Sediment-hosted Deposits Research Articles

Epigenetic gold–platinum-group element deposits: examples from Brazil and Australia

Recent research into anomalous gold–platinum-group element (PGE) occurrences (± uranium) in Brazil and Australia has identified an unusual class of epigenetic Au–PGE deposit, previously poorly understood and little documented in the scientific literature. These epigenetic, sediment-hosted deposits display a strong structural control and are hosted entirely or partly within reducing (graphitic or carbonaceous) rocks. Metal transport in these deposits is interpreted to have been in low-temperature (<300°C), saline and acidic fluids as chloride complexes, as deduced from the common metasomatic alteration haloes rich in hematite and sericite. Few, if any, sulphides (pyrite ± chalco-pyrite) are found disseminated within the orebodies and their enveloping host rocks. Within the ore there is a distinct Au>Pd>Pt abundance association, other PGE displaying only minor enrichment (<0.5 ppm). The precious-metal mineralization tends to occur as Au-Pd or Au phases, which contain variable base-metal concentrations, and as more distinct phases of Pd–Pt or Pt. Some ore minerals contain anomalous Se and occur, in places, as complex precious-metal selenide phases. Additionally, Co, Ni, U and LREE, among other trace elements, commonly reach anomalously high concentrations (>20 ppm). These metal associations resemble those of the Fe–oxide Cu–Au deposit group.

Biogeochemical Prospecting of Sulphide Minerals in Winder Valley, Balochistan, Pakistan

Abstract: The Mor Range is mountainous terrain in southern Balochistan dominantly occupied by exposures of rocks of the Ferozabad Group which is comprised of Lower‐Middle Jurassic carbonates and siliciclastics. This group contains syngenetic and epigenetic Zn–Pb–Ba mineralization classified as Stratiform Sediment‐Hosted (SSH) and Mississippi Valley Type (MVT) deposits. Three important flora of the Saharo–Sindian, phytogeographical region of Pakistan, namely Tamarix aphyl–la, Salvadora oleoides and Acacia arabica have been studied as a mineralization prospecting tool. Samples of these species were collected from Winder Valley adjoining the Mor Range to prospect for new areas of mineralization, besides the known sulphide deposits. This study revealed the Draber, Thorar, Raj, Mithi and Kanrach localities as new prospect. Quantitative estimation of biophile elements (Zn–Cu–Pb) from the three species was made. A number of new Zn, Cu and Pb anomalies were distinguished in the area. The variation in the constituents of these species from different localities possibly varies with nature and distance from bedrock, mobility, climate, average abundance in the plant ash and exclusion mechanism of the flora. Acacia arabica with deep penetrating roots shows the best results and could be used to delineate new horizons of mineralization. In these species Zn and Cu are relatively concentrated whereas Pb shows limited enrichment. Lead therefore is the element best suited for pinpointing the mineralization owing to its relatively less mobile character.Relationships among Zn–Cu–Pb have been established using scattergrams and triangular variation diagram, which also demonstrate their genetic affiliation. Statistical analyses, such as determination of mean, mode, median, standard deviation, and coefficient of correlation, were also made to improve the raw geochemical data. These biogeochemical methods appear to be suitable in arid climate of Balochistan, if proper attention is paid to species selection.

