Strata-bound Sulfide Deposits Research Articles

Recognizing the evolution of the stratabound polymetallic massive sulfide deposits in Tongling mineralization cluster, east China through colloform pyrite

Stratabound Cu-Au sulfide deposits are widely distributed in the Middle -Lower Yangtze River Mineralization Belt, eastern China. These deposits are strictly distributed along the Carboniferous stratum, but occur in close proximity to the Cretaceous diorite rocks. The simultaneous presence of sedimentary and magmatic evidence leads the genesis of these stratabound massive sulfide deposits to be an open question. Colloform pyrite (Py-c) is widely distributed in stratabound sulfide deposits and its genesis remains controversial which contrarily limits the establishment of a metallogenic model for the stratabound sulfide deposits. In this study, the mineral composition, morphology, microstructure, and organic composition of Py-c were systematically studied using optical microscopy, powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), micro-area laser Raman spectroscopy, and gas chromatography-mass spectrometry (GC–MS). Results reveal three types of textures of Py-c: oolitic, concentric-banded, and micritic (no other characteristic texture). The concentric-banded texture shows alternating light and dark bands which are composed of fine and coarse Py-c grains, respectively. All the Py-c ores are similar in phase composition, where the main phase is pyrite, the minor phases are siderite, dolomite, and quartz, and the trace phases are clay minerals (e.g., illite) and organic matter. The size of pyrite crystals varies from 50 nm to 1.4 μm. Most of the pyrite crystals are euhedral and subhedral, where the euhedral crystals show cubic and spheroidic shapes. The surfaces of cubic crystals are smooth and stripless. Detrital and authigenic quartz are found in Py-c ores. Detrital quartz grains are highly round. Py-c mold impressions are widespread on the grain surface. Py-c around the detrital quartz did not recrystallize. Authigenic quartz grains are euhedral and subhedral, cover a size range of hundreds of nanometers to several micrometers, and aggregate to form crystal clusters. In addition, small amounts of organic matters are found co-existing with Py-c. Results of n-alkanes analysis show that organic matters in Py-c consist mainly of microorganisms and terrestrial higher plants, indicating a possibility of forming Py-c by microorganisms which used terrestrial organic matters as electron donors. These systematic analyses indicate that Py-c in Tongling stratabound sulfide deposits was formed through biogeochemical-mediated sedimentation processes in a semi-enclosed sea basin. In Yanshanian period (Jurassic-Cretaceous), Py-c was transformed to macrocrystalline pyrite (Py-m) through recrystallization or to pyrrhotite and chalcopyrite through thermal metamorphism and mineral-fluid reaction. Py-c ores, which have high porosity, high chemical activity, and organic matter, could have played key roles in precipitating Cu and Au from Yanshanian magma-derived hydrothermal fluids and constrained the formation of stratabound sulfide orebodies in the Tongling mineralization cluster.

Genesis of Strata‐bound Sulfide Orebodies in the Tongling Polymetallic Mineralization Cluster, SE China: Evidence from Colloform Pyrite

AbstractColloform pyrite (CPy) is widely distributed in the Tongling mineralization cluster of the Middle–Lower Yangtze River Mineralization Belt (MLYRMB), China. There have many debates as to whether such CPy is associated with Late Mesozoic igneous or Carboniferous sedimentation. CPy from the Xinqiao deposit, a representative of the stratabound sulfide deposits in the MLYRMB, was studied by powder X‐ray diffraction (XRD), field‐emission scanning electron microscopy (SEM), and high‐resolution transmission electron microscopy (TEM). The results show that CPy mainly comprises pyrite, pyrrhotite, quartz, and illite. Pyrite in CPy shows cubic, globule, and xenomorphic morphologies. No octahedral or pyritohedron was observed. Most of the quartz crystals display xenomorphic morphology, where pyrite mold are popular on the surface. Organic matter (OM), which is usually bound to illite, is an important component in CPy. Morphological investigations which exhibit detrital features of quartz and clay minerals indicate that they were derived from continental weathering. Specially, some hexagonal pyrrhotite nanoparticles which show mackinawite morphology are coexisted with OM. The results indicate that the transformation process of sulfides possibly is mackinawite (the precursor) —hexagonal pyrrhotite—pyrite. Thus, compositional and micro‐textural characteristics of CPy in Xinqiao deposit suggest it to be a sedimentary origin rather than a hydrothermal origin which is associated with Yanshanian magmatism. Moreover, the coexistence of CPy and stratabound sulfide orebodies in the MLYRMB suggests a causal link between the two. It is considered that CPy might have served as a Cu mineralization geochemical barrier for the Cu‐bearing ore‐forming fluids, which originated from the Mesozoic magma in the MLYRMB.

