Volcanic-hosted Massive Sulfide Deposits Research Articles

The Ballynoe Stratiform Barite Deposit, Silvermines, County Tipperary, Ireland

The Ballynoe barite deposit is a conformable, mineralised horizon of Lower Carboniferous age overlying a diastem and mass faunal extinction demarking the transition from a quiet water environment to one of dynamic sedimentation. The geometry of the barite orebody correlates with the palaeotopography of the footwall, which acted as an important control over the lateral extent, thickness, and nature of the mineralisation. Sedimentary features within the barite horizon suggest that it was precipitated in the form of a cryptocrystalline mud which underwent major diagenetic modification resulting in extensive stylolitisation, recrystallisation, and remobilisation. There is abundant and compelling geological and isotopic evidence for early local exhalation from the presence of a hydrothermal vent fauna consisting of delicately pyritised worm tubes and haematised filaments of apparent microbial origin. The worm tubes are remarkably similar to examples from modern and ancient volcanic-hosted massive sulphide deposits, and the filamentous microfossils have similarities to modern Fe-oxidising bacteria. Strontium in the barite has an 87Sr/86Sr ratio indistinguishable from seawater between 350 and 344 Ma whilst oxygen isotopes from barite and chert suggest a diagenetic origin in equilibrium with such seawater around 60–70 °C. Fluid inclusion studies have shown that, in general, low temperature inclusions are very saline (20%–25%) whilst at higher homogenisation temperatures they are more dilute (9%–12%).

In Situ LA-ICP-MS U-Pb Geochronology, Sr-Nd-Hf Isotope and Trace Element Analysis of Volcanic Rocks from the Gacun Volcanic-Hosted Massive Sulfide Deposit in Sichuan, China

The Gacun deposit is a typical Volcanic Hosted Massive Sulfide (VHMS) associated with Late Triassic seafloor calc-alkaline felsic volcanics. Studies of zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf isotope geochemistry of volcanic rocks from the Northern Yidun arc were undertaken in this paper. We reshaped the Gacun magmatic system activity time, defined the origin of magma evolution, and proposed a metallogenic model of the deposit. Whole-rock major element compositions of the magmatic rocks in the Northern Yidun island arc indicate that they are a complete basalt–andesite–dacite–rhyolite assemblage, showing three obvious stages of composition evolution. They are enriched in large-ion lithophile and light rare earth elements, but depleted in high field-strength and heavy rare earth elements, with weak-to-negligible Eu anomalies (obvious in rhyolite). These geochemical features indicate that the Northern Yidun island arc is a magmatic arc based on ancient continental crust. The Ganzi–Litang oceanic subduction induced mantle melting and produced calc-alkaline basaltic magma, while the MASH processes at the bottom of the crust produced andesitic magma. Part of the andesite magma erupted to form andesite lava. The remaining part was mixed with magma produced via anatexis of ancient crust (approximately 20%–40% of the ancient crustal component), forming the ore-bearing rhyolite. Zircon U-Pb age data defines Gacun magmatic–hydrothermal mineralization sequence of events: At 238 Ma, arc magmatism led to the formation of andesite in the eastern part of the deposit. At 233 Ma, in the arc zone (the western part of Gacun deposit), a large-scale bimodal magmatism formed the main ore-bearing rock series of Gacun deposit, rhyolitic volcanic rocks. At 221 Ma, volcanic eruptions tended to end and sub-volcanic intrusion occurred, forming a lava dome, which was located under the ore-bearing rhyolitic volcanic rocks. The lava dome acted as a thermal engine and promoted hydrothermal circulation. The hydrothermal activity reached a peak at 217 ± 1 Ma, and the Gacun VHMS deposit was formed.

Exceptional preservation of organic matter and iron-organic colloidal mineralization in hydrothermal black smoker-type sulfide mineralization from the Paleoarchean seafloor

