfides but represent a separate, high-silver period of mineralization. 相似文献
14.
The Pemali tin deposit is located in a Triassic granite pluton the magmatic evolution of which is characterized by a decrease of compatible Ca, Mg, Ti, P and Zr in the sequence: medium- to coarse-grained biotite granite, megacrystic medium-grained biotite granite, two-mica granite/muscovite granite. The tin mineralization is confined to the two-mica granite and consists of disseminated cassiterite as well as greisen-bordered veins. The highly evolved muscovite granite is tin-barren and is distinguished from the two-mica granite by its low mica content and low loss-on-ignition values. The fluid inclusions in quartz and fluorite of the two-mica granite and of the greisen homogenize in the 115–410 °C temperature range; the salinities are in the range of 0.4–23 equiv wt% NaCl and the CO2 concentrations are < 2 mole%. 相似文献
15.
Fluid inclusion and stable isotope (O, H, C, and S) constraints on the genesis of the Serrinha gold deposit, Gurupi Belt, northern Brazil 总被引：1，自引：0，他引：1
Evandro L. Klein Chris Harris Christophe Renac André Giret Candido A. V. Moura Kazuo Fuzikawa 《Mineralium Deposita》2006,41(2):160-178
The Serrinha gold deposit of the Gurupi Belt, northern Brazil, belongs to the class of orogenic gold deposits. The deposit is hosted in highly strained graphitic schist belonging to a Paleoproterozoic (∼2,160 Ma) metavolcano-sedimentary sequence. The ore-zones are up to 11 m thick, parallel to the regional NW–SE schistosity, and characterized by quartz-carbonate-sulfide veinlets and minor disseminations. Textural and structural data indicate that mineralization was syn- to late-tectonic and postmetamorphic. Fluid inclusion studies identified early CO2 (CH4-N2) and CO2 (CH4-N2)-H2O-NaCl inclusions that show highly variable phase ratios, CO2 homogenization, and total homogenization temperatures both to liquid and vapor, interpreted as the product of fluid immiscibility under fluctuating pressure conditions, more or less associated with postentrapment modifications. The ore-bearing fluid typically has 18–33mol% of CO2, up to 4mol% of N2, and less than 2mol% of CH4 and displays moderate to high densities with salinity around 4.5wt% NaCl equiv. Mineralization occurred around 310 to 335°C and 1.3 to 3.0 kbar, based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions. Stable isotope data on quartz, carbonate, and fluid inclusions suggest that veins formed from fluids with δ18OH2O and δDH2O (310–335°C) values of +6.2 to +8.4‰ and −19 to −80‰, respectively, which might be metamorphic and/or magmatic and/or mantle-derived. The carbon isotope composition (δ13C) varies from −14.2 to −15.7‰ in carbonates; it is −17.6‰ in fluid inclusion CO2 and −23.6‰ in graphite from the host rock. The δ34S values of pyrite are −2.6 to −7.9‰. The strongly to moderately negative carbon isotope composition of the carbonates and inclusion fluid CO2 reflects variable contribution of organic carbon to an originally heavier fluid (magmatic, metamorphic, or mantle-derived) at the site of deposition and sulfur isotopes indicate some oxidation of the originally reduced fluid. The deposition of gold is interpreted to have occurred mainly in response to phase separation and fluid-rock interactions such as CO2 removal and desulfidation reactions that provoked variations in the fluid pH and redox conditions. 相似文献
16.
The Gemericum is a segment of the Variscan orogen subsequently deformed by the Alpine–Carpathian orogeny. The unit contains abundant siderite–sulphide and quartz–antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano-sedimentary sequences. The siderite–sulphide veins and siderite replacement deposits of the Gemericum represent one of the largest accumulations of siderite in the world, with about 160 million tonnes of mineable FeCO3. More than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern, reflecting the distribution of regional metamorphic zones. Siderite–sulphide veins are typically contained in low-grade (chlorite zone) sedimentary, volcano-sedimentary or volcanic Lower and Upper Paleozoic rocks. Quartz–antimony veins are hosted by higher-grade units (biotite zone). Siderite–sulphide veins are dominated by early siderite followed by a complex set of stages, including quartz–sulphide (chalcopyrite, tetrahedrite), barite, tourmaline–quartz, and sulphide-remobilization stages. The temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes, within-vein replacement and recrystallization. Siderite–sulphide veins show considerable vertical (up to 1200 m) and lateral (up to 15 km) extent, and a thickness typically reaching several metres. Carbonate-replacement siderite deposits of the Gemericum are hosted by a Silurian limestone belt and are similar to stratabound siderite deposits of the Eastern Alps (e.g., Erzberg, Austria).Based on a review of geological, petrological and geochronological data for the Gemericum, and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals, the siderite–sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense. The deposits were formed during several stages of regional crustal-scale fluid flow. Isotope (S, C, Sr, Pb) fingerprinting identifies the metamorphosed rock complexes of the Gemericum as a source of most components of hydrothermal fluids. Fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types, and the existence of contrasting P–T conditions during vein evolution. A high-δ18O, medium- to high-salinity, H2O-type fluid is the most important component during siderite deposition, whereas H2O–CO2-type fluid inclusion containing dense liquid CO2 and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline–quartz stage. Younger quartz–ankerite(±siderite)–sulphide stages are characterized by high-salinity (17 to 35 wt.% NaCl equivalent) and low-temperature (Th=90 to 180 °C) H2O-type fluids.The vein deposits are interpreted as a result of multistage hydrothermal circulation, with Variscan and Alpine mineralization phases. Based on available indirect data, the most important mineralization phase was related to regional fluid flow during the uplift of a Variscan metamorphic core complex, producing siderite–sulphide (±barite) mineralization, while tourmaline–quartz stage and sulphide remobilization stages are related to Alpine processes. Two phases of vein evolution are evident from two groups of 87Sr/86Sr isotope ratios of Sr-rich, Rb-poor hydrothermal minerals: 0.71042–0.71541 in older barite and 0.7190–0.7220 in late-stage celestine and strontianite. 相似文献
17.
