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1.
The chemical composition of native gold and electrum from auriferous vein and gold-silver vein deposits in Japan has been analyzed and summarized. The Ag/Au ratios of native gold and electrum from these two types of deposits are distinct, i.e., 10–20 Ag at % (auriferous vein) and 30–70 Ag at % (gold-silver vein). Thermochemical calculations suggest that the Ag/Au ratio of native gold and electrum should decrease with increasing chloride concentration and temperature. This is consistent with analytical results of native gold and electrum and fluid inclusion studies. Based on the Ag content of native gold and electrum, the Fe content of sphalerite, and the estimated temperatures, it is deduced that the sulfur activity for auriferous vein-type systems was lower than that of gold-silver vein-type systems. 相似文献
2.
Stannite and sphalerite coexisting with iron sulfides (pyrite and/or pyrrhotite) from Japanese ore deposits associated with tin mineralization were analyzed. Based on the iron and zinc partitioning between stannite and sphalerite, the formation temperature and sulfur fugacity for this mineral assemblage were estimated. A good correlation between stannite-sphalerite temperatures and filling temperatures of fluid inclusions and sulfur isotope temperatures was obtained. This good correlation suggests that the stannite-sphalerite pair is a useful indicator of temperature and sulfur fugacity. It is deduced that the formation temperatures are not different for skarn-type, polymetallic vein-type and Sn-W vein-type deposits, whereas the sulfur fugacities are different; sulfur fugacities increase from the skarn-type through the Sn-W vein-type to the polymetallic vein-type deposits. 相似文献
3.
Gold mineralization related to a sulphide-bearing quartz vein within Triassic phyllites has been identified in a former study of an abandoned Cu–Zn–Pb mine in southern Tuscany–northern Latium. The vein-hosted mineral assemblage includes pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, and tetrahedrite; the gold content reaches up to 20 ppm. The mineralization was related to fluids characterised by moderate temperatures and moderate salinities (5–15% NaCl equivalent) and represents the highest grade of Au in the ore deposits of southern Tuscany. The difference in the estimated values of Te of primary, L+V, pseudo-secondary L+V, secondary L+V fluids inclusions reflects different components present in the solutions.Based on these results one can conclude that euhedral quartz and pyrite deposition was related to fluids having variable salinity and chemical components.The origin of gold in Ponte San Pietro (PSP) deposit may be related to leaching of metapelites/phyllites derived from pelitic rocks containing some amount of sulphur-bearing organic matter; it is worthy noting that quartz–marcasite bearing lenses, interstratified in Triassic phyllites, contain significant amount of visible gold. The Co, Ni, Mn, as well as the δ34S data, suggest a different origin for the phyllite-hosted marcasite, the PSP quartz-vein hosted pyrite, and for gold occurrences. 相似文献
4.
A. R. Campbell 《Mineralium Deposita》1987,22(1):42-46
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
5.
The Emarat deposit, with a total proved reserve of 10 Mt ore grading 6% Zn and 2.26% Pb, is one of the largest Zn–Pb deposits in the Malayer–Esfahan belt. The mineralization is stratabound and restricted to Early Cretaceous limestones and dolomites. The ore consists mainly of sphalerite and galena with small amounts of pyrite, chalcopyrite, calcite, quartz, and dolomite. Textural evidence shows that the ore has replaced the host rocks and thus is epigenetic.Sulfur isotopes indicate that the sulfur in sphalerite and galena has been derived from Cretaceous seawater through thermochemical sulfate reduction. Sulfur isotope compositions of four apparently coprecipitated sphalerite–galena pairs suggest their precipitation was under equilibrium conditions. The sulfur isotopic fractionation observed for the sphalerite–galena pairs corresponds to formation temperatures between 77 °C and 168 °C, which agree with homogenization temperatures of fluid inclusions.Lead-isotope studies indicate that the lead in galena has been derived from heterogeneous sources including orogenic and crustal reservoirs with high 238U/204Pb and 232Th/204Pb ratios. Ages derived from the Pb-isotope model give meaningless ages, ranging from Early Carboniferous to future. It is probable that the Pb-isotope model ages that point to an earlier origin than the Early Cretaceous host rocks are derived from older reservoirs in the underlying Carboniferous or Jurassic units, either from the host rocks or from earlier-formed ore deposits within these units.This research and other available data show that the Emarat Zn–Pb deposit has many important features of Mississippi Valley-type (MVT) lead–zinc deposits and thus we argue that it is an MVT-type ore deposit. 相似文献
6.