Fossil organic matter (OM) in Paleoarchean rocks is an invaluable tracer of ancient life, yet it is often contentious due to low preservation potentials of its original organic molecular characteristics under generally high metamorphic grades. This study reports on exceptionally preserved OM within black smoker-type sulfide mineralizations from the 3.24 billion-years-old Sulphur Springs volcanic-hosted massive sulfide deposit, East Pilbara Terrane, northwestern Australia. Fine scale mineralogical and organic molecular variations – documented by SEM and TEM techniques, Raman and FTIR Spectroscopy, as well as ToF-SIMS analysis – trace the formation of millimetre-scale pyritic hydrothermal orifices. Overmature OM (≳200–250 °C) enriched in aromatic hydrocarbons occurs within earliest formed colloform-banded pyrite that could have precipitated at high temperature upon mixing of hot hydrothermal fluids with cold seawater. In contrast, mature OM (within the catagenesis window; ∼100–150 °C) with lower proportions of aromatic hydrocarbons occasionally occurs alongside pyrite within barite near the inner paleofluid conduits of the hydrothermal orifices. A deposition of this OM generation from cooler fluids enriched in sulfate during waning hydrothermal activity is corroborated by its association with original mineralogy, particularly the occurrence of isolated nanoinclusions of OM within hydrothermal barite away from grain boundaries or fissures and cracks, which renders impossible an origin from post-depositional hydrocarbon migration. When compared to the overmature OM within colloform-banded pyrite, the better-preserved inner OM not only contains less abundant, smaller, and less ordered aromatic hydrocarbons, but also higher amounts of aliphatic molecules with longer chain lengths and higher proportions of carbonyl and amide functional groups. Although the latter characteristics are consistent with contributions by microbial biomass, abiotic origins are equally plausible because hydrothermal processes can produce OM with similar organic molecular attributes. Irrespective of these uncertainties on the ultimate sourcing of OM, common intergrowths of the cooler OM with nanoscopic iron oxides – hematite and lesser magnetite – hint at its hydrothermal deposition from colloidal particles that have been stabilized by iron-organic complexation. Further research is required to ascertain the significance of this process for the hydrothermal cycling and release of organic carbons and bound iron into the Paleoarchean oceans. Collectively, our results show that ancient marine hydrothermal systems can preserve OM with differing degrees of thermal degradation, which allows for insights into its sourcing, cycling, and deposition.

A mineralogical and geochemical application for determining hydrothermal alteration zones associated with volcanic-hosted massive sulphide deposits at Tasik Chini area, Peninsular Malaysia

The volcanic-hosted massive sulphide (VHMS) deposits of the Tasik Chini area, Peninsular Malaysia are hosted by Permian felsic volcanic rocks (∼ 290 Ma) of rhyolitic to rhyodacitic composition and comprise two deposits (Bukit Botol and Bukit Ketaya). Although the ores and the felsic volcanic host rocks of these deposits have metamorphosed to greenschist facies and were affected by a complex deformation history, the hydrothermal alteration zones associated with mineralization are preserved and show systematic mineralogical and geochemical changes with increasing proximity to the ore bodies. The geometry of alteration assemblages of the Bukit Botol and Bukit Ketaya deposits shows they occur as semi-conformable or stratabound zones around the ore lenses. According to petrography study and short wavelength infrared (SWIR) spectra measurements supported by X-Ray diffractometer (XRD), the Bukit Botol deposit is characterised by proximal quartz-sericite-pyrite and distal quartz-sericite alteration zones, whereas distal quartz-chlorite-sericite-pyrite-pyrophyllite ± kaolinite and proximal quartz-chlorite-pyrite ± carbonate ± pyrophyllite form the alteration assemblages of the Bukit Ketaya deposit. The geochemical data is coherent with the mineralogical characteristics, using the Ishikawa alteration (AI) and chlorite-carbonate-pyrite (CCPI) indexes, showing the important of chlorite and sericite formation in the ore proximal and ore distal zones. In addition, the molar elemental ratios of Na2O/Al2O3 versus K2O/Al2O3 and MgO/Al2O3 versus K2O/Al2O3 support that the abundance of muscovite (sericite) and chlorite controlled the intensity of alteration at the both deposits. The results of this study indicate that the combination of mineralogy and geochemistry studies help in defining specific alteration styles and related mineral geochemical variations of the VHMS deposits in Tasik Chini area.

Mineralogy and geochemistry of clastic sulfide ores from the Talgan VHMS deposit, South Urals, Russia: Signatures of diagenetic alteration

Diagenetically altered thin-layered sulfide ores from the Talgan volcanic-hosted massive sulfide deposit (South Urals) represent an intercalation of thin sulfide and hyaloclastite layers up to 3 and 0.5 cm thick. Their mineralogy and trace element, including rare earth elements (REE), and S, C and O isotopic geochemistry are considered in this work. They occur at the slopes of ore bodies and pinch outside them within hyaloclastites and their thickness increases towards the flanks of the ore bodies. The sulfide layers are mostly composed of authigenic pyrite, which forms zoned nodules, anhedral aggregates, euhedral crystals and pseudomorphs after pyrrhotite crystals. The pyrite nodules and anhedral aggregates host inclusions of authigenic chalcopyrite, sphalerite, galena, barite, cotunnite, native gold, stromeyerite, tellurobismutite and hessite. The trace element content of sulfide layers widely varies and depends on the amount of non-sulfide minerals (chlorite, illite, calcite, quartz, rutile, scheelite), which formed as a result of alteration of dacitic hyaloclastite components mixed with sulfide clasts. The sulfide layers are enriched in REEs (up to 561 ppm) due to the presence of REE-bearing minerals (bastnaesite, parisite, synchysite, galgenbergite, REE-bearing xenotime), which do not occur in massive sulfide ores. A narrow range of δ34S values of pyrite (1.41–3.27 ‰) from sulfide layers is explained by equilibrium or kinetic dissolution–precipitation processes of redeposition of sulfide ore clasts during diagenetic processes. The δ13C values of bulk samples of sulfide layers (–5.29 to –17.89 ‰) indicate that the authigenic carbonates formed from pore fluids circulated in former organic-rich hyaloclastite layers. The δ18O values of the same samples (+4.00 to + 8.36 ‰) are higher compared to seawater δ18O values due to diagenetic isotopic exchange between the hyaloclastitic components and pore fluids at elevated temperatures of burial process. The positive δ34S–Mn, δ13C–Mn, δ13C–As, δ13C–Sb, δ18O–Mn and δ18O–U correlations reflect organic matter consumption during the evolution of authigenic minerals. Data presented in this study helps to define the origin of diagenetically altered layered sulfide ores in volcanic-hosted massive sulfide deposits and are important for understanding diagenetic REE accumulations in sulfide-rich sediments of these deposits.

