Facies analysis and correlations in complex mineralised submarine volcanic successions : Mount Read volcanics, Western Tasmania

Volcanic-hosted massive sulfide (VHMS) deposits provide a significant contribution to world zinc, copper, lead, silver, and gold and continue to be a target for significant base metal exploration. The Middle to Late Cambrian Mount Read Volcanics in western Tasmania are altered, deformed and metamorphosed submarine volcanic facies interfingered with sedimentary rocks, comprising one of the richest VHMS provinces worldwide. The Mount Read Volcanics host six world-class VHMS deposits ranging from polymetallic Zn-Pb-Cu-rich (Hellyer, Que River, Rosebery and Hercules) to Cu-rich (Mount Lyell) and Au-rich (Henty). Previous studies and recent high-precision dating conducted in the Mount Read Volcanics have led to the hypothesis that ore formation occurred during a single chronostratigraphic interval. However, the Hellyer, Que River, Rosebery and Hercules VHMS deposits in the northern Mount Read Volcanics occur in a range of local geological settings and within very different host volcanic successions, hampering stratigraphic correlations both on regional and local scales. These major deposits occur at two stratigraphic levels; the stratigraphically higher level (Hellyer and Que River) is associated with polymictic, basaltic to dacitic, volcanic breccia and coarse-grained sandstone of the mixed sequence of the Que-Hellyer Volcanics (Mount Charter Group), and the lower level (Rosebery and Hercules) is associated with feldspar-phyric fiamme breccias and interbedded pumice-rich sandstone and siltstone of the Hercules Pumice Formation (upper Central Volcanic Complex). Furthermore, the Hellyer and Que River VHMS deposits are accepted to have formed dominantly by seafloor massive sulfide deposition whereas several different genetic models have been proposed for the formation of the Rosebery and Hercules VHMS deposits, both by syn-volcanic sub-seafloor replacement and seafloor massive sulfide accumulation. In this study, the integration of diamond drill core logging, geological mapping, facies analysis, thin-section petrography and whole-rock compositional data of the upper Central Volcanic Complex and overlying western volcano-sedimentary sequences (Mount Charter and Dundas Groups) is used to establish local and regional lithostratigraphic correlations in the Sock Creek-Burns Peak area with the adjacent VHMShosting areas to the NE (Hellyer-Mount Charter) and SSW (Rosebery-Howards Road). These techniques are combined with the examination of the different genetic models, local geological settings and host volcanic successions of the Hellyer, Que River, Rosebery and Hercules VHMS deposits in order to constrain the most prospective stratigraphic position in the Sock Creek-Burns Peak area. The Sock Creek-Burns Peak area of the northern Mount Read Volcanics comprises the upper Central Volcanic Complex and the Mount Charter Group. The stratigraphy of the Sock Creek-Burns Peak area can be considered in terms of seven regional stratigraphic units (USB), one of which (USB4b, polymictic volcanic breccia and sandstone) has been correlated with the mixed sequence of the Que-Hellyer Volcanics. This unit overlies dacitic facies that are possible equivalents of lithologies occurring in the footwall succession of the Hellyer and Que River VHMS deposits, and underlies black mudstone (USB5) and polymictic quartz-bearing, mass flow-type volcaniclastic units that are likely equivalents of lithologies occurring above the Hellyer, Que River, Rosebery and Hercules VHMS deposits. Assuming that a syn-volcanic, sub-seafloor replacement model best accounts for the formation of the Rosebery and Hercules VHMS deposits, this study proposes that the emplacement of USB4b in the Sock Creek-Burns Peak North area occurred immediately before, to at the same time as, the formation of the Hellyer and Que River seafloor and the Rosebery and Hercules sub-seafloor VHMS deposits in the northern Mount Read Volcanics. This major VHMS-forming event coincided with a period of quiescence in volcanic activity during which deposition of black mudstone (USB5 and possible correlates) occurred in the northern Mount Read Volcanics. Prior to the formation of the VHMS deposits, at approximately 503 Ma, major explosive eruptions generated the rhyolitic pumice breccia of the Footwall Member of the Hercules Pumice Formation in the Rosebery-Hercules area (SSW of Sock Creek-Burns Peak), and feldspar-phyric rhyolitic to dacitic and minor feldspar-quartz-phyric rhyolitic lavas, domes and minor cryptodomes were emplaced in the Sock Creek-Burns Peak (USB1) and White Spur-Howards Road area (correlated with the Central Volcanic Complex), and presumably also in the Hellyer-Mount Charter area (NE of Sock Creek-Burns Peak). Dacitic-andesitic volcanic centres in the Sock Creek-Burns Peak area and basaltic-andesitic-dacitic volcanic centres in the Hellyer-Mount Charter area developed probably at the same time as the Host-rock Member of the Hercules Pumice Formation and back mudstone accumulated slowly over an approximately 3-million-year period in the Rosebery-Hercules area. The VHMS deposits mark the only chronostratigraphic horizon at approximately 500 Ma. The mixed sequence of the Que-Hellyer Volcanics accumulated in the Hellyer-Mount Charter area and its correlate (USB4b, polymictic volcanic breccia and sandstone) was deposited in the Sock Creek-Burns Peak North area while black mudstone continued accumulating in the Rosebery-Howards Road area. Regional, basinwide deposition of black mudstone followed in the Hellyer-Mount Charter (Que River Shale) and Sock Creek-Burns Peak (USB5) areas at the same time as black mudstone was still accumulating in the Rosebery-Howards Road area. Following the major phase of massive sulfide formation, explosive eruptions produced the volcanic quartz-bearing units of the Hangingwall Volcaniclastics in the Rosebery-Hercules area and the White Spur Formation in the White Spur-Howards Road area. Similar volcanic quartz-bearing lithofacies were deposited at approximately the same time in the Sock Creek and Burns Peak areas (USB7) while the Que River Shale continued accumulating in the Hellyer-Mount Charter area. Despite the lack of evidence for the presence of economically significant mineral deposits in the Sock Creek-Burns Peak area, USB4b represents the lithostratigraphic unit in the Sock Creek-Burns Peak area favorable for the formation of Hellyer-Que River-like seafloor VHMS deposits, and the volcano-sedimentary succession in the Sock Creek-Burns Peak area below the top of USB4b is potentially prospective for Rosebery-Herculeslike sub-seafloor VHMS deposits. This stratigraphic interval includes dominantly coherent feldspar-quartzphyric rhyolite and feldspar-phyric dacite, and monomictic rhyolite and dacite breccia facies of USB1 (upper Central Volcanic Complex) that are equivalent to the stratigraphic level at which the Rosebery and Hercules sub-seafloor VHMS deposits occur. Tracing USB4b to the SW and further investigating the volcanosedimentary succession below the top of USB4b in the Sock Creek-Burns Peak area may assist in future VHMS exploration. The volcanic footwall and host successions to the VHMS deposits at Hellyer and Que River (feldspar-phyric andesites and basalts, and polymictic volcanic breccia, sandstone and mudstone) and at Rosebery and Hercules (feldspar-phyric fiamme breccia, and pumice-rich sandstone and siltstone) are very different in character and do not constitute good exploration guides to other VHMS deposits elsewhere in the northern MRV. They were deposited synchronously with the emplacement of partly extrusive rhyolite and dacite cryptodomes, domes and lavas, and andesitic to basaltic lavas and domes in the Sock Creek-Burns Peak area. However, the hanging wall successions at all deposits comprise black mudstone and mass flow-type, volcanic quartz-bearing volcaniclastic units that are excellent stratigraphic markers for VHMS exploration.