Mineralogical characteristics of colloform pyrite from Tongguanshan ore field and its implications for mineralization of the stratabound sulfide deposit in Tongling mineralization cluster, Anhui Province

Mineralogical characteristics of colloform pyrite from Tongguanshan ore field and its implications for mineralization of the stratabound sulfide deposit in Tongling mineralization cluster, Anhui Province

Fluid sources and metallogenesis in the Blackbird Co-Cu-Au-Bi-Y-REE district, Idaho, U.S.A.: insights from major-element and boron isotopic compositions of tourmaline

Tourmaline is a widespread mineral in the Mesoproterozoic Blackbird Co–Cu–Au–Bi–Y–REE district, Idaho, where it occurs in both mineralized zones and wallrocks. We report here major-element and B-isotope compositions of tourmaline from stratabound sulfide deposits and their metasedimentary wallrocks, from mineralized and barren pipes of tourmaline breccia, from late barren quartz veins, and from Mesoproterozoic granite. The tourmalines are aluminous, intermediate in the schorl–dravite series, with Fe/(Fe + Mg) values of 0.30 to 0.85, and 10 to 50% X -site vacancies. Compositional zoning is prominent only in tourmaline from breccias and quartz veins; crystal rims are enriched in Mg, Ca and Ti, and depleted in Fe and Al relative to cores. The chemical composition of tourmaline does not correlate with the presence or absence of mineralization. The δ 11 B values fall into two groups. Isotopically light tourmaline (−21.7 to −7.6‰) occurs in unmineralized samples from wallrocks, late quartz veins and Mesoproterozoic granite, whereas heavy tourmaline (−6.9 to +3.2‰) is spatially associated with mineralization (stratabound and breccia-hosted), and is also found in barren breccia. At an inferred temperature of 300°C, boron in the hydrothermal fluid associated with mineralization had δ 11 B values of −3 to +7‰. The high end of this range indicates a marine source of the boron. A likely scenario involves leaching of boron principally from marine carbonate beds or B-bearing evaporites in Mesoproterozoic strata of the region. The δ 11 B values of the isotopically light tourmaline in the sulfide deposits are attributed to recrystallization during Cretaceous metamorphism, superimposed on a light boron component derived from footwall siliciclastic sediments ( e.g. , marine clays) during Mesoproterozoic mineralization, and possibly a minor component of light boron from a magmatic–hydrothermal fluid. The metal association of Bi–Be–Y–REE in the Blackbird ores suggests some magmatic input, but involvement of granite-derived fluids cannot be conclusively established from the present database.

Gis modeling for mineral potential mapping of carbonate-hosted pb-zn deposits

Regional exploration datasets from carbonate-hosted Pb-Zn deposits in the Neoproterozoic Irece Basin (Brazil) were modeled by GIS-based (Geographic Information System) predictive, probabilistic tools that allow fuzzy logic and weights-ofevidence methods to be applied. These deposits are hosted within dolomitic shallow-water facies in the carbonate sequences of the Una-Bambui Group, and show remarkable stratigraphic control at the end of the first regressive cycle (the so-called B1 Unit, Salitre Formation). The datasets used for this study include a geological map (1:100.000 scale), airborne radiometric grids, a soil geochemical survey and two LANDSAT TM images. Twenty known stratabound Pb-Zn sulfide deposits were employed as training points. Thirteen evidential themes derived by GIS processing based on the exploration model were grouped into five factors: stratigraphic, structural, geochemical, geophysical and multispectral factor. Weights (W + , W - ) and contrast (C=W + -W - ) calculations guided the data-driven modeling, and an exploration model supported the choice of fuzzy membership functions for the knowledge-driven modeling. The most predictive factor was the geochemical factor. The geophysical maps, although not having sufficient spatial resolution to predict deposits, provide important regional screening variables.

