Massive Sulfide Lenses Research Articles

Geochemistry of the Ganesh Himal zinc-lead deposits, central Nepal Himalaya

A dolomite hosted strata-bound high-grade (19-25%) zinc-lead sulphide deposit occurs, between 4,000 m and 5,100 m over an area of 5 km2 in the ~ 7 km wide MCT zone of the Ganesh Himal area of central Nepal. The host crystalline milky white sugary dolomite occurs in a repeated sequence of garnet-mica schist, quartzite, calc-schist and concordant amphibolite of older Lesser Himalayan sequence (Upper Nawakot Group), all showing ductile deformation. The rocks along with the ore have undergone at least three phases of deformation. A series of overturned, steep northerly dipping and NE to ENE plunging anticlines and synclines form the dominating structure, characterised by disharmonic shape of the dolomite host rock because of apparent squeezing out from limbs into the axial regions. Compared to their strike lengths, the ore bodies and the host rock bodies have long extension along the plunge direction. The ore has a very simple composition of sphalerite-galena-pyrite with a little pyrrhotite, magnetite and chalcopyrite. Chemically, it consists mainly of zinc along with iron, lead, silver and very low silica, silicates and alumina. Concentrations of trace and rare elements are very low. Ore body types vary from dissemination and bands to massive sulphide lenses, arranged en echelon parallel to the schistosity/ bedding. The rocks of the area were subjected to almandine-amphibolite facies of metamorphism, to 750±150 MPa pressure and 500 °C to 750 °C temperature conditions. The latest thermal event was as young as ~12 Ma. The lead isotope data are interpreted to establish an age of 875 to 785 Ma for the Ganesh Himal deposits, while sulphur isotope data imply an age greater than 650 Ma. The Ganesh Himal deposit appears to be Vindhyan equivalent in the Himalaya, showing highest metal values as on date. Only two out of six occurrences in the area have been explored so far. The ore reserve estimates stand at 2.4 million tonnes with 14.66% zinc, 3.01 % lead, and 23.5 g/t silver. Taking into account all the occurrences the Ganesh Himal basin might have had 861,000 tonnes zinc and 182,000 tonnes lead at the minimum. The lithologic sequence represents a shallow marine facies of deposition. The δ34 S values for Ganesh Himal sulphides indicate that the sulphur was probably produced by biogenic reduction of contemporaneous seawater sulphate. The lead isotope ratios of Ganesh Himal deposit fall on a single-stage growth curve, on which also fall many big deposits of the world, indicating that the Ganesh Himal zinc-lead deposit has high potential. The primary control of the mineralisation is stratigraphic, the present ore body configuration being controlled by south vergent folding related with the metamorphism and thrusting along the MCT. Pyrite framboids and geochemical characters of ore indicate that the mineralisation is syngenetic sedimentary, and it may have been deposited in association with a bioherm or reef in an anoxic environment. The source of metal ions is not clear yet, but the contemporaneous basic rock bodies providing the metal ions could be a possibility.

Downhole seismic imaging of a massive sulfide orebody with mode‐converted waves, Halfmile lake, New Brunswick, Canada

Multioffset, multiazimuth downhole seismic data were acquired at Halfmile lake, New Brunswick, to image known massive sulfide lenses and to investigate the potential existence of a steeply dipping mineralized zone connecting them. The massive sulfide lenses, which have significantly higher elastic impedances than host rocks, produce strong scattering. The downhole seismic data show prominent scattered (P‐P and S‐S) and mode‐converted (P‐S and S‐P) waves originating from the deposit. Such complex scattering from massive sulfide ore was not observed previously in vertical seismic profiling data. Migration of the scattered and mode‐converted waves from several shot points imaged different parts of the deepest lens. The scattered S‐waves and mode‐converted waves provide additional imaging capabilities that should be considered when selecting downhole seismic methods for mining exploration.

Geology and volcanic stratigraphy of the Canatuan and Malusok volcanogenic massive sulfide deposits, southwestern Mindanao, Philippines

The Canatuan and Malusok massive sulfide deposits are located near Siocon, Zamboanga del Norte, in southwestern Mindanao, Philippines. The Canatuan–Malusok area is underlain by the Jurassic–Cretaceous Tungauan schists, which form much of the Zamboanga Peninsula. The volcanic strata at Canatuan and Malusok can be traced for >7 km along strike and is host to at least three discrete massive sulfide bodies: Canatuan, Malusok and SE Malusok. Basal basaltic andesite volcanic rocks are generally chemically uniform and show only moderate alteration. The massive sulfide deposits occur in overlying rhyolitic to rhyodacitic volcanic rocks that are altered to a schistose assemblage of quartz, sericite, chlorite and pyrite. The alteration is texturally destructive but graded clastic beds are locally observed. Despite tropical saprolitic weathering, four lithogeochemical subunits of the felsic package are identified. Stratigraphic interleaving, however, has made correlation of these units over any significant distance difficult. The sulfide lenses are overlain by a few metres of felsic schists which locally contain manganese-bearing silicates and oxides that serve as a stratigraphic marker. Hangingwall andesitic volcaniclastic rocks are discontinuously preserved, although where present, they consist of regularly bedded mafic volcanic sandstones. The lateral continuity of a manganese-bearing marker and flanking felsic volcaniclastic intervals indicate that locally the volcanic strata form a homoclinal sequence. The Canatuan Au–Ag–Cu–Zn deposit consists of a gossan overlying a massive sulfide lens. The sulfides and gossan are flat lying and hosted within felsic volcanic rocks. The gossan is gold–silver-rich, and was formed by a combination of oxidation and volume collapse of the original sulfide lens. The sulfide minerals present below the current water table, are auriferous massive pyrite with base metal sulfides, with some supergene chalcocite. The transition from gossan to sulfides is very sharp, occurring at the water table. Massive sulfide deposits at Malusok are hosted in the same felsic sequence as Canatuan and they have similar base and precious metal contents. Only limited gossan has been found at Malusok. The bimodal nature of the volcanic rocks at Canatuan, together with their low HFSE contents, near-flat REE patterns and tholeiitic affinities, suggest that they formed in an intra-oceanic arc setting above a depleted mantle source. Mafic and felsic volcanic rocks of similar composition have been recovered from the Tonga-Kermadec and Izu-Bonin-Marianas island-arc systems in the western Pacific. Mafic rocks at Canatuan show no evidence for LILE enrichment that characterizes melts derived from metasomatized mantle under more mature arcs, suggesting that they are the product of a nascent, rather than a mature arc. There is no evidence from the REE, or other incompatible trace elements, that continental crust or evolved arc crust was involved in the generation of the Canatuan-Malusok volcanic rocks. Although it has been proposed that the Zamboanga metamorphic complex comprises microcontinental fragments of Eurasian affinity, our data do not support an evolved crustal setting for the Canatuan-Malusok volcanic rocks, which we suggest were derived from an intra-oceanic arc and subsequently accreted to the eastern Mindanao terrane.