The Wangfeng gold deposit is located in Western Tian Shan and the central section of the Central Asian Orogenic Belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Caledonian granites and is structurally controlled by the Shenglidaban ductile shear zone. The gold orebodies consist of gold-bearing quartz veins and altered mylonite. The mineralization can be divided into three stages: quartz–pyrite veins in the early stage, sulfide–quartz veins in the middle stage, and quartz–carbonate veins or veinlets in the late stage. Ore minerals and native gold mainly formed in the middle stage. Four types of fluid inclusions were identified based on petrography and laser Raman spectroscopy: CO2–H2O inclusions (C-type), pure CO2 inclusions (PC-type), NaCl–H2O inclusions (W-type), and daughter mineral-bearing inclusions (S-type). The early-stage quartz contains only primary CO2–H2O fluid inclusions with salinities of 1.62 to 8.03 wt.% NaCl equivalent, bulk densities of 0.73 to 0.89 g/cm3, and homogenization temperatures of 256 °C–390 °C. Vapor bubbles are composed of CO2. The middle-stage quartz contains all four types of fluid inclusions, of which the CO2–H2O and NaCl–H2O types yield homogenization temperatures of 210 °C–340 °C and 230 °C–300 °C, respectively. The CO2–H2O fluid inclusions have salinities of 0.83 to 9.59 wt.% NaCl equivalent and bulk densities of 0.77 to 0.95 g/cm3, with vapor bubbles composed of CO2, CH4, and N2. Fluid inclusions in the late-stage quartz are NaCl–H2O solution with low salinities (0.35–3.87 wt.% NaCl equivalent) and low homogenization temperatures (122 °C–214 °C). The coexistence of inclusions of these four types in middle-stage quartz suggests that fluid boiling occurred in the middle-stage mineralization. Trapping pressures estimated from CO2–H2O inclusions are 110–300 MPa and 90–250 MPa for the early and middle stages, respectively, suggesting that gold mineralization mainly occurred at depths of about 10 km. In general, the Wangfeng gold deposit originated from a metamorphic fluid system characterized by low salinity, low density, and enrichment of CO2. Depressurized fluid boiling caused gold precipitation. Given the regional geology, ore geology, fluid-inclusion features, and ore-forming age, the Wangfeng gold deposit can be classified as a hypozonal orogenic gold deposit. 相似文献
18.