Sm-Nd dating of the giant Sullivan Pb-Zn-Ag deposit, British Columbia

We report here Sm and Nd isotope data for hydrothermal tourmalinites and sulfide ores from the giant Sullivan Pb-Zn-Ag deposit, which occurs in the lower part of the Mesoproterozoic Purcell (Belt) Supergroup. Whole-rock samples of quartz-tourmaline tourmalinite from the footwall alteration pipe yield a Sm-Nd isochron age of 1470 ± 59 Ma, recording synsedimentary B metasomatism of clastic sediments during early evolution of the Sullivan hydrothermal system. Data for variably altered (chloritized and/or albitized) tourmalinites from the hanging wall of the deposit, which are believed to have formed originally ca. 1470 Ma, define a younger 1076 ± 77 Ma isochron because of resetting of Sm and Nd isotopes during Grenvillian metamorphism. HCl leachates of bedded Pb-Zn ore yield a Sm-Nd isochron age of 1451 ± 46 Ma, which is consistent with syngenetic-exhalative mineralization ca. 1470 Ma; this age could also reflect a slightly younger, epigenetic hydrothermal event. Results obtained for the Sullivan deposit indicate that the Sm-Nd geochronometer has the potential to directly date mineralization and alteration in stratabound sulfide deposits that are not amenable to dating by other isotope methods.

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Paraná state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irecê and Nova Redenção (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Significance of tourmaline-rich rocks in the North Range Group of the Cuyuna Iron Range, east-central Minnesota

Concentrations of tourmaline in Early Proterozoic metasedimentary rocks of the Cuyuna iron range, east-central Minnesota, provide a basis for redefinition of the evolutionary history of the area. Manganiferous iron ore forms beds within the Early Proterozoic Trommald Formation, between thick-bedded granular iron-formation having shallow-water alepositional attributes and thin-bedded, nongranular iron-formation having deeper water attributes. These manganese-rich units were previously assumed to be sedimentary in origin. However, a reevaluation of drill core and mine samples from the Cuyuna North range has identified strata-bound tourmaline and tourmalinite, which has led to a rethinking of genetic models for the geology of the North range. We interpret the tourmaline-rich rocks of the area to be a product of submarine-hydrothermal solutions flowing along and beneath the sediment-seawater interface. This model for the depositional environment of the tourmaline is supported by previously reported mineral assemblages within the Trommald Formation that comprise aegirine; barium feldspar; manganese silicates, carbonates, and oxides; and Sr-rich barite veins.In many places, tourmaline-rich metasedimentary rocks and tourmalinites are associated locally with strata-bound sulfide deposits. At those localities, the tourmaline-rich strata are thought to be lateral equivalents of exhalative sulfide zones or genetically related subsea-floor replacements. On the basis of the occurrence of the tourmaline-rich rocks and tourmalinites, and on the associated minerals, we suggest that there is a previously unrecognized potential for sediment-hosted sulfide deposits in the Cuyuna North range.

Lead isotope systematics of strata-bound sulfide deposits in the Caledonides of Norway

A lead isotope investigation has been carried out on approximately 40 strata-bound sulfide deposits in the Caledonides of Norway. The deposits occur in a wide range of palcotectonic enviromnents and lithostratigraphie units. A plot of their lead isotope data ( 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb) displays a linear trend, with volcanogenie deposits in an ocean-floor environment at the least radiogenic end and sediment-hosted deposits in an intraplate continental environment at the radiogenic end. This trend is probably a result of mixing mantle-derived lead with lead derived from an older radiogenic basement. Within the upper allochthon there is a general decrease in radiogenicity of the lead with higher teetono-stratigraphic position. This is paralleled by an increase in the proportion of volcanic rocks and is interpreted to reflect the diminished input of radiogenic lead from continentally derived sediments.The uniform character of the radiogenic end member indicates that lead in sediments, derived from a continental domain, was the immediate source and that the lead was homogenized during sedimentary transport. This is in agreement with the observed lithologies in the footwall sequences of the sampled deposits.Early obduetion of ophiolites and island ares onto Laurentia has been proposed as a model for the Scandinavian Caledonides. Although not ruled out by the lead isotope data from Norway, these data give no reason for invoking derivation of continental lead from the Laurentian side of the Iapetus ocean.A compilation of similar lead isotope data from the whole Caledonian-Appalachian orogen shows regional differences in the slopes of the mixing lines, reflecting different basement (continental) sources of lead. The lead isotope compositions of massive sulfide deposits may therefore become a useful tool in interpreting the origin of exotic terranes.

