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1.
The Tyndrum Pb+Zn veins, hosted by late Proterozoic quartzites, were probably generated in the Tournaisian (360 Ma). By determination of sulphur isotopic ratios of vein minerals three aspects of the Tyndrum mineralization were addressed, (i) sulphate sulphur sources; (ii) reduced sulphur source; (iii) isotopic equilibrium in the vein system including geothermometry. Twelve galenas have δ34S values ranging from +3.55 ‰ to +6.38 ‰ (this excludes one value of +11.21 ‰ from a large but nearly barren quartz vein). Other sulphides are enriched or depleted in 34S in the sense expected for isotopic equilibrium although there is no evidence for isotopic equilibrium between the vein minerals. The sulphide sulphur source was probably in the Dalradian metasediments where disseminated pyrite averages +6 ‰. Baryte had δ34S values averaging 14 ‰ and was therefore not in isotopic equilibrium with sulphides: a continental groundwater source is most likely.  相似文献   

2.
The weathered surface expression of the Lady Loretta lead-zinc-silver deposit outcrops almost continuously over a distance of about 4 km. Lead isotope ratios have been determined for samples, some of which contain > 4000 ppm Pb, from geochemically anomalous areas in order to assign drilling priorities to the anomalies.Twelve of the 43 samples analyzed contain isotopically homogeneous lead with ratios (207Pb/206Pb 0.9532–0.9549) similar to the expected targets for major ore deposits in the Mount Isa-McArthur River metallogenic province (207Pb/206Pb 0.955–0.962). Three main zones of interest can be outlined on the basis of lead isotopic data. The top priority is assigned to a 100-m-long section of the ironstone which coincides with the known outcrop of the ore. Lower priority for drilling is given to the two other zones which outcrop over smaller areas and have more radiogenic lead in their immediate surroundings. Several samples with anomalously high lead contents (> 1000 ppm) contain more radiogenic lead (207Pb/206Pb < 0.950) which was most likely derived from the weathering country rocks by supergene leaching and redeposition. Of the 22 samples containing ≥ 380 ppm Pb, lead isotopic analyses would have reduced the number deserving further attention to 10. Lead isotopic analyses in the early stages of exploration could have assisted in minimizing exploration expenditure at Lady Loretta.  相似文献   

3.
Sulphur isotopic compositions of sulphides within garnet-rich rocks and high-grade ore from the Broken Hill deposit, New South Wales, Australia, have been determined and show a range of values of –3.3 to +6.7 per mil. Thermochemical considerations, including the spread of values of 34S, suggest that the deposit was derived from a mixed source of sulphur in which seawater, reduced by inorganic processes, mixed with magmatic sulphur or that sulphate from contemporaneous seawater was reduced biogenically at low temperatures. Thermochemical considerations also suggest that pyrrhotite formed by desulphidation of pyrite so that the original Fe-S-O assemblage was pyrite ± magnetite.34S measurements show a broad range which is considered to be due to isotopic reequilibration during retrograde metamorphism and analytical and sampling technique. These data should not be used to indicate original temperatures of deposition or metamorphic temperatures associated with the various metamorphic events.  相似文献   

4.
Sulphur isotope data from coexisting sulphides and sulphates from the Taolin Pb-Zn ore deposit have been used to estimate the temperatures of sulphur mineral precipitation. The data indicate that sulphide was the dominant sulphur species in solution at high temperatures and that sulphate was dominant at low temperatures. Also the data show that the δ34S value of total sulphur in solution was close to zero at high temperatures (~325°C) but had high positive values (+15%.) at low temperatures (~250°C). We interpret this phenomenon in terms of the effects of mineral precipitation on the isotopic composition of the solution. The increase in the δ34S value of total sulphur with decreasing temperature was brought about by the removal from the system, by precipitation, of isotopically light sulphides.  相似文献   

5.
Most sulphides from various rock types and mineralization of Archean age exhibit sulphur isotope values (δ34S) near 0‰. This is due to a general absence of conditions suitable for the oxidation-reduction reactions essential for isotopic fractionation. However, some important occurrences of Archean Au mineralization do display significant variations in δ34S). One such occurrence with unusual isotopic compositions is the Hemlo gold deposit: pyrite from the ore typically has δ34S < −6‰ to a minimum value of −17.5‰ The isotopic composition of the pyrite is correlated with the content of Au, indicating that they are genetically related.The sulphur isotope data suggest that sulphur compounds experienced redox reactions in the hydrothermal system at Hemlo and that sulphate was present prior to the Au mineralization. This sulphate may be of exogenic origin from a restricted basin, or of endogenic origin from magmatic-hydrothermal fluids. Other characteristic features of the deposit, such as enrichment in Sb, Tl and Hg, may also be explained by ore deposition under moderately oxidizing conditions.Distinctive δ34S values were also observed in pyrite from Au mineralization at Heron Bay, 30 km west of Hemlo, and from baritic strata 21 to 27 km west and on approximate strike from Hemlo. These occurrences also represent sulphate-bearing hydrothermal systems and, as such, give distant information on the unusual environment of the mineralization at Hemlo. Since the isotopes of sulphur are sensitive to oxidation conditions they may be used to identify hydrothermal activities characteristic of this type of Au mineralization and to explore for other unusual areas of sulphate accumulation in Archean terrains.  相似文献   