New production in the Idaho Cobalt Belt: a unique metallogenic province

The Idaho Cobalt Belt (ICB) is a unique example of sea-floor hydrothermal brines that vented along an ancient rift within a deep-water setting, providing for an interesting and distinctive geochemistry. The ICB is a northwest-trending zone of Co-Cu-Au occurrences, at least 64 km long and up to 10 km wide, centred on the Blackbird mine. It is an exceptional metallogenic province in which cobalt occurs in sufficiently high concentrations to make it the primary metal in the deposits. The deposits are mostly strata-bound and appear to have formed during a mafic volcanogenic-exhalative mineralizing event on a palaeo-sea-floor ca 1600 m.y. ago. Regional metamorphic events have remobilized Cu and, to a lesser extent, Co in varying degrees throughout the belt. The deposits are unusually rich in Co, Cu, Fe, As, Au, B, Bi and light rare earths, but low in Ni (relative to Co), Ag, Pb, and Zn. The synsedimentary controls, mineralogy and geochemistry are unusual among ore deposits, though similar features in some other deposits are known. Sediment-hosted deposits are major ore producers in the world, but most tend to be Pb- and Zn-rich. Some sediment-hosted deposits that contain Cu also contain Co—notably, the deposits of the Zambian Copperbelt. Significant differences exist between deposits in the Zambian Copperbelt and the ICB, whereas many intriguing similarities are found between ICB deposits and Sullivan, Canada, and Mount Isa and Broken Hill, Australia.The ICB is currently known to host 18 Co-Cu deposits within eight sequences of metamorphosed, mafic volcaniclastic rocks. Additional exploration is expected to discover more deposits as only a small portion of the belt has undergone systematic exploration. Formation Capital's Idaho cobalt project encircles the inactive Blackbird mine and encompasses the Ram and Sunshine deposits as well as more than 30 prospects and six deposits. The project's total resource base exceeds 5 000 000 t in four deposits, all of which remain open in at least two directions. To date, only two of the deposits have been included in pre-feasibility studies—Ram and Sunshine. The project is in the final feasibility, permitting and pre-development phase.

Rare-Earth-Element and Isotopic Evidence for the Genesis of the Bajiazi Stratabound Sulfide Ore Bodies of Northeast China

This study examines the rare-earth-elemenl and isotopic geochemistry of sulfide ores and associated rocks in the Bajiazi sulfide deposits of northeastern China. The distribution and concentration of rare earth elements (REE) in sulfide ores and associated rocks from the Bajiazi deposits have been determined by inductively coupled plasma spectroscopy (ICP) methods. Birdwing-shaped rare-earth-element profiles are only observed in granitic rocks at Bajiazi. Diverse rare-earth-element profiles of the ores and dolostones are different from those of granite and are interpreted to reflect an early sedimentary-diagenetic mineralization stage during which rare earth elements were added to the ores and host rocks. The characteristic features of the abundances of REE in the Bajiazi samples, including granite and sedimentary host rocks, are demonstrated by a variety of Eu anomalies in the chondrite-normalized REE pattern. Eu is the most mobile element, exhibiting increasingly negative anomalies in granite rock samples relative to the depletion of the sulfide ores and associated sedimentary host rocks, which are considered to be caused by preferential scavenging of Eu from sea water to sediments. The δ34S values of sulfide minerals at Bajiazi generally range from −12.3 to 14.2%, suggesting reduction both of marine sulfate and biogenic sources. The δ34S values from different ore types, sulfide minerals, and mines indicate a sedimentary exhalative origin, although they were, to some extent, homogenized during late overprinting. The late-stage sulfides from Bajiazi are isotopieally remarkably homogeneous and are significantly, although slightly, enriched in δ34S. These results imply multiple sources of sulfur derived from biogenic reduction, and/or sulfate in oceanic and/or connate waters, or from marine evaporites, and/or from magmatic hydrothermal sources. Isotopic temperatures from intersulfide fractionations (pyrite, sphalerite, and galena) range from 75° to 542°C, indicating the Bajiazi sulfide minerals have experienced different mineralizing stages. Galena in the Bajiazi Proterozoic sediment-hosted Pb-Zn sulfide deposits has a very uniform Pb-isotope composition, with 206Pb/204Pb = 16.07 to 16.58, 207Pb/204Pb = 15.00 to 15.66, and 208Pb/204Pb = 36.13 to 36.92, suggesting that it all formed from a common mineralizing fluid. The variation of lead-isotopic values of ores is similar to that of their host sedimentary strata. The lead in the various ore types and host rocks within the Bajiazi district is virtually identical. Lead-isotope signatures, which generally plot very close to model crustal growth curves, characterized by a model age of ∼1350 Ma, indicate a normal, nonradiogenic origin and an Early Proterozoic upper-crustal source for the lead in the ores. That lead was unrelated to the radiogenic lead in the Mesozoic granite. The δ13C values range from +1 to −5% PDB, and the δ18O values from −6 to −15% PDB. Later crystallization generations are enriched in the light isotopes of carbon and oxygen compared to early generations. The characteristics of carbon and oxygen isotopes of the Bajiazi deposits indicate that: (1) primary ores were precipitated in the Proterozoic marine environment; (2) both ores and host carbonates from the Bajiazi district are remarkably homogeneous in their carbon- and oxygen-isotope compositions during later tectonic, conlact-metamorphic stages; (3) although the variations in isotopic composition of carbon and oxygen are relatively small, a significant evolution toward a lighter isotopic composition with advancing diagenetic, tectonic, and contact metamorphic processes is observed; and (4) this evolution is independent of the presence or absence of ore minerals.

