Base Metal Sulfide Deposits Research Articles

The link between hydrothermal epigenetic copper mineralization and the Caçapava Granite of the Brasiliano Cycle in southern Brazil

Base-metal deposits in the Caçapava do Sul Copper Province are hosted by both volcanosedimentary rocks of the Bom Jardim Group and by metamorphic rocks of the Passo Feio Formation, and show a spatial relationship to the Caçapava Granite. These associations have led to much controversy about the genesis of the base-metal deposits, which has been at least partly resolved by precise dating using SHRIMP (Sensitive High Resolution Ion Microprobe) U/Pb zircon studies combined with S, Pb, and Sr isotope trace studies. The Passo Feio Formation is Neoproterozoic in age and was derived from a complex continental source, as shown by the presence of xenocryst zircons of Archaean, Paleoproterozoic, and Neoproterozoic ages. It was metamorphosed at ca. 700 Ma. The syntectonic Caçapava Granite that intruded the supracrustal rocks of the Passo Feio Formation at 562 Ma was derived from an old sialic basement. Lead-isotope data are consistent with a 562 Ma age for the base-metal sulphide deposits sited in the Passo Feio Formation. The least-radiogenic compositions lie between the field of the isotopic compositions of the Caçapava Granite and rocks of the Passo Feio Formation, suggesting that Pb in the sulphide deposits may have been derived from both sources. The Pb, like that in the Caçapava Granite and Passo Feio Formation, was derived from a primitive crustal source. Sulphur isotope data from the base-metal sulphide deposits in the Passo Feio Formation are compatible with a mixed sedimentary and magmatic source. The most logical model for ore genesis, based on the isotopic data and spatial relationships, is that magmatic metal-bearing fluids from Caçapava Granite leached metals from the Passo Feio Formation and that the deposited sulphides therefore show mixed isotopic signatures. However, there is also some isotopic evidence from the Caçapava Granite itself that suggests assimilation of S-bearing rocks of the Passo Feio Formation during emplacement. Thus, isotopic signatures could have been inherited from assimilated metal sulphides at this stage, and deposition could have been entirely from Caçapava Granite-derived magmatic fluids. Importantly, the inferred 562±8 Ma age for the deposits in the Passo Feio Formation is younger than the well-constrained age of 594±5 Ma for the Camaquã/Santa Maria deposits. Thus, the epigenetic sulphides in the Passo Feio Formation cannot be the source of these deposits as previously suggested. Other isotopic data also argue against such a model.

Ore Group Classification - A Case Study from Ingaldhal Sulphide Deposit, Karnataka Craton, India

All varieties of base metal sulphide deposits may be classified using base metal ratios viz. Copper Ratio (CR=100Cu/{Cu+Zn}), Zinc Ratio (ZR=100Zn/{Zn+Pb}) and Lead Ratio (PR=100Pb/{Pb+Cu}). Based on this ore group classification an attempt has been made to identify ore groups present in the late Archaean volcanic associated Ingaldhal Sulphide Deposit (ISD) of Karnataka Craton. Using base metal ratios (CR, ZR and PR) five distinct fields are suggested: Cu, Cu-Zn, Cu-Zn-Pb, Zn-Cu and Zn-Cu-Pb/Zn-Pb-Cu with their associated geological and geochemical characteristics. A transitional change in the ore groups is observed from the North Block (Cu rich groups; CR>50) to South Block (Zn and Pb dominated groups; CR<50) and the Main Block is represented by all the five ore groups. This classification may be useful as a pointer in the exploration of polymetallic base metal deposits. Copper rich groups (Cu, Cu-Zn and Cu-Zn-Pb) exhibit high concentration of Bi and Co, and low CaO and Cd whereas the Zn rich groups (Zn-Cu and Zn-Cu-Pb/Zn-Pb-Cu) show high CaO and Cd, and low Bi and Co. The CR of most of the ore groups exhibit a sympathetic relation with Co and Ag. A preliminary attempt has been made for better understanding of ore group genesis of ISD by integrating ore group characteristics with deduced geothermometry.