V-RICH MINERALS IN CONTACT-METAMORPHOSED SILURIAN SEDEX DEPOSITS IN THE POBLET AREA, SOUTHWESTERN CATALONIA, SPAIN

The Lower Llandoverian metasedimentary rocks of the Prades Mountains, southwestern Catalonia, Spain, consist of interstratified anorthite-rich beds, chert beds, phosphate beds, sulfide-rich black shale (with mainly pyrrhotite and minor chalcopyrite), massive sulfide lenses and calc-silicate beds. These metasedimentary rocks are anomalously enriched in V and Cr, and have disseminated minerals of precious metals: mainly sperrylite and palladian lollingite. The V- and Cr-rich associations are the result of an isochemical metamorphism of V- and Cr-rich protoliths. The contact metamorphism of V-rich shale produced metapelitic hornfels with V-rich aluminosilicates and V–Cr oxides. The occurrence of V oxides in apparent equilibrium with V-rich silicates suggests a limit for the V and Cr contents in these aluminosilicates at the conditions of thermal metamorphism. On the other hand, metamorphism of anorthite- and V-,Cr-rich sedimentary rocks produced V- and Cr-rich silicates, such as goldmanite, V-rich amphiboles, V-rich titanite and V-rich allanite.

The Distribution of Platinum Group Elements in the Insizwa Lobe, Mount Ayliff Complex, South Africa: Implications for Ni-Cu-PGE Sulfide Exploration in the Karoo Igneous Province

The Mount Ayliff Complex of the Eastern Cape province of South Africa is a layered intrusion of some 800 km2 surface area and up to 1,200 m thickness. On the basis of compositional similarities and spatial association, it is generally interpreted to form part of the Karoo igneous province. Similarities between the Mount Ayliff Complex and the staging chambers and feeder conduits to flood basalts that host magmatic sulfide ores elsewhere in the world suggest that the Mount Ayliff Complex may have an enhanced potential for Noril’sk-Talnakh–type massive Ni-Cu sulfide ores, an idea that is supported by the well-known sulfide occurrence at Waterfall Gorge. Here, we present major-, trace-, and noble-element data for 30 samples of cumulate rocks from a continuous 1,200 m drill core through the Insizwa lobe of the complex, as well as six samples of a footwall sill, the Taylor’s Koppie dike, that was considered a possible feeder zone to the lobe, and through a massive sulfide lens in the footwall of the lobe at Ndzongiseni. We show that most of the Insizwa cumulate rocks contain small amounts of cumulus sulfides. The sulfides, including those at Waterfall Gorge, can be explained by a model in which the sulfides were segregated from magma having chilled-margin composition. On the basis of the composition of the chilled margins, the Insizwa magma contained no entrained sulfides and was depleted in platinum group elements (PGE), although not in Cu and Ni, before emplacement. These compositional characteristics are also observed in lavas of the central Karoo igneous province and suggest only limited sulfide segregation from these magmas at depth. Our findings thus indicate relatively little potential for economic magmatic Ni-Cu-PGE sulfide deposits in the Insizwa lobe.

