Volcanic-hosted Massive Sulfide Research Articles

Petrology and geochemistry of sphalerite from the Cambrian VHMS deposits in the Rosebery-Hercules district, western Tasmania: Implications for gold mineralisation and Devonian metamorphic-metasomatic processes

Sphalerite is the major ore mineral in the Zn-rich, volcanic-hosted massive sulphide deposits of western Tasmania. These deposits have been affected by regional metamorphism to upper greenschist facies, and associated tectonic deformation related to the Devonian Tabberabberan Orogeny. The southern end of the Rosebery deposit has undergone metasomatic replacement related to a post-orogenic Devonian granite intrusion.

Productivity of volcanic-hosted massive sulfide districts: New constraints from the δ18O of quartz phenocrysts in cogenetic felsic rocks

A correlation has been established between Zn productivity of volcanic-hosted massive sulfide districts and δ 18 O of quartz phenocrysts from cogenetic rhyolitic rocks. In highly productive districts, ores are associated with rhyolitic rocks, of possible S-type affinity, containing high δ 18 O quartz phenocrysts. In less-productive districts, ores are associated with I-type rhyolitic rocks with low δ 18 O phenocrysts. This correlation may be caused by (1) low-temperature isotopic alteration that produced 18 O-rich overgrowths on phenocrysts, and (2) emplacement of transitional S-type intrusions at greater depths than I-type intrusions, resulting in larger hydrothermal cells and allowing leaching of more Zn. The latter hypothesis is favored.

The subsea-floor formation of volcanic-hosted massive sulfide; evidence from the Ansil Deposit, Rouyn-Noranda, Canada

The Ansil Cu-Zn volcanic-hosted massive sulfide deposit is located approximately 14 km north-northwest of Rouyn-Noranda, Quebec, within the Blake River Group of the Archean southern Abitibi greenstone belt. The deposit is the second lowest within the Central Mine sequence, a series of bimodal rhyolite-andesite formations that fill in the Noranda cauldron, host to 17 volcanic-hosted massive sulfide deposits. The deposit consists of a single massive sulfide lens situated at the contact between rhyolite of the Northwest formation and overlying andesite of the Rusty Ridge formation. The rhyolite flow forms a ridge up to 500 m high which was inundated by the andesite flows. The orebody is hosted within a unit of quartz porphyritic, finely layered, felsic volcaniclastite (Cranston tuff) that fills in an east-west-trending graben formed on the upper surface of the rhyolite flow. At the east end of the graben the Cranston tuff is interlayered with a thin unit of dacite flows. The deposit lies within an extensive discordant alteration zone characterized by feldspar destruction and Na depletion. The zone extends from 400 m below the deposit at the contact of the synvolcanic Flavrian Intrusive Complex to over 400 m above the deposit. This alteration zone is controlled by a fault system that served as a focus for hydrothermal fluid upflow from the beginning of cauldron formation until the end of the first cauldron cycle. Within the Na depletion zone there are three distinct alteration facies associated with the orebody. Early hydrothermal activity is defined by hydrothermal explosion breccias that formed along the base of the east-west trending graben walls. They were filled with finely banded quartz-albite- sphalerite-pyrite, and the wall rock altered to sericite-quartz, with finely disseminated sphalerite and pyrite- pyrrhotite. The graben was then filled with the Cranston tuff, which was selectively altered and mineralized by the Zn-rich fluid. Subsequent development of northerly trending faults was accompanied by the upflow of Cu-rich fluid that selectively chloritized the footwall rhyolite below the graben floor and formed a ...

