University of Tasmania
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Hydrothermal alterations in the Que-Hellyer VHMS District, Tasmania : implications for exploration

posted on 2023-05-27, 13:39 authored by Wu, WMS
The Hellyer, Fossey, Que River, and Mount Charter volcanic-hosted massive sulfide (VHMS) deposits occur within the Que-Hellyer Volcanics of the Mount Read Volcanics in western Tasmania, Australia. The Mount Read Volcanics are a Middle to Late Cambrian, predominantly submarine succession of rhyolitic to basaltic volcanic and intrusive rocks with variable proportions of intercalated sedimentary rocks. The VHMS deposits occur along a 6 km north-northwest strike and are hosted within the same stratigraphic sequence. Following the discovery of the Fossey deposit in 2007, there was renewed interest in improving the current understanding of the geochemical features of the hydrothermal alteration zones around these VHMS systems. This thesis provides new geologic information on the Fossey and Mount Charter deposits and advances the overall understanding of the whole Que-Hellyer district. The compilation of a semi-continuous spread of regional and deposit-scale data over the 6 km strike length provides a unique opportunity to review the entire mineralising system and develop a better understanding of the alteration systematics using whole-rock geochemistry, alteration mineral chemistry and spectral analyses. The Hellyer deposit is a high grade polymetallic deposit with an irregular elongate shape, offset by the steep, north-south striking Jack Fault. It represents a well preserved example of a seafloor vent deposit with a focussed stringer system. Hellyer is the largest deposit of the four, with a pre-mining resource of 16.5 Mt at 0.4% Cu, 7.2% Pb, 13.9% Zn, 169 g/t Ag and 2.6 g/t Au. The Fossey deposit is located approximately 100 m south of the Hellyer orebody. It is an irregular, stratiform zone of barite with minor glassy silica-pyrite mineralisation associated with areas of high grade base metal sulfide. Fossey had a pre-mining resource of 740 kt at 0.5% Cu, 6.8%Pb, 12.4% Zn, 121 g/t Ag and 2.4 g/t Au. The Que River deposit is 3 km south of Hellyer and consists of five steeply dipping ore lenses (PQ, P North, QR 32, Nico and S). The four lenses that lie below the extrusive dacite have been interpreted to lie within the same stratigraphic horizon above the S lens and the current geometric distribution of these lenses is a result of folding and faulting. Que River had a pre-mining resource of 3.2 Mt at 0.7% Cu, 7.6% Pb, 13.6% Zn, 204 g/t Ag and 3.5 g/t Au. The Mount Charter deposit is about 3 km south of the Que River deposit and consists of a network of barite veins and breccia zones with minor amounts of pyrite, sphalerite, galena, and quartz. No mining has taken place at Mount Charter and but it has a resource of 6.1 Mt at 1.2 g/t Au and 36 g/t Ag. The Mount Charter mineralisation is hosted entirely within the dacitic lavas and breccias of the mixed sequence of the Que-Hellyer Volcanics. These units are strongly altered by pervasive K-feldspar-quartz-sericite-pyrite with areas of strong chlorite alteration overprinted by K-feldspar and quartz. Coherent lavas contain weak to moderate barite-pyrite-base metal sulfide veining, the clastic sections are more strongly mineralised with barite-pyrite-base metal sulfide in the breccia matrix. Pervasive K-feldspar alteration suggests a lower temperature hydrothermal system than the Hellyer, Fossey and Que River deposits. The Fossey deposit also occurs within the mixed sequence, with largely andesitic footwall and basaltic hanging wall units like Hellyer. The main alteration pipe at the Fossey deposit is characterised by strong inner quartz-sericite-pyrite alteration and outer sericite-quartz-pyrite alteration zones. Intense Mg-rich chlorite-pyrite alteration is closely associated with massive sulfide mineralisation and is restricted to 10 m away from mineralisation. Minor K-feldspar alteration occurs in the polymict breccia adjacent to the massive barite mineralisation and also at the margins of the main alteration pipe. The Fossey East deposit occurs in the footwall sequence of the Fossey deposit and is connected to the Fossey deposit by a massive to semi-massive to barite body. The presence of a fine volcaniclastic layer within the massive barite mineralisation suggests that the Fossey East