MINERALIZAÇÃO E ALTERAÇÃO HIDROTERMAL NO DEPÓSITO CUPRO-AURÍFERO DE SERRA VERDE, PROVÍNCIA MINERAL DE CARAJÁS

Among known cupro-auriferous ore deposits of the Itacaiunas shear belt (Carajas Mineral Province), the Serra Verde deposit is the only one that is iron oxide-poor. Located halfway between the cities of Curionopolis and Parauapebas, it is hosted by the Archean Rio Novo Group and consists mainly of two massive sulfide lenses that are concordant to subconcordant with the stratigraphy of the metavolcano-sedimentary host pile. These metric lenses have been thermally metamorphosed by the Estrela granitic complex (EGC), dated at 2.763 Ga, and locally deformed by shear zones. The ore is composed of chalcopyrite with lesser amounts of pyrrhotite, pyrite, cubanite, molybdenite, sphalerite and mackinawite, in this order of abundance. Gold occurs mostly within sulfide grains but its contents are generally below 1 ppm. Metals with more expressive concentrations are silver (8-11 ppm), cobalt (243-935 ppm) and nickel (773-2623 ppm). Rare-earth element totals are up to 42 ppm. Quartz, Mg-hornblende, fluorapatite and ilmenite are the most common gangue minerals. In the deposit area and surroundings three major hydrothermal events have been recorded. The oldest event (I) was contemporaneous with the formation of the host rocks in a submarine environment. The other two are related respectively to the emplacement of the EGC (event II) and the development of shear zones (event III). Successive fluid circulation caused the host rocks to be hydrothermally altered to mineral assemblages that characterize propylitization, silicification, tourmalinization and amphibolitization, in addition to local potassification and albitization. Mineralogical and chemical data allow to estimate at 480-300 o C and 270- 200 o C, respectively, the prevalent temperature intervals of the events II and III. The massive nature of the sulfide ore, the concordant to subconcordant attitude of the lenses in relation to the host rocks, the temporal relationships between the lenses and the EGC, the occurrence of stratiform tourmalinite beds associated with the lenses, and the types of hydrothermal alteration are consistent with a volcanogenic massive sulfide typology. Geochronological data, albeit preliminary, corroborate this interpretation since they point to similar ages for both the mineralization and host rocks. On the other hand, the overwhelming dominance of chalcopyrite over iron sulfides and the uncommon occurrence of fluorapatite and ilmenite do not fit well the proposed model. A compromising hypothesis is to envisage a hydrothermal system in which fluids rich in Cu, P and Ti, derived from a hidden magmatic source, mixed with sea-water and debouched on to the seafloor where sulfides + Au were deposited.

Explanation for many of the unusual features of the massive sulfide deposits of the Iberian pyrite belt

Newly published fluid-inclusion data from quartz in stockwork veins beneath seven massive sulfide lenses in the Iberian pyrite belt suggest that the lenses were formed from fluids that on reaching the sea reversed buoyancy and ponded in basins. Sulfides quenched in the resulting brine pool would have settled to form a sulfide mud. This process provides a relatively efficient trapping mechanism for metal in the fluids and effectively excludes ambient seawater, accounting for the deposits tending to have the characteristics of large size, sheet-like form, absence of relict chimney structures, and a mineral content characterized by pyrite-arsenopyrite, and absence or scarcity of barite, marcasite, and Fe oxides. If total S was less than total metals in the stockwork fluids, some or all of the more soluble Zn and Pb could have been swept from the basin at the overflow, accounting for the variable but generally low Zn and Pb contents of the ores. The lack of sedimentary source for the high salinities implicates magmatic intrusions, possibly similar to those related to Sn-W mineralization.

Geology and Host-Rock Alteration of the Henty and Mount Julia Gold Deposits, Western Tasmania

The Mount Julia-Henty gold deposits are a series of small tonnage ( 20 Mt) of quartz-sericite altered volcanics. The alteration system is hosted in Cambrian submarine volcaniclastic and hyaloclastic dacites associated with interbedded carbonates and calcareous volcaniclastic sandstones located at the Mount Read Volcanics Tyndall Group-Central Volcanic Complex boundary. The alteration zone forms a subvertically dipping tabular sheet over 3 km in length and between 10 and 100 m in width. Although this zone is broadly strata bound in the north, it crosscuts stratigraphy and is hosted 50 to 300 m deeper to the south, well below the Central Volcanic Complex-Tyndall Group contact. The alteration zone is bound updip by the South Henty fault and downdip by abruptly decreasing alteration intensity. Mineralization is confined to lenses within the subvertically dipping alteration zone. The alteration zone is strongly deformed due to reactivation of the South Henty fault. The sericitic alteration assemblage is strongly foliated and mylonitized and the orebody is now located on the steeply west dipping, overturned limb of a broad, shallowly south plunging syncline. Alteration is distinctly asymmetric and can be subdivided into three categories: (1) footwall alteration, (2) alteration associated with mineralization (A zone), and (3) hanging-wall alteration. Alteration in the southern area (Mt. Julia) is similar to the north (Zone 96) but has a much lower sulfide content. Footwall alteration consists of intensely sericite ± pyrite ± carbonate altered schistose rhyolitic and dacitic volcanics. The main mineralized zone (A zone) is also zoned from intensely leached, massive quartz alteration, to quartz-sericite alteration, to an outer quartz-sericite-pyrite-chlorite alteration. Minor massive pyrite and massive sulfide lenses are located at the top of the A zone. Hanging-wall alteration consists of chlorite-albite-quartz alteration in andesitic volcaniclastic rocks and albite-quartz alteration of rhyolitic volcaniclastic rocks and lavas. Bedded carbonates and calcareous volcaniclastic rocks are mainly associated with the upper parts of the A zone alteration zone but are also found toward the footwall and well up into the overlying sequences. The footwall and A zone alteration zones are intensely Na2O depleted and K2O enriched due to feldspar destruction and sericitization. The hanging-wall alteration is strongly Na2O enriched due to intense albitization. Aluminum and K2O have been strongly depleted from the massive quartz alteration within the A zone, suggesting leaching by highly acidic fluids. Mineralization consists of pyrite and chalcopyrite with lesser galena and sphalerite and with minor gold, electrum, galenobismuth, and native bismuth. Gold, copper, and bismuth are mainly confined to the massive quartz and quartz-sericite alteration zones, whereas the outer quartz-sericite-chlorite alteration halo is dominated by pyrite. Metal zonation extends from a gold-silver-rich core associated with copper, lead, and bismuth, to a proximal halo of copper, lead, and bismuth, and then to a distal halo of zinc. Modeling of oxygen and carbon stable isotopes suggests that the carbonates formed from an early phase of basin-wide magmatic CO2 devolatilization that commenced early in the hydrothermal event and continued for a long time period. The carbonates precipitated due to mixing of small amounts of magmatic CO2-rich fluid with seawater at and below the seawater interface. An alternative model suggests that the observed carbon and oxygen stable isotope signature results from fluid-rock interaction between a preexisting carbonate and a later hydrothermal fluid. The Au-Cu-Bi-Ag association and alteration zonation suggests that the Henty-Mount Julia system formed from an evolving submarine Cambrian hydrothermal system, with some low-sulfidation epithermal characteristics. Phase separation of magmatic volatiles from metalliferous magmatic brines formed the alteration halo first, followed by the mineralizing event. Deposition occurred through mixing of the magmatic fluid with bicarbonate/H2S-rich seawater circulating through the unconsolidated Lynchford Member and Central Volcanic Complex volcanics. Mineral deposition was less efficient at Mount Julia where the magmatic volatile phase was dominant and the late, reduced seawater influx less focused, resulting in discontinuous, lower grade mineralization and extensive areas of barren massive quartz and quartz-sericite alteration. The metal zonation is a result of both sulfur availability and decreasing temperature. The Henty-Mount Julia alteration system possibly represents a gold-rich end member of a large, regional, submarine magmatic copper-gold hydrothermal event. This mineralizing event is probably responsible for most of the deposits found on the southeastern side of the Henty fault, including the Mount Lyell deposits.