Gold mineralisation throughout about 45 Ma of Archaean orogenesis: protracted flux of gold in the Golden Mile, Yilgarn craton, Western Australia 总被引：1，自引：0，他引：1
The Golden Mile deposit was discovered in 1893 and represents today the largest Archaean orogenic lode gold system in the world (50 M oz produced gold). The Golden Mile deposit comprises three major styles of gold mineralisation: Fimiston, Oroya and Charlotte styles. Fimiston-style lodes formed at 250 to 350 °C and 100 to 200 MPa and are controlled by brittle–ductile fault zones, their subsidiary fault zone and vein networks including breccias and open-cavity-infill textures and hydrothermally altered wall rock. Fimiston lodes were formed late D1, prior to D2 regional upright folding. Hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite, an increase in sericite and in Fe content of carbonates. Lodes contain siderite, pyrite, native gold, 17 different telluride minerals (Au–Ag tellurides contain ~25% of total gold), tourmaline, haematite, sericite and V-rich muscovite. Oroya-style lodes formed at similar P–T conditions as the Fimiston lodes and are controlled by brittle–ductile shear zones, associated dilational jogs that are particularly well developed at the contact between Paringa Basalt and black shale interflow sedimentary rocks and altered wall rock. The orebodies are characterised by micro-breccias and zones of intense shear zone foliation, very high gold grades (up to 100,000 g/t Au) and the common association of tellurides and vanadian mica (green leader). Oroya lodes crosscut Fimiston lodes and are interpreted to have formed slightly later than Fimiston lodes as part of one evolving hydrothermal system spanning D1 and D2 deformation (ca. 2,675–2,660 Ma). Charlotte-style lodes, exemplified by the Mt Charlotte deposit, are controlled by a sheeted vein (stockwork) complex of north-dipping quartz veins and hydrothermally altered wall rock. The Mt Charlotte orebody formed at 120 to 440 °C and 150 to 250 MPa during movement along closely spaced D4 (2,625 Ma) and reactivated D2 faults with the quartz granophyre in the Golden Mile Dolerite exerting a strong lithological control on gold mineralisation. Veins consist of quartz–carbonate–minor scheelite, and wall-rock alteration comprises chlorite destruction and growth of ferroan carbonate–sericite–pyrite–native gold. Pyrite–pyrrhotite is zoned on the scale of vein haloes and of the entire mine, giving a vertical temperature gradient of 50–100 °C over 1,000 vertical metres. The structural–hydrothermal model proposed consists of four major stages: (1) D1 thrusting and formation of Fimiston-style lodes, (2) D2 reverse faulting and formation of Oroya-style lodes, (3) D3 faulting and dissecting of Fimiston- and Oroya-style lodes, and (4) D4 faulting and formation of Mt Charlotte-style sheeted quartz vein system. The giant accumulation of gold in the Golden Mile deposit was formed due to protracted gold mineralisation throughout episodes of an Archaean orogeny that spanned about 45 Ma. Fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing, efficiency and duration. In addition to the long-lasting fluid plumbing system, the wide variety of transient structural and geochemical traps, multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world.Editorial handling: B. Lehmann 相似文献
19.
Liangliang Yu Deru Xu Weixin Chen Bo Zhang Lihui Tian Qiang Shan 《Resource Geology》2020,70(4):336-347
The late Triassic Baolun gold deposit hosted by Silurian phyllites is a large‐scale high‐grade gold deposit in Hainan Island, South China. The ores can be classified into quartz‐vein dominated type and less altered rock type. Three mineralization stages were recognized by mineral assemblages. The early stage, as the most important mineralization stage, is characterized by a quartz–native gold assemblage. The muscovite?quartz?pyrite?native gold assemblage is related to the intermedium mineralization stage. In late mineralization stage, native gold and Bi‐bearing minerals are paragenetic minerals. Microthermometry analyses show that the early mineralization stage is characterized by two types of fluid inclusions, including CO2‐rich inclusions (C‐type) and aqueous inclusions (W‐type). C‐type inclusions homogenize at 276–335°C with an averaged value of 306°C and have salinities of 1.0–10.0 wt% NaCl equivalent (mean value of 4.9 wt% NaCl equivalent). W‐type inclusions homogenize at 252–301°C (mean value of 278°C) with salinity of 4.0–9.7 wt% NaCl equivalent (mean value of 7.4 wt% NaCl equivalent). In intermedium mineralization stage, C‐type and W‐type inclusions homogenize at 228–320°C (mean value of 283°C) and 178–296°C (mean value of 241°C), with salinities of 2.4–9.9 wt% NaCl equivalent (mean value of 6.5 wt% NaCl equivalent) and 3.7–11.7 wt% NaCl equivalent (mean value of 7.7 wt% NaCl equivalent), respectively. No suitable mineral, such as quartz or calcite, was found for fluid inclusion study from late mineralization stage. In contrast, only aqueous inclusions were found from post‐ore barren veins, which yielded lower homogenization temperatures ranging from 168–241°C (mean value of 195°C) and similar salinities (2.6–12.6 wt% NaCl equivalent with averaged value of 7.2 wt% NaCl equivalent). The different homogenization temperatures and similar salinities of C‐type and W‐type from each mineralization stage indicate that fluid immiscibility and boiling occurred. The Baolun gold deposit was precipitated from a CO2‐bearing mesothermal fluid, and formed at a syn‐collision environment following the closure of the Paleo‐Tethys. 相似文献
20.
The Dalradian and Ordovician–Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base‐metal sulphide‐bearing vein deposits associated with kilometre‐scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H2O–CO2–salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base‐metal deposition and occurred at temperatures between 220 and 360°C and pressures of between 1.6 and 1.9 kbar. Caledonian mineralization in Ordovician–Silurian metamorphic rocks occurred at temperatures between 300 and 360°C and pressures between 0.6 and 1.9 kbar. A later, probably Carboniferous, stage of mineralization was associated with base‐metal sulphide deposition and involved a low to moderate temperature (Th 70 to 240°C), low to moderate salinity (0 to 20 wt% NaCl eq.), H2O–salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long‐term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献