Toward an understanding of the metallogeny of the Tasman fold belt system

The Tasman fold belt system which makes up the eastern third of the Australian continent is a composite of five Paleozoic orogenic belts. The major mineral provinces of the Tasman fold belt system include provinces of volcanic-hosted massive sulfide deposits, porphyry-granitoid-associated Cu-Au and Au deposits, and granitoid- associated Sn and W deposits as well as provinces of structurally controlled Au and Cu-Pb-Zn deposits. The localization of these provinces is discussed in terms of the nature of the magmatism which is a source of heat and in many cases of fluids and metals, the fracture systems which facilitate the transfer of heat, magmas, and/or aqueous fluids through the crust, and the timing and nature of the stress regimes that permit these fracture systems to be open to fluid flow. The available data indicate that the magmatism of the Cu-Au province of Central-West New South Wales originated in the mantle. Magma systems linked with the deposits of the volcanic-hosted massive sulfide province of western Tasmania, the Sn-W provinces of the Tasman fold belt system, and the porphyry-associated Au deposits of north Queensland had their roots in crust mantle interactions. Crystal fractionation was an important process in the magma systems related to the development of these provinces. A feature of the Tasman fold belt system is the prevalence of major north-south-trending fault systems. More subtle west- northwest structural trends can be appreciated at the continent scale and these Proterozoic structural trends appear to have exerted a significant control on the distribution of major mineral provinces within the Tasman fold belt system. Extension reactivation of basement structures late in tectonic cycles was a major mechanism for creating permeable domains in the upper crust and focusing fluid flow and hydrothermal mineralization. It is suggested that the Permo-Carboniferous Au and Sn provinces of north Queensland developed.

Target vectoring using lithogeochemistry: applications to the exploration for volcanic-hosted massive sulphide deposits

Target vectoring using lithogeochemistry: applications to the exploration for volcanic-hosted massive sulphide deposits

Selenium and its importance to the study of ore genesis: the theoretical basis and its application to volcanic-hosted massive sulfide deposits using pixeprobe analysis

Distribution coefficients for the partitioning of selenium between aqueous fluids and sulfide minerals have been estimated from existing thermodynamic data. These data indicate that selenium levels in pyrite increase at lower temperature for a given fluid H 2Se H 2S ratio. Consideration of the aqueous speciation of selenium and sulfur indicates that above 200°C, H 2Se H 2S approximates Σ Se Σ S in most hydrothermal fluids. Using pixeprobe data for pyrite from eastern Australian volcanic-hosted massive sulfide deposits, the Σ Se Σ S ratio of zinc-rich ores is below 1.5 × 10 −6, whereas the Σ Se Σ S ratio of copper-rich ores is 7–300 × 10 −6. Using Σ Se Σ S ratios of 10 −7 for sea water and 10 −4 for fluids with magmatic-derived sulfur, zinc-rich ores contain minimal (< 1.5%) magmatic-derived sulfur, copper-rich ores contain much higher (> 7%) magmatic-derived sulfur.

Trace elements in sulfide minerals from eastern Australian volcanic-hosted massive sulfide deposits; Part I, Proton microprobe analyses of pyrite, chalcopyrite, and sphalerite, and Part II, Selenium levels in pyrite; comparison with delta 34 S values and implications for the source of sulfur in volcanogenic hydrothermal systems

Part I. Pyrite, chalcopyrite, and sphalerite from six volcanic-hosted massive sulfide (Mount Chalmers, Rosebery, Waterloo, Agincourt, Dry River South, and Balcooma) deposits in eastern Australia were analyzed using a proton microprobe to determine trace element abundances. In pyrite, trace elements can be divided into three groups according to the most likely occurrence of the element: (1) elements that occur mainly as inclusions (Cu, Zn, Pb, Ba, Bi, Ag, and Sb), (2) elements that occur as nonstoichiometric substitutions in the lattice (As, Tl, Au, and possibly Mo), and (3) elements that occur as stoichiometric substitutions for Fe (Co and Ni) or S (Se and Te). Hydrothermal and metamorphic recrystallization cleans pyrite of group 1 and group 2 elements, but does not appear to affect the concentrations of group 3 elements. Colloform pyrite grains have the highest levels of As and Au (up to 200 ppm), suggesting that rapid precipitation is important in incorporating Au into auriferous pyrite. Elements that occur as inclusions in chalcopyrite include Pb, Bi, Zn (P), and Ba. The occurrence of As and Sb is unresolved, although consistently high values of As in some samples suggest that As may substitute into the lattice of chalcopyrite. Elements that substitute into the lattice include Ag (for Cu), In, Sn and Zn (?) (for Fe), and Se (for S). Lead, Ba, Sb, possibly, and in some cases, Cu, occur commonly as inclusions in sphalerite. Lattice substitutions in sphalerite include Fe, Cd, Cu (to 4,500 ppm), Ni, In, Ag, Te, Ga and possibly Mo. In addition, consistently high (2,000-4,000 ppm) levels of As in the Rosebery barite zone may indicate As lattice substitution. Part II. The Se content of pyrite in volcanic-hosted massive sulfide deposits varies as follows: in Cu-poor, Zn-rich deposits, Se levels are low (mainly <5 ppm) throughout; in Cu-rich deposits, Se levels are highest (10-200 ppm) in stringer zones and the lower part of the massive sulfide lens, and decrease toward the top of the massive sulfide lens and into peripheral altered rocks. Metamorphic recrystallization does not affect these variations. Although delta 34 S values also vary systematically in individual deposits, no systematic differences were noted between Cu-rich and Zn-rich deposits...