Volcanic Sequence and Alteration at the Parys Mountain Volcanic-Hosted Massive Sulfide Deposit, Wales, United Kingdom: Applications of Immobile Element Lithogeochemistry

Lenses of massive Zn-Pb-Cu sulfides at the Parys Mountain deposit in Anglesey, northwestern Wales, occur at and near the contact between Ordovician shales and overlying rhyolites. The rhyolites, which have been dated as Lower Silurian (Parrish, 1999), have a thickness of a few hundred meters and are overlain by Lower Silurian shales. The rhyolites and shales strike northeasterly for 2 to 3 km and dip to the north. Minor basalts are present but there are no intermediate rocks. Cu-bearing stockwork veins occur in the upper portion of the Ordovician shales, and Cu-bearing mineralization was mined from the Lower Silurian shales in the 18th century. Precambrian basement rocks are present to the south and north of Parys Mountain. Application of immobile element lithogeochemical techniques to >1,000 drill core and outcrop samples allows division of the rhyolites into five main chemical types (A, B, C, D1, and D2), the shales into three main types, and the mafic rocks into two types. The distribution of these units has been mapped across the property. Where individual volcanic chemical units have lateral continuity, they are considered to represent singular geologic events and thus are time markers. Correlations based on volcanic facies alone are of limited use, because a chemical unit can pass laterally through different volcanic facies, whereas in strongly altered zones the original volcanic facies cannot be recognized at all. The chemostratigraphic units, on the other hand, can be traced through even the most altered zones. Consequently they provide an improved means of correlating volcanic units on local to property scales. The first volcanic rocks to accumulate above the Ordovician shales were thin flows and volcaniclastic beds of rhyolite B and a laterally adjacent lens of largely massive rhyolite C. The main massive sulfide lenses in the western part of the property are associated with these volcanic events. These units were covered by thick flows of rhyolite A in the western part of the property. Rhyolites D and A are the main chemical types in the eastern part. Rhyolite D has two subtypes: D1 is generally massive and flow banded, whereas D2 is pyroclastic. It is suggested that rhyolite D1 was erupted first, followed by less fractionated rhyolite D2, which was the last volcanic event prior to the accumulation of the Silurian shales. The main vents for rhyolite A in the west and D in the east were separated by about 1 km. The alteration minerals at Parys Mountain are sericite, chlorite, quartz, pyrite, and Fe-bearing carbonates. Mass changes calculated for the rhyolites and the shales show a general property-scale increase in alteration in the downdip direction. FeO and MgO additions of up to 10 and 20 wt percent, respectively, are present in footwall shales up to 50 m below the massive sulfide lenses (in areas where stockwork veins are present). Similar FeO and MgO gains occur in rhyolites up to 50 m above the ore lenses. In rhyolites with Fe + Mg gains, Si and K have generally been lost. In areas lateral to the massive sulfides, shales and rhyolite B have gained large amounts of silica. Most Parys Mountain rhyolites have moderately high Nb contents and slightly inclined rare earth element (REE) patterns, whereas the basalts are enriched in the high field strength elements (HFSE) including the light REE. The primary geochemical features of the volcanic rocks, together with regional geologic relationships in northwestern Wales, suggest that volcanism occurred during intraplate rifting of a small marine basin, which was underlain by Precambrian continental crust. The small volumes of all rhyolite types (and basalt) at Parys Mountain suggest that they were derived from small, discrete pods of magma, which were tapped by basement faults. Most of the massive sulfides accumulated immediately before, and during, the first outpourings of rhyolitic lava.

New Mapping and Interpretations of the Mount Lyell Mining District, Tasmania: A Large Hybrid Cu-Au System with an Exhalative Pb-Zn Top