Metallogenic epochs in the Inner Zone of southwest Japan

The principal metallogenic epochs in the Inner Zone of southwestern Japan are classified by considering mainly radiometric and microfossil age data, as follows—(1) Early Cambrian Epoch: Characterized by podiform-type chromite deposits hosted in ophiolite complexes; these are the earliest-formed mineral deposits in the Japanese Islands, and probably formed in the shallow mantle of island-arc settings; (2) Carboniferous to Permian Epoch: Characterized by Besshi-type base metal sulfide deposits mainly associated with submarine lavas and tuffs of basaltic composition including their metamorphosed equivalents, and by bedded cupriferous pyrite deposits associated with submarine acidic tuffs. Both types of deposits would have been formed in a back-arc basin environment, geochemically influenced by an island-arc; (3) Middle Triassic to Jurassic Epoch: Characterized by most of the bedded manganese deposits associated with bedded cherts, including their metamorphosed equivalents, which would have formed by hot spring activity in an ocean basin environment with a limited current circulation. The manganese deposits and their host rocks are the result of seafloor accretion; (4) Late Cretaceous to Paleogene Epoch: Characterized by extensive continental-arc magmatism (volcanic and plutonic), resulting in formation of various mineral deposit types, related mainly to ilmenite series (Late Cretaceous) or magnetite series (Paleogene) granitic rocks. Related to the former are polymetallic skarns and veins containing Cu, Sn and W; also related to this metallogenic activity are stratabound hydrothermal clay deposits of kaolin, pyrophyllite, and sericite associated with pre-ilmenite series granitoids subaerial felsic pyroclastic rocks. Molybdenite-bearing veins and replacement-type sericite deposits are characteristics of the magnetite series terranes, with local skarns and vein occurrences. As with ilmenite — series granitoids occurring in southwestern Japan, eastward younging is also recognized in these mineral deposits, regardless of ore type; (5) Neogene Epoch: Characterized by intense submarine volcanism of acidic composition and associated Kuroko deposits in the Green Tuff region, a major tectonic division and a major metallogenic province of the Neogene Tertiary of Japan. These mineral deposits would be related to Japan Sea opening. In the `Non-Green Tuff region,' important mineralization events are represented by sedimentary uranium deposits, and veins characterized by Au, Sb or Hg occurrences; (6) Plio-Pleistocene Epoch: Characterized by epithermal Au–Ag and Au–Ag–Cu veins associated with subaerial andesitic to dacitic volcanic rocks of Plio-Pleistocene age. Volcanism of this and following epochs is characteristic of an island-arc setting; and (7) Quaternary Epoch: Characterized by volcanic sulfur deposits and minor iron sand deposits.

Regional alteration systems associated with volcanogenic massive sulfide mineralization at Panorama, Pilbara, Western Australia

Outstanding exposure across the Panorama district of the Archean Pilbara block of Western Australia reveals a cross section through a ca. 3.24 Ga massive sulfide-bearing volcanic pile and an underlying coeval subvolcanic intrusion, in an area of low metamorphic grade and very low strain. This has provided a rare opportunity to map a complete regional-scale hydrothermal alteration system associated with volcanogenic massive sulfide mineralization and to assess the role of a subvolcanic intrusion in driving such a system.Four major alteration facies can be defined in the Panorama district. These are feldspar-bearing background alteration and feldspar-sericite-quartz alteration, and completely feldspar-destructive sericite-quartz and chlorite-quartz alteration. These alteration facies affect all of the volcanic pile and the upper parts of the underlying subvolcanic intrusion. Regional semiconformable alteration zones, which are predominantly feldspar-bearing, are crosscut by broad corridors of transgressive feldspar-destructive alteration. A convective hydrothermal model is invoked to explain this distribution of alteration facies, where semiconformable alteration zones represent seawater recharge, and transgressive feldspar-destructive alteration zones mark the path of evolved seawater discharging back to the sea floor. The interpreted discharge sites are coincident with base metal sulfide deposits at the top of the volcanic pile.Geologic relationships, and the distribution of alteration zones, clearly implicate the Strelley Granite as the subvolcanic intrusion which drove the hydrothermal alteration systems. Granophyric textures and miarolitic cavities in the upper parts of the intrusion suggest that a magmatic fluid phase was exsolved early in its crystallization history. Granite-hosted copper-zinc-tin veins, near the top of the intrusion, are interpreted to be a product of this magmatic fluid. This raises the possibility of a substantial contribution of magmatically derived metal to the overlying volcanogenic massive sulfide hydrothermal alteration systems.