Geochemistry of the Mount Windsor Volcanics; implications for the tectonic setting of Cambro-Ordovician volcanic-hosted massive sulfide mineralization in northeastern Australia

The Mount Windsor subprovince is an important relic of Upper Cambrian to Lower Ordovician sedimentation and volcanism in the northern part of the Tasman orogenic zone. Volcanic-hosted massive sulfide mineralization occurs at several stratigraphic horizons within the volcano-sedimentary package and one major mine is operational within the belt. Major and trace element data and Nd isotope ratios are presented for the least altered coherent units from the three major volcanic-bearing formations in the Mount Windsor subprovince. The data are used to discriminate four major phases of volcanism and related intrusive activity derived from three isotopically discrete sources and to assess the geodynamic setting in which the volcanism occurred. The earliest phase of mafic volcanism has minor and trace element characteristics suggesting an alkaline intraplate or rift association and it was probably produced by partial melting of attenuated subcontinental lithospheric mantle. The overlying Mount Windsor Formation silicic volcanics have Nd isotope characteristics (epsilon (sub Nd(480 Ma)) = -4.7 to -12.8) that suggest they were produced by partial melting of underlying Precambrian crustal rocks. Mafic volcanics of the overlying Trooper Creek Formation include a low TiO 2 suite and a high TiO 2 suite with a range of distinguishing chemical characteristics but similar Nd isotope ratios (epsilon (sub Nd(480 Ma) = 3.8-2.3), which indicate derivation from relatively depleted asthenospheric mantle variably modified by subduction processes. The high TiO 2 suite is also represented by abundant intrusions within the underlying volcanic package. The more silicic volcanics in the Trooper Creek Formation appear to be cogenetic with their mafic associates but have varying Nd isotope ratios, which suggest progressive crustal interaction with increasing SiO 2 content. Comparisons with modern volcanic compositions and ore depositional environments suggest that the volcanic and sedimentary units within the Mount Windsor subprovince were deposited in a back-arc basin developed by extension of continental lithosphere along the eastern Australian margin in the Late Cambrian and Early Ordovician. Mineralization and volcanic deposits of similar age farther north in the Tasman orogenic zone suggest that this basin may have had a north-south orientation, although there is no clear evidence remaining of the original arc front deposits.

Zincian staurolite in the Dry River South volcanic-hosted massive sulfide deposit, northern Queensland, Australia: an assessment of its usefulness in exploration