6.
Luiswishi is a Congo-type Neoproterozoic sediment-hosted stratiform Cu–Co ore deposit of the Central Africa Copperbelt, located northwest of Lubumbashi (DRC). The ores form two main Cu–Co orebodies hosted by the Mines Subgroup, one in the lower part of the Kamoto Formation and the other at the base of the Dolomitic Shales Formation. Sulphides occur essentially as early parallel layers of chalcopyrite and carrolite, and secondarily as late stockwork sulphides cross-cutting the bedding and the early sulphide generation. Both types of stratiform and stockwork chalcopyrite and carrolite were systematically analyzed for sulphur isotopes, along the lithostratigraphic succession of the Mine Series. The quite similar δ34S values of stratiform sulphides and late stockwork sulphides suggest an in situ recrystallization or a slight remobilization of stockwork sulphides without attainment of isotopic equilibrium between different sulphide phases (chalcopyrite and carrolite). The distribution of δ34S values (−14.4‰ to +17.5‰) combined with the lithology indicates a strong stratigraphic control of the sulphur isotope signature, supporting bacterial sulphate reduction during early diagenesis of the host sediments, in a shallow marine to lacustrine environment. Petrological features combined with sulphur isotopic data of sulphides at Luiswishi and previous results on nodules of anhydrite in the Mine Series indicate a dominant seawater/lacustrine origin for sulphates, precluding a possible hydrothermal participation. The high positive δ34S values of sulphides in the lower orebody at Luiswishi, hosted in massive chloritic–dolomitic siltite (known as Grey R.A.T.), fine-grained stratified dolostone (D.Strat.) and silicified-stromatolitic dolomites alternating with chloritic–dolomitic silty beds (R.S.F.), suggest that they were probably deposited during a period of regression in a basin cut off from seawater. The variations of δ34S values (i.e. the decrease of δ34S values from the Kamoto Formation to the overlying Dolomitic Shales and then the slight increase from S.D.2d to S.D.3a and S.D.3b members) are in perfect agreement with the inferred lithological and transgressive–regressive evolution of the ore-hosting sedimentary rocks [Cailteux, J., 1994. Lithostratigraphy of the Neoproterozoic Shaba-type (Zaire) Roan Supergroup and metallogenesis of associated stratiform mineralization. In: Kampunzu A.B., Lubala, R.T. (Eds.), Neoproterozoic Belts of Zambia, Zaire and Namibia. Journal of African Earth Sciences 19, 279–301].  相似文献   

7.
Rajlich  P.  Legierski  J.  Šmejkal  V. 《Mineralium Deposita》1983,18(2):161-171

Tertiary epigenetic lead ± zinc and copper mineralizations occur in the Mesozoic carbonate cover and stibnite, barite and copper mineralization in the Precambrian to Palaezoic basement in the Eastern High Atlas, Morocco. The carbon isotope data from host carbonates range from +3 to -1‰ PDB. The data are typical for marine carbon. There is no difference between surrounding dolomites and younger vein dolomites. The oxygen isotopic composition of the samples is enriched in 16O; the range is from -3 to -11‰ PDB with respect to that of carbonates in equilibrium with marine water. It is postulated that the isotopic composition was changed during dolomitization being caused by transition of connate formation waters diluted to some degree by meteoritic water. The bacteriogenically reduced sulphur from syngenetic sulphides and Mesozoic marine sulphate mobilized during Tertiary orogenesis are thought to be the source of H2S and SO4. Cambro-Ordovicien sulphate is also supposed to be the source of sulphur in the Jebel Zelmou barite deposit in the basement. Isotopic composition of lead from galenas in the Tertiary deposits from the Mesozoic cover are strikingly homogenous. Model ages vary between 230–180 Ma. It is suggested that important mobilization processes are responsible for Tertiary metallogeny in Morocco.