The influence of 17-β estradiol on the mechanical and electrical function of guinea-pig's myocardium

The Upper Proterozoic (?) metamorphic rocks on the western edge of the Bohemian Massif are subdivided into two distinct groups (Variegated Group, Monotonous Group) which sharply differ from each other with respect to their lithology and metallogenic significance. The somewhat older Monotonous Group metamorphics originated from arenaceous and argillaceous clastic rocks which were laid down in a shallow marine rift basin (1-st order basin). There is some evidence for placer-like gold concentrations to have been re-mobilized by subsequent LP/HT regional metamorphism. The 2-nd order basin opened during advanced stages of rifting and received the various volcanic and sedimentary rocks of the ‘Variegated Group’. Chemical discrimination allows assigning the bimodal volcanic rocks to within plate volcanism. There is no argument for collission-related volcanic activity. Sedimentary rocks consisted of limestones, marls and graywackes that reflect rapid subsidence of the basin. Except for graphite this series is of little economic importance. Most of the concentrations of U, W, and Sn present, have to be classified as “protore” rather than “ore”. By contrast, the 3-rd order basin gains metallogenetic significance, for it hosts the massive sulfides of Bodenmais Kies Belt. These sediment-hosted FeZnCuPb sulfides show a rough zonation into proximal FeZnCu and distal PbBa mineralization. Zn concentration in these stratabound ores involved precipitation of Fe-rich sphalerite, zincian spinel and Zn staurolite. Silver was enriched during retrograde metamorphism, when Ag-rich fahlores, schapbachite, and native silver were formed. By analogy to sediment-hosted deposits elsewhere in the world and especially in view of the “barite fringe” these stratiform ore deposits are classified as Sullivan-type or Meggen-type deposits. Moreover, this interpretation is supported by Pb and S isotopes.

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.

The Ca 2+ antagonist action in CCI 4 liver toxicosis and recovery in rats

The Upper Proterozoic (?) metamorphic rocks on the western edge of the Bohemian Massif are subdivided into two distinct groups (Variegated Group, Monotonous Group) which sharply differ from each other with respect to their lithology and metallogenic significance. The somewhat older Monotonous Group metamorphics originated from arenaceous and argillaceous clastic rocks which were laid down in a shallow marine rift basin (1-st order basin). There is some evidence for placer-like gold concentrations to have been re-mobilized by subsequent LP/HT regional metamorphism. The 2-nd order basin opened during advanced stages of rifting and received the various volcanic and sedimentary rocks of the ‘Variegated Group’. Chemical discrimination allows assigning the bimodal volcanic rocks to within plate volcanism. There is no argument for collission-related volcanic activity. Sedimentary rocks consisted of limestones, marls and graywackes that reflect rapid subsidence of the basin. Except for graphite this series is of little economic importance. Most of the concentrations of U, W, and Sn present, have to be classified as “protore” rather than “ore”. By contrast, the 3-rd order basin gains metallogenetic significance, for it hosts the massive sulfides of Bodenmais Kies Belt. These sediment-hosted FeZnCuPb sulfides show a rough zonation into proximal FeZnCu and distal PbBa mineralization. Zn concentration in these stratabound ores involved precipitation of Fe-rich sphalerite, zincian spinel and Zn staurolite. Silver was enriched during retrograde metamorphism, when Ag-rich fahlores, schapbachite, and native silver were formed. By analogy to sediment-hosted deposits elsewhere in the world and especially in view of the “barite fringe” these stratiform ore deposits are classified as Sullivan-type or Meggen-type deposits. Moreover, this interpretation is supported by Pb and S isotopes.

