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1.
The sheeted quartz–sulfide veins of the Radzimowice Au–As–Cu deposit in the Kaczawa Mountains are related to Upper Carboniferous post-collisional potassic magmatism of the composite Zelezniak porphyry intrusion. Multiple intrusive activity ranges from early calc-alkaline to sub-alkaline and alkaline rocks and is followed by multiple hydrothermal events. Early crustally derived dacitic magma has low mg# (<63) and very low concentrations of mantle-compatible trace elements, high large-ion lithophile elements (LILE), moderate light rare-earth elements (LREE), and low high-field-strength elements (HFSE). Later phases of more alkaline rocks have higher mg# (60–70), and LILE, LREE, and HFSE characteristics that indicate mafic magma contributions in a felsic magma chamber. The last episode of the magmatic evolution is represented by lamprophyre dikes which pre-date ore mineralization and are spatially related to quartz–sulfide–carbonate veins. The dikes consist of kersantite and spessartite of calc-alkaline affinity with K2O/Na2O ratios of 1.1–1.9, mg# of 77–79, and high abundances of mantle-compatible trace elements such as Cr, Ni, and V. They have high LILE, low LREE, and low HFSE contents suggesting a subduction-related post-collisional arc-setting. The mineralization started with arsenopyrite that was strongly brecciated and overprinted by multiple quartz–carbonate phases associated with base-metal sulfides and Au–Ag–Bi–Te–Pb±S minerals. The sulfur isotope composition of sulfides ranges from –1.1 to 2.8 34S and suggests a magmatic source. At least two generations of gold deposition are recognized: (1) early refractory, and (2) subsequent non-refractory gold mineralization of epithermal style. Co-rich arsenopyrite with refractory gold and pyrite are the most abundant minerals of the early stage of sulfide precipitation. Early arsenopyrite formed at 535–345°C along the arsenopyrite–pyrrhotite–loellingite buffer and late arsenopyrite crystallized below 370°C along the arsenopyrite–pyrite buffer. Non-refractory gold associated with base-metal sulfides and with Bi–Te–Ag–Pb–S mineral assemblages has an average fineness of about 685, and is represented by electrum of two generations, and minor maldonite (Au2Bi). Fluid inclusions from various quartz generations co-genetic with base-metal sulfides and associated with carbonates, tellurides and non-refractory gold indicate fluids with moderate salinity (9–15 wt% NaCl equiv.) and a temperature and pressure drop from 350 to 190°C and 1.2 to 0.8 kbar, respectively. According to the result of the sulfur isotope fractionation geothermometer the temperature of base-metal crystallization was in the range from 322 to 289°C. Preliminary results of oxygen isotope studies of quartz from veins indicate a gradual increase in the proportion of meteoric water in the epithermal stage. The gold to silver ratio in ore samples with >3 ppm Au is about 1:5 (geometric mean). Hydrothermal alteration started with sericitization, pyritization, and kaolinitization in vein selvages followed by alkaline hydrothermal alteration of propylitic character (illitization and chloritization), albitization and carbonatization. The mineralization of the Radzimowice deposit is considered as related to alkaline magmatism and is characterized by the superposition of low-sulfidation epithermal mineralization on higher-temperature and deeper-seated mesothermal/porphyry style.Editorial handling: B. Lehmann  相似文献   

