Hellyer is a large (16.2 million metric tons), high-grade (13.9% Zn, 7.1% Pb, 0.4% Cu, 168 g/t Ag, 2.5 g/t Au), sea-floor, mound-style, polymetallic volcanic-hosted massive sulfide (VHMS) deposit located in the Mount Read Volcanics of western Tasmania. The deposit is hosted by the Que-Hellyer Volcanics, a sequence of late Middle Cambrian mafic to felsic coherent volcanics and polymict volcaniclastics.
Hydrothermal alteration occurs in the regional footwall, immediate footwall, and hanging wall. Alteration in the regional footwall is confined to patchy quartz, albite, and chlorite, with minor sericite, epidote, and hematite. Underlying Hellyer is a zoned alteration pipe with a central siliceous core (quartz-sericite), which passes into zones of chlorite, chlorite-carbonate, sericite-chlorite, and finally sericite-quartz (stringer envelope zone) on the margin. Overlying the central part of the deposit, within the hanging-wall basalt, is a distinctive and zoned alteration plume. Five alteration zones have been identified: fuchsite, chlorite, carbonate, quartzalbite, and sericite. Fuchsite-dominated alteration occupies the central portion of the hanging-wall alteration plume. Chlorite and carbonate alteration surrounds the fuchsite zone with carbonate zones forming near to the ore deposit and chlorite zones extending above and lateral to the carbonate. Outward is quartz-ablite alteration, which extends laterally into distal sericite alteration.
Mass-change calculations for the footwall and hanging wall indicate that, in general, the footwall alteration zones are depleted in CaO, Na2O, La, Sr, Ni, Cr, and V but have enrichments of Fe2O3, MnO, MgO, K2O, S, and most metals. Compared to the host basalt, the hanging-wall alteration has gained CaO, K2O, Na2O, CO2, S, in Rb, Ba, Ag, As, Mo, Sb, Cs, and Tl, while Fe2O3, MnO, MgO, P2O5, La, Sr, Pb, Zn, Th, U, Cd, and Nd are depleted. CaO, Na2O, Cr, V, and Ni are depleted in the footwall andesite but enriched in the hanging-wall alteration plume. This relationship suggests that these elements were sourced from the breakdown of feldspars, pyroxenes, and andesitic groundmass of the footwall lithologies and transported in the hydrothermal fluid into the overlying basalt and precipitated as albite, calcite, and white micas in the hanging-wall alteration.
The development of alteration associated with the Hellyer VHMS deposit occurred in three stages. Stage 1 regional footwall alteration was formed by unfocused hydrothermal convection of seawater down into the recently deposited volcanic pile at temperatures between approximately 250 degrees celsius and 200 degrees celsius and at low to moderate water/rock ratios. Stage 2 alteration formed by structurally controlled fluid flow from a deep intensifying hydrothermal convection system and created the footwall alteration pipe. Decreasing water/rock ratios and temperatures, over a range of 350 degrees celsius to less than 200 degrees celsius, led to the development of the concentric alteration pipe mineral zones. Based on modeling of whole-rock d18Ofr values, geochemical modeling, and mineral assemblages, the siliceous core is interpreted to have formed at temperatures near 350 degrees celsius, the chlorite-rich alteration zone at 300 degrees celsius to 250 degrees celsius, and the outer sericite-rich alteration at temperatures of 250 degrees celsius to 150 degrees celsius. The final, synmineralization fluid-flow event, stage 3, created the hanging-wall alteration plume. After rapid burial of the deposit by basalt, continuation of upward hydrothermal fluid flow created the zoned hanging-wall alteration. Distribution of hanging-wall alteration assemblages suggests a temperature gradient from approximately 250 degrees celsius for the fuchsite zone to lower temperatures (150 degrees celsius) for the distal quartz-albite and sericite alteration zones.
Alteration mineralogy, mineral chemistry, lithogeochemistry, and stable isotope characteristics of the footwall and hanging-wall alteration have been combined into a comprehensive set of vectors, which can be used in exploration for VHMS deposits in similar geologic settings.