Lead isotopic investigation and metallogenetic evolution of the sediment-hosted sulphide deposits of Brazil

Lead isotopic investigation and metallogenetic evolution of the sediment-hosted sulphide deposits of Brazil

N 2-Ar-He compositions in fluid inclusions: Indicators of fluid source

A quadrupole mass spectrometer was used to measure bulk samples of conservative gas species N 2, Ar, and He in fluid inclusions from a variety of hydrothermal systems. Analyses of these tracer elements help determine 1. (1) if gases extracted by bulk inclusion analyses can provide accurate measurement of N 2-Ar-He in active and fossil geothermal systems, 2. (2) if hydrothermal fluids associated with paleogeothermal systems in a continental setting follow N 2ArHe systematics similar to th the western Pacific Rim active geothermal systems, specifically New Zealand, and 3. (3) whether different deposit types systematically vary with regard to N 2ArHe. The N 2ArHe ratios of fluid inclusion volatiles released from recently deposited minerals from the Valles system are similar to those of present day Valles thermal waters. Those inclusion samples from deep within the Valles system, below a regional aquitard, increase in N 2. Compositions for inclusions from the Questa and Copper Flat-porphyry deposits are N 2-rich, similar to those of arc-related volcanic gases, whereas those from Taylor Creek Sn deposit appear to be mixtures of magmatic and crustal components. N 2-Ar-He ratios of the Precambrian Tribag deposit suggest a basalt source, but significant levels of self-generated He from U and Th in the inclusion fluids are also possible. Inclusions from two epithermal deposits with low-salinity inclusions have N 2-Ar-He ratios trending towards air-saturated meteoric waters (ASW), and those inclusions with higher salinities indicate minor to no ASW component. The N 2-Ar-He ratios in Fresnillo and Cochiti inclusions, which have magmatic helium isotopic ratios, indicate additions of magmatic gases to meteoric fluids. Inclusions from sediment-hosted deposits that contain hydrocarbon-bearing brines are He-rich, as are meteoric waters with a long residence time in the crust. At relevant pressure-temperature-composition conditions, Henry's Law constants of N 2, Ar, and He are similar, and thus, the relative amounts of these species trapped under boiling conditions will not vary appreciably from solubility-controlled amounts in coexisting liquid. Furthermore, there is no evidence of He loss or gain in inclusions by diffusion process. Data from the various systems that were examined indicate that fluid inclusion N 2-Ar-He compositions may be related to magmatic fluid, sedimentary brine, deep circulating meteoric fluid, and shallow circulating meteoric fluid sources, which we modified from those proposed by Giggenbach (1986). Relationships between the N 2-Ar-He tracer gases and other measurable quantities in fluid inclusions are indicated to be helpful in understanding fluid mixing processes in paleogeothermal systems.

Detailed Sulphur Isotope Studies of Lower Palaeozoic-Hosted Pyrite Below the Giant Navan Zn + Pb Mine, Ireland: Evidence of Mass Transport of Crustal S to a Sediment-Hosted Deposit

Detailed Sulphur Isotope Studies of Lower Palaeozoic-Hosted Pyrite Below the Giant Navan Zn + Pb Mine, Ireland: Evidence of Mass Transport of Crustal S to a Sediment-Hosted Deposit

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.