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8.
The Song Hien Rift basin is considered as one of the important regions for gold deposits in North East Vietnam. Host rocks of a number gold deposits in the Song Hien Rift basin are mainly in Lower Triassic sedimentary formations. However, there is the Hat Han gold deposit hosted in fined-grained mafic magmatic rocks with similar characteristics as gold deposit hosted in the Triassic sediments. Sulphur isotopic compositions of sulphide are similar to those in carbonaceous shale, suggesting that the sulphur was ‘borrowed’ from sedimentary rocks in filling the rift basin. Gold-bearing sulphides (pyrite and arsenopyrite) are the main form of Au presence in the ore. Gold in pyrite is present as Au+ 1, and a minor amount of as nanoparticles of native Au (Au0); whereas in arsenopyrite, gold is chemically bound as the octahedral complex AuAs2. Analysis of geology, as well as geochemical and isotopic studies show that the genesis of the Hat Han gold deposit is not related to the Cao Bang mafic magmatism; instead the latter only serves as (ore) host rock. The geochemical results presented above suggest that the gabbro host rock only supplies iron needed for sulphide formation. With regard to ore genesis, the Hat Han gold deposit in the Song Hien rift basin was generated in the similar way as sediment-hosted gold deposit. There are many similar typomorphic features between the Hat Han deposit and Carlin-like deposits in the Nanpanjang sedimentary basin in China.  相似文献   

9.
Correlative fractionation relationships of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ38S versus Δ34S diagrams are deciphered theoretically. Taking into account dissolved H2S and SO42- in hydrothermal fluids during precipitation of both sulphate and sulphide minerals, a 4-species closed system is suggested for describing the conservation of mass among all sulphur-bearing species on the δ-Δ diagrams. The covariation in the δ34S values of both sulphate and sulphide is ascribed to isotopic exchange between oxidized and reduced sulphur species during mineral precipitation. The isotopic exchange could be a thermodynamic process due to simple cooling of high temperature fluids, which results in an equilibrium fractionation, or a kinetic process due to mixing of two sulphur reservoirs, which leads to a disequilibrium fractionation. The δ34S value of total sulphur in a hydrothermal system could change due to the precipitation of minerals, or due to the escape of H2S and/or SO42-. Sulphur isotope data for anhydrite and pyrite pairs from the Luohe porphyrite iron deposit in the Yangtze River Valley is used to illustrate the mixing responsible for the disequilibrium fractionation.  相似文献   

10.
The isotope ratios 33S/32S and 34S/32S have been measured in sulphur fractions extracted from samples of the meteorites Allende and Eagle Station by leaching at successively greater acid concentrations and higher temperatures. On a three isotope plot of δ33Svsδ34S most of the data lie on or close to the mass fractionation line. The last fraction of sulphur extracted from a bulk Allende sample lies off the line and has an approximately 1%. excess in the 33/32S ratio.Previous searches for anomalous abundance patterns of 32S, 33S, 34S and 36S have been reported by HULSTON and THODE (1965a,b), THODE and REES (1971), and REES and THODE (1972). No isotope abundance variations were found, in the meteorite and lunar samples studied, which could not be explained on the basis of either mass dependent isotope fractionation or, in the special case of iron meteorites, cosmic ray production of 33S and 36S. We report here preliminary results of a renewed search for isotopically anomalous sulphur in which we are concentrating on the Allende and Eagle Station meteorites, both of which contain anomalous oxygen (CLAYTON etal., 1973, 1976). In a first attempt to distinguish between normal sulphur and any possible anomalous sulphur, we have leached both bulk samples and hand separated components of these meteorites with hydrochloric acid.CLAYTON and RAMADURAI (1977) suggested that the presence of isotopically anomalous sulphur would be evidence for the existence of presolar grains which are relics of nucleosynthesis in certain zones of supernova expansion. In particular they suggested that sulphides of titanium are good candidates for isotopic analysis. These are not expected to exist in conventional solar equilibrium condensation sequences, but might be abundant in condensates from silicon burning shells of supernovae. Our chemical procedures were already completed when CLAYTON and RAMADURAI'S suggestions came to our attention and it must be stressed that so far, in all cases but one we have examined only sulphur from sulphides which are decomposed by HC1. Thus we may not have sampled sulphides of the type suggested by CLAYTON and RAMADURAI.All samples of the Allende meteorite were ground finer than 50μm before acid extraction of sulphur. Samples of sulphur were extracted from the various phases of the meteorites by using successively stronger hydrochloric acid leaches, longer times and higher temperatures of reaction. Sulphur initially released as H2S was successively converted to CdS, Ag2S and SF6, this latter compound being analysed mass spectrometrically (THODE and REES, 1971). Analyses of nine SF6 samples prepared from Ag2S originally derived from Canyon Diablo troilite were also performed in order to monitor fluorination and mass spectrometry precision and to establish the zero points ofthe isotope variation scales. The results are shown in Table 1. The sulphur contents of the various samples were determined gravimetrically as Ag2S. The bulk and matrix samples are probably a few percent low because of mechanical losses. The percentages of sulphur in each fraction of a sample extracted during each leaching stage are given in the table. The total sulphur content in the bulk and matrix samples of the Allende meteorite i.e., the sum of the sulphur contents of the individual fractions, varies from 1.8 to 2.08%, the highest percentage being in the matrix. These values compare with about 2 to 2.1% obtained by CLARKE etal. (1970).  相似文献   

