The Thalanga sequence : facies architecture, geochemistry, alteration and metamorphism of felsic volcanics hosting the Thalanga massive sulphide deposit (Early Ordovician, Northern Queensland, Australia)

The Thalanga sequence is a felsic, lava-dominated, submarine volcanic succession in the Cambra-Ordovician Mount Windsor Subprovince (northern Queensland, Australia). The sequence consists of altered rhyolite lavas, syn-volcanic intrusions and volcanielastic facies (footwall), conformably overlain by a lithologically diverse unit (Favourable Horizon) containing the Thalanga massive sulphide deposit, succeeded by a volcanosedimentary facies association dominated by wealdy altered dacite lavas (hangingwall). This package is `~`400 to `~`800 m thick, has a subvertical dip due to regional deformation and youngs towards the south. Massive, Zn-Pb-Cu-rich sulphide-ore lenses (total 6 Mt) are stratiform, occur over a strike length of `~`3 km and are invariably underlain by a broad,continuous alteration zone which extends at least 300 m into the footwall. Regional metamorphism in the Thalanga area generated upper greenschist facies (450 to 500°C; ≤3.5 kbar), foliated biotite-muscovite±chlorite assemblages. Subsequent contact metamorphism, associated with the intrusion of a voluminous, post-tectonic diorite pluton, was of similar metamorphic grade. Six drill core cross sections (total 15 km of core) in the Thalanga mine area and one outcrop section located 5 km to the west of the deposit have been studied. Reconstruction of the facies architecture indicates that the massive sulphides f01med in a below-storm-wave-base depositional environment on top of an elevated, lava-dominated, rhyolitic volcanic centre. Effusive eruptions persisted after formation of the massive sulphide lenses but involved compositionally and texturally distinct dacite. A modern analogue for the setting of the Thalanga deposit is the P ACMANUS hydrothermal field on the crest of the dacite lava-dominated Pual Ridge in the eastern Manus back-arc basin (Papua New Guinea). Rhyolite in the footwall alteration zone includes both genuine volcaniclastic facies and apparent elastic facies formed by domainal or multi-stage hydrothermal alteration of coherent rhyolite. A comparison of the distribution of elastic and coherent facies with the geometry of pyrite-rich, intensely altered, discordant zones in the footwall suggests that intense hydrothermal fluid flow was independent of the facies arrangement. Alteration facies at Thalanga are texturally and mineralogically diverse and ten different facies have been defined on the basis of dominant mineral assemblages and general alteration intensity. Alteration facies representing moderate to intense hydrothermal alteration (quartz-K-feldspar, disseminated tremolite, quartz-pyrite, chlorite-pyrite, carbonate-chlorite-tremolite and phyllosilicate-rich, mottled alteration facies) are exclusive to the footwall. The mottled facies, characterised by muscovitebiotite-chlorite±pyrite assemblages, represents the bulk of the footwall alteration zone and is related to diffuse upflow of hydrothermal fluids causing destruction of primary feldspar, formation of hydrothermal phyllosilicates and precipitation of pyrite. It envelopes discordant zones of quartz-pyrite alteration facies connected to massive sulphides in the Favourable Horizon. Variations in the distribution of some alteration facies along strike suggest that parts of the hydrothermal system exp~rienced particular alteration processes. Low intensity alteration facies ( epidote, phyllosilicate, and albite alteration and hematite dusting) are common in the hangingwall dacite and probably represent the results of diagenetic alteration and low-grade sub-seafloor metamorphism. The geochemical effects of alteration were examined using whole rock major and trace element data and electron microprobe analyses of chlorite, biotite and muscovite. Mass balance calculations show that hydrothermal alteration in the footwall is characterised by substantial gains in Fe, S and Mg and Na loss. Furthermore, most alteration facies gained Si, and K was conserved or added during hydrothermal activity. The concentrations of Ba, Rb, Sr, As and Bi vary according to qunatitative differences in major mineral phases among alteration facies (eg. abundances of barite, muscovite, alkali feldspar, plagioclase, tremolite, epidote and pyrite). Light REE were added to footwall rhyolite during hydrothermal alteration whereas heavy REE remained immobile. In general, Eu was progressively leached from the footwall with increasing intensity of hydrothermal alteration. The geochemical data and in particular the Mg-rich character of the footwall alteration zone indicate that the hydrothermal fluids at Thalanga were seawater-derived. Several compositional features of the footwall alteration zone show systematic changes with distance from the ore defining a geochemical halo. These geochemical proximity indicators may assist in prospect- and mine-scale exploration for VHMS deposits in similar geological settings.