New detailed mapping of the Mount Lyell mines area has resulted in a better understanding of the large Cambrian volcanic-hosted alteration-mineralization system. The mapping integrates the exhalative top and silica-rich upper part of the system at Lyell Comstock, the large displaced mass of silica-rich schists with high-grade bornite ores at North Lyell, and the deeper levels of the system exemplified by the low-grade Prince Lyell deposit. The 6-km-long alteration system has a core zone of pyrite-rich sericite-chlorite-silica schists, which contains most of the copper orebodies, flanked by marginal zones of pyrite-poor sericite and chlorite schists. The alteration has been focused along the Great Lyell fault, a major reverse-type growth fault, which forms the contact between the volcanic rocks and the younger (Late Cambrian) siliciclastic conglomerate sequence of the Owen Group. Small lead-zinc-rich massive sulfide lenses, together with lenses of fossil-rich limestone, characterize the exhalative zone, which is hosted within submarine volcaniclastic breccias and sandstones in the basal part of the late Middle Cambrian Tyndall Group at Comstock. The upper part of the alteration zone immediately below this is characterized by numerous cherty silica masses or heads contained within sericite-pyrite-silica schists, culminating in the very large Comstock chert body that appears to have formed a siliceous cap to the system. Ore lenses of disseminated chalcopyrite-pyrite, with some bornite, were located below the silica cap. At North Lyell, a large displaced mass of schists from the upper part of the system has numerous silica heads and a large cap-like silica body, the North Lyell chert, similar to the Comstock chert, which has been generally misinterpreted as silicified Owen conglomerate. Bornite-chalcopyrite ores were located immediately beneath this chert body, and several chalcopyrite-pyrite lenses are present at slightly deeper levels. The schist mass was apparently emplaced into the Owen basin by collapse from the scarp of the Great Lyell fault, after the alteration zone was uplifted and unroofed by erosion. The sulfide- and chert-rich mass was exposed, oxidized, and partially eroded during its slow, nappe-like emplacement, and gossan-like hematite-barite bodies and hematite-chert breccias were developed along the contact in places and interfinger with the contemporaneous Middle and Upper Owen sediments. There appears to be a zonation in the copper ores from disseminated chalcopyrite-pyrite in the deeper levels, as at Prince Lyell, to chalcopyrite with minor bornite in the silica heads zone (exemplified by Comstock and Western Tharsis), to bornite-rich ores with lesser chalcopyrite in the upper levels against the silica cap, as at North Lyell. Pyrophyllite is associated with the bornite mineralization, which appears to represent a high-sulfidation phase, possibly associated with increased or renewed magmatic activity, rather late in the development of this large, hybrid, magmatic-seawater system. Much of the upper part of the system, probably including some rich orebodies, was apparently eroded off during deposition of the Owen Group.

Magnetite-rich calc-silicate alteration in relation to synvolcanic intrusion at the Ansil volcanogenic massive sulfide deposit, Rouyn-Noranda, Quebec, Canada

The Ansil Cu–Au volcanogenic massive sulfide deposit is located within an Archean-age cauldron infill sequence that contains the well-known Noranda base metal mining district. The deposit is unusual in that 17% of the massive pyrrhotite–chalcopyrite orebody is replaced by semi-massive to massive magnetite. Temporally associated with the magnetite formation are several calc-silicate mineral assemblages within the massive sulfide lens and the underlying sulfide stockwork vein system. Coarse-grained andradite–hedenbergite and ferroactinolite–ilvaite alteration facies formed in the immediate footwall to the massive magnetite–sulfide lens, whereas an epidote–albite–pyrite-rich mineral assemblage overprints the margins of the chlorite-rich stockwork zone. The epidote-rich facies is in turn overprinted by a retrograde chlorite–magnetite–calcite mineral assemblage, and the andradite–hedenbergite is overprinted first by ferroactinolite–ilvaite, followed by semi-massive to massive magnetite. The footwall sulfide- and magnetite-rich alteration facies are truncated by a late phase of the Flavrian synvolcanic tonalite–trondhjemite complex. Early phases of this intrusive complex are affected to varying degrees by calc-silicate-rich mineral assemblages that are commonly confined to miarolitic cavities, pipe vesicles and veins. The vein trends parallel the orientation of synvolcanic faults that controlled volcanism and hydrothermal fluid migration in the overlying cauldron succession. The magnetite-rich calc-silicate alteration facies are compositionally similar to those of volcanic-hosted Ca–Fe-rich skarn systems typical of oceanic arc terranes. Tonalite–trondhjemite phases of the Flavrian complex intruded to within 400 m of the base of the earlier-formed Ansil deposit. The low-Al trondhjemites generated relatively oxidized, acidic, Ca–Fe-rich magmatic–hydrothermal fluids either through interaction with convecting seawater, or by assimilation of previously altered rocks. These fluids migrated upsection along synvolcanic faults that controlled the formation of the original volcanogenic massive sulfide deposit. This is one of the few documented examples of intense metasomatism of a VMS orebody by magmatic–hydrothermal fluids exsolved from a relatively primitive composite sub-seafloor intrusion.