11.
The Konkola deposit is a high grade stratiform Cu–Co ore deposit in the Central African Copperbelt in Zambia. Economic mineralisation is confined to the Ore Shale formation, part of the Neoproterozoic metasedimentary rocks of the Katanga Supergroup. Petrographic study reveals that the copper–cobalt ore minerals are disseminated within the host rock, sometimes concentrated along bedding planes, often associated with dolomitic bands or clustered in cemented lenses and in layer-parallel and irregular veins. The hypogene sulphide mineralogy consists predominantly of chalcopyrite, bornite and chalcocite. Based upon relationships with metamorphic biotite, vein sulphides and most of the sulphides in cemented lenses were precipitated during or after biotite zone greenschist facies metamorphism. New δ34S values of sulphides from the Konkola deposit are presented. The sulphur isotope values range from −8.7‰ to +1.4‰ V-CDT for chalcopyrite from all mineralising phases and from −4.4‰ to +2.0‰ V-CDT for secondary chalcocite. Similarities in δ34S for sulphides from different vein generations, earlier sulphides and secondary chalcocite can be explained by (re)mobilisation of S from earlier formed sulphide phases, an interpretation strongly supported by the petrographic evidence. Deep supergene enrichment and leaching occurs up to a km in depth, predominantly in the form of secondary chalcocite, goethite and malachite and is often associated with zones of high permeability. Detailed distribution maps of total copper and total cobalt contents of the Ore Shale formation show a close relationship between structural features and higher copper and lower cobalt contents, relative to other areas of the mine. Structural features include the Kirilabombwe anticline and fault zones along the axial plane and two fault zones in the southern limb of the anticline. Cobalt and copper behave differently in relation to these structural features. These structures are interpreted to have played a significant role in (re)mobilisation and concentration of the metals, in agreement with observations made elsewhere in the Zambian Copperbelt.  相似文献   

12.
《Sedimentary Geology》1999,123(3-4):255-273
This study investigates the sulphur source of gypsum sulphate and dissolved groundwater sulphate in the Central Namib Desert, home to one of Africa's most extensive gypsum (CaSO4·2H2O) accumulations. It investigates previously suggested sulphate precursors such as bedrock sulphides and decompositional marine biogenic H2S and studies the importance of other potential sources in order to determine the origin of gypsum and dissolved sulphate in the region. An attempt has been made to sample all possible sulphur sources, pathways and types of gypsum accumulations in the Central Namib Desert. We have subjected those samples to sulphur isotopic analyses and have compiled existing results. In addition, ionic ratios of Cl/SO4 are used to determine the presence of non-sea-salt (NSS) sulphur in groundwater and to investigate processes affecting groundwater sulphate. In contrast to previous work, this study proposes that the sulphur cycle, and the formation of gypsum, in the Namib Desert appears to be dominated by the deposition of atmospheric sulphates of phytoplanktonic origin, part of the primary marine production of the Benguela upwelling cells. The aerosol sulphates are subjected to terrestrial storage within the gypsum deposits on the hyper-arid gravel plain and are traceable in groundwater including coastal sabkhas. The hypothesis of decompositional marine biogenic H2S or bedrock sulphide sources, as considered previously for the Namib Desert, cannot account for the widespread accumulation of gypsum in the region. The study area in the Central Namib Desert, between the Kuiseb and Omaruru rivers, features extensive gypsum accumulations in a ca. 50–70 km wide band, parallel to the shore. They consist of surficial or shallow pedogenic gypsum crusts in the desert pavement, hydromorphic playa or sabkha gypsum, as thin isolated pockets on bedrock ridges and as discrete masses of gypsum selenite along some faults. The sulphur isotopic values (δ34S ‰CDT) of these occurrences are between δ34S +13.0 and +18.8‰, with lower values in proximity to sulphuric ore bodies (δ34S +3.1 and +3.4‰). Damaran bedrock sulphides have a wide range from δ34S −4.1 to +13.8‰ but seem to be significant sources on a local scale at the most. Dissolved sulphate at playas, sabkhas, springs, boreholes and ephemeral rivers have an overall range between δ34S +9.8 and +20.8‰. However, they do not show a systematic geographical trend. The Kalahari waters have lower values, between δ34S +5.9 and +12.3‰. Authigenic gypsum from submarine sediments in the upwelling zone of the Benguela Current between Oranjemund and Walvis Bay ranges between δ34S −34.6 to −4.6‰. A single dry atmospheric deposition sample produced a value of δ34S +15.9‰. These sulphur isotopic results, complemented by meteorological, hydrological and geological information, suggest that sulphate in the Namib Desert is mainly derived from NSS sulphur, in particular oxidation products of marine dimethyl sulphide CH3SCH3 (DMS). The hyper-arid conditions prevailing along the Namibian coast since Miocene times favour the overall preservation of the sulphate minerals. However, sporadic and relatively wetter periods have promoted gypsum formation: the segregation of sulphates from the more soluble halite, and the gradual seaward redistribution of sulphate. This study suggests that the extreme productivity of the Benguela Current contributes towards the sulphur budget in the adjacent Namib Desert.  相似文献   