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
7.
Two tin-polymetallic vein-type deposits widely separated in time and space but with strong similarities in terms of mineralization style, ore mineralogy and chemistry have been studied comparatively with the aim of understanding the mineralogical evolution of In-rich hydrothermal systems. The Tosham deposit, Bhiwani district, Haryana, India, is of Neoproterozoic age and constitutes a Sn–Cu prospect with unusually high In content. The disseminated, crude stockwork and vein mineralization is hosted by greisenised metasedimentary rocks intruded by a porphyritic granite stock and by later rhyolitic effusives. The Goka deposit, Naegi district, Japan is probably of uppermost Cretaceous age and occurs close to a well fractionated ilmenite series granitoid body. The tin-polymetallic vein in the Goka deposit is hosted by a welded tuff unit close to a subvolcanic granodiorite porphyry.The main host minerals of indium in the Tosham and Goka ores are sphalerite, stannite, unidentified Zn–Cu–Fe–In–Sn–S phases and chalcopyrite. Up to 0.48 wt.% In has been noted in the Goka chalcopyrite, whereas at Tosham, the mineral has a maximum In concentration of 1220 ppm. At Goka the sphalerite contains up to 1.89 wt.% In, whereas In-bearing stannite carries up to ca. 9 wt.% of the metal. Roquesite is the other indium mineral present in the Tosham ores, but is absent in Goka. The mineral chemistry of the Tosham and Goka ores suggest that the In-bearing minerals belong to a multi-component Zn–Cu–Fe–(Ag)–Sn–In–S system. Based on various triangular plots of the atomic proportions of the main metals, it is inferred that there are end-member phases, roquesite and stannite, in the Tosham ores co-existing with chalcopyrite. The sphalerite is both pure end-member and Cu–In-bearing in both the Tosham and Goka ores. Some of the analysed stannite grains in Tosham ores could possibly be petrukite. The Zn–Cu–Fe–Sn–In–S system in the two ores has a Sn-poor, high-In solid solution phase and also a Sn-rich, low-In solid solution phase. It seems possible that these two solid solutions were the first to form during hydrothermal ore deposition at high temperatures from a disordered solid solution located at the (Cu + Ag):(Zn + Fe):(In + Sn) = 3:5:2 intersection in the (Cu + Ag)–(Zn + Fe)–(In + Sn) field. With decreasing temperatures, the Sn-poor, In-rich solid solution exsolved the Zn–In-mineral of Ohta [Ohta, E., 1980. Mineralization of Izumo and Sorachi veins of the Toyoha mine, Hokkaido, Japan. Bulletin, Geological Survey of Japan 31, 585–597. (in Japanese with English abstract).] and sphalerite, while the Sn-rich, In-poor solid solution was broken down to stannite and relatively-Cu-rich sphalerite. 相似文献
8.
本文阐述了我国几个不同成因矿床闪锌矿的化学成分、晶胞参数和物理性质,如硬度、密度、反射率、吸收边能量、磁化率等的特征;根据闪锌矿的内部电子结构和微量元素的晶体化学特征讨论了成分、结构与物理性质之间的相互关系;并探讨了这些标型特征在矿床成因和地质找矿上的意义。 相似文献
9.
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
10.