Tectonic settings of Archaean (3325-2775 Ma) crustal-supracrustal belts in the West Pilbara Block

The Archaean Pilbara Block comprises six fault-bounded tectonostratigraphic domains. The West Pilbara Block contains, from east to west, Domains 4, 5 and 6. Review of stratigraphic, magmatic, structural and geochronologic data, and new field mapping, indicates that the crustal and supracrustal rock records correlate to three megacycles. Megacycle IVU (3325-3135 Ma) is recorded by an intra-arc basin and arc granitoids in Domain 6, and a back-arc basin in Domains 4 and 5. The domains were an arc-continent orogen from ∼ 3200-3155 Ma, during which the outboard are setting was accreted to the back-arc continental margin. Although Domain 6 is allochthonous with respect to Domains 5 and 4, it was not exotic with respect to the tectonic settings of the Pilbara continent. Megacycle VL (3135-2955 Ma) is recorded by, in decreasing age: (i) a back-arc basin and arc granitoids in Domain 5: (ii) a deep-marine, retroarc or remnant-ocean basin in Domain 4 and a plutonic belt in Domain 5: (iii) a plutonic belt in Domains 5 and 6, and possibly also in Domain 4: and (iv) craton-wide deformation belts, coeval with the Whim Creek basin and granitoid plutons in Domain 5. The Whim Creek basin was a half-graben that developed at a releasing bend (or overstep) along the boundary fault to Domains 4 and 5, and, to the northeast. Domains 4 and 6. The basin was closed by sinistral transpression as the boundary fault between Domains 4 and 6 propagated to the southwest from the releasing bend, and thereby generating a restraining bend geometry. Fundamental strike-slip faults, which are interpreted to have been tectonic-escape responses to a cause external to the preserved Pilbara Block, juxtaposed domains craton-wide. The initial stage of Megacycle VU (2955-2775 Ma) is recorded by the eroded remnants of a rift province. Preserved elements include the Negri basin (Domain 5 only), mafic-ultramafic sub-volcanic layered intrusions, and granitic plutons and related high-level sills. Two periods of granitoid intrusion are preserved. Base-metal sulphide deposits, hosted by the Whim Creek basin, were probably sourced from extensional geofluids in the rift province. A plume origin is likely for the komatiitic parent magmas to sub-volcanic intrusions, and to basalts of the Negri basin. Plume magmatism is interpreted to have been coincident with, but not the cause of, lithospheric extension because plumerelated volcanism post-dates the first period of plutonic intrusion by ∼ 25 million years. Extension was followed by orogenic deformation that converted Domains 4, 5 and 6 to a hinterland fold-thrust belt. Deformation is interpreted to have been caused by collision orogeny during terrane accretion along the western Pilbara margin. Collision orogeny was the last event of a long history of Pacific-type tectonics along the margins of the Archaean Pilbara continent.

Mineral chemistry and boron isotopic composition of tourmaline from the Devonian metallogenic district of Shanyang-Zhashui, eastern Qinling

Toumaline is widespread in the host strata of strata-bound base metal sulphide deposits in the Devonian metallogenic district around Shanyang-Zhashui in eastern Qinling. As a member of the schorl-dravite series, the tourmaline is characterized by Mg > Fe and Na > Ca, showing apparent chemical zonation which records the geochemistry during its formation and subsequent regional metamorphism and hydrothermal overprint. The close similarity in chemical and isotopic constitutions between the tourmaline of the main metallogenic epoch in this district [FeO/(FeO + MgO)=0.34 − 0.39 and δ11B=−7.6‰ − − 8.8‰] and those related to massive sulphide deposits typical of submarine (exhalative) hydrothermal sedimentation may add further support to a similar mechanism of mineralization for the strata-bound deposits in the district.

Spatial–temporal Frame, Evolution and Mineralization of the Northern Qilian Metallogenic Province

AbstractFour metallogenic epochs occurred in different tectonic environments during the evolution of the Northern Qilian metallogenic province through the geological time. The Middle Proterozoic metallogenic epoch witnessed the tectonic environment of crustal breakup caused by mantle diapirism, in which ultramafic–mafic rocks were intruded along deep fault belts and the superlarge Jinchuan magmatic Cu–Ni sulphide deposit was formed. In the Middle–Late Proterozoic metallogenic epoch the crust was further broken to form an intracontinental rift, in which the Chenjiamiao style massive Cu–Fe sulphide deposits hosted by basic volcanic tuff were formed in the lower volcano–sedimentary sequence, while the large sedex type Jingtieshan style Fe–Cu deposits were formed within the upper abyssal carbon–rich argillaceous sedimentary sequence. The Early Palaeozoic saw the aulacogen environment, within which the Baiyinchang style superlarge massive base and precious metal sulphide deposits hosted by quartz keratophyric tuff were formed in the Middle–Late Cambrian rifted island arc and the massive Cu–Zn sulphide deposits and magmatic chromite deposits associated with the ophiolite suite were formed in the Early–Middle Ordovician, and the Honggou style massive Cu–Fe sulphide deposits hosted by spilite were formed in the Late Ordovician back–arc basin environment. In the Late Palaeozoic–Meso–Cenozoic, the metallogenic province went into an intracontinental orogenic stage characterized by compressive tectonic environment, in which there occurred carbonate–quartz vein type and tectono–alteration gold deposits associated with ductile–shear structures.