2.
The Lewis Ponds Zn–Pb–Cu–Ag–Au deposit, located in the eastern Lachlan Fold Belt, central western New South Wales, exhibits the characteristics of both volcanic-hosted massive sulphide and carbonate-hosted replacement deposits. Two stratabound massive to disseminated sulphide zones, Main and Toms, occur in a tightly folded Upper Silurian sequence of marine felsic volcanic and sedimentary rocks. They have a combined indicated resource of 5.7 Mt grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au. Main Zone is hosted by a thick unit of poorly sorted mixed provenance breccia, limestone-clast breccia and quartz crystal-rich sandstone, whereas Toms Zone occurs in the overlying siltstone. Pretectonic carbonate–chalcopyrite–pyrite and quartz–pyrite stringer veins occur in the footwall porphyritic dacite, south of Toms Zone. Strongly sheared dolomite–chalcopyrite–pyrrhotite veins directly underlie the Toms massive sulphide lens. The mineralized zones consist predominantly of pyrite, sphalerite and galena. Paragenetically early framboidal, dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate occur throughout the breccia and sandstone beds that host Main Zone, but are rarely preserved in the annealed massive sulphide in Toms Zone. Main and Toms zones are associated with a semi-conformable hydrothermal alteration envelope, characterized by texturally destructive chlorite-, dolomite- and quartz-rich assemblages. Dolomite, chlorite, quartz, calcite and sulphides have selectively replaced breccia and sandstone beds in the Main Zone host sequence, whereas the underlying porphyritic dacite is weakly sericite altered. Vuggy and botryoidal textures resulted from partial dissolution of the dolomite-altered sedimentary rocks and unimpeded growth of base metal sulphides, carbonate and quartz into open cavities. The intense chlorite-rich alteration assemblage, underlying Toms Zone, grades outward into a weak pervasive sericite–quartz assemblage with distance from the massive sulphide lens. Limestone clasts and hydrothermal dolomite at Lewis Ponds are enriched in light carbon and oxygen isotopes. The dolomite yielded 13CVPDB values of –11 to +1 and 18OVSMOW values of 6 to 16. Liquid–vapour fluid inclusions in the dolomite have low salinities (1.4–7.7 equiv. wt% NaCl) and homogenization temperatures (166–232°C for 1,000 m water depth). Dolomitization probably involved fluid mixing or fluid–rock interactions between evolved heated seawater and the limestone-bearing facies, prior to and during mineralization. 34SVCDT values range from 2.0 to 5.0 in the massive sulphide and 3.9 to 7.4 in the footwall carbonate–chalcopyrite–pyrite stringer veins, indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater. The sulphide mineral assemblages at Lewis Ponds are consistent with moderate to strongly reduced conditions during diagenesis and mineralization. Low temperature dolomitization of limestone-bearing facies in the Main Zone host sequence created secondary porosity and provided a reactive host for fluid-rock interactions. Main Zone formed by lateral fluid flow and sub-seafloor replacement of the poorly sorted breccia and sandstone beds. Base metal sulphide deposition probably resulted from dissolution of dolomite, fluid mixing and increased fluid pH. Pyrite, sphalerite and galena precipitated from a relatively low temperature, 150–250°C hydrothermal fluid. In contrast, Toms Zone was emplaced into fine-grained sediment at or near the seafloor, above a zone of focused up-flowing hydrothermal fluids. Copper-rich assemblages were deposited in the Toms Zone footwall and massive sulphide lenses in Main and Toms zones as the hydrothermal system intensified. During the D1 deformation, fracture-controlled fluids within the Lewis Ponds fault zone and adjacent footwall volcanic succession remobilized sulphides into syntectonic quartz veins. Lewis Ponds is a rare example of a synvolcanic sub-seafloor hydrothermal system developed within fossiliferous limestone-bearing facies. The close spatial association between limestone, hydrothermal dolomite, massive sulphide and dacite provides a basis for new exploration targets elsewhere in New South Wales.Editorial handling: D. Lentz  相似文献   

3.
Four epithermal vein deposits (i.e. Dawn, Central Extended, Rose's Pride and Klondyke) in the Cracow gold field, central Queensland were investigated in terms of paragenesis, mineralogy, vein textures, fluid inclusions and stable isotopes. The Cracow epithermal field is confined to an area approximately 6 by 5 kilometers. All the deposits are hosted by the massive Camboon Andesite of Upper Carboniferous to Lower Permian age, occur as open-space vein fillings, and have similar paragenesis. However, significant variations in mineralogy, textures of quartz and adularia, and fluid geochemistry were found for a main mineralisation stage (Stage II) of each individual deposits. At Rose's Pride and Klondyke, basemetal sulphides are virtually absent, but significant amounts of calcite and quartz with minor adularia are widely distributed. Replacement textures are distinct, and mineralisation temperature is less than 220 °C and salinity less than 0.2 wt%. The 18O values of quartz and calcite range from –2.65 to –2.06 and from –6.66 to –6.34%. respectively, and calculated 18OH2O value is about –17%. which represents a nearly unshifted palaeo-meteoric water. Gold mineralisation is best developed at Central Extended among the studied deposits, where patches rich in electrum are often observed in polished thin sections and where gold grades exceeding 10 g/t are frequently indicated by assays. Base-metal sulphides are only present locally and rarely exceed 5 volume percent of the vein samples. Quartz is the dominant gangue mineral, but significant amounts of rhombic adularia and chlorite are widely distributed. Various primary and recrystallisation textures possibly inherited from silica gel are well developed and widespread. At individual sites where crustiform bands developed from both walls of a fissure, temperatures could drop sharply from 275 °C to less than 220 °C. The ore-forming fluid at Central Extended, compared with that at Rose's Pride and Klondyke, was isotopically shifted from meteoric water with 18OH2O value of –13.5 calculated in equilibrium with quartz ( 18O values of –3.09 to –1.44%.). The orebodies at Dawn are rich in base-metal sulphides which are commonly coarse-grained and form up to 20 volume percent of the vein materials. Quartz is the predominant gangue mineral, and commonly shows a coarse comb texture. The ore-forming fluid was 275 ± 10 °C and low salinity (0.4 to 0.7 wt%). The 18O values of quartz range from –3.97 to –3.22%., and calculated 18OH2O value is about –12, indicating large isotopic shifts from palaeo-meteoric water. A depth zoning in typical boiling epithermal systems, corresponding to different fluid compositions, wall rock permeability and boiling behaviors, was invoked to explain different characteristics of these selected epithermal veins.  相似文献   