The major, minor and trace element chemistry of chlorite were evaluated as a tool for mineral exploration in the propylitic environment of porphyry ore deposits. Chlorite from eighty propylitically altered samples, located up to 5 km from the Batu Hijau Cu–Au porphyry deposit in Indonesia, was analyzed using electron microprobe and laser ablation inductively-coupled plasma mass spectrometry. The results show that a variety of elements, including K, Li, Mg, Ca, Sr, Ba, Ti, V, Mn, Co, Ni, Zn and Pb, are probably incorporated in the chlorite lattice and display systematic spatial variations relative to the porphyry center. Ti, V and Mg decrease exponentially in concentration with increasing distance, whereas the others increase. Ratioing the former to the latter provides a variety of ratios that vary up to four orders of magnitude, providing sensitive vectoring parameters. Chlorite geothermometry suggests that Ti is substituted into chlorite as a function of crystallization temperature and thus maps out the thermal anomaly associated with the mineralized center. By contrast, Mn and Zn display a maximum in chlorite at a distance of ~ 1.3 km that mirrors the whole rock anomaly for these metals, reflecting their lateral advection into the wall rocks by magmatic-hydrothermal fluids. The recognizable footprint defined by chlorite compositions extends to at least 4.5 km, significantly beyond the whole rock anomalism (≤ 1.5 km) and thus represents a powerful new exploration tool for detecting porphyry systems. Variations in chlorite chemistry are very systematic in the inner propylitic zone (to distances of ~ 2.5 km), thereby providing a precise vectoring tool in a domain where other tools are typically ineffective. In this zone, equations of the form:x=lnRabcan be formulated, where the distance to center, x, is predicted based on a variety of element ratios in chlorite R, and where a and b are exponential fit parameters. Importantly, distal chlorite compositions in porphyry-related propylitic alteration systems are also shown to be distinct from metamorphic chlorite, allowing the external fringes of porphyry-related hydrothermal systems to be distinguished from "background" regional metamorphism or geothermal alteration.
The Gosowong epithermal Au-Ag deposit, located on Halmahera Island, eastern Indonesia, has a resource of 0.99 million metric tons (Mt) at 27 g/t Au and 38 g/t Ag. Host rocks consist of Miocene shallow marine, intermediate-basic volcanic and volcaniclastic rocks. The structural setting consists of an elongate dome with associated tensional fracturing trending parallel to the long axis of the dome. Gold mineralization is hosted within multiphase, epithermal quartz-adularia and quartz-chlorite fissure veins, breccias, and stockwork veining within two gently south plunging ore shoots along a 400-m section of the north-striking, east-dipping Gosowong fault.
Alteration is zoned around the vein from proximal silicic to argillic to vein-related propylitic to distal regional propylitic alteration. Prior to the onset of the Gosowong deposit hydrothermal system the host rocks in the district were altered to a regional propylitic (epidote, Fe chlorite, albite, pyrite) assemblage. As hydrothermal fluids progressed up the Gosowong fault, vein-related propylitic (epidote, Fe chlorite, smectite, calcite, pyrite) alteration formed within approximately 50 m of the deposit. As the hydrothermal system intensified argillic (illite, quartz, chalcedony, adularia, Mg-Fe chlorite, smectite, pyrite) alteration formed within a few 10s of meters of the Gosowong fault and overprinted the vein-related and regional propylitic alteration. Silicic (quartz, chalcedony, adularia, illite, Mg chlorite, pyrite) alteration overprints all other types of alteration and is confined to within a few meters of the vein.
Analysis of the whole-rock geochemistry of altered volcanic rocks indicates there are varying element concentrations depending on location: (1) at surface, Hg, Au, Ag, Pb, Mo, Tl, As, K2O, and Li are enriched and the Alteration Index (AI = 100(MgO + K2O)/(MgO + K2O + CaO + Na2O) has high values; CaO, MgO, Fe2O3, Sr, and Na2O are depleted, and the chlorite-carbonate-pyrite index (CCPI = 100(MgO + FeO)/(MgO + FeO + K2O + Na2O) has low values; (2) proximal to the deposit, Au, Ag, As, Cu, and Pb are enriched, whereas Na2O is depleted; (3) in the alteration halo, K2O, Mo, Tl, As, and S are enriched, and the AI has high values; Na2O and Sr are depleted and the CCPI has low values; and (4) along the downward extension of the Gosowong fault zone, K2O, Tl, Ba, and Li are enriched and the S/Na2O ratio has high values. These mineralogical and geochemical features provide a set of exploration vectors for use in the Gosowong goldfield.