HIGH-PRECISION CA-ID-TIMS AGE CONSTRAINTS ON THE NIBLACK Cu-Zn-Au-Ag DEPOSITS: A NEOPROTEROZOIC VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSIT IN THE NORTH AMERICAN CORDILLERA

Abstract The Neoproterozoic-Cambrian Wales Group and Ordovician-early Silurian Moira Sound unit of Prince of Wales Island, Alaska, USA, host numerous volcanic-hosted massive sulfide (VHMS) deposits and occurrences, including the Niblack VHMS deposits. Previous attempts to determine the age of the felsic volcanic host rocks in the Niblack area have resulted in conflicting results and interpretations. We have utilized chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology to acquire highly precise crystallization and maximum depositional ages for a total of six samples of felsic volcanic and intrusive rocks from Niblack. This study establishes age constraints for the Niblack felsic succession of (1) crystallization ages of 565.1 ± 0.9 and 564.8 ± 1.0 Ma for coherent rhyolite flows, (2) maximum depositional ages of 565.3 ± 0.9 and 565.2 ± 0.9 Ma for felsic volcaniclastic rocks, (3) a crystallization age of 565.2 ± 0.9 Ma for a quartz-feldspar-phyric subvolcanic sill, and (4) a crystallization age of 564.8 ± 1.0 Ma for a felsic dike that crosscuts the Niblack felsic succession. These results indicate that the ~200-m-thick Niblack felsic succession and VHMS deposits formed during one episode of felsic volcanism at ca. 565.1 ± 0.9 Ma and are thus confirmed as part of the Neoproterozoic Wales Group. Results of this study provide the first chronostratigraphic framework for felsic volcanism associated with VHMS deposit formation at Niblack and have implications for mineral exploration on Prince of Wales Island and elsewhere in the Alexander terrane.

Zinc, cadmium and sulphur isotopic compositions reveal biological activity during formation of a volcanic-hosted massive sulphide deposit

Zinc and Cd isotopes fractionate strongly within a range of hydrothermal systems; consequently, metal stable isotopes have been used to understand the formation of hydrothermal ore deposits. We investigated the Zn–Cd–S isotopic compositions of sulphides from the Carboniferous-Permian Xiaobaliang Cu–Au deposit, Inner Mongolia, China. This ancient seafloor hydrothermal system is the only known Cyprus-type volcanic-hosted massive sulphide (VHMS) deposit in the Central Asian Orogenic Belt. The S isotopic compositions of sulphide minerals are highly variable with δ34SCDT values ranging from −21.2‰ to 25.2‰. We infer that negative δ34SCDT values reflect mixing of volcanogenic S and S derived from bacterial sulphate reduction (BSR), and that positive δ34SCDT values reflect mixing of volcanogenic S with S derived from thermochemical sulphate reduction (TSR). Zinc isotopic compositions are relatively consistent at the deposit; δ66Zn values range from 0.10‰ to 0.70‰. These values are similar to those reported for modern seafloor sulphides. We infer that consistency of δ66Zn values relates to the Zn species ([ZnCl4]2−) present in the ore-forming fluids during sulphide precipitation. All of the sulphides are enriched in the light Cd isotopes; δ114/110Cd values range from −0.74‰ to −0.08‰. Zinc and Cd can be affected by biological activity, and we infer that biological activity caused the negative δ114/110Cd values at this deposit because: (1) biological processes (e.g., bacterial activity) enrich fluids in the light Cd isotopes, leading to the precipitation of sulphides with negative δ114/110Cd values; and (2) the Cd isotopic composition of ore-forming fluid is readily modified because the concentration of Cd is much lower than that of Zn in ore and basalt. Our results indicate that Cd isotopes might record ancient biological activity, and that organisms might play a more important role than previously thought in the formation of VHMS systems.