4.
Several important mineral deposits of Sn, Zn, Cu, Pb, and other metals associated with Devonian sediments and Yanshanian (Cretaceous) granitic rocks are known in the Dachang district (Guangxi). Early genetic hypotheses related the origin of the deposits entirely to the Yanshanian granites. Recently, it was suggested that in Devonian times an earlier syngenetic metal concentration may have occurred, later overprinted by the Yanshanian metallogeny. This contribution is aimed at placing constraints on the physicochemical conditions during the Yanshanian ore formation-remobilization by studying the sulfide chemistry (arsenopyrite, sphalerite, stannite) and fluid inclusion data on the two major deposits in the area, i.e., the polymetallic cassiterite deposit of Changpo and the Zn-Cu skarn deposit of Lamo. Sphalerite and arsenopyrite are quite abundant in both deposits; stannite is minor, but fairly widespread at Changpo, and quite rare at Lamo. They are accompanied by pyrite, pyrrhotite, galena, chalcopyrite, cassiterite, fluorite, and a large variety of other sulfides and sulfosalts. The main compositional data for sphalerite and arsenopyrite are summarized as follows:Changpo: arsenopyrite associated with pyrrhotite 31.4–36.1 at% As; Associated with pyrite 31.9–33.1 at% As; sphalerite associated with pyrrhotite 18.3–22.2 mol% FeS; associated with pyrite 10.6–18.6 mol% FeS.Lamo: arsenopyrite associated with pyrrhotite 32.9–35.3 at% As; associated with pyrite 30.3–31.7 at% As; sphalerite associated with pyrrhotite, 17.2–24.4 mol% FeS; associated with pyrite 4.2–19.6 mol% FeS.Partitioning of Fe and Zn between coexisting sphalerite and stannite from Changpo indicates temperatures of 300°–350°C. For Lamo, the following fluid inclusion data are available: fluorite, salinities of 0–9.5 equiv. wt% NaCl, and homogenization temperatures between 160°C and 250°C; quartz, moderate salinities (0–4.6 equiv. wt% NaCl), and homogenization temperatures of 208°–260°C. Combining the mineralogical evidence with the compositional and fluid inclusion data, it is suggested that the evolution of the environment during the Yanshanian event was characterized by the following parameters: pressure was relatively low (on the order of 1–1.5 kb); temperature may have been as high as 500°C during deposition of the As-richest arsenopyrites, but eventually dropped below 200°–250°C in the latest stages; with an increase in sulfur activity and/or the decrease in temperature pyrrhotite was no longer stable in the latest stages of mineralization.  相似文献   

5.
The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, Republic of Korea, occurs in a broadly folded and reverse-faulted terrain of Paleozoic sedimentary rocks: the Taebaeksan basin. The orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks. The Pb-Zn(-Ag) mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals, ore textural relationships and their composition. Based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions, the estimated temperatures for the stages I, II, III and IV reach 330 to 350 °C, 270 to 340 °C, 230 to 250 °C, and <220 °C, respectively. The sulphur activity (atm) of ore formation at the Taebaek deposit was estimated for each stage as 10–11 to 10–11.5, 10–9.5 to 10–13, 10–13.5 to 10–15, and <10–15, respectively. Even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages, sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure (about 1 kbar).  相似文献   

6.
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   

7.
A sulfur isotopic study of the San Cristobal tungsten-base metal mine,Peru   总被引：1，自引：0，他引：1  
The San Cristobal tungsten-base metal deposit differs from other quartz-wolframite vein deposits in that it has a major period of base metal mineralization consisting of pyrite, chalcopyrite, sphalerite, and galena. Homogenization temperatures of primary and pseudosecondary inclusions were measured in augelite (260–400°C), quartz (230–350°C) and sphalerite (180–220°C). The 34S values of H2S in solution in equilibrium with the vein minerals range from 1.6 to 9.0 permil increasing through the paragenesis. The relatively heavy values suggest a nonmagmatic source for the sulfur. Evaporitic sulfates are a likely source of heavy sulfur and sedimentary anhydrite is known to occur near the San Cristobal region. In contrast to San Cristobal are three similar quartz-wolframite vein deposits, Pasto Bueno, Panasqueira, and Tungsten Queen. They each have an average 34S value for sulfides of about 0 permil, suggesting a sulfur of magmatic origin. At San Cristobal an influx of sedimentary sulfur could not only account for the distinctive isotopic signature of the sulfides but also for the presence of the base metal mineralization.  相似文献   

8.
The Quesnel River gold deposit (1.2 million tonnes grading 5.22 g/t Au in three separate zones) occurs within Takla Group volcanic rocks of Upper Triassic age proximal to an alkalic stock. The deposit occurs in amphibole-augite phyric, fragmental, basaltic rocks. Alteration has produced an assemblage of epidote-chloritetremolite-calcite-quartz with lesser pyrite, chalcopyrite, pyrrhotite, sphalerite, marcasite, galena, arsenopyrite and gold.The West Zone comprises a tabular, conformable sulfide body underlain by bedded, variably altered fragmental basaltic rocks and overlain by siltstone and argillite. In the Main Zone, highest gold grades occur adjacent to a sharp discordant alteration front with barren, strongly carbonatized, pyritic basaltic lapilli-tuff. It is overlain by siltstone and argillite and bounded to the east and a depth by a west dipping reverse fault. To the west the auriferous, propylitically altered, rocks grade laterally into lower grade and barren basaltic rocks.Oxygen(18O = + 9 to + 15) and carbon (13O= -14 to –7) isotopic signatures of calcite from carbonate-altered and propylitically altered rocks are similar. However, sulfur isotopic values for pyrite are different, with gold-associated pyrite (34S = –7 to –3) distinct from pyrite in carbonate altered rocks with (34S = + 8 to + 13).The carbonization occurred before complete induration of the basaltic fragmental rocks, whereas propylitization and gold plus sulfide precipitation is clearly epigenetic.  相似文献   