Bulk compositions of intimate intergrowths of chalcopyrite and sphalerite and their genetic implications 总被引：1，自引：0，他引：1
To determine the bulk chemical compositions of chalcopyrite containing starlike sphalerite and sphalerite including dotlike chalcopyrite, specimens from various types of ore deposits in Japan were used for modal and electron microprobe analyses. According to the analytical results, most of the measured zinc contents in chalcopyrite containing starlike sphalerite are less than 0.8 at%, corresponding to the maximum solubility of zinc in chalcopyrite as determined experimentally at 400°C. However, specimens from the Maruyama deposit in the Tsumo mine contain 1.2–1.4 at% Zn, which are within the solubility limit of an intermediate solid solution (ISS) above 400°C. It is therefore concluded that starlike sphalerite in chalcopyrite are exsolution products derived from primary chalcopyrite solid solution and/or zincic ISS. Measured copper contents in sphalerite including dotlike chalcopyrite yield considerably higher values, i.e., 1.5–6.0 at%, which exceed the solubility limits of copper in sphalerite solid solution as determined experimentally. This result suggests that not all the chalcopyrite dots were exsolved from sphalerite, but that most of them are the product of some other mechanisms. 相似文献
11.
The Laloki and Federal Flag deposits are two of the many (over 45) polymetallic massive sulfide deposits that occur in the Astrolabe Mineral Field, Papua New Guinea. New data of the mineralogical compositions, mineral textures, and fluid inclusion studies on sphalerite from Laloki and Federal Flag deposits were investigated to clarify physiochemical conditions of the mineralization at both deposits. The two deposits are located about 2 km apart and they are stratigraphically hosted by siliceous to carbonaceous claystone and rare gray chert of Paleocene–Eocene age. Massive sulfide ore and host rock samples were collected from each deposit for mineralogical, geochemical, and fluid inclusion studies. Mineralization at the Laloki deposit consists of early‐stage massive sulfide mineralization (sphalerite‐barite, chalcopyrite, and pyrite–marcasite) and late‐stage brecciation and remobilization of early‐stage massive sulfides that was accompanied by late‐stage sphalerite mineralization. Occurrence of native gold blebs in early‐stage massive pyrite–marcasite‐chalcopyrite ore with the association of pyrrhotite‐hematite and abundant planktonic foraminifera remnants was due to reduction of hydrothermal fluids by the reaction with organic‐rich sediments and seawater mixing. Precipitation of fine‐grained gold blebs in late‐stage Fe‐rich sphalerite resulted from low temperature and higher salinity ore fluids in sulfur reducing conditions. In contrast, the massive sulfide ores from the Federal Flag deposit contain Fe‐rich sphalerite and subordinate sulfarsenides. Native gold blebs occur as inclusions in Fe‐rich sphalerite, along sphalerite grain boundaries, and in the siliceous‐hematitic matrix. Such occurrences of native gold suggest that gold was initially precipitated from high‐temperature, moderate to highly reduced, low‐sulfur ore fluids. Concentrations of Au and Ag from both Laloki and Federal Flag deposits were within the range (<10 ppm Au and <100 ppm Ag) of massive sulfides at a mid‐ocean ridge setting rather than typical arc‐type massive sulfides. The complex relationship between FeS contents in sphalerite and gold grades of both deposits is probably due to the initial deposition of gold on the seafloor that may have been controlled by factors such as Au complexes, pH, and fO2 in combination with temperature and sulfur fugacity. 相似文献
12.
Yong-Kwon Koh Seon-Gyu Choi Chil-Sup So Sang-Hoon Choi Etsuo Uchida 《Mineralium Deposita》1992,27(1):58-65
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
13.