Geochemistry of Sphalerite from the Permian Volcanic-Hosted Massive Sulphide (VHMS) Deposits in the Tasik Chini Area, Peninsular Malaysia: Constraints for Ore Genesis

The Bukit Botol and Bukit Ketaya deposits are two examples of volcanic-hosted massive sulphide (VHMS) deposits that occur in the Tasik Chini area, Central Belt of Peninsular Malaysia. The mineralisation is divided into subzones distinguished by spatial, mineralogical, and textural characteristics. The primary sulphide minerals include pyrite, chalcopyrite, sphalerite, and galena, with lesser amounts of Sn- and Ag-bearing minerals, with Au. However, pyrrhotite is absent from both deposits. This study presents the results of sphalerite chemistry analysed by using an electron microprobe. Two types of sphalerite are recognised: sphalerite from the Bukit Botol deposit reveals a range of <DL to 24.0 mole% FeS, whereas sphalerite from the Bukit Ketaya deposit shows a range of <DL to 3 mole% FeS. Significant variations are shown in Zn, Cu, Cd, and Ag levels. Although the sphalerite has a wide variation in composition, a discernible decreasing Fe trend is exhibited from the stringer zone towards massive sulphide. This compositional variation in sphalerites may in part reflect variable temperature and activity of sulphur in the hydrothermal fluids during ore formation. Alternatively, the bimodal composition variations suggest that mineral chemistry relates to contrasting depositional processes. The Zn/Cd ratios for sphalerite from both these deposits are similar to those exhibited by volcano−sedimentary deposits with a volcanic origin. Therefore, the consistently low Cd concentrations and moderate to high Zn/Cd ratios suggest mixing of seawater and minor magmatic fluids controlling the chemistry of sphalerite at both deposits during their formation.

Nd and Hf isoscapes of the Yilgarn Craton, Western Australia and implications for its mineral systems

The Yilgarn Craton, Western Australia is endowed with significant gold, nickel‑copper, Cu-Pb-Zn volcanic-hosted massive sulphide (VHMS) and iron ore deposits. Detailed knowledge of the spatial relationships between mineral systems and the crustal framework of the Yilgarn Craton may be powerful tools in the predictive exploration of undiscovered mineral systems. Both Nd and Hf isotopes have the potential to image crustal residence, mantle tapping structures, and juvenile input over both space and time. This study uses a recent compilation of Hf (1236 zircon analyses) and Nd (462 whole rock analyses) isotopes to image the architecture of the crust and compare this to the distribution of gold, nickel‑copper-platinum group elements (Ni-Cu-PGEs), Cu-Pb-Zn VHMS and iron ore deposits. Spatial correlation plots reveal a statistically significant relationship between Nd and Hf isotopic domains and the occurrence of gold, Ni-Cu-PGE, base metals, and iron confirmed by the coefficient of determination (R2) which measures how differences in metal occurrences can be explained by differences in the isotopic signature. Nickel occurrences show the strongest antithetic spatial correlation (R2 = 0.80) with Hf model ages, followed by gold (R2 = 0.58) and base metals (R2 = 0.35), which implies higher metal endowment in areas of more juvenile crust. In contrast, there is a significant positive spatial correlation (R2 = 0.49) between iron ore occurrence and older model ages. Isotopic data can also be plotted as Hf or Nd time-slices, where the strongest spatial correlation between gold, nickel, and base metal occurrence and juvenile input occurs at ca. 2700–2550 Ma. Generally, a model can be proposed where (1) base metals are more prevalent in juvenile crust because of its thinner nature, greater mantle input, high-heat flow, increased water-magma interaction; (2) iron ore is concentrated in regions of old, mature, and stable crust; (3) komatiite-hosted Ni-Cu-PGE systems are focused in juvenile rift zones at the edge of old crustal blocks; and (4) gold is also localised in juvenile crust at craton edges where several factors are localised, including orogenesis, occurrence of previously fertilised crust (i.e. Fe-rich lithologies, sanukitoids), favourable structures (potential inversion of rift structures), and ‘late’ alkaline magmatism. In gold, Ni-Cu-PGE, and base metal mineral systems, the ability of magmas to access higher crustal levels unimpeded appears vital in regional prospectivity and is itself a function of crustal configuration.

Review of Late Cretaceous volcanogenic massive sulfide mineralization in the Eastern Pontides, NE Turkey

The production of Cu-Zn from volcanogenic massive sulfide (VMS) deposits in the eastern Pontides began in the early 1900s, with the exploitation of high-grade ores scattered across the district. The district still possesses economically important blind VMS and associated sulfide deposits. Careful descriptive documentation of the typical features of these VMS ores illustrated the geological characteristics that are important in identifying ore localities and can be used to define exploration targets. The eastern Pontide VMS deposits are examples of volcanic-hosted massive sulfide deposits that exhibit many of the characteristics typical of bimodal-felsic- type VMS mineralization. Nearly all known VMS deposits in the region are hosted by the Kızılkaya Formation, which is characterized by Late Cretaceous dacitic/rhyolitic volcanic rocks that are typically located at the top contact of the dacitic/rhyolitic pile or within the lower part of the overlying polymodal sequence containing various proportions of volcanic and sedimentary facies. Most VMS deposits are composed of a mound of high-grade massive sulfides formed above a zone of lower-grade stringer veins and disseminated mineralization. The dominant sulfide minerals in most deposits are pyrite, chalcopyrite, and sphalerite. Au also occurs in some deposits. The hydrothermal ore facies are diagnostic of subaqueous emplacement of the Pontide massive sulfide deposits that were deposited on the Cretaceous ocean floor. The immediate host lithologies associated with VMS mineralization have typically experienced intense and widespread alteration. The trace element geochemical signatures of the host rocks indicated that the Pontide VMS deposits likely formed in an extensional tectonic regime during subduction. Major lineaments and circular structures exerted fundamental controls on the locations of the VMS deposits in the eastern Pontide district. Age determinations indicated that almost all of the deposits in this region formed in a restricted time interval between ca. 91.1 and 82 Ma. The sulfur isotope compositions of the ore-forming fluids were consistent with those of fluids derived from modified seawater.