Staurolite from the Dry River South volcanic-hosted massive sulfide deposit in northern Queensland, Australia is enriched in ZnO (2.5–6.8%) only within the massive sulfide lens or in highly pyritic biotite-chlorite schist just below the massive sulfide lens. In the footwall alteration zone, staurolite ZnO levels vary between 0.4 and 1.3%, whereas in hanging wall metagraywackes and metapelites, the ZnO content is mainly below 1.0%. As staurolite from metapelitec rocks contains up to 7.9% ZnO, high ZnO levels in staurolite do not necessarily indicate a relationship to Zn-rich massive sulfide. Staurolite grains from the Dry River South amd other massive sulfide lenses have low TiO 2 concentrations (mainly <0.4%) relative to staurolite grains from metasediments and alteration zones (mainly >0.4%). The low concentration of TiO 2 in staurolite from the massive sulfide lens results from the low initial Ti concentration in exhalative ores. High ZnO and low TiO 2 values are indicative of staurolite associated with Zn-rich massive sulfide. Zincian staurolite is a potential exploration indicator at both prospect and reconnaissance scales. At the prospect scale, zinc levels have potential in distinguishing true from false gossans, distinguishing low grade portions of massive sulfide lenses from barren massive pyrite bodies, and characterizing alteration zones. At the reconnaissance scale, staurolite can be collected in the heavy mineral fraction of stream sediments, and multiple grains can be analyzed rapidly using modern electron microprobes.

OVERVIEWS: Geophysical signatures of Western Australian mineral deposits: an overview

Geophysical exploration in Western Australia is hindered by a mantle of conductive and magnetic weathered rocks that covers much of the State. This has required the adaptation of most geophysical methods for successful application in Western Australian conditions, and has led to the development and widespread use of, for instance, high-resolution aeromagnetics and time-domain electromagnetic methods. However, these difficulties have not prevented geophysics from being an integral part of exploration for base metal, diamond, gold, iron ore, manganese, nickel and uranium deposits in Western Australia. Mississippi Valley-type base-metal deposits are difficult geophysical targets and direct detection of the ore is not usually possible. However, gravity and magnetic data can be used to locate basement highs associated with such deposits and, on a semi-regional scale, induced polarisation surveys have been used to locate marcasite halos associated with the orebodies. Volcanic-hosted massive sulphide base-metal deposits have variable geophysical responses. Physical property contrasts with their host are highly variable and thus methods such as magnetics, induced polarisation and electromagnetics may fail to generate recognisable responses. Mise-a-lamasse surveys have proved highly successful for mapping such mineralisation on a prospect scale, once it has been intersected by drilling. The only example of a sedimentary exhalative deposit in the State for which data are available has distinct gravity, magnetic and time-domain electromagnetic anomalies. Diamonds in Western Australia mainly occur in lamproite pipes. These pipes have variable magnetisations but can usually be detected using high-resolution aeromagnetic surveys. The pipes can also be conductive and mapped using electromagnetic techniques if the host rocks are suitably resistive. The major geophysical method utilised in gold exploration is high-resolution aeromagnetics which is used to map favourable structures and rock types. Electrical and electromagnetic methods can also be used where gold is associated with sulphides. Geophysics has been comparatively little used in exploration for iron ore. Exploration for supergene-enriched deposits mainly uses aeromagnetics, to map favourable structures and to detect magnetite destruction and replacement associated with mineralisation, and gamma-ray logging for stratigraphic correlation purposes. The major technique used in manganese exploration is the gravity method, taking advantage of the positive density contrast between ore and host rocks. The mineral sands industry uses aeromagnetic data to map placer deposits containing ilmenite, but the relatively low cost of drilling limits the use of geophysical exploration methods. Nickel sulphide mineralisation can be directly detected using induced polarisation and electromagnetic techniques. Gravity and magnetic surveys are also used, but mainly in a mapping role. Carbonatitic intrusions associated with rare-earth-element mineralisation give rise to large magnetic anomalies. Radiometric and gravity anomalies can also occur. Uranium mineralisation has been directly detected using radiometric data, but some deposits are concealed below cover. Magnetic, electromagnetic, electrical and gravity surveys can be used to locate the rocks and structures which host mineralisation.