AU/AG RATIO VARIATIONS AT MINA III, MINA NOVA AND MINA INGLESA GOLD DEPOSITS, CRIXÁS GREENSTONE BELT, BRAZIL

The Mina III and Mina Nova gold deposits occur at the transition between metabasic volcanic rocks and chemical and detrital metasedimentary rocks, while Mina Inglesa occurs within metakomatiites, all in the Archean Crixás Greenstone Belt. Mina III contains three main mineralized zones: the Upper (UOZ-ms), Lower (LOZ-qv and LOZ-cs) and Garnet (GOZ) Ore Zones. The UOZ-ms is composed of massive sulfide lenses within Fe-dolomitic marbles and Fe and Al-rich rocks. A concordant quartz vein within carbonaceous schists represents the LOZqv, while the LOZ-cs consists of an arsenopyrite-pyrrhotite-bearing carbonaceous schist close to the quartz-vein contact. The GOZ comprises concordant quartz veins within quartz-chlorite-muscovite-garnet schist. At Mina Nova, two main ore zones occur: the Orebody 1 (OB1-cs), consisting of an arsenopyrite-pyrrhotite-bearing carbonaceous schist and the Orebody 2 (OB2-cm), represented by a carbonate-muscovite schist lenses within carbonaceous schist). Mina Inglesa has one mineralized concordant quartz vein (MOB) within talc schists. Mineralogical characterization of the orebodies indicates gold association with arsenopyrite, pyrrhotite, chalcopyrite, carbonate, quartz, micas and oxides at the UOZ; with quartz, carbonaceous matter and arsenopyrite, and less frequently with carbonate, pyrrhotite and micas at the (LOZ-qv and LOZcs); mainly with quartz, micas and arsenopyrite, and more rarely with chalcopyrite and oxides at the GOZ; with arsenopyrite, pyrrhotite and carbonate at the OB1; with carbonate, muscovite and arsenopyrite at the OB2; and with quartz and galena at the MOB. Microprobe analysis in individual gold grains of all ore zones indicate very low values of other elements (less than 0.5 wt. %). The identified Au/Ag ratio variations are intimately associated with host minerals or mineral associations of each ore zone. Initial Au/Ag ratios in source-rocks, transport (hydrosulfide and chloride complexes) and deposition mechanisms (fluid-rock interactions and fluid immiscibility) may have had great importance in the observed chemical variations.

AU/AG RATIO VARIATIONS AT MINA III, MINA NOVA AND MINA INGLESA GOLD DEPOSITS, CRIXÁS GREENSTONE BELT, BRAZIL

The Mina HI and Mina Nova gold deposits occur at the transition between metabasic volcanic rocks and chemical and detrital metasedimentary rocks, while Mina Inglesa occurs within metakomatiites, all in the Archean Crixas Greenstone Belt. Mina III contains three main mineralized zones: the Upper (UOZ-ms), Lower (LOZ-qv and LOZ-cs) and Garnet (GOZ) Ore Zones. The UOZ-ms is composed of massive sulfide lenses within Fe-dolomitic marbles and Fe and Al-rich rocks. A concordant quartz vein within carbonaceous schists represents the LOZ- qv, while the LOZ-cs consists of an arsenopyrite-pyrrhotite-bearing carbonaceous schist close to the quartz-vein contact. The GOZ comprises concordant quartz veins within quartz-chlorite-muscovite-garnet schist. At Mina Nova, two main ore zones occur: the Orebody 1 (OB1-cs), consisting of an arsenopyrite-pyrrhotite-bearing carbonaceous schist and the Orebody 2 (OB2-cm), represented by a carbonate-muscovite schist lenses within carbonaceous schist). Mina Inglesa has one mineralized concordant quartz vein (MOB) within talc schists. Mineralogical characterization of the orebodies indicates gold association with arsenopyrite, pyrrhotite, chalcopyrite, carbonate, quartz, micas and oxides at the UOZ; with quartz, carbonaceous matter and arsenopyrite, and less frequently with carbonate, pyrrhotite and micas at the (LOZ-qv and LOZ- cs); mainly with quartz, micas and arsenopyrite, and more rarely with chalcopyrite and oxides at the GOZ; with arsenopyrite, pyrrhotite and carbonate at the OB1 ; with carbonate, muscovite and arsenopyrite at the OB2; and with quartz and galena at the MOB. Microprobe analysisin individual gold grains of all ore zones indicate very low values of other elements (less than 0.5 wt. %). The identified Au/Ag ratio variations are intimately associated with host minerals or mineral associations of each ore zone. Initial Au/Ag ratios in source-rocks, transport (hydrosulfide and chloride complexes) and deposition mechanisms (fluid-rock interactions and fluid immiscibility) may have had great importance in the observed chemical variations.

The role of geophysics in the discovery and delineation of the Cosmos Nickel Sulphide Deposit, Leinster area, Western Australia

Cosmos is a typical Kambalda-style, komatiite-hosted, highly conductive and magnetic, massive pyrrhotite-pentlandite sulphide deposit. It is located approximately 40 km south of the Mt. Keith nickel mine in the northern Yilgarn Craton of Western Australia. It was discovered in mid-1997 using a combination of geological, geochemical and geophysical exploration techniques during routine evaluation of a prospective ultramafic belt. The discovery hole was the initial test of a short strike-length, late-time transient electromagnetic (TEM) anomaly in an area that had previously been partially drill tested by other companies. The original moving-loop TEM anomaly, recognised on two 150 m spaced lines, was defined in detail using a large, fixed transmitter loop survey. Modelling of the late-time fixed-loop data indicated that the source was a steeply east dipping conductor at about 50-75 m depth. Oregrade massive nickel sulphides were intersected in the predicted position in the first diamond hole drilled to test the conductor. The limited depth extent of the conductor suggested by the modelling was confirmed by subsequent drilling and downhole TEM surveys. Detailed ground and aeromagnetic data subsequently collected over the deposit defined the host ultramafic well, but did not clearly distinguish the magnetic massive sulphide zone. The significance of the Cosmos TEM surveys was their ability to quickly and accurately focus drilling on the small but high-grade massive sulphide lens within the much larger mineralised halo.