Geology and geochemistry of volcanogenic massive sulfide deposits and related igneous rocks, Prince William Sound, south-central Alaska

The Prince William Sound region is located approximately 100 km east-southeast of Anchorage, Alaska, and contains the geologically diverse Late Cretaceous to Eocene Valdez and Orca Groups, which comprise a portion of the accretionary prism complex of south-central Alaska. The rocks host more than 600 known massive sulfide deposits and prospects. Historically, the deposits have been described as epigenetic vein, replacement, and shear zone-hosted ores. We define the Prince William Sound massive sulfide deposits in terms of two genetically related, yet geochemically distinct, types of deposits, both related to submarine volcanism. The first type is hosted in submarine pillow basalt, massive basalt flows, and mafic tuffs and is analogous to Cyprus-type deposits. The second type is hosted in intercalated slate and graywacke and is analogous to Besshi-type deposits. Both types are similar morphologically and are characteristically stratiform to strata bound. The deposits vary in size from less than 1 million tons to greater than 5 million tons, with the majority of the largest deposits forming in sediment-dominated Besshi-type environments.The massive sulfide deposits are Fe-Cu-Zn systems dominated mineralogically by pyrite, pyrrhotite, chalcopyrite, and sphalerite. Minor phases include cubanite, arsenopyrite, tetrahedrite, and secondary marcasite. Silicate gangue is comprised predominantly of quartz and chlorite, with minor sericite, talc, and calcite. Primary sedimentary features preserved within massive sulfide are ubiquitous within the deposits and include planar bedding, crossbedding, and graded bedding.Fluid inclusion microthermometry of quartz gangue from the massive sulfide deposits is consistent with hydrothermal fluid temperatures found in genetically related modern seafloor hydrothermal environments. Minimum trapping temperatures of fluid inclusions in quartz from the stockwork feeder zone of the volcanic-hosted deposits generally range from 210 degrees to 260 degrees C. Fine-grained anhedral quartz gangue in massive sulfide layers also contains sparse primary fluid inclusions; typically the homogenization temperature is lower in the sediment-hosted deposits. Hydrothermal fluids in the volcanic-hosted settings exited the volcanic pile directly into seawater and were rapidly quenched, thus fluid inclusions only record high-temperature fluid conditions. Similar hydrothermal fluids in the sediment-hosted settings exited the volcanic pile into warm, seawater-saturated sediments and were cooled more slowly, in an environment where lower temperature fluid inclusions were formed and preserved.Sulfur and oxygen isotope data support a submarine origin for the deposits. Sulfur isotope values range from delta 34 S GDT = 0.8 to 5.9 per mil for volcanic-hosted deposits, and from 2.7 to 9.2 per mil for sediment-hosted deposits. The higher values for sediment-hosted ores reflect increased amounts of reduced seawater sulfate in the hydrothermal fluid, with sulfate reduction occurring in the seawater-hydrothermal fluid mixing zone within the sediment pile below the seawater-sediment interface. Oxygen isotope values for quartz gangue associated with massive sulfide also reflect the lower temperature deposition that is characteristic of the sediment-hosted deposits: delta 18 O quartz = 7.0 to 7.8 per mil (SMOW) for volcanic-hosted deposits and delta 18 O quartz = 9.6 to 16.9 per mil for sediment-hosted ores.Whole-rock geochemical analyses of the basalts and related igneous rocks from both the Valdez and Orca Groups show that these rocks are consistent with formation in midocean ridge and/or seamount environments. The geochemistry of the Valdez Group rocks suggests that sediment contamination of basaltic magma has occurred, imparting a primitive arclike signature. Discrimination diagrams show that the rocks from both groups have normal midocean ridge basalt, enriched midocean ridge basalt, and low K arc tholeiite affinities, and plot along the tholeiitic-calc-alkaline boundary.The fluid inclusion, stable isotope, and whole-rock data collected in this study are all consistent with the hypothesis that the massive sulfide deposits in the Prince William Sound district are sea-floor volcanogenic massive sulfide deposits that formed in both sediment-starved and sedimented midocean ridge settings.

