Stratiform Ore Research Articles

Re[sbnd]Os and S isotopic constraints on the origins of two mineralization events at the Tangdan sedimentary rock-hosted stratiform Cu deposit, SW China

The Tangdan stratiform Cu deposit in the Kangdian region of South China is hosted in sedimentary rocks of the late Paleoproterozoic to early Mesoproterozoic Dongchuan Group. Orebodies mainly occur in dolostone and black shale above a thick sequence of hematitic sandstone and siltstone. Mineralization consists mainly of bedding-parallel stratiform ores, with minor amounts of discordant vein-type ores. Both stratiform and vein-type ores are composed of bornite and chalcopyrite with minor chalcocite and digenite, associated with gangue minerals including ankerite, dolomite, calcite, and quartz.Cu-sulfides from vein-type ores contain ~122–293ppb Re, much higher than those of stratiform ores (~0.6–14ppb Re). They have highly radiogenic Os with ReOs model ages ranging from 1413 to 1457Ma and a weighted mean ReOs model age of 1432±19Ma (2σ, MSWD=0.30), which is within uncertainty in agreement with isochron ages of 1401±30Ma (2σ, MSWD=0.67) and 1397±71Ma (2σ, MSWD=0.22) using 187Re/188Os versus 187Os/188Os and 187Re versus 187Os regression, respectively. In contrast, sulfides from stratiform ores did not yield meaningful model ages due to the extremely low concentrations of Re and radiogenic Os. However, early studies have shown that the stratiform ores were likely formed at ~1700Ma during the diagenesis of the host strata.Sulfides from the vein-type ores have δ34S values ranging from −15‰ to −8‰, significantly lower than those of stratiform ores (−3‰ to +10‰), suggesting that organic matter may have been involved in the formation of the vein-type ores. Contrasting ReOs and S isotopes suggest that the two types of ores at the Tangdan deposit were likely formed from different ore-forming fluids. We propose that stratiform ores were precipitated from basinal fluids during the early evolution of the rift basin, whereas the vein-type ores were formed due to a much younger thermal event during which hydrothermal fluids interacted with overlying black shale to account for the high Re and negative δ34S values. This study demonstrates that ReOs isotopes of sulfides can be used to study the origin of hydrothermal system.

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of ∼500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1–50 μm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, fO2, of ∼3 log units below the quartz–fayalite–magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950°C at subsolidus conditions, were overgrown by more oxidized host chromite (fO2 ≈ QFM) in association with microcrystal inclusions of silicates (plagioclase An86, clinopyroxene, and pargasite) that were formed between 950 and 1050°C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to ∼0·1°C a−1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.

The Woxi W–Sb–Au deposit in Hunan, South China: An example of Late Proterozoic sedimentary exhalative (SEDEX) mineralization

The Woxi W–Sb–Au deposit in Hunan, South China is hosted by Proterozoic metasedimentary rocks, a turbiditic sequence of graywacke, siltstone, sandy slate and slate. The deposit has two principal styles of mineralization: stratiform and stockwork. The stratiform ores, which account for about 75% of the total proven ore reserves, are composed predominantly of interbedded, massive to laminated Au-bearing quartz, stibnite, scheelite, pyrite, and silty clays. Fine layering, at the macro- to microscale, is the most striking feature of the ores, with individual mm- to cm-scale bands traceable over distances of several meters to a few hundred meters along strike. Structures indicative of soft-sediment deformation due to slumping and synsedimentary sliding are commonly observed in the banded ores and the barren quartz–carbonate beds in the adjacent host rocks. The stockwork mineralization accounts for about 25% of the total ore reserves and is restricted to a 3–10m-thick zone immediately beneath the stratiform ores. It is characterized by numerous quartz+pyrite±scheelite stringer veins that are either subparallel or near perpendicular to the overlying stratiform ore layers. Alteration at Woxi occurs as semiconformable, tabular blankets asymmetrically enveloping the orebodies along their entire length, and, is dominated by a quartz+sericite+chlorite±carbonate assemblage. The footwall alteration is generally more intense and somewhat thicker than the hanging-wall alteration. Geochemical data show that, with distance above and below the mineralized horizons, contents of W, Au, As, and Sb systematically decrease, whereas concentrations of other trace elements such as Hf, Sc, Th, Ta, Y, Zr, Nb and REEs gradually increase. This may reflect a decreasing hydrothermal input in the sediments and an increasing dominance of detrital and seawater-derived components. The chondrite-normalized REE patterns of both bulk ores and individual minerals are comparable to those of many SEDEX-type deposits and associated exhalites, implying similar mineralizing fluids for these deposits. Sulfides from the ore have negative and variable δ34S values (from −12.3‰ to −1.2‰), suggesting a dominant sulfur contribution of bacterial sulfate reduction. A syngenetic (SEDEX) model is thus proposed for the formation of the Woxi deposit. Convective seawater circulated through the sediment pile and episodically vented metalliferous hydrothermal fluids onto the sea floor. Each exhalative episode was preceded by a period dominated by chemical and turbiditic sedimentation (footwall alteration blanket), followed by a peak release of hot metalliferous brines (stratiform ores and associated stringers), and terminated by a waning influx of hydrothermal fluids accompanying burial by later turbidites (hanging-wall alteration blanket).