Facies analysis of a 1.9 Ga, continental margin, back-arc, felsic caldera province with diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe oxide deposits, Bergslagen region, Sweden

The Bergslagen mining district in south-central Sweden has been a major metal producer for more than 1,000 years. In geologic terms, Bergslagen is the intensely mineralized part of a large, Early Proterozoic (mainly 1.90-1.87 Ga), felsic magmatic region of mainly medium to high metamorphic grade in the Baltic Shield. The district contains a diverse range of ore deposit types, including banded iron-formation, magnetitecalc-silicate skarn, manganiferous skarn- and carbonate-hosted iron ore, apatite-bearing iron ore, stratiform and strata-bound Zn-Pb-Ag-(Cu-Au) sulfide ores, and W skarn. Most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns.This study is a field-oriented, regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration, deformation, and metamorphism. The region is interpreted as an extensional, probably back-arc, active continental margin magmatic region. It evolved through stages of intense magmatism, thermal doming, and crustal extension, followed by waning extension, waning volcanism, thermal subsidence, reversal from extension to compressional deformation, regional metamorphism, and structural inversion. The volcanic successions comprise interfingering proximal (near vent), medial (volcano flanks), and distal (volcano margin) facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite-rhyolite complexes. Nonwelded to poorly welded pyroclastic flow and fallout units, and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies. Subvolcanic porphyritic intrusions and cryptodomes are also abundant. The pyroclastic debris was shed from mainly shallow-water and subaerial proximal areas to shallow- and moderately deep-water (above and below storm wave base) distal areas.Most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash-siltstone, limestone, and vitric crystal sandstone and breccia. These mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base, but locally above the storm wave base. Water depths are interpreted to have been mainly 10 to 500 m, with local deeper areas. Although the ore deposits occur in medial to distal facies associations, many occur above, or laterally adjacent to, subsided volcanic vent complexes. This suggests a spatial, temporal, and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers. Except for the apatite-bearing iron deposits, it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes. This is possible, especially for the base metal sulfide deposits, which have intense localized footwall alteration in felsic volcanic rocks. However, an important role of the volcanic centers may have been to provide strong positive pertubations (+ or - magmatic hydrothermal input) to the regional convective geothermal system.The main Zn-Pb-Ag-(Cu-Au) ore deposits span a range between two end-member types: stratiform ash-siltstone-hosted sea-floor deposits that have similarities to some stratiform sediment-hosted Zn-Pb-Ag deposits, especially the Broken Hill-type deposits; and strata-bound volcanic-associated limestone-skarn subsea-floor replacement deposits that more closely resemble felsic volcanic-associated massive sulfide deposits. Facies models are presented for both. The banded iron-formations are interpreted as chemical suspension deposits, rhythmically interbedded with rhyolitic ash-siltstone. The skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements. Apatite-bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite-rhyolite volcanic center.

Organic matter and clay anomalies associated with base-metal sulfide deposits

Organic matter and clay mineral anomalies surround the Polaris, Gays River and Gaspé Mines base-metal sulfide deposits. The Polaris mine, in the Canadian Arctic, is an MVT deposit formed by low temperature solutions (homogenization temperatures, T h: 105–120°C) and is characterized by a zone of pure illite above a kaolinite envelope surrounding the deposit. These clay mineral assemblages are laterally replaced by kaolinite-illite and chlorite-corrensite zones, while the background assemblage in the host rocks is illite-interstratified illite / smectite-chlorite. The organic matter reflectance ( R o) shows two trends depending on the permeability and chemical reactivity of the host rock: (1) an increase with depth above the ore zones in the overlying impermeable shaly cap rock. R o reaches a maximum value of 1.3% above the ore, compared to 0.8% in equivalent country rocks; (2) an anomalous decrease with depth in the mineralized carbonate horizon that hosts the deposit. The Gays River mine, Nova Scotia, is a medium temperature ( T h: 250°C) carbonate-hosted deposit associated with the replacement of well-crystallized detrital illite and chlorite by smectite forming a halo around the main ore zone and by kaolinite and chlorite-corrensite within the ore zone. Reflectance and its absolute variability increase in the host rocks toward the ore zone adjacent to the deposit (0.8 to 1.88%) and shows a relatively low R o in the ore. The economic mineralization at Gaspé Mines, Québec, is a typical high-temperature skarn and porphyry-copper deposit with the concomitant clay mineral alteration assemblages: chlorite-corrensite-illite, smectite-chlorite-illite and almost pure illite with traces of chlorite. The first assemblage is observed within the ore zone and extends slightly outside the procellanite halo. Smectite is also found near the mineralization, but its limit of occurrence is wider than the chlorite assemblage. The illite assemblage is found outside the chlorite zone and is present as irregular tongues. R o increases (1.2 to 15%) approaching ore over a lateral distance of 60 km. In the bleached zone, the organic matter exhibits mosaic and degasification textures and isotropic alteration rims with very low R o. These three deposits, as well as others studied by the authors, are centrally located within thermal aureoles whose size and intensity correlate directly with increasing temperature of formation. The decreasing values of R o in the ore zones of these deposits and the associated authigenic clay minerals reflect changes in chemical conditions due to precipitation of sulfides and to reaction of the ore and post-ore fluids with host rocks.