A stable isotope study of the magnesite deposits associated with the alpine-type ultramafic rocks of Yugoslavia

Veins and stockworks of fine-grained (microcrystalline to cryptocrystalline) massive magnesite are common in the ultramafic masses of the ophiolite zone of the Inner Dinarides. Strata-bound deposits of the same kind of magnesite and/or dolomite in Miocene fresh-water basins lie above and beside the ultramafic masses. A carbon and oxygen isotope study of 12 magnesite deposits in Yugoslavia addresses their genesis and bearing on a possible relationship between epigenetic and syngenetic magnesites. All but one of the vein deposits have delta 13 C PDB values between -10 and -15 per mil (the exception, Oshve, has values averaging 3.1ppm), and each deposit has a characteristic value. All sediment-hosted deposits have delta 13 C values between -9 and +5 per mil, again with relatively restricted, characteristic fields for each. The two stockworks analyzed have delta 13 C values ranging from -10.6 to +4.3 per mil, but only one of these has values that coincide with the associated sediment-hosted magnesite deposits. With the exception of the Oshve vein deposit, delta 18 O SMOW values range from approximately 24 to 36 per mil and correlate positively with the delta (super /13) C values (r = 0.90, n = 42). Some of the sediment-hosted magnesite (Bela Stena) and dolomite (Braneshci, Cherenje) deposits were deposited in lakes (assumed temperature of 20 degrees C) with a calculated water delta 18 O value of -2 per mil, suggestive of meteoric water; lower lake temperatures would yield more negative values. Assuming that meteoric water with this delta 18 O value was the mineralizing solution, then a temperature of deposition of approximately 70 degrees C is indicated in those veins not intimately related to dikes.The delta 13 C values of or = 75 degrees C. The suggested low temperatures are consistent with the monomineralic purity of the vein magnesites. At these low temperatures gravity-driven circulating carbonated meteoric waters would react with harzburgite or serpentinite dissolving magnesium, which precipitates as magnesite on pressure drop as CO 2 escapes. The higher delta 13 C values in certain sediment-hosted and stockwork magnesites indicate the involvement of atmospheric CO 2 and possibly carbonate released to the hydrothermal system during contact metamorphism. This latter process is particularly attractive for the Oshve vein deposit with the unusual delta 18 O and delta 13 C values of 19 and 3.1 per mil, respectively, as this is the one vein which is intimately related to a dacite dike.

Chemical basin analysis of metalliferous “variegated metamorphics” of the Bodenmais ore district (F.R. of Germany)

The Upper Proterozoic (?) metamorphic rocks on the western edge of the Bohemian Massif are subdivided into two distinct groups (Variegated Group, Monotonous Group) which sharply differ from each other with respect to their lithology and metallogenic significance. The somewhat older Monotonous Group metamorphics originated from arenaceous and argillaceous clastic rocks which were laid down in a shallow marine rift basin (1-st order basin). There is some evidence for placer-like gold concentrations to have been re-mobilized by subsequent LP/HT regional metamorphism. The 2-nd order basin opened during advanced stages of rifting and received the various volcanic and sedimentary rocks of the ‘Variegated Group’. Chemical discrimination allows assigning the bimodal volcanic rocks to within plate volcanism. There is no argument for collission-related volcanic activity. Sedimentary rocks consisted of limestones, marls and graywackes that reflect rapid subsidence of the basin. Except for graphite this series is of little economic importance. Most of the concentrations of U, W, and Sn present, have to be classified as “protore” rather than “ore”. By contrast, the 3-rd order basin gains metallogenetic significance, for it hosts the massive sulfides of Bodenmais Kies Belt. These sediment-hosted FeZnCuPb sulfides show a rough zonation into proximal FeZnCu and distal PbBa mineralization. Zn concentration in these stratabound ores involved precipitation of Fe-rich sphalerite, zincian spinel and Zn staurolite. Silver was enriched during retrograde metamorphism, when Ag-rich fahlores, schapbachite, and native silver were formed. By analogy to sediment-hosted deposits elsewhere in the world and especially in view of the “barite fringe” these stratiform ore deposits are classified as Sullivan-type or Meggen-type deposits. Moreover, this interpretation is supported by Pb and S isotopes.