The geology and geochemistry of the Lumwana Cu (± Co ± U) deposits, NW Zambia

The Lumwana Cu (± Co ± U) deposits of NW Zambia are large, tabular, disseminated ore bodies, hosted within the Mwombezhi Dome of the Lufilian Arc. The host rocks to the Lumwana deposits are two mineralogically similar but texturally distinct gneisses, a granitic to pegmatitic gneiss and a banded to augen gneiss which both comprise quartz–feldspar ± biotite ± muscovite ± haematite ± amphibole and intervening quartz–feldspar ± biotite schist. The sulphide ore horizons are typically developed within a biotite–muscovite–quartz–kyanite schist, although mineralization locally occurs within internal gneiss units. Contacts between the ore and host rocks are transitional and characterized by a loss of feldspar. Kinematic indicators, such as S-C fabrics and pressure shadows on porphyroblasts, suggest a top to the north shear sense. The sulphides are deformed by a strong shear fabric, enclosed within kyanite or concentrated into low strain zones and pressure shadows around kyanite porphyroblasts. This suggests that the copper mineralization was introduced either syn- or pre-peak metamorphism. In addition to Cu and Co, the ores are also characterized by enrichments in U, V, Ni, Ba and S and small, discrete zones of uranium mineralization, occur adjacent to the hanging wall and footwall of the copper ore bodies or in the immediate footwall to the copper mineralization. Unlike typical Copperbelt mineralization, unmineralized units show very low background copper values. Whole rock geochemical analyses of the interlayered schist and ore schist, compared to the gneiss, show depletions in Ca, Na and Sr and enrichments in Mg and K, consistent with replacement of feldspar by biotite. The mineral chemistry of muscovite, biotite and chlorite reflect changes in the bulk rock chemistry and show consistent increases in X Mg as the schists develop. δ34S for copper sulphides range from +2.3 ‰ to +18.5 ‰, with pyrite typically restricted to values between +3.9 ‰ and +6.2 ‰. These values are atypical of sulphides precipitated by bacteriogenic sulphate reduction. δ34S data for Chimiwungo (Cu + Co) show a broader range and increased δ34S values compared to the Malundwe (Cu) mineralization. The Lumwana deposits show many characteristics which distinguish them from classical Copperbelt mineralization and which suggests that they are formed by metasomatic alteration, mineralization and shearing of pre-Katangan basement. Although this style of mineralization is reported elsewhere in the Copperbelt, sometimes associated with the more widely reported stratiform ores of the Lower Roan, none of the previously reported occurrences have so far developed the tonnages of ore reported at Lumwana.

Stratiform ore of the large Lengshuikeng silver deposit, China: Mineralogy, fluid inclusions, stable O and C isotopes

The composition of minerals from stratiform ores of the Lengshuikeng deposit, China was studied using contemporary techniques. Barite, enargite, and kutnohorite typical of postvolcanic mineralization have been identified for the first time at the studied deposit. The fluid inclusion study shows that the stratiform ore was formed at a temperature of 340–140°C from homogeneous chloride solution with salinity of 0.2–11.7 wt % NaCl equiv. The O and C isotopic compositions of carbonates from the stratiform ore allowed us to calculate the oxygen isotopic composition of the H2O and the carbon isotopic composition of the CO2 in the ore-forming fluid. The data on ore minerals, fluid inclusions in quartz, and stable O and C isotopes show that two or more fluids derived from different sources could have mixed during formation of the stratiform ores.