9.
Mineral assemblages and chemical compositions of ore minerals from the Boroo gold deposit in the North Khentei gold belt of Mongolia were studied to characterize the gold mineralization, and to clarify crystallization processes of the ore minerals. The gold deposit consists of low‐grade disseminated and stockwork ores in granite, metasedimentary rocks and diorite dikes. Moderate to high‐grade auriferous quartz vein ores are present in the above lithological units. The ore grades of the former range from about 1 to 3 g/t, and those of the latter from 5 to 10 g/t, or more than 10 g/t Au. The main sulfide minerals in the ores are pyrite and arsenopyrite, both of which are divisible into two different stages (pyrite‐I and pyrite‐II; arsenopyrite‐I and arsenopyrite‐II). Sphalerite, galena, chalcopyrite, and tetrahedrite are minor associated minerals, with trace amounts of bournonite, boulangerite, geerite, alloclasite, native gold, and electrum. The ore minerals in the both types of ores are variable in distribution, abundance and grain size. Four modes of gold occurrence are recognized: (i) “invisible” gold in pyrite and arsenopyrite in the disseminated and stockwork ores, and in auriferous quartz vein ores; (ii) microscopic native gold, 3 to 100 µm in diameter, that occurs as fine grains or as an interstitial phase in sulfides in the disseminated and stockwork ores, and in auriferous quartz vein ores; (iii) visible native gold, up to 1 cm in diameter, in the auriferous quartz vein ores; and (iv) electrum in the auriferous quartz vein ores. The gold mineralization of the disseminated and stockwork ores consists of four stages characterized by the mineral assemblages of: (i) pyrite‐I + arsenopyrite‐I; (ii) pyrite‐II + arsenopyrite‐II; (iii) sphalerite + galena + chalcopyrite + tetrahedrite + bournonite + boulangerite + alloclasite + native gold; and (iv) native gold. In the auriferous quartz vein ores, five mineralization stages are defined by the following mineral assemblages: (i) pyrite‐I; (ii) pyrite‐II + arsenopyrite; (iii) sphalerite + galena + chalcopyrite; (iv) Ag‐rich tetrahedrite‐tennantite + bournonite + geerite + native gold; and (v) electrum. The As–Au relations in pyrite‐II and arsenopyrite suggest that gold detected as invisible gold is mostly attributed to Au+1 in those minerals. By applying the arsenopyrite geothermometer to arsenopyrite‐II in the disseminated and stockwork ores, crystallization temperature and logfs2 are estimated to be 365 to 300 °C and –7.5 to –10.1, respectively.  相似文献   

10.
The economic mineralization of Pd at the Lac des Iles mine occurs in the gabbroic rocks of the Mine Block Intrusion in the 2.69 Ga Lac des Iles Intrusive Complex. The complex intruded the tonalitic rocks of the Lac des Iles greenstone belt in the Wabigoon Subprovince of the Superior Province of Canada. We conducted a detailed study on the Pd mineralization in the southern Roby Zone and the Twilight Zone. Sulphide minerals commonly display exsolution textures where pentlandite and chalcopyrite are exsolved from pyrrhotite. Sulphur contents from these zones display positive correlations with the contents of platinum group elements (PGE), Se, and Te, suggesting a magmatic origin of the mineralization where PGE were concentrated in immiscible sulphide melt in the parental magmas. The average ratios of Se/S (703±192×10–6) and Te/S (192±104×10–6) in the two zones are higher than the primitive mantle values of ~300×10–6 and ~48×10–6, respectively. The high ratios are consistent with the derivation of their parental magmas from a depleted mantle source. The High Grade Zone forms a narrow northwest-trending zone in the margin of the Roby Zone, and is hosted by an intensely altered clinopyroxenite/melanogabbroic unit. It contains two mineral assemblages; millerite + siegenite ± chalcopyrite ± pyrite co-existing with hornblende + plagioclase ± quartz ± carbonate, and pyrite ± chalcopyrite with chlorite + actinolite ± albite ± quartz ± carbonate. The ore is high in Pd (mean Pd/Pt ratio of 16.5; up to 25) compared to the southern Roby Zone and Twilight Zone where the Pd/Pt ratios are ~8. It shows positive correlations between Se and Te and between Se and immobile metals, such as Ni and Co. The data suggest a primary magmatic origin of mineralization of the High Grade Zone, but there is substantial scatter on diagrams involving S, such as the plot between S and Se. The evidence suggests that the primary magmatic mineralization was followed by hydrothermal transport of mobile elements. Using the relationships between Se and metals, the ore most likely had 0.8–2 ppm Pt and 8–21 ppm Pd during the primary mineralization. The subsequent hydrothermal activity resulted in the enrichment of Pd by up to 40 ppm. The lack of fluid pathways in the High Grade Zone and the distribution of the zone are consistent with magmatic-hydrothermal activity by aqueous fluids exsolved from the parental magmas of the Roby Zone and High Grade Zone. Sulphide minerals from the southern Roby Zone, Twilight Zone, and High Grade Zone have similar 34S values, ranging from 0.0 to +1.5. The data are consistent with the derivation of S from the mantle. In individual samples from the southern Roby Zone and High Grade Zone, pyrite shows lower 34S than chalcopyrite, suggesting isotopic disequilibrium of S. This likely reflects the crystallization and re-crystallization of sulphide minerals over a wide range of temperatures.Editorial handling: B. Lehmann  相似文献   