Geochemistry and metamorphism of the Prieska Zn-Cu deposit, South Africa

The Prieska orebody is a 50-million-ton massive sulfide Zn-Cu deposit situated near the eastern border of the middle Proterozoic Namaqua province of southern Africa.The strata-bound, stratiform deposit lies in a terrane which has been affected by high-grade metamorphism and intense deformation. Textures now seen in the ore and country rocks are not primary but reflect the complex tectono-metamorphic history. The deposit is contained in a peraluminous unit underlain by a metadacite and overlain by a composite sequence containing metapelites, metabasic rocks, and a variety of reworked volcanic rock types. Amphibolite layers that originated as intrusive basaltic dikes and sills are found in all of these rock types.It is suggested that the massive sulfide ore formed above hydrothermally altered metadacite. The rock types thus derived include a gedrite-bearing assemblage, the metamorphic equivalent of a chloritic alteration pipe, and a quartz-perthite-sillimanite gneiss representing silicified and sericitized sediments at the floor of the discharge basin. Overlying these rocks is the massive sulfide unit. Calcite-anhydrite-barite rocks immediately above the ore may have formed a sulfate-carbonate capping during sulfide deposition.The Prieska deposit and other smaller sulfide deposits in the same region have geochemical features (including metal composition and Pb and S isotope characteristics) that distinguish them from the important massive sulfide deposits of the central part of the Namaqua province. Whereas the Prieska deposits are related to hydrothermal action in a volcano-sedimentary sequence, no metamorphic equivalents of alteration pipes have been found at the sediment-hosted Aggeneys-Gamsberg deposits.

Anorogenic felsic magmatism, rift sedimentation, and giant Proterozoic Pb-Zn deposits

The giant lead deposits that developed after ∼1.8 Ga in Australia, North America, and Africa are absent in the earlier geologic record. These Proterozoic lead (+ zinc) deposits were all preceded by major outbreaks of anorogenic felsic magmatism that brought significant amounts of lead-rich material (>30 ppm) to the surface of Proterozoic cratons. Erosion of these volcanics apparently transferred major quantities of the lead to rift basins where giant lead deposits were generated. All known giant Proterozoic sediment-hosted lead-zinc deposits (more than 50 Mt of lead metal) are explained by a model that is supported by Sm/Nd data relevant to mantle separation ages of volcanics and the metasedimentary source rocks for the lead ores.

Mineralogy, fluid characteristics, and silver distribution at Real de Angeles, Zacatecas

The Real de Angeles low-grade, bulk tonnage deposit is located in the southeastern part of Zacatecas, on the western edge of the Mesa Central in transition with the Sierra Madre Occidental volcanic province. Carbonaceous sandstones and siltstones of Cretaceous age host the mineralization, but no associated intrusions are known. Production of Ag, Pb, Zn, and Cd at 10,000 tons per day began early in 1982 by open-pit methods. Calculated reserves are 85 million metric tons at 75 g/metric ton Ag, 1.0 percent Pb, 0.92 percent Zn, and recoverable amounts of Cd.The ore minerals occur in thin (1-3 cm wide), discontinuous veinlets related to sedimentary deformation and diagenesis, and to a lesser extent, in fractures and faults of Laramide age. Cavity-filling textures are common. Sulfide mineralization is overlain by a clay alteration assemblage that is overprinted by oxidation. Alteration within the sulfide zone consists of silicification and thin envelopes of propylitic and potassic alteration around the veinlets. Minor occurrences of hornfelsic alteration are observed in the deeper zones of the orebody.Pyrrhotite, sphalerite, and galena are the most common sulfides, with lesser amounts of arsenopyrite and pyrite. Silver occurs primarily as a solid solution in galena of up to 0.77 wt percent and reflects the coupled substitution 2Pb = Ag + Sb + or - Bi. Other silver phases of microscopic size and trace occurrence include freibergite ((Ag,Cu) 10 (Fe,Zn) 2 Sb 4 S 13 ), stephanite (Ag 5 SbS 4 ), and argentitc (Ag 2 S). The silver, lead, and zinc minerals were emplaced in one mineralizing event.Major gangue minerals include adularia, quartz, fluorite, and calcite. Although adularia is the most abundant, quartz and fluorite accompany ore deposition, and calcite is both pre- and postore.Analysis of fluid inclusions in adularia, quartz, fluorite, and calcite indicates ore and gangue precipitation from hydrothermal solutions of 1.4 to 21.0 equiv wt percent NaCl. Quartz and fluorite fluid inclusion homogenization temperatures indicate a formation temperature of about 290 degrees C.Sulfur isotope variations are from -4 to -9 per mil delta 34 S for sphalerite, pyrrhotite, and galena. A mixing model is proposed of approximately 60 percent sedimentary sulfur with 40 percent magmatic sulfur in the hydrothermal fluid. Comparison of temperatures calculated from sulfur isotope pairs and temperatures of formation from fluid inclusions suggests isotopic disequilibrium during ore formation. This is common in sediment-hosted deposits where there is a mixing of sedimentary and magmatic sulfur.Ore-stage base metal deposition occurred under reducing conditions within the pyrrhotite stability field, although the f (sub O 2 ) -pH conditions of the fluid fluctuated both before and after the ore stage. Sulfur was transported as H 2 S. Temperatures of formation and salinity, textures of ore and gangue minerals, and hydrothermal alteration indicate an epithermal to mesothermal, fissure vein-stockwork deposit.