Protolith interpretation in metamorphic terranes: a back-arc environment with Besshi-type base metal potential for the Quha Formation, Natal Province, South Africa

A scheme for protolith interpretation of high-grade gneisses is presented which integrates field, petrographic and geochemical criteria to interpret not only the nature of the original rock types, but also the geological setting in which they formed. The amphibolite to granulite-grade supracrustal gneisses of the ∼ 1.2 Ga Quha Formation from the Mzumbe Terrane, Natal Metamorphic Province, South Africa, represent a metamorphosed sequence of predominantly volcanic and volcaniclastic rocks of K-feldspar-absent basaltic to andesitic compositions, along with penecontemporaneous (greywacke) sedimentary rocks derived from their erosion. Interlayered, finely laminated quartz-garnet-pyrite rocks (coticules) point to localised volcanogenic exhalative activity, which may indicate the existence of Besshi-type base metal sulphide deposits. Two types of nearly concordant amphibolite layers represent metamorphosed dykes of pre- and late-tectonic age. Geochemical parameters including ACF, TAS and REE plots show strong calc-alkaline volcanic-arc signatures, while bulk compositional data can be successfully matched with proposed protolith mineral compositions. UPb dating of selected zircon separates from Quha metavolcanic volcanic rocks suggest an age of crystallisation of ∼ 1200 Ma, indistinguishable from that of the pre-tectonic, arc-related Mzumbe tonalite-trondhjemite with which the Quha Formation is intimately associated. However, none of the Quha rocks are cogenetic with the Mzumbe Suite.

Comparison of hydrothermal alteration of carboniferous carbonate and siliclastic rocks in the Valles caldera with outcrops from the Socorro caldera, New Mexico

Continental Scientific Drilling Program (CSDP) drill hole VC-2B [total depth 1761.7 m (5780 ft); maximum temperature 295 °C] was continuously cored through the Sulphur Springs hydrothermal system in the western ring-fracture zone of the 1.14 Ma Valles caldera. Among other units, the hole penetrated 760.2 m (2494.1 ft) of Paleozoic carbonate and siliciclastic strata underlying caldera fill and precaldera volcanic and epiclastic rocks. Comparison of the VC-2B Paleozoic rocks with corresponding lithologies within and around the 32.1 Ma Socorro caldera, 192 km ( 119 miles) to the south-southwest, provides insight into the variability of alteration responses to similar caldera-related hydrothermal regimes.The Pennsylvanian Madera Limestone and Sandia Formation from VC-2B preserve many of the sedimentological and diagenetic features observed in these units on a regional basis and where unaffected by high temperatures or hydrothermal activity. Micrites in these formations in VC-2B are generally altered and mineralized only where fractured or brecciated, that is, where hydrothermal solutions could invade carbonate rocks which were otherwise essentially impermeable. Alteration intensity (and correspondingly inferred paleopermeability) is only slightly higher in carbonate packstones and grainstones, low to intermediate in siltstones and claystones, and high in poorly cemented sandstones. Hydrothermal fracture-filling phases in these rocks comprise sericite (and phengite), chlorite, allanite, apatite, an unidentified zeolite and sphene in various combinations, locally with sphalerite, galena, pyrite and chalcopyrite. Terrigenous feldspars and clays are commonly altered to chlorite and seriate, and euhedral anhydrite “porphyroblasts” with minor chlorite occur in Sandia Formation siltstone. Fossils are typically unaltered, but the walls of some colonial bryozoans in the Madera Limestone are altered to the assemblage chlorite-sericite-epidote-allanite. La, Ce and Nd are present in an unidentified hydrothermal mineral occurring throughout much of the VC-2B Pennsylvanian sequence.Carboniferous carbonate and siliciclastic formations within and around the Socorro caldera show a similar style of alteration and mineralization to their Valles caldera counterparts, but by contrast locally host commercial, caldera-related, base-metal sulfide deposits. As in the Valles rocks, mineralization and alteration in those of the Socorro caldera were strongly controlled by porosity. Unless disrupted by fractures, breccias, or karst cavities ( not identified in Valles caldera drill holes), the rocks remained relatively unaltered. Where these features allowed ingress of mineralizing hydrothermal solutions, base-metal sulfides and rare-earth-element-bearing minerals were precipitated.