11.
The Ixtahuacan Sb-W deposits are hosted by upper Pennsylvanian to Permian metasedimentary rocks of the central Cordillera of Guatemala. The deposits consist of gold-bearing arsenopyrite, stibnite and scheelite. Arsenopyrite and scheelite are early in the paragenesis, occurring as disseminations in pyritiferous black shale/sandstone and in argillaceous limestone, respectively. Some stibnite is disseminated, but the bulk of the stibnite occurs as massive stratabound lenses in black shales and in quartz-ankerite veins and breccias, locally containing scheelite.Microthermometric measurements on fluid inclusions in quartz and scheelite point to a low temperature (160–190°C) and low to moderate salinity (5–15 wt% NaCl eq.) aqueous ore fluid. Abundant vapour-rich inclusions suggest that the fluid boiled. Carbon dioxide was produced locally as a result of interaction of the aqueous fluid with the argillaceous limestone. Bulk leaching experiments and SEM-EDS analyses of decrepitated fluid inclusion residues indicate that the ore-bearing solution was NaCl-dominated. The 18O values of quartz, ankerite and scheelite from mineralized veins range from 19.7 to 20.5, 18.1 to 20.0 and 7.0 to 8.4 respectively. The average temperature calculated from quartz-scheelite oxygen isotopic fractionation is 170°C. The oxygen isotopic composition of the fluid, interpreted to have been in equilibrium with these minerals, ranged from 5.7 to 7.6, and is considered to represent an evolved meteoric water. Diagenetic or syngenetic pyrite has a sulphur isotopic composition of 0.5±0.3 which is consistent with bacterial reduction of sulphate. The 34S values of arsenopyrite and stibnite range from –2.8 to 2.0 and –2.7 to –2.3 respectively, and are though to reflect sulphur derived from pyrite.The Ixtahuacan deposits are interpreted to have formed at low temperature (<200°C) and a depth of a few hundred metres from a low fO2 (10–49–10–57), high pH (7–8) fluid. Arsenic was probably transported as arsenious acid, antimony and gold as thio-complexes and tungsten as the complex HWO 4 – .A model is proposed in which a meteoric fluid, heated by a felsic intrusion at depth, was focused to shallow levels along faults. The interaction of the fluid with pyritiferous beds caused the deposition of arsenopyrite as a result of sulphidation and/or decreasing fO2; gold probably co-precipitated with As or was adsorbed onto the arsenopyrite. The precipitation of stibnite was caused by boiling. Scheelite deposited in response to the increase in Ca2+ activity which accompanied interaction of the ore fluid with the argillaceous limestones.  相似文献   

12.
The assemblage paragonite + quartz is encountered frequently in low- to medium-grade metamorphic rocks. With rising grade of metamorphism they react mutually to yield the condensed assemblage albite + Al2SiO5.The univariant curve pertaining to the equilibrium paragonite + quartz=albite + andalusite + H2O has been located experimentally. The reversed P H 2 O-T data are: 1 kb: 470–490° C 2 kb: 510–530° C 3 kb: 540–560° C 4 kb: 560–580° C 5 kb: 590–600° C The univariant curve pertaining to the equilibrium paragonite + quartz=albite + kyanite + H2O runs through the following P H 2 O-T-intervals: 5 kb: 570–625° C 6 kb: 600–630° C 7 kb: 620–640° C Thermodynamic calculations of S 298 0 , H f,298 0 and G f,298 0 of the phase paragonite from the experimental data presented above and those obtained from the equilibria of the reaction paragonite=albite + corundum + H2O (Chatterjee, 1970), agree within the limits of uncertainty. This prompts the idea that Zen's (1969) suggestion of a possible error of approximately 7 kcal in G f,298 0 of the Al2SiO5 polymorphs may in fact be due to an error of similar magnitude in G f,298 0 of corundum.A best estimate of S 298 0 , H f,298 0 and G f,298 0 of paragonite based on these considerations yield: S 298 0 : 67.61±3.9 cal deg–1 gfw–1 H f,298 0 : –1411.4±2.7 kcal gfw–1 G f,298 0 : –1320.9±4.0 kcal gfw–1 These numbers will be subject to change when better thermochemical data on corundum and albite are available.In medium-grade metamorphic rocks the assemblage paragonite + quartz is commonly found in stable coexistence with such other phases as muscovite, staurolite, andalusite, kyanite, but not with cordierite or sillimanite. However, the assemblage paragonite-sillimanite has been reported to be stable in the absence of quartz. All these petrologic observations can be explained on the basis of the stability data of the phases and phase assemblages concerned.  相似文献   