Factors controlling the precipitation of copper and cobalt minerals in sediment-hosted ore deposits: Advances and restrictions

An assessment is presented of existing data on the most important copper and cobalt complexes and sulphides applied to reactive transport modelling. The most important complexes in ore-forming hydrothermal solutions at 150°C are CoCl(H2O)5+, CoCl4−2 and CoCl2(H2O)2(aq) for cobalt, and CuCl2− and CuCl3−2 for copper. Reactive transport modelling was carried out to simulate the transport and precipitation of copper and cobalt sulphides in sediment-hosted stratiform ore deposits, such as the Central African Copperbelt. A limitation is the lack of thermodynamic data for carrollite (CuCo2S4). Initially chalcopyrite (CuFeS2) and cattierite (CoS2) precipitate in a reducing host rock. Pyrite dissolves in the horizon where both minerals form and it precipitates in adjacent rocks. The continuous supply of copper and cobalt causes formation of the metal-rich sulphides bornite (Cu5FeS4) and linnaeite (Co3S4). Chalcopyrite and cattierite precipitate further in the flow direction. In this model, the dissociation reaction of chalcopyrite and bornite are:CuFeS2(s)+2H+=Cu2++Fe2++2HS−Cu5FeS4(s)+2H+=4Cu++Cu2++Fe2++4HS−However, if the dissociation reactions only include Cu+ and Fe3+, simulations show the precipitation of only chalcocite (Cu2S), which is explained by the low concentration of trivalent iron in solution. The dissociation reactions are:CuFeS2(s)+2H+=Cu++Fe3++2HS−Cu5FeS4(s)+2H+=5Cu++Fe3++4HS−

Geology and genesis of Kafang Cu–Sn deposit, Gejiu district, SW China

Kafang is one of the main ore deposits in the world-class Gejiu polymetallic tin district, SW China. There are three main mineralization types in the Kafang deposit, i.e., skarn Cu–Sn ores, stratiform Cu ores hosted by basalt and stratiform Cu–Sn ores hosted by carbonate. The skarn mainly consists of garnet and pyroxene, and retrograde altered rocks. These retrograde altered rocks are superimposed on the skarn and are composed of actinolite, chlorite, epidote and phlogopite. Major ore minerals are chalcopyrite, pyrrhotite, cassiterite, pyrite and scheelite. Sulfur and Pb isotopic components hint that the sources of different types of mineralization are distinctive, and indicate that the skarn ore mainly originated from granitic magma, whereas the basalt-hosted Cu ores mainly derived from basalt. Microthermometry results of fluid inclusions display a gradual change during the ore-forming process. The homogenization temperature of different types of inclusions continuously decreases from early to late mineralization stages. The salinities and freezing temperatures exhibit similar evolutionary tendencies with the T homogenization, while the densities of the different types keep constant, the majority being less than 1. Oxygen and hydrogen isotopic values (δ18O and δD) of the hydrothermal fluids fall within ranges of 3.1 to 7.7‰ with an average of 6.15‰, calculated at the corresponding homogenization temperature, and −73 and −98‰ with an average of −86.5‰, respectively. Microthermometry data and H–O isotopes indicate that the ore-forming fluid of the Kafang deposit is mainly derived from magma in the early stage and a mixture of meteoric and magmatic water in late stage. Molybdenite Re–Os age of the skarn type mineralization is 83.4±2.1Ma, and the stratiform ores hosted by basalt is 84.2±7.3Ma, which are consistent with the LA-ICP-MS zircon age of the Xinshan granite intrusion (83.1±0.4Ma). The evidence listed above reflects the fact that different ore styles in the Kafang deposit belong to the same mineralization system.