13.
Fourteen stratiform, stratabound and vein-type sulphide occurrences in the Upper Allochthon of the Central–North Norwegian Caledonides have been studied for their sulphur, oxygen and hydrogen isotope composition. Depositional ages of host rocks to the stratabound and stratiform sulphide occurrences range from 590 to 640?Ma. The sulphides and their host rocks have been affected by polyphase deformation and metamorphism with a peak temperature of 650?°C dated to 432?Ma. A total of 104 sulphide and 2 barite samples were analysed for δ34S, 16 whole-rock and quartz samples for δ18O and 12 samples of muscovite for δD. The overall δ34S values range from ?14 to +31‰ with the majority of sampled sulphides lying within a range of +4 to +15‰. In most cases δ34S within each hand specimen behaves in accordance with the equilibrium fractionation sequence, δ34Sgn<δ34Scp<δ34Ssph<δ34Spy. A systematic increase in δ34S from the vein sulphides (?8‰) through schist/amphibolite-hosted (+6‰) and schist-hosted (+7 to +12‰) to dolomite-hosted (+12 to +31‰) occurrences is documented. The δ34S averages of the stratiform schist-hosted sulphides are 17 to 22‰ lower than in the penecontemporaneous seawater sulphate. The Bjørkåsen (+4 to +6‰) occurrence is a volcanogenic massive sulphide (VMS) transitional to sedimentary massive sulphide (SMS), exhalative, massive, pyritic deposit of Cu–Zn–Pb sulphides formed by fluids which obtained H2S via high-temperature reduction of seawater sulphate by oxidation of Fe2+ during the convective circulation of seawater through underlying rock sequences. The Raudvatn, volcanic-hosted, disseminated Cu sulphides (+6 to +8‰) obtained sulphur via a similar process. The Balsnes, stratiform, ‘black schist’-hosted, pyrite–pyrrhotite occurrence (?6 to ?14‰) is represented by typical diagenetic sulphides precipitated via bacteriogenic reduction of coeval (ca. 600?Ma) seawater sulphate (+25 to +35‰) in a system open to sulphate supply. The δ34S values of the Djupvik–Skårnesdalen (+7 to +12‰), Hammerfjell (+5 to 11‰), Kaldådalen (+10 to +12‰) and Njallavarre (+7 to +8‰) stratiform, schist-hosted, massive and disseminated Zn–Pb (±Cu) sulphide occurrences, as well as the stratabound, quartzite-hosted, Au-bearing arsenopyrite occurrence at Langvatnet (+7 to +11‰), suggest that thermochemically reduced connate seawater sulphate was a principal sulphur source. The Sinklien and Tårstad, stratabound, dolomite- and dolomite collapse breccia-hosted, Zn (±Cu–Pb) sulphides are marked by the highest enrichment in 34S (+20 to +31‰). The occurrences ?are?assigned to the Mississippi-Valley-type deposits.?High δ34S values require reduction/replacement of contemporaneous (ca. 590?Ma) evaporitic sulphate (+23 to +34‰) with Corg-rich fluids in a closed system. The Melkedalen (+12 to +15‰), stratabound, fault-controlled, Cu–Zn sulphide deposit is hosted by the ca. 595?Ma dolomitised Melkedalen marble. The deposit is composed of several generations of ore minerals which formed by replacement of host dolomite. Polyphase hydrothermal fluids were introduced during several reactivation episodes of the fault zone. The positive δ34S values with a very limited fractionation (<3‰) are indicative of the sulphide-sulphur generated through abiological, thermochemical reduction of seawater sulphate by organic material. The vein-type Cu (±Au–W) occurrences at Baugefjell, Bugtedalen and Baugevatn (?8 to ?4‰) are of hydrothermal origin and obtained their sulphur from igneous sources with a possible incorporation of sedimentary/diagenetic sulphides. In a broad sense, all the stratiform/stratabound, sediment-hosted, sulphide occurrences studied formed by epigenetic fluids within two probable scenarios which may be applicable separately or interactively: (1) expulsion of hot metal-bearing connate waters from deeper parts of sedimentary basins prior to nappe translation (late diagenetic/catagenetic/epigenetic fluids) or (2) tectonically driven expulsion in the course of nappe translation (early metamorphic fluids). A combination of (1) and (2) is favoured for the stratabound, fault-controlled, Melkedalen and Langvatnet occurrences, whereas the rest are considered to have formed within option (1). The sulphides and their host rocks were transported from unknown distances and thrust on to the Fennoscandian Shield during the course of the Caledonian orogeny. The displaced/allochthonous nature of the Ofoten Cu–Pb–Zn ‘metallogenetic province’ would explain the enigmatically high concentration of small-scale Cu–Pb–Zn deposits that occur only in this particular area of the Norwegian Caledonides.  相似文献   