Archean, deep-marine, volcanic eruptive products associated with the Coniagas massive sulfide deposit, Quebec, Canada

The mafic-dominated volcanic and related volcaniclastic sedimentary rocks, which host the Archean Coniagas Zn–Pb–Ag massive sulfide deposit, are inferred to be the result of submarine explosive and effusive eruptions at depths of approximately 1000 m, as suggested by the presence of volcaniclastic turbidites, the absence of wave-induced sedimentary structures, pillowed lava flows, the sulfide deposit itself, and the incipient arc setting. The rock assemblage includes massive, pillowed and brecciated, basaltic to andesitic flows, massive, andesitic to rhyodacitic lapilli tuffs, andesitic stratified lapilli tuffs, and bedded tuffs. Preserved fragments and delicate volcanic textures, such as angularity of clasts, chilled clast margins, and clast vesicularity, and sedimentary structures are consistent with a subaqueous hydroclastic origin for the volcaniclastic sedimentary rocks. Explosive degasification of magma and (or) lava, in conjunction with fragmentation due to the interaction of magma–water, or nonexplosive hydroclastic fragmentation can account for the observed characteristics in the volcaniclastic deposits.The 280 m thick Coniagas volcano-sedimentary succession, used to reconstruct the volcanic history of the deposit, records two explosive–effusive volcanic cycles. The initial stage of each cycle is envisaged to have commenced with a small fire fountain or boiling-over eruption. Transport and deposition of the fragmented debris along the flanks of the volcanic edifice is attributed to high-concentration particulate gravity flows. The massive lapilli tuffs are interpreted as laminar debris flows, whereas the stratified lapilli tuffs may reflect turbulent flow deposits. The bedded tuffs were produced during the waning eruptive stages or elutriated from high-concentration syneruption flows. Ingestion of water, causing hydroclastic fragmentation, occurred during the eruptive and (or) the transport process. Calm, effusive mafic volcanism, characterized by massive, pillowed and brecciated flows and reworked counterparts, terminates each volcanic cycle. The massive, felsic lapilli tuffs, which host the mineralization, are inferred to represent locally reworked hydroclastic products of explosive or nonexplosive origin. The Coniagas mine deposit may serve as a guide for future exploration of small Archean volcanic-hosted massive sulfide deposits with a restricted alteration halo.

Geophysical Signatures of Western Australian Mineral Deposits: An Overview

Geophysical exploration in Western Australia is hindered by a mantle of conductive and magnetic weathered rocks that covers much of the State. This has required the adaptation of most geophysical methods for successful application in Western Australian conditions, and has led to the development and widespread use of, for instance, high-resolution aeromagnetics and time-domain electromagnetic methods. However, these difficulties have not prevented geophysics from being an integral part of exploration for base metal, diamond, gold, iron ore, manganese, nickel and uranium deposits in Western Australia. Mississippi Valley-type base-metal deposits are difficult geophysical targets and direct detection of the ore is not usually possible. However, gravity and magnetic data can be used to locate basement highs associated with such deposits and, on a semi-regional scale, induced polarisation surveys have been used to locate marcasite halos associated with the orebodies. Volcanic-hosted massive sulphide base-metal deposits have variable geophysical responses. Physical property contrasts with their host are highly variable and thus methods such as magnetics, induced poiarisation and electromagnetics may fail to generate recognisable responses. Mise-a-la-masse surveys have proved highly successful for mapping such mineralisation on a prospect scale, once it has been intersected by drilling. The only example of a sedimentary exhalative deposit in the State for which data are available has distinct gravity, magnetic and time-domain electromagnetic anomalies. Diamonds in Western Australia mainly occur in lamproite pipes. These pipes have variable magnetisations but can usually be detected using high-resolution aeromagnetic surveys. The pipes can also be conductive and mapped using electromagnetic techniques if the host rocks are suitably resistive. The major geophysical method utilised in gold exploration is high-resolution aeromagnetics which is used to map favourable structures and rock types. Electrical and electromagnetic methods can also be used where gold is associated with sulphides. Geophysics has been comparatively little used in exploration for iron ore. Exploration for supergene-enriched deposits mainly uses aeromagnetics, to map favourable structures and to detect magnetite destruction and replacement associated with mineralisation, and gamma-ray logging for stratigraphic correlation purposes. The major technique used in manganese exploration is the gravity method, taking advantage of the positive density contrast between ore and host rocks. The mineral sands industry uses aeromagnetic data to map placer deposits containing ilmenite, but the relatively low cost of drilling limits the use of geophysical exploration methods. Nickel sulphide mineralisation can be directly detected using induced polarisation and electromagnetic techniques. Gravity and magnetic surveys are also used, but mainly in a mapping role. Carbonatitic intrusions associated with rare-earth-element mineralisation give rise to large magnetic anomalies. Radiometric and gravity anomalies can also occur. Uranium mineralisation has been directly detected using radiometric data, but some deposits are concealed below cover. Magnetic, electromagnetic, electrical and gravity surveys can be used to locate the rocks and structures which host mineralisation.

