Isotopic Compositions Of Sulfides Research Articles

Lead Isotopic Composition and Lead Source of the Huogeqi Cu-Pb-Zn Deposit, Inner Mongolia, China

The Huogeqi orefield located on the northern side of Mt. Langshan, Inner Mongolia occurs in the Middle Proterozoic Langshan Group metamorphic rocks, and the orebodies are stratiform. In the past twenty years, many Chinese geologists have conducted researches on the Huogeqi Cu-Pb-Zn deposit, but there has been still a controversy on its origin. Some advocate that the deposit is of sedimentary-metamorphic reworking origin, some hold that it is of sea-floor SEDEX origin, and others have a preference for magmatic superimposition origin. The crux of the controversy is that there is no common understanding about the source of ore-forming materials. In this paper, the Pb isotopic compositions of regional Achaean-Early Proterozoic basement rocks, various types of sedimentary-metamorphic rocks and volcanic rocks in the mining district, Late Proterozoic and Hercynian magmatic rocks are introduced and compared with the ore-lead composition, so as to constrain the source of the ore lead. The result indicates that (1) sulfides in the ores have homogeneous Pb isotopic compositions, showing a narrow variation range. Their 206Pb/204Pb ratios are within a range of 17.027–17.317; 207Pb/204Pb ratios, 15.451–15.786 and 208Pb/204Pb ratios, 36.747–37.669; (2) the Pb isotopic compositions of the regional Achaean-Early Proterozoic basement rocks are characteristic of the old Pb isotopic composition at the early-stage evolution of the Earth, which varies over a wider range, reflecting significant differences in Pb isotopic compositions of the ores. All this indicates that the source of ore lead has no bearing on the basement rocks; (3) the sedimentary-metamorphic rocks in the mining district are characterized by highly variable and more radiogenic Pb isotopic compositions and their Pb isotopic ratios are obviously higher than those of ores, demonstrating that ore lead did not result from metamorphic reworking of these rocks; (4) Pb isotopic compositions of Late Proterozoic diorite-gabbro and Hercynian granite are higher than those of ores. Meanwhile, the Pb isotopic compositions of sulfides in the small-sized strata-penetrating mineralized veinlets formed at later stages are completely consistent with that of sulfides in stratiform-banded ores, suggesting that these veinlets are the product of autochthonous reworking of the stratiform-banded ores during the period of metamorphism and the late magmatic superimposition-mineralization can be excluded; (5) amphibolite, whose protolith is basic volcanic rocks, has the same Pb isotopic compositions as ores, implying that ore lead was derived probably from basic volcanism. So, the source of ore-forming materials for the Huogeqi deposit is like that of the volcanic massive sulfide (VMS) deposits. However, the orebodies do not occur directly within the volcanic rocks, and instead they overlie the volcanic rocks, showing some differences from those typical VMS-type deposits.

10.1007/s11720-008-1003-y

More than 200 analyses of the sulfur isotopic composition of sulfides from various terrigenous and intrusive host rocks, metasomatically altered wall rocks, and gold lodes of the Upper Kolyma region are presented. In accessory pyrite of the metaterrigenous rocks, δ34S varies from −23.1 to +5.7‰ δ34S of pyrite and arsenopyrite from gold-quartz mineralization is within the range −10.6 to −0.4‰ and is close to the average δ34S of pyrite from the metaterrigenous rocks (−4.4‰). In the intrusive rocks, δ34S of pyrite varies from −3.8 to +2.6‰ (+0.7‰, on average) and drastically differs from δ34S of arsenopyrite from postmagmatic gold-rare-metal mineralization (−7.9 to −2.7‰; −5.2‰, on average). The comparison of the δ34S of accessory sulfides from the host rocks with δ34S of sulfides from the gold deposits suggests that sulfur mobilized from the terrigenous sequences participated in the hydrothermal process. The results obtained are consistent with the metamorphic model of the formation of gold-quartz deposits in the Upper Kolyma region.

Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn-Pb Deposits: Evidencefrom Mineralogy, Chemistry, and Isotopic Composition of Sulfides at the LisheenMine

Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has δ 34S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The δ 34S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low δ 34S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low δ 34S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have δ 34S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The δ 34S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5‰, 1 σ ) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in δ 34S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or δ 34S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

Middle Proterozoic ocean chemistry: Evidence from the McArthur Basin, northern Australia

We report on the iron-sulfur systematics and S-isotopic compositions of upper Paleoproterozoic carbonaceous shales from the Wollogorang and lower Reward formations, McArthur Basin, northern Australia. Iron speciation in these rocks is similar to that of modern euxinic Black Sea sediments and distinct from sediments depositing from oxygenated water, providing persuasive evidence of euxinic depositional environments in a mid-Proterozoic marine basin. High values of Degree of Pyritization (DOP) also indicate anoxic bottom-water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides. The isotopic composition of the sulfides is, however, quite enriched in 3 4 S and for the lower Reward Formation similar to contemporaneous seawater sulfate. These 3 4 S-enriched sulfides are consistent with extensive, or even complete, sulfate depletion within the euxinic basins in which the sediments were deposited. Based on analogy with modern euxinic environments and further constrained by model results, middle Proterozoic seawater sulfate concentrations were probably within the range of 0.5 to 2.4 mM. Taken together, our results indicate a relatively low-sulfate late Paleoproterozoic ocean and sulfidic deep waters in the McArthur seaway, a condition that could have encompassed much larger regions of the ocean.

Isotope fractionation by sulfate-reducing natural populations and the isotopic composition of sulfide in marine sediments

Isotope fractionations during sulfate reduction by natural bacterial populations were measured in seven different marine sediments and compared with the isotopic composition of solid-phase sulfides in the same sediments. The measured fractionations during sulfate reduction could explain only between 41% and 85% of the 34S depletion in the sedimentary sulfides. This result directly demonstrates that the depletion of 34S in solid sulfides is an expression of the combined activity of sulfate-reducing organisms with additional fractionations accumulated during the oxidative part of the sulfur cycle. The only known process to significantly augment the fractionations created during sulfate reduction is the microbial disproportionation of the intermediate sulfur compounds: elemental sulfur, thiosulfate, and sulfite. In a simple model, we show how important each of these disproportionation processes could be in generating the isotopic composition of sedimentary sulfides. The sulfate reduction rates in the sediments studied varied by a factor of 1000 and did not correlate with the fractionation during sulfate reduction. By contrast, a correlation was observed between sulfate- reduction rate and the extent of 34S depletion into sedimentary sulfides, the most 34S-depleted sulfides found in sediments supporting the lowest rates of sulfate reduction. Thus, the fractionations imposed during the disproportionating processes are better expressed in sediments with low sulfate-reduction rates. The direct phototrophic oxidation of sulfide to sulfate, with minimal fractionation, may have been important in the sediments that had a high sulfate-reduction rate.

Sulfur isotopic composition of hydrothermal precipitates from the Lau back-arc: implications for magmatic contributions to seafloor hydrothermal systems

The sulfur isotopic composition of sulfides and barite from hydrothermal deposits at the Valu Fa Ridge back-arc spreading center in the southern Lau Basin has been investigated. Sulfide samples from the White Church area at the northern Valu Fa Ridge have δ34S values averaging +3.8‰ (n= 10) for bulk sphalerite-chalcopyrite mineralization and +4.8‰ for pyrite (n= 10). Barite associated with the massive sulfides exhibits an average of +20.7‰ (n= 10). Massive sulfides from the active Vai Lili hydrothermal field at the central Valu Fa Ridge have much higher δ34S ratios averaging +8.0‰ for bulk sphalerite-chalcopyrite mineralization (n= 5), +9.3‰ for pyrite samples (n= 5), and +8.0‰ and +10.9‰ for a chalcopyrite and a sphalerite separate, respectively. The isotopic composition of barite from the Vai Lili field is similar to that of barite from the White Church area and averages +21.0‰ (n= 8). Sulfide and barite samples from the Hine Hina area at the southern Valu Fa Ridge have δ34S values that are considerably lighter than those observed for samples from the other areas and average −4.9‰ for pyrite (n= 9), −4.0 and −5.7‰ for two samples of sphalerite-chalcopyrite intergrowth, and −3.4‰ for a single chalcopyrite separate. The total spread in the isotopic composition of sulfides from Vai Lili and Hine Hina is more than 20‰ over a distance of less than 30 km. The δ34S values of sulfides at Hine Hina are the lowest values so far reported for volcanic-hosted polymetallic massive sulfides from the modern seafloor. Barite from the Hine Hina field also has unusually light sulfur with δ34S values of +16.1 to +16.7‰ (n= 5). Isotopic compositions of the sulfides at Hine Hina indicate a dramatic decrease in δ34S from ordinary magmatic values and, in the absence of biogenic sulfur and/or boiling, imply a unique 34S-depleted source of probable magmatic origin. Sulfide-barite mineralization in the Hine Hina area is associated with a distinctive alteration assemblage consisting of cristobalite, pyrophyllite, kaolinite, opal-CT, talc, pyrite, native sulfur, and alunite. Similar styles of alteration are typically known from high-sulfidation epithermal systems on land. Alunite-bearing, advanced argillic alteration in the Hine Hina field confirms the role of acidic, volatile-rich fluids, and a δ34S value of +10.4‰ for the sulfur in the alunite is consistent with established kinetic isotope effects which accompany the disproportionation of magmatic SO2 into H2S and H2SO4. The Hine Hina field occurs near the propagating tip of the Valu Fa back-arc spreading center (i.e., dominated by dike injections and seafloor eruptions) and therefore may have experienced the largest contribution of magmatic volatiles of the three fields. The sulfur isotopic ratios of the hydrothermal precipitates and the presence of a distinctive epithermal-like argillic alteration in the Hine Hina field suggest a direct contribution of magmatic vapor to the hydrothermal system and support the concept that magmatic volatiles may be an important component of some volcanogenic massive sulfide-forming hydrothermal systems.