Melt inclusions in quartz from felsic volcanic rocks of the Iberian Pyrite Belt: Clues for magmatic ore metal transfer towards VMS-forming systems

The Iberian Pyrite Belt (IPB) is a major metallogenic province with numerous volcanic-hosted massive sulfide (VMS) deposits. Albernoa, Serra Branca and Neves-Corvo represent three well-known felsic volcanic centers in the Portuguese portion of the IPB. Albernoa has no known VMS mineralization, whereas gossans are well known in Serra Branca. Neves-Corvo is a world-class VMS deposit with high grade and tonnage for massive Cu and Zn sulfide ores as well as for Sn (massive and stringer cassiterite ores; now exhausted). Magmatic-hydrothermal fluids may contribute to the metal budget in some VMS-forming systems. In the IPB, direct evidence for this has been lacking and the magmatic link was not yet fully established. This work presents the first quartz-hosted melt inclusion data of phyric felsic volcanic rocks from the Albernoa, Serra Branca and Neves-Corvo areas in the IPB. Melt inclusions of intermediate composition found in Albernoa felsic volcanics show evidence for early stages of crustal melt assimilation supporting previous petrogenetic models for the IPB. High Sn contents (up to 46 ppm) found in the melt inclusions from all volcanic centers are a primary magmatic feature on these IPB volcanic rocks and are above the background value for the IPB felsic volcanic rocks. Estimated temperature of formation of magmatic quartz, in excess of 800 °C, suggests that all three volcanic centers gathered favorable conditions for sustaining long-lived hydrothermal cells. Highest estimated temperatures (>892 °C) are attributed to one sample from Albernoa and one sample from Neves-Corvo. Their similar, pre-eruptive, Cu and Sn distribution supports the model for the Neves-Corvo deposit indicating that Sn and (at least part of the) Cu decoupled from a common magmatic source in the early stages of the ore-forming process. The occurrence of barren volcanic sequences originating from Sn-rich melts is evidence that a set of concurrent factors are additionally required to generate a high-grade VMS-deposit. Melt inclusion data can be used as a tool to assist future exploration of favorable volcanic centers in the IPB.

The 4D evolution of the Teutonic Bore Camp VHMS deposits, Yilgarn Craton, Western Australia

The Teutonic Bore Camp, comprised of the Teutonic Bore, Jaguar and Bentley deposits, is one of the most significant volcanic-hosted massive sulphide (VHMS) camps in Western Australia. Despite being extensively studied, only recently there have been advances in the understanding of the mechanism that drove the formation of mineralisation. It has been recognized by recent studies that the volcanic-hosted deposits from the Teutonic Bore Camp represent replacement-type VHMS systems, with significant input of fluids and metals from a magmatic source. This paper tests the existing hypothesis that the nearby Penzance granite acted as the metals source and/or thermal engine driving the development of these ore deposits.New age constraints on the formation of the host volcanic sequence at the Bentley deposit and the crystallization of the Penzance granite allows for the construction of a 4D evolutionary model for the ore system. A new U-Pb SHRIMP monazite age of 2681.9 ± 4.5 Ma indicates that the Penzance granite post-dates the host stratigraphy at Bentley (ca. 2693 Ma) and is probably coeval with mineralisation. All zircons (Penzance, Bentley units I and III) have very similar ƐHf(i), with most values between −1 and + 6, slightly higher than the ƐHf(i) of zircons from other granites and volcanics within the Kurnalpi Terrain, and indicative of juvenile sources. The mean Th/U ratios are ~0.7 and ~0.6 for the Penzance and Bentley zircons, respectively. All zircons have similar Ce/Nd(CN) ratios. The chemical similarities between the zircons from the granite and the volcanic rocks at Bentley support a shared magmatic source between the Penzance and the Teutonic Bore Camp sequence. The Penzance granite is the likely source of heat, and potentially metals, which drove the VHMS mineralisation at the Teutonic Bore Camp.