Proterozoic tectonic evolution and metallogenesis in the Aravalli-Delhi orogenic complex, northwestern India—Reply

Abstract The Proterozoic Aravalli-Delhi orogenic complex hosts a large number of economically important stratabound base metal sulphide deposits. A few W-Sn deposits associated with granites are also known in the complex. Based on tectono-lithological features, the following domains are distinguished: the Archaean basement consisting of the Banded Gneissic Complex and granites, the Early Proterozoic Bhilwara, Aravalli, Jharol, and north Delhi belts and the Middle to Late Proterozoic south Delhi belt and the Vindhyan basin. Intra-cratonic rifting of the Archaean basement commencing ∼ 2.2 Ga ago resulted in the rock association of the Bhilwara belt, a thick pile of detritus derived largely from felsic sources and, to a minor extent, from tholeiitic sills with the geochemical characteristics of ocean-floor basalts. Stratiform Zn-Pb(-Cu) sulphide deposits at Rajpura-Dariba, Rampura-Agucha and also possibly those at Pur-Banera and Jahazpur formed 1.8 ± 0.04 Ga ago by convective seawater circulation in zones of crustal extension. The metal content of exhalative brines was precipitated in troughs where biologic activity was prolific. The shelf sediments of the Aravalli belt, characterized by dolomites with stromatolitic phosphorites, were deposited on a passive continental margin at the rifted western edge of the Archaean basement complex. The monotonous pelitic pile of the Jharol belt represents deep-pelagic continental-rise sediments. The Rakhabdev lineament delineates the shelf-rise boundary. The mafic-ultramafic bodies along this lineament represent Aravalli oceanic crust which subducted westward and eventually obducted as a consequence of an initial collision of the Aravalli continental margin with an incipient arc to the west around 1.5 Ga ago. In the Aravalli belt at Zawar, strongly radiogenic stratabound Pb-Zn deposits were formed close to the basement in second-order basins with biologic activity, by hydrothermal solutions convecting through a heterogeneous source. The north Delhi belt comprises three sedimentation domains: the Khetri sub-basin, the Alwar sub-basin and the Lalsot-Bayana sub-basin. Sedimentation in this belt commenced with shelf carbonates, followed by coarse clastic sediments and volcanites. Finally, pelites and semi-pelites were deposited in a multi-lagoonal, shallow-water, locally evaporitic environment. Synkinematic granitic intrusives in the north Delhi rocks are in the age range of 1.7-1.5 Ga. The stratabound Cu sulphide deposits of the Khetri belt and the pyrite-pyrrhotite deposits at Saladipura with a Pb-Pb model age of ∼ 1.8 Ga were formed from hydrothermal seawater and partly bacteriogenic sulphur in small, shallow, rift-related basins. The south Delhi belt, with extensive mafic volcanism, localized felsic volcanism, argillaceous-arenaceous-carbonate accumulations in successive, elongated basins and conspicuous felsic plutonism, is interpreted as an island arc. The geochemistry of pillowed and massive metavolcanites and an ophiolite assemblage indicate subduction in the western side of the arc. Part of the rocks of the Delhi belt may also have formed in rift-related back-arc basins as suggested by the ocean-floor characteristics of metavolcanic rocks. Subduction on the western side of the arc ultimately led to its terminal collision with the Aravalli continental margin around 1.0 Ga, involving intense progressive strain and oblique convergence. Small Cu-(Zn) deposits were formed within sedimentary intercalations in calc-alkaline basalts close to the hypothetical subduction zone. Extensional tectonics in the back-arc basins produced stratiform Zn-Pb-Cu deposits (Ambaji-Deri) around 1.1 Ga ago by high-temperature reduction of seawater convecting through multiple sources. Sn-W mineralizations are apparently related to the Malani felsic igneous phase of 735 Ma Rb-Sr age.

Ore mineralogy of the Applachian-Caledonian stratabound sulfide deposits

The diverse ore mineralogies of more than 100 stratabound sulfide deposits that occur along the length of the Appalachian-Caledonian orogen have been tabulated. The minerals reflect the varying Cu:Zn:Pb ratios that have been used as a means to separate the ore deposits into six distinct types following the IGCP Project 60 guidelines. The ore deposits, each containing at least one million metric tons of ore, are typical of massive sulfide deposits throughout the world and provide a basis for contrasting and comparing this orogen with others. Differing grades of metamorphism have significantly modified the ore-mineral grain sizes and textures, but do not seem to have effectively influenced the number or variety of mineral species present.