Sphalerite from the Sterling Hill Mine, New Jersey: Origin and History

The Sterling Hill and Franklin zinc-iron-manganese deposits, Sussex County, New Jersey, are sedimentary-exhalative in origin but were highly deformed and metamorphosed to granulite grade during the Mesoproterozoic Ottawan orogeny. They are unique because the major zinc minerals, zincite, franklinite, and willemite, are oxides and silicates rather than sulfides. However, at Sterling Hill minor granular sphalerite occurs in the central black willemite zone associated with loelling and graphite and sparsely elsewhere in the deposit. Based on textural observations and stable isotope geochemistry, sphalerite was present during metamorphism and was most likely part of the protolith mineral assemblage. It likely formed from a fluid containing seawater sulfate that circulated through volcanic rocks and reacted with igneous sulfides; some contribution from biological reduction of sulfate seems likely. This interpretation agrees with the generally accepted model that the protolith formed on the floor of an ancient rift in an environment similar to the present-day Red Sea brine pool sediments. The presence of loellingite and absence of pyrite or arsenopyrite in the black willemite zone requires that fAs2−/fS2− >1 during metamorphism. The position of the black willemite zone, occupying most of the central cross member of the deposit, and the more reduced mineral assemblages of the zone relative to the rest of the deposit are compatible with it being the feeder zone for the stratiform ores that make up the rest of the Sterling Hill deposit, indicating that the deposit is inverted relative to its original position. Pressure-corrected temperatures from fluid inclusion geothermometry and temperatures determined from carbon isotope fractionation factors between coexisting graphite and calcite suggest a postmetamorphic event that occurred at ∼360°–280°C at between ∼820 and 750 Ma. The fluid inclusion and mineral assemblage data also suggest that at least one additional postmetamorphic event affected the sphalerite and loellingite and their coexisting minerals. This event involved the development of secondary fluid inclusions in sphalerite and the formation of a secondary manganese arsenosilicate mineral and may correlate with the pervasive serpentinization-like alteration of the deposit. The complex sequence of vents indicated by the textures, isotopic ratios, and fluid inclusions is compatible with the history of prolonged cooling and uplift experienced by the New Jersey Highlands in the Neoproterozoic.

Metalliferous Manganese Oxide Mineralization Associated with the Boleo Cu-Co-Zn District, Mexico

Apart from its large reserves of Cu-Co-Zn mineralization contained within stratiform ore beds or mantos, the Boleo district of central Baja, Mexico, also contains significant concentrations of Mn oxides (typically 2–3 wt % Mn) interspersed with hydrothermal sulfides and both hydrothermal and weathering-related metal oxide minerals. Within the mantos, Mn oxides occur as concentric nodules, disk-shaped concretions, laminae, granules or aggregates, veins, dendrites, and various replacements. The oxides are composed of varying amounts of hollandite, cryptomelane, todorkite, romanechite, pyrolusite, and X-ray amorphous Mn oxide. Chemically, the manto oxides are unique owing to their extraordinarily high abundances of Co, Cu, Pb, and Zn (typically 2–5 wt %, but up to 23 wt %). Existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of Mn oxides within the mantos. A new MnO 2 -Fe 2 O 3 t-MgO diagram is proposed in order to discriminate among highly metalliferous Mn oxides. Based on the new classification scheme, manto Mn oxides record a complex interaction between hydrothermal and diagenetic processes. To better understand the mechanism of oxide deposition, radiogenic isotopes (Sr and Pb) and REE geochemistry were used to delineate fluid and metal sources. Isotopic and REE constraints indicate that the Boleo Mn oxides are largely hydrothermal in origin; however, they show varying degrees of modification resulting from diagenetic processes. Hydrothermal Mn oxides are syngenetic (synsedimentary to syndiagenetic) in origin and were precipitated in association with sulfides, reflecting localized variations in redox conditions within the manto sequence, due to the downward infiltration of metal-bearing brines. Apart from morphological and stratigraphic evidence (e.g., occurrence of nodular crusts along the upper contact of the manto), a syngenetic origin is supported by a single K-Ar date of 7.0 ± 0.2 Ma, which is within the estimated age range for deposition of Boleo Formation clastic sediments. Interaction between brine and seawater or brine and pore fluid, as it infiltrates the sedimentary pile, produced hydrothermal precipitates with certain hydrogenetic attributes (i.e., positive Ce anomaly, LREE enrichment). On the other hand, oxides with hydrogenetic and with mixed hydrothermal-hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids. By far the greatest degree of remobilization occurred in response to compactional dewatering of the manto, where the dewatering pore waters enriched the uppermost manto strata in Mn, Fe, Zn, Co, and to lesser degrees, Cu. Later diagenetic modifications coincided with the development of a supergene overprint, which typifies Boleo Formation strata at elevations above the present-day water table.