Geochemical and Nd isotopic signature of felsic volcanic rocks as a proxy of volcanic-hosted massive sulphide deposits in the Iberian Pyrite Belt (SW, Spain): The Paymogo Volcano-Sedimentary Alignment

The Iberian Pyrite Belt (IPB), located in the South Portuguese Zone of the Iberian Massif (Spain), is a world-class VHMS province containing numerous, huge mineral deposits hosted by bimodal volcanic sequences. The province can be subdivided in several domains, which differ from each other both in the nature and relative abundance of their volcanic sequences, and in the distribution of associated VHMS deposits. In spite of its geological and economic interest, information available for the northern part of the province is still relatively scarce. We present a new dataset of geochemical and Nd isotopic analyses on the Paymogo Volcano-Sedimentary Alignment (PVSA), an area that is considered to be representative of this northern segment of the IPB.Volcanic sequences in the PVSA contain very scarce mafic rocks. In contrast, felsic rocks are abundant and have been classified in two groups: (i) the Paymogo rhyolitic sequence, hosting most of the VHMS deposits and (ii) a dacitic sequence. Although approximately coeval, these groups of felsic rocks clearly differ in geochemical features and ƐNd signature. In view of these differences, it is suggested that felsic rocks were produced from different mid-crustal domains during a major event of partial melting related to a regional, late Devonian/Early Carboniferous transtensional event. It is further suggested that some volcanic rocks previously described as andesites, are better interpreted as dacites.The age (347.3 ± 0.8 Ma), geochemical features, (Zr < 200 ppm) and ƐNd signature (−3.2 to −2.9) of the Paymogo rhyolites are very similar to those in other parts of the IPB including those hosting the deposits in the Riotinto Mining District, where petrographic, geochemical features and ƐNd signature had already been proposed as a proxy of VHMS mineralization. These similarities between the Riotinto and Paymogo areas suggest that their geochemical features and Nd isotope signature could provide useful exploration criteria throughout the IPB, in order to distinguish potential VHMS-bearing units from barren volcanic successions.

Origin of Fe-Mn ± Si layers associated with the Permian volcanic-hosted massive sulphide deposits in the Tasik Chini district, Peninsular Malaysia

The volcanic-hosted massive sulphide (VHMS) deposits in the Tasik Chini district, Peninsular Malaysia are hosted in Permian volcano-sedimentary rocks and are composed of Fe-Mn ± Si layers which occur stratigraphically above the ore bodies. The oxide layers are 1 cm to 2 m thick, and occur discontinuously throughout the Bukit Botol deposit, whereas layer of oxides up to 10 m thick occurs continuously over a strike length of over 50 m at the Bukit Ketaya deposit. Their mineralogy is dominated by hematite ± (quartz ± barite ± magnetite ± pyrite ± clastic components), and commonly exhibit massive iron-silica banding to microbreccia-like textures. The Fe-Mn ± Si layers from the Bukit Botol and Bukit Ketaya deposits generally have high Si, Fe, Mn and base metals contents, various proportions of Al, Zr and Ti, and high levels of Ba, As, U and V. Such mineralogical and geochemical concentrations and other geological suggest that the associated Fe-Mn ± Si layers were deposited by the mixing of mainly chemical components and to a smaller degree detrital components in a local submarine environment due to hydrothermal activity on the seafloor.

4D history of the Nimbus VHMS ore deposit in the Yilgarn Craton, Western Australia

Due to its unique mineralogy in the Archaean Yilgarn Craton, the origin of the high-grade Nimbus Ag-Zn-(Au) deposit has been contentious for a number of years. Recent interpretations of the deposit as a shallow water and low temperature volcanic-hosted massive sulphide (VHMS) deposit with epithermal characteristics (i.e. a hybrid bimodal-felsic deposit), were based on detailed studies of its volcanology, mineralogy, hydrothermal alteration assemblages, geochemistry, multiple S isotopes and trace element content of sulphide. However, this model has been questioned in favour of a late, epithermal fault-hosted system. Effective greenfields exploration for similar deposits elsewhere requires a robust deposit model, and well understood timing of ore formation. We present a comprehensive multi-disciplinary study to further constrain the processes involved in the evolution of Nimbus and generate a 4D evolutionary model of the system.Host rock dacite formation is well constrained by SHRIMP U-Pb zircon geochronology to c. 2703 Ma. Re-Os ages for the first sulphide phase, gives an imprecise maximum age to the ore formation of 2682 ± 110 Ma. High precision Pb-isotope variations in ore-stage and late galenas track changes in the sulphides over time, complemented by new geochronology. The Pb-isotope data indicates that the polymetallic ore precipitation happened at the same time as the volcanism, with the second generation indicating minor, local remobilisation of Pb in late quartz-carbonate veins at c. 2630 Ma. 40Ar/39Ar dating of sericite-altered plagioclase from foliated dacite, and U-Pb SHRIMP dating of syn- to post-deformation monazites have yielded ages of ~2630 Ma. The late event at c. 2630 Ma is most likely related to widespread late low-Ca granite emplacement across the Eastern Goldfields.This study provides arguments that suggest that Nimbus is a syn-volcanic deposit. The ore timing is consistent with the model of replacement-type VHMS deposit, with the age of the main mineralisation similar to the age of the dacitic host-rocks at 2703 Ma. The late event modifies the pre-existing ore and overprints the previous ore-related alteration ages from the altered plagioclase. High precision, double spiked, Pb isotope constraints from late quartz-carbonate veins provides an opportunity to vector to hidden deposits.