Supra-subduction zone ophiolites as favorable hosts for chromitite, platinum and massive sulfide deposits

Supra-subduction zone ophiolites, as exemplified by the Zambales Ophiolite Complex, host extensive chromitite, volcanic-hosted massive sulfide deposits and, to a lesser degree, platinum-group minerals. In contrast, mid-ocean ridge basalt ophiolites are almost barren of economic mineral deposits. The high degree of partial melting, high water pressure, changes in oxygen fugacity, temperature, pressure and magma mixing could explain the chromitite deposits in zupra-subduction zone ophiolites. Platinum-group minerals are modeled to be derived from multi-stage melting events that also characterize supra-subduction zone ophiolites. The presence of capping rocks and the tectonics of emplacement are believed to be critical in the preservation of volcanic-hosted massive sulfides in ophiolites. Marginal basins (= SSZ ophiolites) are more easily emplaced than the large, open sea oceanic basin ophiolites which are usually subducted. Geochemical and tectonic controls point to supra-subduction zone ophiolites as more promising exploration targets.

Lithogeochemical Exploration of Metasomatic Zones Associated with Volcanic-Hosted Massive Sulfide Deposits Using Pearce Element Ratio Analysis

Reactions between mineralizing hydrothermal fluids and host rocks produce distinct miner- alogically and geochemically zoned alteration haloes in the footwall of many volcanic-hosted massive sulfide (VHMS) deposits, often very much larger than the deposits themselves. Empirical alteration indices have been developed to exploit these features and are widely used as geochemical vectors in the search for new deposits, but only with limited success. The principal limitation of using these indices as lithogeochemical exploration vectors is that they cannot distinguish between the geochemical modifications produced by hydrothermal alteration and the geochemical heterogeneity present in host rocks before the onset of hydrothermal activity. To overcome this problem, a theoretically based lithogeochemical exploration method has been developed that (1) identifies the sources of geochemical variability in VHMS host rocks, (2) isolates the pre-existing geochemical heterogeneity in these rocks through use of linear fr...

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

The effect of alteration and metamorphism on wall rocks to the Balcooma and Dry River South volcanic-hosted massive sulfide deposits, Queensland, Australia

Two types of visibly hydrothermally altered rocks are present in the Cambro-Ordovician Balcooma volcanic-hosted massive sulfide (VHMS) district of northeast Queensland: (1) quartz-chlorite ± biotite schist, and (2) pyritic quartz-muscovite schist. The quartz-chlorite ± biotite schist, which has gained Mg, Fe, Mn, Si, and S, but lost K, Na, Ca, Rb and Sr, occurs in a crosscutting pipe below Cu-rich massive sulfide lenses at the Balcooma deposit. Pyritic quartz-muscovite schist, which has gained Fe, Si, K, Rb, S, Cu, Zn and Pb, but lost Na, Ca and Sr, occurs in extensive zones below the Dry River South sulfide lens and Balcooma Zn-Pb-rich lenses as well as in stratiform zones west of Balcooma. With the possible exceptions of Nb and Y at Dry River South, Al, Ti, Zr, Nb and Y are all immobile in these alteration zones. An outer zone of weak Fe, S, Cu and Zn enrichment, and Na, Ca and Sr depletion surrounds the Balcooma deposit and extends up to distances of 200 m beyond visibly altered rocks. Evidence for syn-tectonic composition changes to these alteration zones exist as 0.1–10 m thick gradational zones between quartz-chlorite ± biotite schist and the outer weakly altered zone at Balcooma. These zones lack muscovite and contain highly poikilitic staurolite and biotite porphyroblasts. These zones gained Mg, Fe, Mn, Si, Ca, Cu and Sr, but lost K and S. These zones also gained Zr, which is considered to be immobile in most geological environments. Major and minor element concentrations in metasediments from the Balcooma district vary systematicaly with Zr/TiO 2 as a result of weathering and deposition during turbitidic sedimentation. Deviations from trends relating Na and Fe to Zr/TiO 2 can be used to define peripheral weak alteration zones not identifiable from geological mapping.