13.
Calcareous hornfelses and marbles all contain calcite+K-feldspar+quartz+sphene±diopside±plagioclase ±scapolite±clinozoisite. In addition, rocks on one side of a fault contain combinations of biotite, amphibole, and muscovite while those on the other side contain combinations of grossular, wollastonite, and axinite. At bars, mineral-fluid equilibria in biotite and amphibole-bearing rocks record T= 440° C and garnet-bearing rocks record T=540° C and Conventional volumetric fluid-rock ratios were calculated using measured progress of prograde decarbonation reactions and the conditions of metamorphism: marbles, 0–0.4; amphibole-bearing hornfelses, 1.0–1.4; garnet-bearing hornfelses, 2.8–6.7. Decarbonation reactions were driven by pervasive infiltration of rock by reactive aqueous fluids. Differences in fluid-rock ratio between interbedded marble and hornfels and lack of correlation between fluid-rock ratio and whole-rock Cl-content, however, argue for channelized fluid flow along lithologic layers. A new analysis of reaction progress allows estimation of time-integrated fluxes for a specified temperature gradient along the direction of flow. Results are: marbles, 0–0.1×105 cm3/cm2; amphibole-bearing hornfelses, 0.8–1.3×105 cm3/cm2; garnet-bearing hornfelses, 1.2–2.5 × 105 cm3/cm2. Fluid flowed from regions of low to regions of high temperature. Using a simple thermal model for the area, the duration of contact metamorphism was estimated as 105 years. Assuming the time of fluid flow was the same as the duration of the thermal event, the first measurements of average metamorphic fluxes (q) and permeabilities (k) are: average marbles, q=0–0.3×10–8 cm/s and k =2×10–6 darcy; hornfels, q=3–8×10–8 cm/s and k =20–53×10–6 darcy. Estimated premeabilities are within the range of values measured for metamorphic rocks in the laboratory. Fluxes, permeabilities, and whole-system fluidrock ratios are similar to those estimated for the Skaergaard hydrothermal system by Norton and Taylor (1979).  相似文献   

14.
Kuroko-type massive sulfide deposits of the Eastern Black Sea province of Turkey are related to the Upper Cretaceous felsic lavas and pyroclastic rocks, and associated with clay and carbonate alteration zones in the footwall and hangingwall lithologies. A complete upward-vertical section of a typical orebody consists of a stringer-disseminated sulfide zone composed mainly of pyrite and chalcopyrite; a massive pyrite zone; a massive yellow ore consisting mainly of chalcopyrite and pyrite; a black ore made up mainly of galena and sphalerite with minor amounts of chalcopyrite, bornite, pyrite and various sulfosalts; and a barite zone. Most of the deposits in the province are associated with gypsum in the footwall or hangingwall. The paragenetic sequence in the massive ore is pyrite, sphalerite, chalcopyrite, bornite, galena and various sulfosalts, with some overlap between the mineral phases. Massive, stringer and disseminated sulfides from eight kuroko-type VMS deposits of the Eastern Black Sea province have a 34S range of 0–7 per mil, consistent with the 34S range of felsic igneous rocks. Sulfides in the massive ore at Madenköy (4.3–6.1 per mil) differ isotopically from sulfides in the stringer zone (6.3–7.2 per mil) suggesting a slightly increased input of H2S derived from marine sulfate with time. Barite and coarse-grained gypsum have a 34S range of 17.7–21.5 per mil, a few per mil higher than the 34S value of contemporaneous seawater sulfate. The deposits may, therefore, have formed in restricted basins in which bacterial reduction of sulfate was taking place. Fine-grained, disseminated gypsum at Kutlular and Tunca has 34S values (2.6–6.1 per mil) overlapping those of ore sulfides, indicating sulfide oxidation during waning stages of hydrothermal activity.  相似文献   