The sources of metals in sulfide deposits in the Helgeland nappe complex, north-central Norway; Pb isotope evidence

The Helgeland Nappe Complex of the uppermost Caledonian allochthons of Norway is host to a number of different sulfide deposits. These occur in epicontinental metasedimentary rocks close to, or at, the contact with the granitoid Bindal batholith. Sixty-eight common Pb analyses for six Zn-Pb sulfide deposits and one As-Au deposit make it possible to recognize three distinct episodes of sulfide ore formation, which can be related to successive stages of the tectonic and petrogenetic evolution of the Helgeland Nappe Complex.Prior to peak Caledonian deformation, Zn-Pb sulfides were deposited from ore-forming solutions that derived their lead from a probable Late Proterozoic continental source, as indicated by low 206 Pb/ 204 Pb and 208 Pb/ 204 pb ratios and high 207 Pb/ 204 Pb ratios. Shortly after peak metamorphism and emplacement of the Bindal batholith, skarn related base metal and As-Au vein deposits were formed (lead isotope ratios of the sulfides are indistinguishable from the initial lead isotope ratios of the Bindal batholith), and the metals and ore-forming fluids were probably derived from the granitoid magmas. During the final uplift of the Caledonian nappes after orogenic collapse, structurally controlled base and precious metal sulfide deposits and occurrences were formed. The radiogenic 206 Pb/ 204 Pb ratios and low 208 Pb/ 204 Pb ratios indicate an ore-forming process involving convection of meteoric water and leaching of metals from the rocks in the brittle zone.This lead isotope survey of the sulfide deposits in the Helgeland Nappe Complex has revealed repeated ore-forming activity at previously mineralized sites which involved introduction of new lead when new metal reservoirs became available as a result of changing tectonic settings.

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Post-recrystallisation mobilisation phenomena in metamorphosed stratabound sulphide ores

AbstractMetamorphosed stratabound iron- and base-metal sulphide deposits often exhibit microtextures in which fractures in cataclastically-deformed pyrite porphyroblasts are filled with matrix sulphides; chalcopyrite, sphalerite, pyrrhotite or galena. Discussions of such textures have mostly centred on whether solid-phase or fluid-phase mechanisms were responsible for the movement of the matrix sulphides.The small Zn-Cu sulphide body at Gressli, in the central Norwegian Caledonides, shows these textural features to an extreme degree. Both chalcopyrite and sphalerite show heavy replacive relations to the cataclastically deformed metablastic pyrite, along fracture walls and grain boundaries. They also occur injected along the opened-up triple junctions of foam-textured pyrite. In addition, parts of the ore show patchy quartz with clear replacive relationship to all three sulphides, a feature not often reported from such ores. Such textures can be interpreted to support a mobilisation sequence chalcopyrite-sphalerite-quartz within the Gressli ore. Their extent and degree of development indicate that fluid-phase mobilisation of the three minerals must have played a dominant role. Chalcopyrite and sphalerite are most likely derived from within the ore-mass itself; an external source for the SiO2 seems most probable, in the form of metahydrothermal solutions moving along retrograde shear zones at or near ore-walls.

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.

Framboid-derived structures in some Tasman fold belt base-metal sulphide deposits, New South Wales, Australia

Base-metal sulphide deposits within the Tasman Fold Belt, New South Wales, Australia, contain a range of pyrite textural types which suggest textural evidence for the transformation of framboidal pyrite into pyrite atoll structures in a variety of sulphide and non-sulphide matrices. The writers suggest that the textural transformation occurred while the sulphidic sediments existed as colloids (gels), rather than by growth in the solid state. The resulting conceptual model provides a scheme for the explanation of a wide variety of sulphide textures in these deposits, and also for comparison with similar textures in other sulphide deposits of related ore genesis.