مراحل تشکیل و تکوین کانسار سولفید توده ای غنی از طلای باریکا، خاور سردشت، شمال باختر پهنه دگرگونه سنندج- سیرجان: بر اساس مطالعه ساخت، بافت و میکروترمومتری سیالات درگیر

Barika gold (and silver) rich massive sulfide deposit is located 18 km east of Sardasht, north western part of Sanandaj-Sirjan metamorphic zone. Rock units outcropped in this area are Cretaceous metamorphosed volcano-sedimentary rocks in green schist facies, and include association of meta-andesite, meta-tuffite, phyllite and slate rocks, where the meta-andesitic unit (Kmv1) is host rock to the deposit. Barika deposit is composed of stratiform ore and stringer zone. Stratiform part contains massive and banded sulfide and barite associated with subordinary silica bands underlain by stringer silicic zone. The massive and banded ore is surrounded by an extensive zone of silicic stringer veins and disseminated mineralization within strongly altered and deformed andesitic volcanic rocks. Ore mineral assemblages in stratiform part of the deposit are quite variable and consist of pyrite, sphalerite, galena, stibnite, a variety of sulfosalt minerals and gold (electrum). Ore mineral assemblages in silicic veins in the stringer zone are simpler and consist of pyrite, galena, sphalerite, tetrahedrrite and trace chalcopyrite. Average gold and silver grades in stratiform ore are 8.3 and 420 g/t respectively and in silicic stringer veins are 0.7 and 30 g/t. Base metal content in both parts of the deposit are less than 1%. Based on our research, Barika is an immature Kuroko type massive sulfide deposit, which contains only black ore. The Barika deposit underwent low-grade metamorphism and high grade deformation after the ore deposition event. Geologic processes appear to control the mode of occurrence of gold in the deposit. In synvolcanism stage, deposition of a gold–bearing low temperature (140-200°C) and low salinity (1-9.6 wt% NaCl) hydrothermal fluid formed a Kuroko black ore type deposit. In the stage of mineralization, invisible gold was concentrated in framboidal pyrite and other sulfide minerals. The main effect of progressive metamorphism (D1) on gold mineralization is recrystallization of the framboidal pyrite and migration of submicroscopic gold to the crystal boundary to form electrum under low to moderate strain. Highly strain and deformation (D2) on the deposit caused the remobilization of gold and accompanied As, Sb, Ag, and Pb minerals and continued to form coarse-grained electrum (up to 3 mm) in open space fractures. The formation of barren silica veins (D3) and high angle normal faults (D4) are the results of retrograde metamorphism.

Biogenic sedimentation factors of mineralization in the Neoproterozoic strata of the Baikal–Patom region

The formation environments of stratiform ore deposits in the Neoproterozoic Baikal–Patom region (BPR) have been considered. A model for the formation of the Sukhoi Log gold ore deposit in the Bodaibo zone has been put forward. The first stage is gold concentration by a chemolithotrophic bacterial community. Independently established facts suggest that bacterial communities may also have contributed to initial metal accumulation in the sediments of the Kholodnaya Pb–Zn deposit. The ore beds occur in the high-carbon sediments of the side and trough of a back-arc basin. Sedimentation (Dal’nyaya Taiga and Zhuya regional horizons) took place during the “back-arc basin–foreland basin” transition. This transition is characterized by increased sediment bioproductivity, which is clearly evidenced from the increased biophile-element content and taxonomic diversity of organic remains. Hundreds of microfossil sites in the Neoproterozoic BPR host littoral benthos (cyanobacteria and brown algae) and plankton (green algae). Most microfossils in the outer shelf, on the basin side, and in its trough belong to chemolithotrophic bacteria. These bacteria are assumed to have accumulated metals in the vent field of the back-arc basin. Studies showed the ability of microorganisms (bacteria, algae, fungi, etc.) to accumulate Fe, Mn, Au, Pb, Zn, and other metals. Bacterial communities are particularly important for metal accumulation in the vent fields of rift zones and areas of arc volcanism. All these conditions were observed in the Neoproterozoic BPR.