Adapting the use of AEM for greenfields VHMS exploration under cover

ABSTRACTElectromagnetics (EM) has been used extensively for Volcanic Hosted Massive Sulphide (VHMS) exploration in Australia. Exploring under conductive cover introduces significant limitations when using EM to identify bedrock conductivity anomalies that may be associated with VHMS deposits. We present an alternative approach, whereby robust geological modelling of the Airborne Electromagnetic (AEM) data plays a major role in the exploration strategy in the Bryah and Yerrida Basins of central Western Australia. The AEM is not only used for the identification of bedrock conductors but also forms a critical dataset constraining a robust basin-wide geological model. This model is used to identify priority areas for follow up surface geophysics and geochemistry. A patchwork of AEM surveys, covering portions of the Bryah and Yerrida basins, has been acquired by various explorers and contractors during the last decade. Systems and system specifications vary greatly. Accordingly, accurate geological interpretation of a basin-scale area, flown using various systems, cannot be derived from either raw data or fast/approximate conductivity products provided by contractors. All datasets require reconciliation with a common workflow and robust modelling strategy. Historic AEM data acquired with different systems along the edges of the tenure have been reprocessed and inverted. The remaining central block awaits the contractor's arrival before the data is subject to the same workflow. The end result will be a seamless basin-wide 3D conductivity model (extending over 6500 km2), which will inform the geological interpretation and subsequent follow-up exploration efforts. The preliminary 3D models already allow clear identification and modelling of the pyritic shale horizons, enabling the anomalous geochemistry and strongly conductive nature of these units to be discounted in the targeting process

Targeting VHMS mineralization at Erayinia in the Eastern Goldfields Superterrane using lithogeochemistry, soil chemistry and HyLogger data

Exploration for volcanic-hosted massive sulfide (VHMS) deposits in the Archaean Yilgarn Craton of Western Australia is hampered by a combination of a paucity of outcrop, deep weathering, and saline groundwaters in geologically complex greenstone belts. We present a detailed account of the geology at Erayinia NW (Murrin Domain) in the southern part of the Kurnalpi Terrane, ~150 km SE of Kalgoorlie and 4 km NW of the King VHMS deposit (Edjudina Domain). We have used a combination of logging, petrography, lithogeochemical, pXRF, hyperspectral and soil datasets to help identify VHMS proximal signatures for future drill testing.Diamond drilling at Erayinia NW has identified widespread hydrothermal alteration and minor base metal mineralization across a strike length of at least 3 km. A lower package of calc-alkaline metabasalt, is overlain by intermediate volcanic and associated volcaniclastic rocks, a mixed sequence of argillaceous sedimentary rocks interbedded with FI to FII affinity dacitic rocks, and calc-alkaline metabasalt. SHRIMP UPb zircon geochronology has constrained the age of the host stratigraphy to 2685 ± 5 Ma. Hydrothermal alteration (quartz-sericite-pyrite) is most prevalent in the 50-75 m thick felsic dominated sequence interbedded with black shales, with local zones of chloritization associated with minor ZnPb mineralization. Massive pyrite was intercepted in the two southernmost holes located approximately 300 m apart.Black shales and hydrothermally altered felsic rocks contain elevated Ag, As, Bi, Cd, Cu, Hg, In, Mo, Pb, Sb, Te, Tl associated with increasing Zn concentrations. Broad halos of Sb, Tl, Eu/Eu* and normative corundum define the stratigraphy of interest. Hydrothermally altered felsic rocks are characterized by higher abundances of white mica and lesser chlorite than surrounding strata. Increased quantities of Mg-rich chlorite, muscovitic (Al-rich) white mica, and Fe‑carbonate (siderite) occur proximal to FeZn mineralization. Significant enrichments in trace elements (Ag, As, Bi, Cd, In, Mo, Ni, Sb, Sn, Te, Tl; plus Alteration Index and K2O; and lower Al, Ba, Sr) in black shales towards drillholes EC154D and EC157D suggest this area is most proximal to VHMS mineralization. Soil metal concentrations vary significantly according to underlying regolith type (e.g. colluvium, alluvium, playa lake). Geochemical data levelled by regolith type reveal several bulls-eye multi-element geochemical anomalies (e.g. Zn, Pb, Cu, As, Bi, Cd, Mo, Sn, Tl, W) over the King deposit (2.15 Mt. at 3.47 wt% Zn), King North occurrence (e.g. 1 m at 6.7 wt% Zn + Pb), with new targets identified at Erayinia NW and elsewhere for future drill testing.