mineralisation may be an offset of the original Fossey deposit. The andesitic and basaltic footwall rocks around the Fossey East deposit are strongly altered by K-feldspar-quartz-sericite-pyrite and quartz-sericite-pyrite±K-feldspar. These alterations zones are surrounded by strong sericite-chlorite-carbonate-pyrite alteration to form the main footwall alteration pipe. A stringer zone dominated by pyrite-quartz±base metal sulfides is concentrated on the eastern side of the Fossey East footwall rocks. While there is sericite-quartz-pyrite alteration, most has been overprinted by K-feldspar and quartz alteration assemblages. At Fossey and Fossey East, sericite (white mica) is ubiquitous in the footwall rocks and is the more dominant alteration mineral than chlorite in most of the footwall alteration zones. Isocon diagrams comparing the various alteration zones at Fossey and Fossey East show that most of alteration are characterised by significant mass gains of K, Ba and Rb and mass loss of Na and Ca. Using whole-rock geochemistry, the rocks in the Que-Hellyer district are divided into eight alteration groups (strong sericite, moderate sericite, weak sericite, sericite-chlorite, chlorite, sericite-albite, albite, K-feldspar) and unaltered lithologies. The strong sericite and K-feldspar groups are concentrated along the linear trend from Hellyer to Mount Charter; whereas the weak sericite, albite and least-altered group rocks occupy peripheral locations. Chlorite alteration is not as dominant as sericite in the Que-Hellyer district, its geochemical features tend to be obscured by co-existing sericite and host rock compositions. The use of the Alteration Index, K2O enrichment and Na2O depletion are the best whole-rock geochemical indicators for hydrothermal alteration associated with VHMS mineralisation. This hydrothermal alteration is accompanied by enrichment of As, Tl, Sb, Mo, W, Rb, and Cs and depletions of Sr and Li. Regional SWIR data show that the VHMS mineralisation and alteration halo are characterised by long wavelength phengitic mica (Al-OH 2210-2218 nm). Fe-OH absorption wavelengths also show systematic variations that suggest more Mg-rich chlorites are formed proximal to the deposits, while relatively Fe-rich chlorites are more common on the peripheries. Chlorite around the Hellyer, Fossey, Que River and Mount Charter deposits and footwall alteration are characterised by Fe-OH wavelength of less than 2252 nm. Mineral compositions of white mica closest to VHMS mineralisation in the Que-Hellyer district is phengitic, with high Mg/(Mg+Fe) and elevated Ba, Tl, Sb, F and locally Zn concentrations. These phengitic micas are also depleted in Na, Ni and Co compared to background (least-altered) samples. Chlorite compositions closest to mineralisation are characterised by high Mg/(Mg+Fe) ratios with elevated Mg, F, Li, Zn and As concentrations and depleted Aliv, Fe, Na, Co, Ni and Cr concentrations. Proximal to the Mount Charter mineralisation, secondary K-feldspar is enriched in K, Ba, Ca, Rb, As, Tl, Sb, Zn and Pb and depleted in Na, Sr and Y. The combined use of regional whole-rock geochemistry, SWIR, and mineral chemistry is an effective way to highlight prospective areas for VHMS mineralisation. The use of alteration mineral chemistry provides a greater understanding on the controls of alteration mineralogy on whole-rock geochemistry. The major and trace element variation trends in white mica, K-feldspar, and chlorite are reflected in whole-rock geochemistry depending on the abundance of the alteration mineral. Regional SWIR data can be used to identify hydrothermal upflow zones in the Que-Hellyer district and whole-rock major and trace metal trends can be used to vector towards the deposits. The three mineralising systems in the Que-Hellyer district are separated by approximately 3 km. This appears to be the optimal periodicity of the hydrothermal convection cells in this district as suggested by the interpreted upflow zones. Based on this research, the areas in the Que-Hellyer district that have similar mineralogical and geochemical footprints as the known deposits, and hence the most prospective, include the areas at depth north of Hellyer, south of Fossey, and along the high strain zone and at depth at Mount Charter.


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Copyright 2014 the author

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