14.
An integrated mineralogical-geochemical and stable isotopic study of Pb-Zn deposits located at Kayar-Ghugra (Zn-Pb ± Ag), Rampura-Agucha (Zn-Pb, Ag), Dariba-Bethumni (Zn-Pb) and Zawar (Pb-Zn ± Cd, Ag) in Rajasthan is presented in this paper. The Kayar Zn-Pb deposit hosted by (i) phlogopite-tremolite bearing dolomitic carbonates and (ii) scapolite bearing calc-silicates, both belonging to Mesoproterozoic Delhi Supergroup exhibit distinctly different δ13C signatures being close to zero permil for the former reflecting deposition in pristine marine environment and much depleted isotopic values for the latter possibly related to post-depositional alterations. The Zn-Pb sulphides of Agucha, hosted in amphibolite facies to lower granulite facies metasedimentary units belonging to the Bhilwara Supergroup have δ34S values that indicate (i) H2S dominated regime characterized by low fO2, low pH, wherein the δ34S(fluid) responsible for mineralisation approximates the δ34S(sulphide); (ii) the role of seawater in the generation of Agucha ores; (iii) the process of a low temperature oxidation of sulphides in the hydrothermal fluids resulting in the formation of sulphate, by the interaction of ground water; (iv) isotopic disequilibrium in sulphatesulphide pairs that explain oxidation of H2S by acid groundwater (low pH) and deposition of sulphides at higher temperatures and (v) equilibrium isotopic fractionation of the coexisting sulphides reflecting in a higher concentration of H2S (>10?5m) in relation to the total metal content in the hydrothermal fluid $\left( {m_{H_2 S} \geqslant mS_{_{metals} } } \right)$ . Accordingly the concentration of sulphide-sulphate in the hydrothermal solution responsible for the mineralization in Agucha exceeds that of total metals. The sulphides of Bethumni-Rajpura-Dariba belt hosted in low to medium grade siliceous carbonates has a marginally positive (mean of +1.5‰) δ13C values. At Sindeswar, broad and widely scattered δ34S values indicate a polymodal sedimentary source of sulphur that recrystallised at rather low temperature of < 50°C possibly during the processes of low temperature bacterial reduction. The C and O-isotopic studies on mineralized and non-mineralized carbonates reveal (i) normal marine depositional signatures for non-mineralized carbonates with possible minor influence of biogenic carbon during deposition and (ii) ore zone carbonates exhibit depleted δ13C values presumably due either to the deeper mantle-like source of carbonates or due to post-depositional equilibration with isotopically light meteoric waters. In Zawar belt, sulphides hosted in dolomitic carbonate indicated (i) near identical δ34S values of disseminated galena and pyrite veinlets and depleted values of ?4.6 ‰ for late veins of massive galena of Zawar Mala (ii) pyritepyrrhotite veinlet having enhanced δ34S values when compared to the PbS-ZnS veinlet in Morchia-Magra, Balaria and Baroi mines. The carbon isotopic values for carbonates of Zawar Mala mine area are mostly depleted and those from Balaria and Baroi mines exhibit values of 13C close to zero. The generally depleted δ 18O clustering around ?15 ‰ tally well with the reported Paleoproterozoic carbonates and is attributed to the post-depositional equilibration reactions with isotopically light meteoric waters. It is summarized that the host carbonates for Zn-Pb deposits occurring in different tectono-stratigraphic units in Rajasthan have largely similar but bimodal distribution of δ 18O and δ13C isotopic ratios that suggest normal marine values and much depleted values. Whereas the former seems to be in general agreement with the nature of distribution in the Palaeoproterozoic carbonates the latter is attributed to (i) depositional conditions of the basins that includes absence or presence of biogenic activity (ii) isotopic re-equilibration under different metamorphic recrystallization events and/or (iii) interaction with isotopically lighter meteoric waters. In contrast to the uniformity in the C and O distribution pattern, the S-isotopic distribution in the deposits of Rampura-Agucha, Bethumni-Rajpura-Darbia and Zawar mine areas show marked variations reflecting complex deposit-specific ore-forming processes in the said deposits.  相似文献   