Distinguishing Ore Deposit Type and Barren Sedimentary Pyrite Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Trace Element Data and Statistical Analysis of Large Data Sets

Faced with ongoing depletion of near-surface ore deposits, geologists are increasingly required to explore for deep deposits or those lying beneath surface cover. The result is increased drilling costs and a need to maximize the value of the drill hole samples collected. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of pyrite is one tool that is showing promise in deep exploration. Since the trace element content of pyrite approximates the composition of the fluid from which it precipitated and the crystallization mechanism, the trace element characteristics can be used to predict the type of deposit with which a pyritic sample is associated. This possibility, however, is complicated by overlapping trace element abundances for many deposit types. The solution lies with simultaneous comparison of multiple trace elements through rigorous statistical analysis. Specifically, we used LA-ICP-MS pyrite trace element data and Random Forests, an ensemble machine learning supervised classifier, to distinguish barren sedimentary pyrite and five ore deposit categories: iron oxide copper-gold (IOCG), orogenic Au, porphyry Cu, sedimentary exhalative (SEDEX), and volcanic-hosted massive sulfide (VHMS) deposits. The preferred classifier utilizes in situ Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Tl, and Pb measurements to train the Random Forests. Testing of the Random Forests classifier using additional data from the same deposits and sedimentary basins (test data set) yielded an overall accuracy of 91.4% (94.9% for IOCG, 78.8% for orogenic Au, 81.1% for porphyry Cu, 93.6% for SEDEX, 97.2% for sedimentary pyrite, 91.8% for VHMS). Similarly, testing of the Random Forests classifier using data from deposits and sedimentary basins that did not have analyses in the training data set yielded an overall accuracy of 88.0% (81.4% for orogenic Au, 95.5% for SEDEX, 90.0% for sedimentary pyrite, 73.9% for VHMS; insufficient data was available to perform a blind test on porphyry Cu and IOCG). The performance of the classifier was further improved by instituting criteria (at least 40% of total votes from the Random Forests needed for a conclusive identification) to remove uncertain or inconclusive classifications, increasing the classifier’s accuracy to 94.5% for the test data (94.6% for IOCG, 85.8% for orogenic Au, 87.8% for porphyry Cu, 95.4% for SEDEX, 98.5% for sedimentary pyrite, 94.6% for VHMS) and 93.9% for the blind test data (85.5% for orogenic Au, 96.9% for SEDEX, 96.7% for sedimentary pyrite, 84.6% for VHMS). The Random Forests classification models for pyrite trace element data can be used as a predictive modeling tool in greenfield terrains by providing an accurate indication of ore deposit type. This advance will assist mineral explorers by allowing early implementation of predictive ore deposit models when prospecting for ore deposits. Furthermore, the ability of the classifier to accurately identify pyrite of sedimentary origin will allow researchers interested in paleoenvironmental conditions of ancient oceans to effectively screen prospective samples that are affected by a hydrothermal overprint.

Boosting for Mineral Prospectivity Modeling: A New GIS Toolbox

With an increasing demand for raw materials, predictive models that support successful mineral exploration targeting are of great importance. We evaluated different machine learning techniques with an emphasis on boosting algorithms and implemented them in an ArcGIS toolbox. Performance was tested on an exploration dataset from the Iberian Pyrite Belt (IPB) with respect to accuracy, performance, stability, and robustness. Boosting algorithms are ensemble methods used in supervised learning for regression and classification. They combine weak classifiers, i.e., classifiers that perform slightly better than random guessing to obtain robust classifiers. Each time a weak learner is added; the learning set is reweighted to give more importance to misclassified samples. Our test area, the IPB, is one of the oldest mining districts in the world and hosts giant volcanic-hosted massive sulfide (VMS) deposits. The spatial density of ore deposits, as well as the size and tonnage, makes the area unique, and due to the high data availability and number of known deposits, well-suited for testing machine learning algorithms. We combined several geophysical datasets, as well as layers derived from geological maps as predictors of the presence or absence of VMS deposits. Boosting algorithms such as BrownBoost and Adaboost were tested and compared to Logistic Regression (LR), Random Forests (RF) and Support Vector machines (SVM) in several experiments. We found performance results relatively similar, especially to BrownBoost, which slightly outperformed LR and SVM with respective accuracies of 0.96 compared to 0.89 and 0.93. Data augmentation by perturbing deposit location led to a 7% improvement in results. Variations in the split ratio of training and test data led to a reduction in the accuracy of the prediction result with relative stability occurring at a critical point at around 26 training samples out of 130 total samples. When lower numbers of training data were introduced accuracy dropped significantly. In comparison with other machine learning methods, Adaboost is user-friendly due to relatively short training and prediction times, the low likelihood of overfitting and the reduced number of hyperparameters for optimization. Boosting algorithms gave high predictive accuracies, making them a potential data-driven alternative for regional scale and/or brownfields mineral exploration.

Targeting VHMS mineralisation in metamorphosed volcanic terranes

Despite having been a target for volcanic-hosted massive sulphide (VHMS) deposits since the 1960s, few resources have been defined in the Archean Yilgarn Craton of Western Australia. Exploration ch...