Metallogeny and lead isotopic composition of base metal sulfide deposits in the Rappen area, northern Sweden

Early Proterozoic volcanic and sedimentary rocks of the Rappen district in northern Sweden were deposited at a destructive plate margin to the south of the Archaean craton of the western Baltic Shield. The volcano-sedimentary suite was intruded by two generations of early Proterozoic granites at ca. 1.89–1.85 Ga and ca.1.82–1.78 Ga, respectively, and metamorphosed at upper amphibolite facies conditions. Small stratabound iron, copper, and zinc deposits occur in felsic to mafic tuffs and arkosic sediments. Small deposits of molybdenum, tungsten, and uranium formed during the emplacement of the younger granites. The lead isotopic compositions of sulfide trace lead from the various deposits are highly heterogeneous. In the 206Pb/204Pb–207Pb/204Pb diagram they fall on mixing arrays between little evolved early Proterozoic lead and highly radiogenic Caledonian lead. The least radiogenic lead isotopic compositions from the various deposits have a wide range of 207Pb/204Pb ratios and thus indicate variable involvement of Archaean crustal lead in the Proterozoic deposits. Deposits hosted by siliciclastic rocks have higher 207Pb/204Pb ratios than deposits hosted in mafic to felsic tuffites. The lead isotopic heterogeneity suggests that the lead in the various deposits was locally derived and, furthermore, that the sedimentary rocks in part originated from the Archaean craton to the north. Lead mixing arrays in the 206Pb/204Pb–207Pb/204Pb diagram demonstrate that in Paleozoic time radiogenic lead was mobilized and transported in the basement. Source ages calculated from the mixing arrays (ca.1.9 Ga and ca.1.8 Ga) correspond to the age of the Early Proterozoic volcanism and metamorphism respectively. One group of deposits includes lead from at least three sources and illustrates that radiogenic lead was multiply mobilized and transported in the Proterozoic basement. It occurs in deposits that occur in zones that became permeable during the reactivations of the basement.

Late Proterozoic outer shelf manganese and iron deposits at Otjosondu (Namibia) related to the Damaran oceanic opening

The Otjosondu manganese mining area is located at the eastern exposed extent of the inland branch of the late Proterozoic Damara orogen. In spite of intense deformation and a high-grade metamorphic overprint, the sedimentological interpretation of the manganeserich rocks and their associated lithologies has led to a paleoenvironmental model which gives insight into ore genesis. At least two manganese silicate ore horizons (manganese formations) are associated with banded iron-formations. This sequence is sandwiched between supermature metaquartzarenites interpreted as a shoreface deposit derived from a cratonic basement and deposited under high-energy conditions.Manganese ore varieties and their country rocks have been classified using their texture, mineralogy, and geochemistry and have thus been interpreted in sedimentological and facies terms. The evaluation of the vertical facies sequences, their lateral distribution and their contact relationships renders a spatial reconstruction of the environmental setting for the manganese ores possible. The development of facies from a near-shore clastic regime to manganese-bearing hemipelagic sediments and ultimately to pelagic manganostones and ironstones reflects a zonation on a marine shelf. The facies evolution indicates transgressive conditions on a large scale onto a tectonically stable but subsiding shelf, starting with the transgressive quartzarenites overlying an unconformity to the pre-Damara basement, followed by chemical sediments of the ore-bearing horizon and overlying carbonates, and ending up with pelitic schists. On a smaller scale both transgressive and regressive conditions prevailed during manganese precipitation. Small-scale sedimentary cycles are interpreted as due to glaciogene-influenced fluctuations of the sea level.The source of manganese and iron is proposed to have been situated in the deeper oceanic parts of the associated basin. Initial spreading in the Khomas sea and the formation of oceanic crust is documented by tholeiitic volcanics and associated base metal orebodies representing more proximal manifestations of the exhalative activity. These base metal sulfide deposits are associated with pyrite-bearing graphitic pelagics and this was the first site to partition iron and other base metals from manganese. Upwelling along the shelf edge was involved and additional separation of Mn and Fe was due to a change in Eh conditions associated with a redox interface intersecting with the outer shelf environment. The evolution of the Khomas sea provided the ore-forming solutions and also initiated the subsidence of the passive shelf margin. Fluctuations in sea level were due to the basin development on one hand and glacial-interglacial cycles on the other. The latter fluctuations controlled the amount of terrigenous input into an otherwise pelagic regime.