15.
The Navia gold belt is located in the West Asturian-Leonese Zone of the Iberian Variscan Orogen. The host rocks of the mineralization are quartzites, sandstones and black shales of Cambro-Ordovician age. The gold belt extends along 35 km and has five major veins: Penedela, Encarnita, Fornaza, Carmina and S. Jose. The ores belong to at least four associations having contrasting mineralogies and textures. The δ34S values for individual mineral phases reflect the polyphase metallogenic history. The older association (Stage 1) is Fe-Mn-rich and is made up of spessartine, grunerite-dannemorite and quartz, with magnetite, pyrrhotite and chalcopyrite as metallic phases. The mineralization of Stage 1 is followed by the As-rich Stage 2 with quartz, arsenopyrite and pyrite. The δ34S values for pyrite range from 14.9 to 19.9 per mil (n = 16), and for arsenopyrite from 13.2 to 17.3 per mil (n = 7). The observed isotopic homogeneity likely implies isotopic equilibrium at the scale of the gold vein. Stage 3 contains a coarse-grained base metal sulphide-rich association. The δ4S values for sphalerite range from 16.4 to 20.6 per mil (n= 16), and for galena from 17.0 to 18.7 per mil (n = 11). δ34Ssp > δ34Sgl suggests that the sulphur isotopic fractionation of the ore-forming system had reached equilibrium. The youngest crosscutting mineral association (Stage 4) consists of Pb-Sb sulphosalts, bornite, electrum and quartz. The δ34S values for sulphosalts range from 9.7 to 15.8 per mil, showing the lightest results of the Navia sulphides.The relatively tight clustering of δ34S values of the Au-related sulphides, and the results of fluid inclusions and paragenetic studies, can be interpreted to indicate that the hydrothermal fluids of the last three stages were dominated by H2S. In the H2S predominant field, sulphide minerals precipitating from solutions would exhibit δ34S values similar to the δ34SΣS value of the ore fluid. The heavy δ34SΣS of the Navia fluids is consistent with leaching of sulphur from the host rocks. The main sulphur source could be diagenetic pyrite from the siliciclastic rocks of the Cabos and Luarca Formations, which exhibit δ34S values from 8.3 to 21.2 per mil. An additional sulphur-source in Stage 3 would be the leaching of disseminated sphalerite and galena present in Cambrian carbonates.  相似文献   

16.
Electron microprobe analyses of gold and associated ore minerals as well as stable isotope analyses of sulphide and carbonate minerals were performed in order to determine the metal and fluid sources and temperature of the mineralizing systems to better understand the genesis of the Atud gold deposit hosted in the metagabbro–diorite complex of Gabal Atud (Central Eastern Desert, Egypt). The gold can be classified as electrum (63.6–74.3 wt.% Au and 24.6–26.6 wt.% Ag) and is associated with arsenopyrite and As-bearing pyrite in the main mineralization (gold-sulphides) phase within the main mineralized quartz veins and altered host rocks. Based on the arsenopyrite geothermometer, As-contents (29.3–32.7 atom%) in arsenopyrite point to deposition in the Log ?S2 and T ranges of ?10.5 to ?5.5 and 305–450°C, respectively, during the main mineralizing phase. Based on the δ34S isotopic compositions of the sulphides, they are originated from magmatic fluids in which the sulphur is either sourced directly from magma or remobilized from the magmatic rocks (gabbroic rocks). On the other hand, calcite formed from fluids having mainly magmatic mixed with variable metamorphic signatures based on its δ13C and δ18O values. This work concluded that the gold-bearing ores at Atud deposit have magmatic sources leaching from the country intrusive rocks during water/rock interactions then remobilized during a metamorphic event. Therefore, the Atud gold deposit is classified as an intrusion-related gold deposit, in which the gabbro–diorite host intrusion acted as the source of metals which were mobilized and deposited as a result of the effects of NW–SE shearing.  相似文献   

17.
Stratiform sediment hosted Zn–Pb–Ag deposits, often referred to as SEDEX deposits, represent an economically important class of ore, that have received relatively little attention in terms of defining lithochemical halos and geochemical vectors useful to exploration. This study concentrates on the Lady Loretta deposit which is a typical example of the class of Proterozoic SEDEX deposits in northern Australia. We examined the major and trace element chemistry of carbonate-bearing sediments surrounding the deposit and defined a series of halos which extend for several hundred metres across strike and up to 1.5 km along strike. The stratiform ore lens is surrounded by an inner sideritic halo [Carr, G.R., 1984. Primary geochemical and mineralogical dispersion in the vicinity of the Lady Loretta Zn–Pb–Ag deposit, North Queensland. J. Geochem. Expl. 22, 217–238], followed by an outer ankerite/ferroan dolomite halo which merges with low iron dolomitic sediments representative of the regional background compositions. Carbonate within the inner siderite halo varies in composition from siderite to pistomesite (Fe0.6Mg0.4CO3), whereas carbonate in the outer ankerite halo varies from ferroan dolomite to ankerite (Ca0.5Mg0.3Fe0.2CO3). Element dispersion around the stratiform ore lens is variable with Pb, Cu, Ba and Sr showing very little dispersion (<50 m across strike), Zn and Fe showing moderate dispersion (<100 m) and Mn and Tl showing broad dispersion (<200 m). Within the siderite halo Cu, Mg and Na show marked depletion compared to the surrounding sediments. The magnitude of element dispersion and change in carbonate chemistry around the Lady Loretta orebody has enabled the development of three geochemical vectors applicable to exploration. Whole rock analyses are used to calculate the three vector quantities as follows: (1) SEDEX metal index = Zn + 100Pb + 100Tl; (2) SEDEX alteration index = (FeO + 10MnO)100/(FeO + 10MnO + MgO); (3) manganese content of dolomite: MnOd = (MnO × 30.41)/CaO. All three vectors increase to ore both across strike and along strike. The manganese content of dolomite (MnOd) exhibits the most systematic pattern increasing from background values of about 0.2 wt% to a maximum of around 0.6 wt% at the boundary between the ankerite and siderite halos. Siderite within the inner halo contains considerably more Mn with MnO values of 0.4 to 4.0 wt%. It is suggested here that the basket of indices defined at Lady Loretta (Zn, Tl, metal index, alteration index, MnOd and MnOs) is applicable in the exploration for stratiform Zn–Pb–Ag deposits in dolomite-rich sedimentary basins generally. The indices defined can firstly assist in the identification of sedimentary units favourable for SEDEX mineralisation, and secondly provide vectors along these units to ore. The alteration index and MnOd, however, should only be used for exploration dolomitic sequences; they are not recommended for exploration in clastic sequences devoid of carbonates.  相似文献   