The Relationship between Hydrocarbon and Stratabound Metal Sulfide Deposits: The Upper Smackover as an Analog

A genetic link between hydrocarbon and stratabound metal sulfide deposits has often been suggested. Both are thought to result from similar processes operating during the evolution of sedimentary basins, yet exploitable hydrocarbon and metal sulfide deposits are not found together. Consequently, the nature of their genetic relationship remains unclear. The Upper Jurassic Smackover Formation of the North Louisiana Salt Basin - a prolific hydrocarbon producer - contains disseminated authigenic sulfide minerals reminiscent of stratabound metal sulfide deposits. The close association of these sulfide minerals with hydrocarbon deposits provides an opportunity to examine the relation between the two. The mineralogy and chemistry of late-stage authigenic phases in the Upper Smackover are similar to ore and 'gangue' minerals of Mississippi Valley-type sulfide deposits. The sulfide minerals consist of replacement or pore-filling sphalerite, galena, pyrite, marcasite, and chalcopyrite. The mineralogy and chemistry of the sulfides and their related minerals vary spatially throughout the basin. These variations reflect local processes and the sources of the sulfide minerals' constituents. The same source rocks from which hydrocarbons are derived are likely sources of base metal ions. Likewise, reduced sulfur is related to hydrocarbons by either a common source or by thermochemical sulfate reduction. Thus, spatial variationsmore » in chemistry and mineralogy of the late-stage authigenic sulfides of the Upper Smackover may be the key to understanding the relationship between hydrocarbons and stratabound metal sulfide deposits.« less

The Kupferschiefer; an overview with an appraisal of the different types of mineralization

The Kupferschiefer; an overview with an appraisal of the different types of mineralization

Sediment geochemistry and base metal sulphide mineralisation in the Quidong area, Southeastern New South Wales, Australia

Geochemically anomalous, pyritic sediments occur directly above a Mid Silurian unconformity in the Quidong area of southeastern New South Wales. The composition of these sediments reflects derivation from a mixture of: (a) feldspar- and mica-depleted detritus reworked from underlying quartz-rich flysch; (b) Mg-rich clay or chlorite precipitated from hydrothermal exhalations; and (c) pyrite formed by reaction of iron in clays or oxides with reduced sulphur derived largely from sea-water sulphate and possibly a magmatic source. Three types of base metal sulphide mineralisation occur at Quidong including: (a) weak syngenetic concentrations in the pyritic sediments; (b) stratabound and fault-controlled bodies of massive sulphides hosted by the pyritic sediments and containing higher grade Pb, Zn and Cu; and (c) small vein and cavity fillings of galena, barite and other minor sulphides in overlying limestones. All types of mineralisation are related to hydrothermal activity which occurred during and after deposition of the pyritic facies. The geochemistry of the immediately underlying basement rocks and Pb isotope data indicate that the source of the metal-bearing fluids was deeper in the crust and probably related to widespread partial melting and magmatic processes. The sulphidic sediments and stratabound sulphide deposits represent syngenetic-epigenetic, sediment hosted mineralisation developed in a shallow marine environment, distal from major volcanic centers. This style of mineralisation has not previously been described from the region. It has some similarities to the Irish-Alpine type spectrum of deposits best known in Europe.

Distribution and controls of platinum group element and base metal mineralization in the Darwendale subchamber of the Great Dyke, Zimbabwe

Three major sulfide zones containing platinum group elements (PGE) have been identified in the upper part of the ultramafic sequence of the Great Dyke. The PGE are at the bases of the sulfide zones; the lower two have sparse sulfide mineralization over a thick interval; the upper zone contains higher grade in a thinner stratigraphic layer. None of the current models for the formation of stratabound sulfide deposits in layered intrusions adequately accounts for the mineralization. The two lower sulfide zones are interpreted as resulting from progressive fractional segregation of sulfide from the silicate magma, whereas the upper zone may have resulted from mixing of a relatively primitive magma at the base of the chamber with an evolved liquid derived from near the roof.