Pb isotopes in sulfides from mid-ocean ridge hydrothermal sites

The authors report Pb isotope ratios of sulfides deposited at seven recently active mid-ocean ridge (MOR) hydrothermal vents. Sulfides from three sediment-starved sites on the Juan de Fuca Ridge contain Pb with isotope ratios identical to their local basaltic sources. Lead in two deposits from the sediment-covered Escanaba Trough, Gorda Ridge, is derived from the sediments and does not appear to contain any basaltic component. There is a range of isotope ratios in a Guaymas Basin deposit, consistent with a mixture of sediment and MOR basalt Pb. Lead in a Galapagos deposit differs slightly from known Galapagos basalt Pb isotope values. The faithful record of Pb isotope signatures of local sources in MOR sulfides indicates that isotope ratios from ancient analogues ca be used as accurate reflections of ancient oceanic crustal values in ophiolite-hosted deposits and continental crustal averages in sediment-hosted deposits. The preservation of primary ophiolitic or continental crustal Pb isotope signatures in ancient MOR sulfides provides a powerful tool for investigation of crustal evolution and for fingerprinting ancient terranes.

Alteration of Late Middle Proterozoic Volcanics and its Relation to Stratabound Copper-Silver-Gold Mineralization Along the Margin of the Kalahari Craton in Swa/Namibia and Botswana

Summary Metavolcanic and metasedimentary rocks of late Middle Proterozoic age are preserved in basins along the northern margin of the Kalahari Craton. Three broad stratigraphic divisions are recognized: a lower acid extrusive unit, a basic extrusive/red bed unit and a shallow marine unit. The basalts of the basic extrusive/red bed unit are interpreted to be continental within-plate basalts. Copper mineralization occurs as native copper in amygdaloidal altered basalt, and as sediment-hosted, stratiform sulphide deposits at the base of the shallow marine unit. The rocks have been strongly altered during regional metamorphism which reached temperatures up to 350°C. Massive domains were metamorphosed to oligoclase-chlorite-epidote-haematite rocks. In permeable zones such as flow top breccias, amygdaloidal zones, and regions of secondary, tectonically induced fracturing, the basalts have been altered to epidote-quartz-chlorite-haematite-muscovite rocks. During this epidote alteration copper, zinc and cobalt were leached from the basalts, thus providing a possible metal source for the sediment-hosted deposits. A model for multiphase ore genesis is presented in which basalt alteration is the main metal source for epigenetic mineralization.

Lead isotope systematics of strata-bound sulfide deposits in the higher nappe complexes of the Swedish Caledonides

55 galena samples from 18 deposits. Volcanite-hosted deposits have a less radiogenic lead isotope composition than sediment-hosted deposits, while galena in quartz segregations and crack fillings shows the most pronounced radiogenic lead isotope composition. Application of the plumbotectonic model to the lead isotope data for the stratiform deposits suggests that the volcanite-hosted deposits show the most significant influence of a mantle-related source.--Modified journal abstract.