18.
A strong link between high Sr/Y arc magmas and porphyry Cu–Mo–Au deposits has been recognized in recent years. The Tongshan and Duobaoshan deposits are representative large Cu–Mo–Au deposits in NE China. We report LA–ICP–MS zircon U–Pb crystallization age of 471.5 ± 1.3 Ma for Tongshan ore-related granitoid. Re–Os isotopic analyses of the two chalcopyrite samples from Tongshan deposit show a model age range of 470.2–477.1 Ma. The Duobaoshan and Tongshan ore-related granitoids show higher Sr/Y and La/Yb ratios. The δ34S values of sulphides from the Duobaoshan and Tongshan deposits vary from −2.3‰ to 0.0‰, belonging to a magmatic-hydrothermal system. The Pb isotopic ratios of the sulphides from the Duobaoshan and Tongshan deposit range from 17.201 to 18.453 for 206Pb/204Pb, 15.445 to 15.551 for 207Pb/204Pb, and 36.974 to 37.999 for 208Pb/204Pb, indicating the addition of lower crustal material. The Duobaoshan and Tongshan granitoids were formed in a subduction-related continental arc setting, produced by partial melting of juvenile hydrous basalts underplating the deep continental crust during the Ordovician.  相似文献   

19.
The present investigation deals with sulphur isotope distribution in Lower Proterozoic iron and sulphide mineralizations in northern Sweden. The contrasting sulphur isotope patterns are indicative of different genesis. Some 267 sulphur isotope analyses of pyrite, pyrrhotite, chalcopyrite, sphalerite, galena and bornite from 23 occurrences have been performed. Some deposits exhibit uniform compositions, although the mean 34S values are clearly different, while other mineralizations have widely fluctuating values.The 34S values in syngenetic, exhalative sedimentary skarn iron ores, quartz-banded iron ores and sulphide mineralizations of the 2.0–2.5 Ga old (Lapponian) Greenstone group show a large spread, supporting the existence of bacteriogenic sulphate reduction processes. The spread of the sulphur isotope values ( 34S = -8 to +25), and the non-equilibrium conditions, point to a biogenic rather than to an inorganic reduction of seawater sulphate.The isotopic composition of the sulphides in the epigenetic Lannavaara iron ores which were formed by a hydrothermal scapolite-tourmalme-related process, indicates a sulphur source similar to that of the Greenstone group. The 34S values of Cu-(Au) sulphide mineralizations in the Malmberget region (e.g. Aitik), which were formed by a similar process and hosted by the volcanics-volcanoclastics of the 1.9 Ga old Porphyry group, are slightly below zero , indicating a magmatic origin. The existence of different sulphur compositions for these mineralization types formed by a similar hydrothermal process, probably reflects the influence of the host rock, the solutions leaching pre-existing sulphides.In southern Norrbotten, epigenetic, Cu-Zn-Pb veintype mineralizations in metavolcanics and metasediments have 34S values close to zero indicating a magmatic origin. The sulphur isotope data of the volcanogenic, massive sulphide ores of the Skellefte district, in particular the ores of the Adak dome, are close to zero .The lead and sulphur isotopic features of the sulphides in northern Sweden show that the ore-forming processes were of a different nature on both sides of the Archean-Proterozoic border, implying differences in the crustal development. Lead isotopes show that lead was mobilized from specific sources on each side of the border. The sulphur of the sulphides in the Greenstone group in NE Sweden and Finland was introduced by sedimentary processes, whereas the sulphur of the sulphide occurrences towards the SW, mainly in the Porphyry group, is dominated by a magmatic sulphur component.  相似文献   

20.
Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (206Pb/204Pb), 15.622 (207Pb/204Pb) and 38.191 (208Pb/204Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (206Pb/204Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.  相似文献   

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