Abstract. There have been two primary sources for industrial indium; one from massive sulfides, while the other is dissemination-veins and skarns, related to felsic igneous rocks. The latter group of the In-bearing deposits is abundant in the Japanese Islands. Indium occurs as In-minerals such as sakuraiite, roquesite, laforetite and many unidentified minerals, but the majority is contained as an impurity in sphalerite, and tin and copper sulfides. Average grades of the ores from which indium has been extracted vary from a few ppm (e.g., Kosaka mine) to more than 300 ppm (Toyoha mine). The amount of indium in all the major basemetal deposits is estimated by analyzing representative samples. The main indium deposits are subvolcanic and tin-poly-metallic vein types. The largest one is Toyoha mine (4,700 tons hi) and the Ashio mine (ca. 1,200 tons In) was found to be the second largest. Many small occurrences, were recognized in the Miocene magnetite-series belt, besides the classic occurrences in the ilmenite-series granitic terrains of SW Japan, including the Ikuno and Akenobe tin(-tungsten) polymetallic veins, located in the northern margin of the late Cretaceous Sanyo ilmenite-series province. Magnetite-series magmas with deep source are necessary to concentrate sulfur in the magma chamber but sedimentary source rocks and their reducing agents are needed to collect and to precipitate indium. The Japanese islands are essentially accretionary terrains intruded by various deep oxidized magmas; thus forming magnetite/ilmenite-series paired belts, which are sometimes mixed. This unique geologic setting may be the most fundamental reason why indium is rich in vein-type deposits of the Japanese Island arcs. 相似文献
14.
Polymetallic Mineralization at the Nakakoshi Copper Deposits, Central Hokkaido, Japan 总被引：6，自引：0，他引：6
Abstract: Polymetallic mineralization at the Nakakoshi deposits, Kamikawa town, central Hokkaido, occur as fracture-filling veins in Cretaceous slate of the Hidaka Supergroup. Ten veins have been recognized in NE-SW and E-W directions. Sericite in altered slate which is the host of the deposits, was dated at 31. 1 Ma, Oligocene in age.
No. 9 vein consists of massive chalcopyrite ore with various kinds of minerals such as pyrite, pyrrhotite, arsenopyrite, sphalerite, tetrahedrite, Ag-minerals and Cu–Zn–Fe–In–Sn–S minerals, quartz and sericite. Chalcopyrite and pyrite contain sphalerite star and sphalerite with chalcopyrite emulsions. Maximum indium contents of sphalerite and the Cu–Zn–Fe–In–Sn–S minerals are 1. 8 and 16. 3 wt%, respectively. The sulfur isotopic ratios, δ34 S of ore minerals, range from –12. 9 to –9. 6%. Formation temperatures of the sulfide minerals are estimated as 300–500°C, based on the paragenesis and chemical compositions of the minerals. 相似文献
No. 9 vein consists of massive chalcopyrite ore with various kinds of minerals such as pyrite, pyrrhotite, arsenopyrite, sphalerite, tetrahedrite, Ag-minerals and Cu–Zn–Fe–In–Sn–S minerals, quartz and sericite. Chalcopyrite and pyrite contain sphalerite star and sphalerite with chalcopyrite emulsions. Maximum indium contents of sphalerite and the Cu–Zn–Fe–In–Sn–S minerals are 1. 8 and 16. 3 wt%, respectively. The sulfur isotopic ratios, δ
15.