Genesis of paleokarst and strata-bound zinc-lead sulfide deposits in a Proterozoic dolostone, northern Baffin Island, Canada; reply

Genesis of paleokarst and strata-bound zinc-lead sulfide deposits in a Proterozoic dolostone, northern Baffin Island, Canada; reply

Fe-Zn-Cu-Pb mineralization in the Salton Sea geothermal system, Imperial Valley, California

The Salton Sea geothermal system is an area of active metamorphism in a Plio-Pleistocene deltaic sedimentary sequence. Within it, hypersaline brines containing up to 250,000 ppm total dissolved solids and temperatures up to 365 degrees C are causing incipient sulfide and oxide mineralization. We have made a detailed analysis of ore mineralization in two boreholes drilled to depths of up to 2,500 m in this system. Major ore minerals observed are, in order of decreasing abundance, pyrite, hematite, sphalerite, chalcopyrite, pyrrhotite, marcasite, and galena. The associated nonore minerals are mainly calcite, quartz, epidote, anhydrite, adularia, and chlorite. Ore mineralization can be divided into three main types. Diagenetic sulfide mineralization, occurring at depths less than 760 m and temperatures less than 250 degrees C, consists of fine-grained stratiform iron sulfides occurring as cement in sandstone and as disseminations and bands in shale. Metamorphic sulfide mineralization, occurring at depths greater than 760 m and temperatures greater than 250 degrees C, includes the progressive development of porphyroblastic pyrite with increasing depth and temperature, its replacement by other Fe-Cu-Zn-Pb sulfides at depths greater than 1,220 m, and its decomposition into skeletal aggregates at depths greater than 1,525 m, concurrent with hornfelsic recrystallization of the host sediments. Vein and related pore-filling ore mineralization is of two contrasting types: hematite-silicate-sulfide + or - sulfate and sulfide-carbonate + or - silicate. The first type occurs in open porous veins in single vertical zones at least 220 m thick in each borehole. The second type occurs in veins that are generally sealed and occur in relatively thin, scattered vertical intervals in both boreholes. Thermodynamic analysis indicates that the modern reservoir brines are near equilibrium with the hematite-silicate-sulfide + or - sulfate vein assemblage at 300 degrees C, with pH = 5.4 and log a (sub O 2 ) = -30. Reactions between hematite, pyrite, and iron-bearing silicates (chlorite, epidote) probably control the redox state of the present brines. Calculated metal-chloride complex solubilities for Fe, Zn, Pb, and Cu agree well with brine analyses. Hematite, pyrite, and chalcopyrite are supersaturated; galena and sphalerite are undersaturated in the modern brines.The phase relations of the presently sealed sulfide-carbonate + or - silicate veins require that earlier fluids, if formed at the same temperature and pH, had to be more reduced and sulfur rich than the present slightly oxidized, sulfur-poor brines. Contrasting fluid redox states may be caused by a late lateral or fault-controlled influx of more oxidized surficial waters. Lack of acid alteration implies boiling has not occurred to produce the oxidized fluids. Early formed diagenetic iron sulfides are commonly replaced by later Cu-Pb-Zn sulfides and iron oxides and enveloped and resorbed by veins, acting as nuclei and sources of S and Fe for later mineralization. Precipitation probably occurs when seismic or hydraulic fracturing allows the metal-rich, sulfur-poor brines to interact with earlier pyrite sulfur in the host rocks. Thus the geothermal system as presently explored is best classified as an incipient stratabound sulfide deposit that is being overprinted by hydrothermal and metamorphic processes.

Lead and sulfur isotopes and a model for the origin of the Ducktown Deposit, Tennessee