Facies architecture of the felsic lava‐dominated host sequence to the Thalanga massive sulfide deposit, Lower Ordovician, northern Queensland

The Thalanga volcanic‐hosted massive sulfide deposit occurs in the Cambro‐Ordovician Mt Windsor Subprovince in northern Queensland. The orebody comprises steeply dipping, stratiform, sheet‐like, polymetallic massive sulfide lenses. Overall, the volcanic facies architecture at Thalanga is dominated by quartz‐ and/or feldspar‐phyric lavas and synvolcanic intrusions that comprise coherent facies and in situ and resedimented autoclastic facies. Systematic phenocryst logging (mineralogy, abundance, size) has been used to discriminate separate emplacement units of rhyolite in the footwall and dacite in the hangingwall. Some of the petrographically different rhyolite and dacite types can also be distinguished using immobile‐element geochemistry. Rhyolitic lavas and intrusions in the footwall are weakly to strongly altered. Apparent clastic textures resulting from hydrothermal alteration and metamorphism are widely developed in the coherent facies. Genuine clastic textures are characterised by clasts with randomly oriented internal laminar or banded fabric (e.g. rotated, flow‐laminated clasts), marked and consistent differences in quartz phenocryst abundance and/or size range between clasts and matrix, and normal grading. Mass‐flow‐emplaced, rhyolitic breccia units delineate palaeo‐sea‐floor positions in the footwall that are potentially prospective for exhalative massive sulfide mineralisation. A comparison of the distribution of clastic and coherent facies with the geometry of strongly altered zones in the footwall indicates that intense hydrothermal fluid flow was independent of the facies arrangement. The massive sulfide lenses conformably overly altered footwall rhyolite and occur in a distinctive facies association which includes coarse quartz‐phenocryst‐rich rhyolitic sills with peperitic contacts and crystal‐rich polymictic breccia. The hangingwall to the orebody consists of largely unaltered dacitic lavas and synvolcanic intrusions and minor dacitic pumice breccia, dacitic breccia and polymictic volcanic breccia. The facies architecture shows that the Thalanga massive sulfide deposit formed in a below‐storm‐wave‐base depositional environment on top of an elevated, lava‐dominated, rhyolitic volcanic centre. A modern analogue for the setting of the Thalanga massive sulfide is the PACMANUS hydrothermal field on the crest of the dacite lava‐dominated Pual Ridge in the eastern Manus backarc basin (Papua New Guinea).

Geology and mineralization of the Cannington Ag-Pb-Zn deposit; an example of Broken Hill-type mineralization in the eastern succession, Mount Isa Inlier, Australia

The Cannington Ag-Pb-Zn deposit represents an important new discovery of Ag-rich Broken Hill-type mineralization in the Eastern succession of the Mount Isa inlier. The deposit is concealed beneath 10 to 60 m of Recent and Cretaceous cover, and there is no oxidation profile preserved at the basement subcrop. Mineralization is hosted by amphibolite facies migmatitic quartzofeldspathic gneisses, and is characterized by intense deformation and metamorphism, with complex metasomatic and retrograde overprints. Lithostratigraphic correlations of the host lithologies with other units in the Eastern succession are unclear. Limited dating of probable stratigraphic equivalents has given an age of 1677 + or - 9 Ma, which is broadly coeval with host depositional ages for Pb-Zn-Ag mineralization at Mount Isa, HYC and Broken Hill.The orebody is divided on the basis of late structural displacement into Northern and Southern zones. The Southern zone is the focus of current development, and mineralization occurs as crudely strata-bound massive sulfide lenses that display complex brittle and ductile disruption. A large-scale isoclinal D 2 synform within the Southern zone appears to control broad repetition patterns between ore lenses. Grade control within individual ore zones can also be related to zones of ductile strain and metasomatism influenced by strain partitioning around the termination of the Core Amphibolite.Mineralization within the Cannington Southern zone is divided into five main economic lode horizons that incorporate 10 mineralization types. These types are defined on the basis of distinctive zonations in Pb/Zn ratios, and Fe-rich versus siliceous gangue lithologies. Fe-rich mineralization types are characterized by coarse-grained, equigranular hedenbergite, Mn-Fe pyroxenoid, magnetite, olivine, and fluorite mineralogies, zones of amphibole, almandine, ilvaite, pyrosmalite-dominant mineralogies with sulfide- and fluorite-rich ductile breccias are associated with extensive postpeak metamorphic metasomatism and retrogression. Siliceous mineralization types represent late-stage metasomatism, and are associated with further modification of mineralization and retrogression of Fe silicates. Siliceous mineralization types exhibit a distinctive low abundance of magnetite and fluorite.Dominant sulfides are galena and sphalerite, which show multiple generations and variable intergrowths. Subordinate magnetite-pyrrhotite with minor arsenopyrite-lollingite-chalcopyrite are characteristic of Fe-rich mineralization types. Pyrite is generally absent and is only locally associated with late structural and low-temperature metasomatic overprints. Extreme Ag enrichment is a consistent association of all mineralization types in the Cannington deposit, and is related to argentiferous galena with freibergite inclusions. High levels of Sb, Cd, As, Cu, and F are also a feature of specific mineralization types. When in full production, Cannington will be one of the world9s largest Ag producers.Cannington shows many similarities with the Broken Hill Main lode (New South Wales), and represents an important new example of a Broken Hill-type classification. However, the Ag enrichment that characterizes Cannington is unusual even for previously considered Ag-rich members of the classification. A genetic model is proposed that involves high-temperature metasomatic zone refining of a preexisting Fe-Ca-Mn-Pb-Zn-Ag-rich mineralized system.