G. N. Baturin 《Lithology and Mineral Resources》2002,37(5):412-428
The behavior of molybdenum and manganese is studied in phosphorite samples from shelves, seamounts, and islands of the ocean. In shelf phosphorites, molybdenum and manganese contents are 2–128 and 12–1915 ppm, respectively, while the Mo/Mn ratio ranges from 0.004 to 4.5. Phosphorites from oceanic seamounts impregnated with ferromanganese oxyhydroxides contain 0.84–14.5 ppm of Mo and 0.1–17% of Mn. The Mo/Mn ratio ranges within 0.0008–0.004. Phosphate-bearing ferromanganese crusts overlying the seamount phosphorites contain 54–798 ppm of Mo and 10–20% of Mn; Mo/Mn ratio varies within 0.002–0.005. Corresponding values for most island phosphorites are 0.44–11.2 ppm, 27–287 ppm, and 0.008–0.20, respectively. Phosphorites from reduced environments are characterized by a relative enrichment in Mo and depletion in Mn, whereas the Mo/Mn ratio reaches maximum values. The ratio decreases with transition to suboxic and oxic conditions. Molybdenum content in recent shelf sediments is commonly higher than that in authigenic phosphorites from these sediments. Recent phosphorite nodules from the Namibian shelf become depleted in Mo and Mn during their lithification, but Pliocene–Pleistocene nodules of similar composition and origin from the same region are enriched in Mo and characterized by a variable Mn content. The higher Mo content in phosphate-bearing ferromanganese crusts is a result of coprecipitation of Mo and Mn from seawater. Nonweathered phosphorites on continents and phosphorites from oceanic shelves are largely enriched in Mo with the Mo/Mn ratio ranging from 0.01 to 1.0. This is an evidence of their formation in reducing conditions. 相似文献
16.
Compounds of Fe, Pb and Zn with mixed and intermediate sulfur valences form ubiquitous inclusions and relics in banded sphalerite, pyrite-melnikovite and galena. Banded sulfides continuously grade into banded compounds with mixed and intermediate sulfur valences, the latter with a fibrous microtexture. A fibrous microtexture is also shown by banded sphalerite and pyrite from Zn-Pb deposits of Belgium and Poland. It is therefore suggested that the fibrous sphalerite inherited such a microtexture, unusual for cubic ZnS, by direct replacement of a fibrous precursor with mixed and/or intermediate sulfur valences. The last band of banded sphalerite is often overgrown by idiomorphic, isometric sphalerite precipitated directly from the solution as ZnS. The following Fe, Pb and Zn compounds with mixed and intermediate sulfur valences were found in carbonate-hosted Zn-Pb deposits of Belgium and Poland: sulfoxylanes (M2+SO2; S2+), subsulfites (M2+S2O4; S3+), sulfites (M2+SO3; S4+), pyrosulfites (M2+S2O5; S4+) and thiosulphates(M2+S2O3; S2– and S6+). 相似文献
17.
The Sekarna Zn–Pb deposit is located in Central Tunisia at the northeastern edge of the Cenozoic Rohia graben. Mineralization comprises two major ore types: (1) disseminated Zn–Pb sulfides that occur as lenses in sedimentary phosphorite layers and (2) cavity-filling zinc oxides (calamine-type ores) that crosscut Late Cretaceous and Early Eocene limestone. We studied Zn sulfide mineralization in the Saint Pierre ore body, which is hosted in a 5-m-thick sedimentary phosphorite unit of Early Eocene age. The sulfide mineralization occurs as replacements of carbonate cement in phosphorite. The ores comprise stratiform lenses rich in sphalerite with minor galena, Fe sulfides, and earlier diagenetic barite. Laser ablation–inductively coupled plasma mass spectrometry analyses of sphalerite and galena show a wide range of minor element contents with significant enrichment of cadmium in both sphalerite (6,000–20,000 ppm) and galena (12–189 ppm). The minor element enrichments likely reflect the influence of the immediate organic-rich host rocks. Fluid inclusions in sphalerite give homogenization temperatures of 80–130°C. The final ice melting temperatures range from −22°C to −11°C, which correspond to salinities of 15–24 wt.% NaCl eq. and suggest a basinal brine origin for the fluids. Sulfur isotope analyses show uniformly negative values for sphalerite (−11.2‰ to −9.3‰) and galena (−16‰ to −12.3‰). The δ34S of barite, which averages 25.1‰, is 4‰ higher than the value for Eocene seawater sulfate. The sulfur isotopic compositions are inferred to reflect sulfur derivation through bacterial reduction of contemporaneous seawater sulfate, possibly in restricted basins where organic matter was abundant. The Pb isotopes suggest an upper crustal lead source. 相似文献
18.