The Gossan Hill volcanic-hosted massive sulfide (VHMS) deposit is an Archean Cu-Zn-magnetite-rich deposit located in the Warriedar fold belt of the Yilgarn craton, Western Australia. The deposit is hosted by redeposited rhyodacitic tuffaceous volcaniclastics of the Golden Grove Formation and is overlain by felsic volcanic rocks of the Scuddles Formation. The deposit consists of two separate subvertical ore zones. The stratigraphically lower Cu-rich ore zone (7.0 Mt @ 3.4% Cu) is strata bound and varies from podiform massive pyrite-chalcopyrite-pyrrhotite-magnetite to sheetlike massive magnetite-carbonate-chlorite-talc. The upper Zn-Cu ore zone (2.2 Mt @ 11.3% Zn, 0.3% Cu, 1.5 g/t Au, and 102 g/t Ag) contains strata-bound massive sphalerite- pyrite-chalcopyrite that overlies discordant, moundlike massive pyrite-pyrrhotite-chalcopyrite-magnetite. Sulfide stockwork connects the upper and lower ore zones. Metal zonation varies from Cu (+-Au) in the lower ore zone to Zn-Cu at the base of the upper ore zone, which grades upward and laterally to Zn-Ag-Au (+-Cu, +-Pb). Multiphase deformation and greenschist facies metamorphism overprint the deposit.
The relationship between host rocks and sulfide indicates that mineralization was broadly codepositional with sedimentation of the Golden Grove Formation. However, gradational and interdigitating contacts between these volcaniclastic rocks and magnetite support the formation of massive magnetite by subsea-floor replacement. Sulfides replace and vein the massive magnetite with inferred synchronous formation of the upper and lower sulfide ore zones. Although these sulfide ore zones formed mainly by subsea-floor replacement, stratiform hydrothermal chert-sulfide-sediment layers within, and adjacent to, the upper Zn-rich ore zone attest to some local exhalation.
The thickest development of massive magnetite, massive sulfide, and stringer stockwork occurs in the north of the deposit and supports a common feeder during massive magnetite and sulfide formation. Furthermore, local chlorite-quartz hydrothermal alteration, massive magnetite, massive sulfide, and stockwork all form asymmetric zones that thin southward. These attributes indicate strong synvolcanic structural control during mineralization; the relict synvolcanic growth structure is obscured and probably occupied and by a dacite dome of the hanging-wall Scuddles Formation. Thermodynamic considerations suggest that massive magnetite and sulfide formed from similar high-temperature (>300 degrees celsius), slightly acidic, low-fO2 hydrothermal fluids. Massive magnetite formed from H2S-poor fluids, whereas massive sulfide formed from relatively rich H2S fluids. Physicochemical changes associated with interaction between upwelling, H2S-bearing fluids and preexisting massive magnetite may have resulted in the subsea-floor precipitation of sulfide in the lower Cu-rich ore zone.
The Gossan Hill VHMS deposit represents a multistage hydrothermal system within an environment characterized by rapid volcaniclastic sedimentation and changing structural and magmatic processes. The six main stages were as follows: (1) initiation of the hydrothermal system, (2) sedimentation, metasomatism, and progressive heating of convecting fluids, (3) deposition of massive magnetite from H2S-poor fluids by subsea-floor replacement above a buried synvolcanic conduit, (4) structural reactivation tapping deeper H2S- and metalbearing fluids, (5) subsea-floor replacement and minor exhalative sulfide mineralization, with (6) burial and preservation of the deposit resulting from proximal felsic volcanism.
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