The Mt. Mulga barite–magnetite–copper–gold mineralisation, Olary Domain, South Australia

The Mt. Mulga barite Cu–Au deposit in the central part of the Olary Domain is hosted by the Palaeoproterozoic Willyama Supergroup of the Curnamona Craton. The lenticular, stratiform ore bodies are hosted by the Ethiudna Subgroup at and near the contact to the underlying Wiperaminga Subgroup. The ore bodies consist of massive, coarse crystalline barite up to 6 m thick interspersed with granular (< 2 cm) magnetite and quartz. In the southern section the massive barite contains Cu–Au mineralisation consisting of chalcopyrite, bornite and native gold. Along strike the mineralised unit grades into a stratiform quartz–magnetite layer which can be traced for several kms. Several smaller (< 1 m thickness) barite lenses occur about 1.5 km north of main barite body in the upper parts of the Ethiudna Subgroup close to the contact to the Saltbush Subgroup. A pronounced positive Eu-anomaly of the mineralisation hosting unit indicates a reducing character of the mineralising fluids and is characteristic for exhalative sediments. 87Sr/ 86Sr-isotope ratios of barite fall into a narrow range of 0.708 to 0.713 suggesting an origin of the mineralising fluids from the underlying Lower Willyama sequence. The δ 34S values of barite from the main ore body range from + 13 and + 16‰, while the barites of the other ore bodies range from + 6 and + 11‰. Chalcopyrite revealed δ 34S values of − 1.3 to + 4.5‰. Barite is suggested to be formed by mixing of a hydrothermal, reduced, hydrothermal Ba-rich fluid with oxidised, sulphate-rich water of a non-marine basin. Although an epigenetic formation of the Cu–Au mineralisation during the Olarian Orogeny is possible, we suggest a cogenetic formation with the barite–iron-oxide mineralisation.

Breithauptite: A rare antimonide in the Dariba-Rajpura-Bethumni belt, Rajsamand district, Rajasthan

Breithauptite (NiSb), a rare antimonide, is generally found associated with hydrothermal vein ores of Ni and Co. In India, it has so far been recorded only from the gold-quartz veins of the Kolar Gold Field. Occurrence of this antimonide in a sediment-hosted milieu however, has seldom been noted. We report here the identification and characterization of this rare phase in both the stratiform and the vein-type ores of the sediment-hosted Zn-Pb sulphide deposits and prospects within the Dariba-Rajpura-Bethumni metallogenic belt in Northwestern India. Metamorphic remobilization from the stratiform ores of the belt has been postulated as a possible mechanism for the conspicuous segregation of breithauptite as a discrete phase within these ores.

The Kuzha barite-base-metal deposit

The Kuzha barite-base-metal deposit is located on the western slope of the southern Urals and is hosted in strongly deformed Riphean rocks. The Main barite zone is related to a tectonic suture accompanied by alkaline and basic dikes. The stratiform base-metal ore mineralization is confined to a dolomite layer conjugated with the suture zone. The ore mineralization was formed as a result of interaction between heated mineralized water ascending along the suture zone and descending sulfate-bearing vadose water. Biogenic sulfate reduction played the main role in sulfide precipitation. The heating of the ore-forming medium up to 120°C terminated sulfate reduction and facilitated deposition of barite. The Kuzha deposit has much in common with other base-metal and barite-base-metal deposits in Russia, Kazakhstan, and the United States.

P.295 The markers of cellular interactions at traumas of maxillary

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.