Aroldo Misi Sundaram S.S. Iyer Carlos Eduardo S. Coelho Colombo C.G. Tassinari Washington J.S. Franca-Rocha Ion de Abreu Cunha Adriana S. Rocha Gomes Tolentino Flvio de Oliveira Joo Batista G. Teixeira Valter Mnaco C. Filho 《Ore Geology Reviews》2005,26(3-4):263-304
The Proterozoic sediment-hosted Zn–(Pb) sulfide and non-sulfide deposits of the São Francisco Craton, Brazil, are partially syn-diagenetic and epigenetic and were probably formed during extensional events. The majority of the deposits occur within shallow water dolomites. The Pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the Plumbotectonic model. Some of the deposits are characterized by highly radiogenic lead (206Pb/204Pb ≥ 21) originating from the highly radioactive crust of the São Francisco Craton. Pb and S isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments, respectively. The relatively high temperatures of formation (100 to 250 °C) and moderate salinity (3% to 20% NaCl equiv.) of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation. The steep paleo-geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids. 相似文献
19.
《International Geology Review》2012,54(5):445-469
The chief host of cadmium in zinc deposits is sphalerite, the cadmium content of which depends on the type of deposit. Sphalerite from Mississippi Valley-type (MVT) deposits has high cadmium concentrations whereas sphalerite from exhalative deposits has low cadmium concentrations. The Cd content of sphalerite depends on the Cd/Zn ratio, ligand activities, and temperature of the ore-forming fluids. The combined effect of variation of temperature, pH, total activity of reduced sulfur, and activity of Cl- cannot by itself account for either Cd depletion (exhalative deposits) or Cd enrichment (MVT deposits). Variations in the Cd/Zn ratio of the fluid have a significant effect in determining that of sphalerite. Basinal brines, which can considered to be the recent equivalents of MVT fluids, have high Cd/Zn ratios, and active exhalative systems are characterized by low Cd/Zn ratios. Probably the differences in sphalerite composition between the different deposit types are less a function of temperature and ligand activities than Cd/Zn ratio of the ore-forming fluids. In the hydrothermal environment, the Cd/Zn ratio is generally not high enough to allow crystallization of cadmium sulfides (greenockite or hawleyite). The abundance of greenockite in the supergene alteration zone of hydrothermal zinc deposits can be explained on the basis of Zn scavenging by crystallizing smithsonite. 相似文献
20.
Uranium in Phosphorites 总被引：1，自引：0，他引：1
The uranium concentration in phosphorites on continents and modern seafloor varies from 0.nto n· 102ppm (average 75 ppm). The average uranium concentration is 4–48 ppm in Precambrian and Cambrian deposits, 20–90 ppm in Paleozoic and Jurassic deposits, 40–130 ppm in Late Cretaceous–Paleogene deposits, 30–130 ppm in Neogene deposits, and 30–110 ppm in Quaternary (including Holocene) deposits. On the whole, the variation range is almost similar for phosphorites of different ages. The U/P2O5ratio in phosphorites ranges from less than unity to 24 · 10–4(average 3.2 · 10–4). Major phosphorite deposits of the world with ore reserves of approximately 250 Gt (or 58 Gt P2O5) contain up to 19 Mt of uranium. Uranium is present in phosphorites in the tetra- and hexavalent, i.e., U(IV) and U(VI) forms, and their ratio is highly variable. At the early diagenetic stage of the formation of marine phosphorites in a reductive environment, U(VI) diffuses from the near-bottom water into sediments. It is consequently reduced and precipitated as submicroscopic segregations of uranium minerals (mainly uraninite) that are probably absorbed by phosphatic material. During the subsequent reaction between phosphorites and aerated water and the weathering in a subaerial environment, uranium is partly oxidized and lost. The uranium depletion also occurs during catagenesis owing to a more complete crystallization of calcium phosphate and replacement of nonphosphatic components. 相似文献