Evaluation of the source-rock control on precious metal grades in volcanic-hosted massive sulfide deposits from western Tasmania

Volcanic-hosted massive sulfide (VHMS) deposits from western Tasmania have significantly higher Au grades than other VHMS deposits from eastern Australia and elsewhere. Huston and Large (1989) have suggested that the dominant controls on Au grades in VHMS deposits are probably temperature and pH of the hydrothermal fluids. However, the concentration of Au in the source rocks being leached by the hydrothermal fluids may also affect the final Au grades in the deposit (e.g., Keays, 1987). The major VHMS deposits of western Tasmania are hosted within the Central Volcanic Complex of the Mount Read Volcanics or overlying andesites of the Que-Hellyer sequence. Other potential source rocks for leaching of metals include late Proterozoic basalts and sediments of the Crimson Creek Formation, ultramafic-mafic rocks from a tectonically emplaced ophiolite complex, and Precambrian schists and quartzites.The background Au concentrations in the least altered Mount Read Volcanics (mainly andesites, dacites, and rhyolites) are in the range 0.9 to 1.3 ppb Au, and the mean values are comparable with those of unaltered modern volcanics of similar composition. Hydrothermally altered equivalents of these volcanics generally have somewhat higher Au concentrations. The Tasmanian ultramafic rocks have significantly lower Au (mean 0.5 ppb) than unaltered periodotite xenoliths, but similar concentrations to ophiolitic ultramafic rocks. High Mg, low Ti volcanics from the western Tasmanian ultramafic-mafic complexes have Au contents (mean 1.6 ppb) similar to the Mount Read mafic rocks. In contrast, relatively high Ti basalts of the Crimson Creek Formation are the most Au-enriched primary volcanics in the area (up to 23 ppb Au). These volcanics were probably erupted during an early rifting phase and may underlie much of the Mount Read Volcanics.Source-rock modeling calculations, based on average Zn concentrations of potential source rocks, suggest that about 70 km 3 of andesitic volcanics would have to be leached to provide the metal content of the Hellyer VHMS deposit. Similar calculations based on Au concentrations suggest somewhat larger leach volumes. Both estimates suggest that some leaching of basement rocks may have occurred during the formation of the VHMS deposits. The relatively high Au concentrations in the Crimson Creek basalts indicate the presence of an anomalous Au-enriched source rock beneath the Mount Read Volcanics which may have contributed to the relatively Au-rich character of the Tasmanian VHMS deposits. However, the temperature, composition (e.g., PH, f (sub O 2 ) , a (sub H 2 S) , m Nacl ), and the consequent solubility of metal species in the hydrothermal fluids will be the major factors which determine the final concentrations of base and precious metals in a specific massive sulfide deposit. Source-rock composition will only be important if the hydrothermal fluid has an appropriate chemistry to allow significant Au solubility.

Geophysical signatures of Australian volcanic-hosted massive sulfide deposits

Geophysical surveys have been used for mineral exploration in Australia since 1928 and have played an important role in exploration, particularly for volcanic-hosted massive sulfide (VHMS) deposits. In recent times geophysics may be credited with the discovery of the Que River, Hellyer, and Benambra deposits. The use of geophysics in VHMS exploration is likely to increase as targets become deeper and downhole methods will be especially emphasized.For the present and foreseeable future, electromagnetic (EM) and particularly drill hole electromagnetic (DHEM) methods will continue to be the most important geophysical techniques in Australian VHMS exploration. However, there exists a class of resistive deposits which do not respond to EM. These are usually iron poor, zinc rich, or fragmented and silicified and many may have been missed. Together with a desire for increasing penetration, this type of target provides an interesting geophysical problem for the future.