The relationship between mineralogical characteristics and isotopic composition of sphalerite, as exemplified by the Huxu deposit, China

The relationship between mineralogical characteristics and isotopic composition of sulfides has not received its proper share of attention from geologists, although many references are available concerning the application of sulfur isotopes to geological problems.

Hydrocarbon-Induced Diagenetic Aureole (HIDA)--Mineralogical and Isotopic Models: ABSTRACT

The Permian red beds that overlie some giant oil fields in southwestern and south-central Oklahoma have undergone extensive mineralogical and chemical diagenesis. The diagenetic minerals occur within a distinctly zoned aureole that delineates the position of the oil field. The geometries of the aureoles strongly reflect the major structural elements that controlled emplacement of hydrocarbons in the underlying rocks. Calcite, ferroan calcite, manganese-rich calcite, dolomite, ankerite, pyrite, marcasite, and native sulfur are the major diagenetic minerals. The innermost zone of each aureole is characterized by abundant carbonate cementation and generally coincides with a major fault system. Pyrite and marcasite cements are commonly associated with carbonate-cemented zones these minerals occur also in the bleached sandstones. ^dgrC13 values of carbonate cements indicate 3 major sources of carbon: (1) an organic source with ^dgrC13 values of approximately -32^pmil vs. PDB, (2) a freshwater source with an average ^dgrC13 value of -8.0 ± 3^pmil, and (3) a hybrid source (freshwater and organic). A mixing model was developed to calculate the proportion of organic carbon in carbonate cement. ^dgrS34 values of pyrite and marcasite average 6.1^pmil and range from -9 to + 16^pmil. The isotopic composition of sulfides is similar to that of oil in the underlying reservoirs. Formation of diagenetic pyrite and marcasite is explained by reduction of iron oxides in red beds by hydrogen sulfide, and by other organic material associated with hydrocarbons. The HIDA concept can be used in exploration for oil and gas, specifically in structurally controlled reservoirs. Table 1. Selected Conversions End_of_Article - Last_Page 319------------

Sulfur isotope fractionation in sulfide mineralization

The isotopic compositions of sulfides and sulfates from various types of sulfide formations were determined. A description of the preparation of sulfurous minerals for mass-spectrometric analysis and of the massspectrometric procedure is given. The results agree in several respects with previous investigations. Various sulfides representing a continuous mineralization process display no significant divergencies if the early and the late minerals are compared. Nor does the material examined indicate any correlation between isotopic composition and zonal arrangement of minerals and metals around emanative centers, in cases where such zoning is evidenced by the regional geological features. A certain correlation between kind of wallrock and isotopic composition may be indicated. If considerable variations of isotopic compositions are found to exist in an ore district, the range of variation seems to be wider in the low-temperature than in the high-temperature sulfides. Supergene oxidation of sulfides to sulfate takes place without change in isotopic composition. A subsequent redeposition of supergene sulfides may involve a displacement of the isotope ratios towards lighter sulfur. Hypogene sulfide mineralization, where both sulfides and sulfates are formed, is apt to cause significant fractionation of the original isotopic composition of the participant sulfur. Local variations indicate that the process maymore » develop differently over short distances. The local character of some isotope exchange reactions is evidenced for instance by considerable variation in the isotope ratios even in a single hand specimen. The range of such variations is generally found to be wider in lowtemperature than in high-temperature mineral assemblages. (auth)« less