Alteration and ore mineral characteristics of the Archean Coniagas massive sulfide deposit, Abitibi belt, Quebec

The Coniagas Mine in the northeastern Abitibi greenstone belt is a small, isolated Archean, volcanic-hosted, massive sulfide deposit rich in Zn-Pb-Ag. The Main lens, which is part of four massive sulfide lenses, is restricted to a 40 m thick massive felsic lapilli tuff unit of the 280 m thick sequence. The massive sulfides, a product of subsurface replacement, have features common to both Mattabi- and Noranda-type deposits. The Coniagas Mine sequence represents part of a small subaqueous volcanic edifice that probably evolved close to an arc or back-arc spreading ridge. A distinct alteration halo of chlorite + sericite ± epidote ± spessartine garnet in the immediate footwall and a hanging wall alteration assemblage of quartz + sericite ± epidote ± chlorite characterize the deposit. The sphalerite + pyrite + galena ± chalcopyrite sulfide mineral assemblage in the Main lens differs significantly from the pyrite + chalcopyrite + sphalerite + pyrrhotite ± galena assemblage in the stringer zone. Chlorite compositions are Fe rich close to the mineralized zone, with an Fe/(Fe + Mg) ratio of 0.38-0.48 in the hanging wall and 0.65-0.70 below the ore. Delicate sulfide textures including colloform pyrite and concentric sphalerite are consistent with a low temperature of formation, whereas higher temperatures are inferred for the stockwork zone. Electron probe microanalysis of sphalerite supports inferred hydrothermal fluid temperatures. The low Fe contents (6.7-10.8 mol% FeS) in sphalerite associated with colloform pyrite of the Main lens contrast with the elevated Fe content (12.7-14.1 mol% FeS) in sphalerite from the stockwork.

Alteration and ore mineral characteristics of the Archean Coniagas massive sulfide deposit, Abitibi belt, Quebec

The Coniagas Mine in the northeastern Abitibi greenstone belt is a small, isolated Archean, volcanic-hosted, massive sulfide deposit rich in Zn-Pb-Ag. The Main lens, which is part of four massive sulfide lenses, is restricted to a 40 m thick massive felsic lapilli tuff unit of the 280 m thick sequence. The massive sulfides, a product of subsurface replacement, have features common to both Mattabi- and Noranda-type deposits. The Coniagas Mine sequence represents part of a small subaqueous volcanic edifice that probably evolved close to an arc or back-arc spreading ridge. A distinct alteration halo of chlorite + sericite ± epidote ± spessartine garnet in the immediate footwall and a hanging wall alteration assemblage of quartz + sericite ± epidote ± chlorite characterize the deposit. The sphalerite + pyrite + galena ± chalcopyrite sulfide mineral assemblage in the Main lens differs significantly from the pyrite + chalcopyrite + sphalerite + pyrrhotite ± galena assemblage in the stringer zone. Chlorite compositions are Fe rich close to the mineralized zone, with an Fe/(Fe + Mg) ratio of 0.38-0.48 in the hanging wall and 0.65-0.70 below the ore. Delicate sulfide textures including colloform pyrite and concentric sphalerite are consistent with a low temperature of formation, whereas higher temperatures are inferred for the stockwork zone. Electron probe microanalysis of sphalerite supports inferred hydrothermal fluid temperatures. The low Fe contents (6.7-10.8 mol% FeS) in sphalerite associated with colloform pyrite of the Main lens contrast with the elevated Fe content (12.7-14.1 mol% FeS) in sphalerite from the stockwork.

World‐class Archaean gold deposits in Canada: An overview

The Archaean Superior and Slave Cratons in Canada contain 14 ‘world‐class’ gold deposits (i.e. > 100 t Au). Most of these occur in greenstone belts, in spatial association with major fault zones, and a few in turbidite sedimentary sequences. These deposits display a significant diversity of geological attributes and timing relations with other geological events. The different styles of deposits include: quartz‐carbonate veins related to shear zones and folds (Dome, Pamour, Hollinger‐McIntyre, Kerr Addison, Kirkland Lake, Sigma‐Lamaque, Con‐Giant, parts of Lupin); zones of disseminated sulfides ± stockworks around porphyry bodies (Hemlo, Malartic); massive sulfide lenses (Home, Bousquet 2, parts of Lupin), sulfide‐rich veins, stockworks and disseminated sulfides (Doyon, Bousquet 1); and carbonate ± quartz veins (Campbell‐A.W. White). Such diversity requires that Canadian Archaean world‐class gold deposits be of multiple geological and genetic types. Three broad categories of models considered to be applicable include the following deposit types: synvolcanic deposits of exhalative, sub‐sea‐floor replacement or shallow‐marine epithermal origins; intrusion‐related deposits as proposed for young arc settings; and syntectonic ‘mesothermal’ vein deposits. An important consequence is that multiple sets of exploration criteria derived from these different models is required in support of future exploration strategies.

Mineralogy of gold in the Elshitsa massive sulphide deposit, Sredna Gora zone, Bulgaria

The Elshitsa volcanic hosted massive sulphide deposit occurs in the central part of the Srena Gora metallogenic zone in Bulgaria. The gold-bearing massive sulphide mineralization is considered to be the product of an island arc volcano-plutonic process and hydrothermal activity that took place during the Late Cretaceous. In addition to the major gold-hosted opaque minerals such as pyrite, chalcopyrite, sphalerite and galena there are minor phases of tennantite, goldfieldite, Se-bearing aikinite, native silver and bornite in the massive sulphide lenses and stringer zones. Most of the sulphide minerals are Se-bearing. All of the six mineral assemblages that were deposited during the pyrite and copper-pyrite stages of mineralization are gold-bearing. The gold tenor as a rule is less than 1 g/t. Native gold and electrum occur as blebs or intergranular particles in the sulphide minerals. Gold in the early massive pyrite is of submicroscopic type ( 100 μm) in the late gold-sulphide assemblages. The electrum fineness in 41 individually studied grains varies between 780 and 992‰ with a mean of 895‰. Native silver was found in association with bornite. Cu, Te, Sb and Bi are the most common trace-elements in gold and electrum. The Cu-Zn-Pb association is most important as a Au-Ag-carrier. A model for gold behaviour during sulphide deformation is proposed involving coarsening of gold grain size from the earlier to the later sulphide mineral assemblages.