Australian volcanic-hosted massive sulfide deposits; features, styles, and genetic models

The Cambro-Ordovician period has yielded the major development of volcanic-hosted massive sulfide (VHMS) deposits in Australia, resulting in about 12 million metric tons of contained metal (copper, lead, and zinc), concentrated in the Mount Read Volcanics (Tasmania) and the Mount Windsor Volcanics (Queensland), The Archean (4 million metric tons of metal) and the Silurian (3.5 million metric tons of metal) constitute the next important episodes while the Devonian and Permian have produced isolated deposits. The deposits range from Cu type to Zn-Cu type to Zn-Pb-Cu type. The Zn-Cu-type deposits are restricted to the Archean and Silurian, whereas the Cu type and Zn-Pb-Cu type occur sporadically throughout the time span from early Archean to Permian. In general terms, the major VHMS-bearing districts are calc-alkaline in character and display a thick basal pile of rhyolitic volcanics (1-3 km thick) including lavas, epiclastics, and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite,dacite, andesite, basalt, and sediments. The major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence. There is a wide range in variability of styles of Australian VHMS deposits including mounds, pipes, sheets, layered deposits, stacked deposits, stockwork and disseminated deposits, distal reworked deposits, and cyclic layered deposits. Although these various styles show a range of features, there is a consistent theme across the spectrum of metal zonation, alteration mineralogy, alteration chemistry, sulfur isotopes, macrotextures, microtextures, and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit. The classic mound-style deposits, such as Hellyer, have a series of key features, subsets of which are represented in the other deposit styles. The mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent. Growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures, leading to zone refining and lead-zinc-silver-gold enrichment in the outer and upper parts of the mound, Departure from the classic mound style of deposit is related principally to three key factors: the chemistry of the ore fluid (salinity, temperature,f02, and aH2S), the nature (permeability and chemistry) of the volcanic pile, and the sea-Hoor environment (sea-Hoor topography and ..seawater depth). These factors control the aspect ratio of the deposit, the extent of stringer zone development, the degree of subsea-floor replacement mineralization, the nature and spatial development of footwall alteration, and the development and style of related distal mineralization. Sulfur isotope studies of Australian VHMS deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit, whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks. This conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences. The input of metals directly from volcanic magma chambers is not precluded by the available data, and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume, rising to mix with convective seawater fluids which contribute lead, zinc, silver, and gold leached from the footwall volcanics and basement rocks. The relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins.

Submarine volcanism; eruption styles, products, and relevance to understanding the host-rock successions to volcanic-hosted massive sulfide deposits

The increasing hydrostatic pressure of the water column with increasing water depth in subaqueous environments limits the ability of superheated volatiles to expand instantaneously against the ambient pressure. Explosive submarine eruptions are only likely in water depths less than 1 km, and generally less than 500 m, thus refuting models of highly explosive, very deep water calderas popularly proposed as the host volcanic centers for the kuroko volcanic-hosted massive sulfide (VHMS) deposits of Japan. Large volumes of blocky breccias can be produced by quench fragmentation during submarine lava eruptions, and such deposits may have been mistaken for pyroclastic deposits in many cases in the past. Pyroclastic debris may occur in the host-rock successions of some VHMS deposits and may even be very voluminous. However, facies characteristics and theoretical constraints indicate that these are usually the products of mass-flow deposition from shallow-water or basin margin volcanic centers. The maximum water depths for submarine explosive eruptions coincide approximately with the minimum pressures and water depths required to prevent boiling of mineralizing hydrothermal fluids in the stockwork before the fluids reach the sea floor. The key elements in evaluating the prospectivity of ancient volcanic successions for VHMS deposits appear to be deep-water sediments and lavas or shallow intrusions in an extensional basin setting. Pyroclastic debris, in many cases at least, appears to be an accidental, externally introduced component. There is little evidence that explosive submarine calderas are essential as host volcanic centers to VHMS deposits.