Mineralogy and genesis of calc-silicates associated with Archean volcanogenic massive sulphide deposits at the Manitouwadge mining camp, Ontario

Skarn-like calc-silicate rocks are reported in spatial association with the Archean Cu–Zn–Ag massive sulphide deposits at the Manitouwadge mining camp, Ontario. Calc-silicates in the footwall of the Willroy mine occur as matrix to breccia fragments of garnetiferous quartzo-feldspathic gneiss and as lenses within garnetiferous quartzo-feldspathic gneiss and are composed of clinopyroxene, garnet, calcic amphiboles, wollastonite, plagioclase, K-feldspar, epidote, quartz, calcite, magnetite, and minor sulphides. Calc-silicates within the main orebody of the Geco mine are characterized by clinopyroxene, calcic amphiboles (Cl–K-rich hastingsitic and ferro-edenitic hornblende, ferro-edenite (up to 4.7 wt.% Cl); and ferroactinolite (6.7 wt.% MnO)), garnet, epidote (including an epidote rich in rare-earth elements and Cl), calcite, quartz, and abundant sulphides. Calc-silicates within the basal 4/2 Copper Zone of the Geco mine contain garnet, gahnite, sphalerite, ferroactinolite (8.5 wt.% MnO), epidote, quartz, biotite, plagioclase, chlorite, muscovite, K-feldspar, and pyrosmalite (with Mn/(Mn + Fe) ratio ranging from 0.21 to 0.61, and up to 3.9 wt.% Cl). The calc-silicates probably represent metasomatic remobilization of dispersed Ca (and Cl) from sea-floor hydrothermal alteration of mafic to intermediate volcanic rocks and are only indirectly related to the hypothesized syngenetic ore-forming processes for the associated base metal sulphide deposits. The calc-silicates formed initially at about 600 °C and 3–5 kbar (1 kbar = 100 MPa) in a mildly reducing environment (from 1 log unit above to 1 log unit below the fayalite–magnetite–quartz buffer) during the upper-amphibolite- to granulite-facies regional metamorphism and were altered subsequently at lower temperatures (&lt;500 °C).

The precious metal-rich, South Hercules mineralization, western Tasmania; a possible subsea-floor replacement volcanic-hosted massive sulfide deposit

South Hercules is a newly delineated, disseminated to semimassive base metal sulfide deposit with significant gold and silver grades. The deposit lies a kilometer along strike south from the Hercules mine, a 2.0-Mt, polymetallic massive sulfide deposit in the Mount Read Volcanics belt, western Tasmania. The mineralization and alteration at the South Hercules deposit has been divided into two major facies: the mineralized sulfide facies and the carbonate facies. The carbonate facies interfingers with and surrounds the upper part of the sulfide facies. The mineralized sulfide facies includes three zones: a massive pyrite + or - barite zone, a siliceous + or - stringer sulfide zone, and a sphalerite-galena + or - pyrite zone. The carbonate facies also consists of three zones: a blebby carbonate-chlorite zone, a massive carbonate zone, and a cherty carbonate zone.The massive pyrite + or - barite zone contains significant gold grades and occurs mostly at the top of the sulfide facies. This zone is characterized by colloform aggregates of pyrite and compact massive pyrite in chert. The siliceous + or - stringer sulfide zone is a silicified and sericitized, tuffaceous shale with disseminated to stringer Pb-Zn mineralization. The massive sphalerite-galena + or - pyrite zone contains subordinate gold and silver and occurs as thin lenses toward the base of the sulfide facies. Significant zinc and lead values occur throughout the mineralized sulfide facies, with the highest values occurring along stratigraphically controlled layers. Copper values are conspicuously low throughout the South Hercules deposit, rarely exceeding 0.5 percent. The carbonate facies is conspicuous by its lack of significant precious metal and base metal grades.Mineragraphic investigations indicate that electrum occurs in two general associations: with pyrite, and with other sulfides such as sphalerite, galena, and tetrahedrite. The electrum locked in the pyrite euhedra is generally fine grained (<5-40 mu m), whereas the electrum associated with other sulfides displays a range between 5 and 200 mu m. Microprobe analysis indicates that the fineness (1,000 Au/Au + Ag wt %) for gold in the deposit ranges from 427 to 965.The delta 18 O isotope composition of the carbonate varies from 9.8 to 16.7 per mil, and the delta 13 C values range from -3.5 to +0.6 per mil. The calculated isotopic composition of the ore fluid at 150 degrees and 250 degrees C is consistent with derivation from evolved seawater. Sulfur isotope analyses of selected samples from the mineralized facies give delta 34 S values of 8.2 to 14.1 per mil; which is in the same range as values from the Rosebery deposit and consistent with values of sulfur derived from Cambrian seawater.The form, texture, mineralogy, stable isotopes, and fluid inclusion data for the South Hercules deposit suggest that it was formed largely by a subsea-floor replacement process from relatively low-temperature (150 degrees -250 degrees C), near-neutral fluids. The very low copper content and relatively high (Au + Ag)/(Pb + Zn) ratio of the ores support a low-temperature environment in which gold was transported as the Au(HS) 2 (super -) complex and Pb and Zn were transported as chloride complexes.