15.
Lead-zinc-fluorite-barite veins in the lower and middle Benue Trough (Nigeria) are located within the Lower Cretaceous (Albian) carbonaceous shales, limestones, and arkosic sandstones of this intracontinental rift structure. The veins in the lower Benue Through consist of sphalerite + galena+marcasite ± chalcopyrite ± barite in a gangue of siderite and quartz hosted by carbonaceous shales, whereas in the middle Benue Trough, fluorite, barite, quartz, and similar sulfide minerals are hosted by limestone and sandstone. Fluid inclusion temperatures in vein minerals range from 95°C to 200°C (without pressure corrections) and salinities range from 14 to 24 equiv. wt% NaCl. Oxygen isotope compositions of limestone wall rocks (middle Benue) have been lowered from premineralization 18O values of about 25 per mil to approximately 16 per mil. Fluid in equilibrium with vein calcite has a calculated 18O of +2.6 per mil at 130°C. The 87Sr/86Sr ratio of this calcite (0.71497) suggests that strontium and calcium had a considerably more radiogenic source than the Cretaceous limestone or evaporite did (87Sr/86Sr=0.7073–0.7078). Observed strontium data, lead isotope compositions of galena, and REE patterns in fluorite suggest that the Lower Paleozoic basement rocks in the trough or their weathered equivalents are likely sources for the Benue Trough ore components. Sulfur isotope data suggest that the sulfur was probably contributed from the Cretaceous evaporites in the trough.Our data favor a basinal brine source for the ore-forming fluid. Fluid criculation probably resulted from high geothermal gradients accompanying continental rifting. Brine interaction with the clastic, carbonate, and evaporite rocks led to metal and sulfur leaching and later deposition in fractures accompanying the Cenomanian deformation and uplift in the Benue Trough.  相似文献   

16.
Abstract: The Bulawan deposit is located in the porphyry copper belt of southwest Negros island, Philippines. Propylitic, K–feldspar, sericitic, and carbonate alteration types can be distinguished in the deposit. Propylite alteration occurs mainly in Cretaceous-Eocene andesitic lavas and agglomerates while K–feldspar, sericite and carbonate alteration types occur mostly in the Middle Miocene dacite porphyry breccia pipes and stocks which were intruded into the andesites. K-feldspar zones occur in the inner parts of the sericitized zone. Sericite alteration overprinted the propylitized and K-feldspar alteration zones, at lower temperature than epidote and chlorite in the propylitized zone. Carbonate alteration is associated with the mineralization in the center of the breccia pipes and along faults. Mineralization consists of gold-silver telluride ores that are hosted by the carbonate– and sericite-altered dacite porphyry breccia pipes. The Bulawan ores occur mainly as disseminations, but unlike many epithermal gold deposits, lack classical epithermal colloform and crustiform quartz veins. The ore minerals are sphalerite, galena, chalcopyrite, pyrite and tetrahedite-tennantite with minor amounts of electrum, calaverite, petzite, sylvanite, hessite, tellurobismuthite, coloradoite, altaite, and rucklidgeite. Electrum and telluride minerals are associated mostly with calcite and dolomite-ankerite minerals. Fluid inclusions in quartz and calcite in clasts of propylitized andesite in the breccia pipes homogenize from about 300° to 400°C while fluid inclusions in quartz, calcite and sphalerite within the dacite porphyry breccia pipes homogenize between 300° to 310°C. The ores were formed around 300°C from hydrothermal solutions with salinity of about 6. 6 wt % NaCl equivalent. The presence of sylvanite and calaverite as intergrowths with each other, and the Ag content of calaverite are consistent with the above temperature estimate. Based on paragenesis, the Bulawan deposit formed in a pyrite-stable environment, with pH between 3. 4 and 5. 5, fO2 between 10-32 to 10-30 atm, fS2 between 10-9.8 to 10-7.8 atm, fTe2 between 10-8.9 to 10-6.5 atm, and total sulfur content about 10-2.8 molal. The dominant reduced sulfur species in the ore solutions may have been H2S(aq), and the likely aqueous tellurium species were H2Te(aq) and H2TeO3(aq). The ore minerals in the Bulawan deposit were probably formed by mixing of slightly saline and low salinity fluids.  相似文献   

17.
A thermodynamic analysis of the intermediate solid solution (Iss) of near-cubanite composition has been attempted by considering an Fe–Zn exchange equilibrium between Iss and sphalerite. The interchange free-energy parameter of Fe–Zn mixing in Iss (WIss) and the free energy of the exchange equilibrium (G1,T ) have been deduced at 500, 600, 700 and 723° C using the compositional data of sphalerite and Iss from phase equilibrium experiments and by the standard method of linear regression analysis. For sphalerite, two independent activity-composition models have been chosen. The extracted values of G1,T and WIss, using both models, are compared. Although the values match, the errors in the extracted parameters are relatively larger when Hutcheon's model is used. Both G1,T and WIss show linear variations with temperature, as given by the following relations: G1,T = –35.41 + 0.033 T in kcal (SE=0.229)WISS= 48.451 – 0.041 T in kcal (SE=0.565) Activity-composition relations and different mixing parameters have been calculated for the Iss phase. A large positive deviation from ideality is observed in Iss on the join CuFe2S3–CuZn2S3. No geothermometric application has been attempted in this study, even though Iss of cubanite composition (isocubanite) in association with sphalerite, pyrite and pyrrhotite is reported from seafloor hydrothermal deposits. This is due to the fact that: (a) the temperatures of formation of these deposits are significantly lower than 500° C, the lower limit of appropriate experimental data base; (b) microprobe data of the coexisting isocubanite and sphalerite in the relevant natural assemblages are not available.Symbols a J i activity of component i in phase J - G1, T standard free energy change of reaction (cal) - GIM free energy of ideal mixing (cal) - GEM free energy of excess mixing (cal) - G M ex free energy of mixing (cal) - G i excess free energy of mixing at infinite dilution (cal) - i J activity coefficient of component i in phase J - i J, 0 standard chemical potential of component i in phase J (cal) - ; i J chemical potential of component i in phase J (cal) - R universal gas constant (1.98717 cal/K·mol) - T temperature in degree (K) - WJ interchange free energy of phase J in (cal) - X J i mole fraction of component i in phase J  相似文献   