Contrasting REE Signatures on Manganese Ores of Iron Ore Group in North Orissa, India

The distribution pattern of Rare Earth Elements (REE) in three categories of manganese ores viz. stratiform, stratabound-replacement, and detrital of Precambrian Iron Ore Group from north Orissa, India was reported. These categories of Mn-ore differed in their major and trace chemistry and exhibited contrasting REE signature. The stratiform ores were relatively enriched in Σ REE content (697 μg · g −1) and their normalized pattern showed both positive Ce and Eu anomalies, whereas the stratabound-replacement types were comparatively depleted in Σ REE content (211 μg · g −1) and showed negative Ce and flat Eu signatures. The detrital categories showed mixed REE pattern. The data plotted in different discrimination diagrams revealed a mixed volcaniclastic and chemogenic source of material for stratiform categories, and LREE (Light Rare Earth Elements) and HREE (Heavy Rare Earth Elements) are contributed by such sources, respectively. In contrast, the stratabound ore bodies were developed during the remobilization of stratiform ores, and associated Mncontaining rocks under supergcne condition followed by the redeposition of circulating mineralized colloidal solutions in structurally favorable zones. During this process, some of the constituents were found only in very low concentration within stratabound ores, and this is attributed to their poor leachability/mobility. The detrital ores did not exhibit any significant characteristic in respect of REE as their development was via a complex combination of processes involving weathering, fragmentation, recementation, and burial under soil cover.

The Upper Paleozoic preflysch and overthrusting in the Türkstan-Alay ranges, southern Fergana

The Upper Paleozoic preflysch sedimentary rocks in the Turkstan-Alay ranges are combined in a common section with limestone of autochthon and synsedimentation carbonate nappes, thus forming the upper-most layers of the stratigraphic section of the latter. By their lithology, relatively small thickness, facies variability, and position at the boundary between carbonate and flysch-olistostrome sequences (in the autochthon), these rocks correspond to a certain extent to preflysch of the Urals and the Mediterranean Alpine Belt. This association of clayey, carbonate, and terrigenous rocks is strictly constrained in stratigraphy (the upper portion of the lower Moscovian substage and the lower portion of the upper Moscovian substage) and localization (the southern slopes of carbonate platforms). The formation of this rock association immediately predated the Late Paleozoic overthrusting and deposition of terrigenous flysch. In paleotectonic terms, preflysch is an indicator of the initial stage of tectonic and magmatic activation that led to the replacement of carbonate sedimentation with deposition of terrigenous and clayey sediments, coeval volcanism, and stratiform ore formation. The following sequence of events has been outlined in the Early and Middle Carboniferous: (1) thrusting of volcanic-sedimentary rocks filling troughs over the northern margins of carbonate platforms, (2) lateritic weathering and deposition of marine bauxite in the Bashkirian and early Moscovian, (3) repeated overthrusting and deposition of preflysch on southern slopes of platforms, (4) invasion of the frontal flysch trough from the south, (5) scouring of preflysch and the underlying limestone, and (6) formation of flysch-olistostrome sequences and tectonic and gravity nappes in the late Moscovian time. This interpretation takes into account the southward vergence of thrust sheets and nappes, the structure and localization of allochthonous fragments of marginal zones of carbonate platforms, and the pre-Bashkirian thrusting of volcanic and sedimentary rocks over the condensed pelagic deposits of the Shalan Group. It is suggested that bauxite and preflysch materials had the same source and were deposited in the Middle Carboniferous on the offshore carbonate shoals.

Rare earth element and yttrium analyses of sulfides from the Dachang Sn-polymetallic ore field, Guangxi Province, China: Implication for ore genesis