Strata-bound sulfide deposits at Ducktown, Tennessee, have many characteristics of volcanogenie exhalative deposits but occur in an overwhelmingly sedimentary environment. Lead isotope ratios of galenas and massive pyritic and pyrrhotitic ores fall into three distinct groups that are not related to wall-rock type, type of ore, or to orebody. A positive correlation between the lead and sulfur isotope ratios of galena indicates a relation between sulfur and lead that does not seem to be an effect of metamorphism but is rather an original feature of the ore. The groupings of lead isotope ratios and a possible bimodality of sulfur isotope compositions, as well as the above-mentioned positive correlation, suggest that Ducktown lead and sulfur represent the mixing of two sources. The dominant source of lead was probably U/Pb-enriched clastic sediments. Seawater sulfate may have dominated sulfur contributions from this source. The second source could have been a deep-seated, mafic igneous source tapped by a deep fault associated with late Precambrian rift basin development. The deep source could have provided heat to drive the hydrothermal cell in the sediments, a relatively nonradiogenic component of lead, sulfur with delta 34 S (sub Sigma ) [asymp] 0 per mil, a considerable amount of Cu and Zn, as well as the small amphibolite pods at Ducktown and along strike.Lead isotope studies of contemporaneous rift volcanics, the Mount Rogers Formation, indicate that lead in Ducktown ores could not have been derived from a similar source. Mount Rogers felsic volcanics contain lead derived from a relatively U/Pb-poor, Th/Pb-enriched source that appears to have been (granulitized?) Grenville-age basement.Lead-isotope ratios of galenas from the Gossan Lead are similar to those at Ducktown. Other Blue Ridge massive sulfide deposits have less radiogenic 206 Pb/ 204 Pb values, but none contain any discernable basement component similar to that of Mount Rogers. Reconnaissance sulfur isotope ratios from other Blue Ridge massive sulfides suggest a tendency toward heavier values than those at Ducktown.

Lateral metal zonation and stringer zone development, reflecting fissure-controlled exhalations at the Stekenjokk-Levi strata-bound sulfide deposit, central Scandinavian Caledonides

The volcanogenic strata-bound sulfide deposit at Stekenjokk occurs in a felsic-dominated, rifted-arc sequence. An already spilitized volcanic pile was altered during mineralization, involving loss of sodium and a marked enrichment in magnesium and potassium. The massive strata-bound ore is capped by black phyllite with gabbro intrusions. A thinner postore sequence of felsic and mafic volcanics lacks the mineralization alteration described above and is of the normal spilitic type, similar to the preore volcanics away from the mineralization. Subsequent deformation led to regional stratigraphic inversion and rocks attained greenschist-grade regional metamorphism.Unfolding of the fold stack reveals lateral dimensions for the Stekenjokk-Levi ore layer on the order of 0.4 X 10 km. In most places, a massive (FeS 2 + ZnS + CuFeS 2 ) ore component is present, up to ca. 8 m thick. It is generally underlain (stratigraphically) by an irregularly disseminated type of ore (FeS + CuFeS 2 ) of more variable thickness (0-25 m). Locally, the latter extends beyond the massive ore layer; in places and especially at the southern end of the orebody, it may be absent. Metal zonation occurs in both vertical and lateral directions; higher Pb and lower Cu proportions as well as higher Ag values occur stratigraphically upward and southward along the elongated ore layer.It is inferred that the disseminated ore is indicative of a stringer zone whereas the massive ore is exhalative, both ore types being produced by hydrothermal solutions. These were controlled by a fissure system along which exhalations were expelled at several locations (feeder chimneys). The fissure system also controlled the bottom topography, creating local troughs in which the brines deposited their iron and base metal sulfides. The period of sulfide deposition occurred at a late stage of the arc development, probably during an extensional regime related to the incipient intra-arc rifting. The lateral metal zonation indicates a general southward migration of the expelled brines. Even if most of the mineralization indicates proximal conditions, the southernmost, Pb- and Ag-rich parts probably represent more distal deposition.

The metamorphism of the supracrustal rocks at Garpenberg, south central Sweden

Abstract The supracrustal sequence at Garpenberg, south central Sweden, consists mainly of felsic gneisses with subordinate intercalations of amphibolites and stratabound sulfide and iron deposits. During the Svecokarelian orogeny this sequence was isoclinally folded, metamorphosed and intruded by synorogenic granitoids. The history of these supracrustals is characterized by at least two successive stages of metamorphism, followed by a stage of alteration at low temperature and pressure. A first stage (M1) of regional, low pressure, deformative metamorphism in the amphibolite facies was followed by retrogressive, low pressure, non-deformative metamorphism in the greenschist facies (M2). M1 is considered to be related to the Svecokarelian orogeny, whereas M2 represents a late or post Svecokarelian event. Later low temperature, low pressure, alteration is represented by cross-cutting fractures filled with mineral assemblages such as prehnite-epidote-calcite or chloriteprehnite. Petrologic and mineralogic evidence supports a metamorphic temperature for M1 of ca 550°C and a total pressure lower than 3.5 kbar.