18.
Mesothermal gold mineralization at the Samdong mine (5.5–13.5 g/ton Au), Youngdong mining district, is situated in massive quartz veins up to 1.2 m wide which fill fault fractures within upper amphibolite to epidote-amphibolite facies, Precambrian-banded biotite gneiss. The veins are mineralogically simple, consisting of iron- and base-metal sulfides and electrum, and are associated with weak hydrothermal alteration zones (<0.5 m wide) characterized by silicification and sericitization. Fluid inclusion data and equilibrium thermodynamic interpretation of mineral assemblages indicate that the quartz veins were formed at temperatures between 425 and 190°C from relatively dilute aqueous fluids (4.5–13.8 wt. % equiv NaCl) containing variable amounts of CO2 and CH4. Evidence of fluid unmixing (CO2 effervescence) during the early vein formation indicates approximate pressures of 1.3–1.9 kbars, corresponding to minimum depths of 5–7 km under a purely lithostatic pressure regime. Gold deposition occurred mainly at temperatures between 345 and 240 °C, likely due to decreases in sulfur activity accompanying fluid unmixing. The 34S values of sulfide minerals (-3.0 to 5.3 ), and the measured and calculated O-H isotope compositions of ore fluids (18O = 5.7 to 7.6; = –74 to –80) indicate that mesothermal gold mineralization at the Samdong mine may have formed from dominantly magmatic hydrothermal fluids, possibly related to intrusion of the nearby ilmenite-series, Kimcheon Granite of Late Jurassic age.  相似文献   

19.
The Milin Kamak gold-silver deposit is located in Western Srednogorie zone, 50 km west of Sofia, Bulgaria. This zone belongs to the Late Cretaceous Apuseni-Banat-Timok-Srednogorie magmatic and metallogenic belt. The deposit is hosted by altered trachybasalt to andesitic trachybasalt volcanic and volcanoclastic rocks with Upper Cretaceous age, which are considered to be products of the Breznik paleovolcano. Milin Kamak is the first gold-silver intermediate sulfidation type epithermal deposit recognized in Srednogorie zone in Bulgaria. It consists of eight ore zones with lengths ranging from 400 to 1000 m, widths from several cm to 3–4 m, rarely to 10–15 m, an average of 80–90 m depth (a maximum of 200 m) and dip steeply to the south. The average content of gold is 5.04 g/t and silver – 13.01 g/t. The styles of alteration are propylitic, sericite, argillic, and advanced argillic. Ore mineralization consists of three stages. Quartz-pyrite stage I is dominated by quartz, euhedral to subhedral pyrite, trace pyrrhotite and hematite in the upper levels of the deposit. Quartz-polymetallic stage II is represented by major anhedral pyrite, galena, Fe-poor sphalerite; minor chalcopyrite, tennantite, bournonite, tellurides and electrum; and trace pyrrhotite, arsenopyrite, marcasite. Gangue minerals are quartz and carbonates. The carbonate-gold stage III is defined by deposition of carbonate minerals and barite with native gold and stibnite.Fluid inclusions in quartz are liquid H2O-rich with homogenization temperature (Th) ranging from 238 to 345 °C as the majority of the measurements are in the range 238–273 °C. Ice-melting temperatures (Tm) range from −2.2 to −4.1 °C, salinity – from 3.7 to 6.6 wt.% NaCl equiv. These measurements imply an epithermal environment and low- to moderate salinity of the ore-forming fluids.δ34S values of pyrite range from −0.49 to +2.44‰. The average calculated δ34S values are 1.35‰. The total range of δ34S values for pyrite are close to zero suggesting a magmatic source for the sulfur.  相似文献   

20.
Gold-silver mineralization in the Nelson area of southern Nevada was controlled by structures associated with intrusion of an east-west oriented pluton. Flatlying breccias formed during intrusion have allowed passive flooding of highly permeable zones and deposition of mineralized quartz and calcite. Steep fractures were formed in the pluton and immediate country rock during cooling, and later reactivated by north-south extension. These fractures have channelled fluids, and some have been the sites of hydrothermal eruptions which produced further brecciation and deposition of mineralized quartzcalcite veins. The mineralizing fluid was water which was boiling at or near 100 °C. The calcite deposited by this water has 13C = –5.4 to –7.1, and 18O = +5.8 to +11.3, and the water was probably meteoric in origin. Mineralization had an epithermal style, with strong local structural control, rather than deep-sourced regional detachment-related hydrothermal origin.  相似文献   

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