The Dachang Sn-polymetallic ore field (Guangxi, China) is one of the largest tin deposits in the world with a tin reserve of above 106 tons. Several types of orebody occur in this giant ore field, including mainly the Lamo Cu-Zn skarns at the contact of Devonian limestone and the Cretaceous Longxianggai granite, the Changpo-Tongkeng stratiform Sn-Pb-Zn ores in Devonian siliceous rocks, and the Gaofeng massive sulfide ore in Devonian reef limestone. Rare-earth element (REE) and yttrium concentrations in sulfides from different orebody types have been determined by ICP-MS. Sulfides from the Lamo skarn-type Cu-Zn mines display significant LREE-enriched pattern with large negative Eu anomalies. This is in agreement with the mineralizing fluid being of magmatic hydrothermal origin. However, sulfides from massive and stratiform orebodies show significantly different REE distributions. The Gaofeng massive sulfides have very low total REE concentrations and REE patterns range from (1) being strongly enriched in light REE with positive Eu anomalies, to (2) relatively flat patterns with strongly negative Ce and positive Eu anomalies. Pattern 1 resembles REE distributions in modern submarine hydrothermal fluids. Pattern 2 exhibits both seawater and hydrothermal vent-fluid characteristics. Sulfides from stratiform orebodies at Changpo-Tongkeng are LREE-enriched with small or no negative Eu anomalies. REE patterns of sulfides from different stratiform orebodies are slightly different, but they are similar to those of each host rock. It indicates that the sulfides and host rocks are synsedimentary and that their REE patterns are controlled by the local physical-chemical environments. Y/Ho atomic weight ratios of granite, skarn and sulfides at Lamo vary from 26.3 to 33.7, close to the chondritic Y/Ho ratio. But sulfides from massive and stratiform ores have more variable Y/Ho ratios (28.4-134) that are similar to those of many modern hydrothermal fluids. Hence, the ore-forming fluids responsible for mineralization of the stratiform and massive ores at Dachang were likely to be of exhalative hydrothermal origin. It is suggested that the Dachang deposits are formed through a two-stage hydrothermal process, i.e., the Devonian submarine hydrothermal ore-forming stage and the Yanshanian magmatic-hydrothermal ore-forming stage.

HERMIONE, EVOLUTION OF ATe-BEARING EPITHERMAL MINERALIZATION, ARGOLIS, HELLAS

The Cu-Te-bearing pyrite deposits of Hermione, Argolis are hosted in Miocenic ophiolites. The ophiolites are overlain by a shale-sandstone formation with intercalations of limestones and manganiferous sedimentary rocks. The ore deposits form irregular lenticular or stratiform ore bodies, and veins. These ore bodies are related to volcanic activity in an arc-related rift at the margins of a palaeocontinent. Late N- to NNE-trending, sinistral, milky quartz-pyrite-calcite veins cut the host ophiolites. Alteration haloes of quartz-calcite, albite-sericitechlorite, and chalcedony-epidote-clay minerals are developed in the lavas as concentric shells, or as envelops that parallel the quartz veins. The telluriumbearing mineralization is developed in two successive stages, characterized by the assemblages: pyrite-(pyrrhotite)-magnetite-chalcopyrite-sphalerite (Stage I) and galena-sphalerite-freibergite-marcasite-chalcocite (Stage II), followed by a supergene stage. The cobaltiferous pyrite-chalcopyrite geothermometer defined two ranges of last-equilibration temperatures: 220° to 250°Cfor Stage I, and 120° to 195°Cfor Stage II. The calculated δ18 Ο and SD compositions of the mineralizing fluids, at 200° and 250°C, reflect the dominance of a magmatic component. The calculated δ SH2S fluid values reveal a magmatic source for the sulphur, with minor contribution from submarine sediments, whereas tellurium is proposed to be derived from a mafic-ultramafic source.

Discovery of seismites in Upper Sinian stratiform lead-zinc deposits in Dadu River valley, Sichuan, China: Implication for ore genesis

A number of lead-zinc sulphide deposits have been discovered in recent years in dolomitic rocks of the Upper Sinian Dengying Formation in the Dadu River valley region, Sichuan Province, China. The lead-zinc mineralization is closely associated with a black siliceous exhalite that has been traced over 50 km of strike. This makes Dadu River valley the most extensive stratiform non-ferrous sulphide horizon having been discovered to date in China. In a previous study by the first author (LIN), these deposits were recognised as belonging to the broad spectrum of lead-zinc sedimentary exhalative (SEDEX) deposits. In this paper, seismites and seismogenetic structures occurring in close association with the Dadu River valley mineralized zone are described. The close spatial and temporal relationship between the seismites and mineralization provides strong evidence for the hypothesis that the lead-zinc SEDEX deposits formed in direct response to earthquake activities around the Sinian-Cambrian boundary. The earthquakes are postulated to be an effective dynamic mechanism for triggering the upward migration of deep circulating ore fluids along synsedimentary growth faults and their exhalation onto the sea-floor to form the extensive deposits of stratiform lead-zinc sulphide ore.