Style and setting of volcanic-hosted massive sulphide mineralisation in the early Permian Berserker beds, Mount Chalmers, Queensland

Mount Chalmers is a Kuroko-style volcanic-hosted massive sulphide (VHMS) deposit that is hosted within the Early Permian Berserker beds of central, eastern Queensland. The Berserker beds occur within tectonic units that comprise the New England Orogen. During the Early Permian horst-graben style extension occurred throughout the New England Orogen and across the back arc region. These fault basins accumulated marine sediments, commonly diamictites and volcanics. It is in one of these extensional basins that the sediments. volcaniclastics and volcanics that form the Berserker beds were probably deposited. The Berserker beds are a complex unit of graded, mass flow emplaced, pumiceous breccias, volcanolithic sandstone units, graded polymict, feldspar phyric pumiceous breccias, rhyolite intrusives, coherent to auto-brecciated rhyolite flows and their autoclastic products, andesitic intrusives, lavas and their autoclastic products. In the vicinity of the Mount Chalmers, mine the Berserker beds are represented by a dynamic and constantly evolving stratigraphic succession of proximal and distal volcanics interbedded with distally derived turbidites. A significant feature of this evolving and variable stratigraphy is the cyclic nature of the volcanism that is represented by the change in volcanism from rhyolite dominant to andesitic dominant and back to rhyolite again. The differing volcanic facies indicate that both subaerial and submarine volcanism were occurring within the general vicinity of each other. The Berserker beds contain a diverse invertebrate fauna and trace fossil assemblage. The invertebrate fauna contains bryozoans, echinoderms, brachiopods and molluscs. The fossil assemblage and its mode of preservation are typical of a shallow shelf (near wave base) sand and silt environment. The faunal assemblage indicates a water depth in the range of 50 - 300 m. The trace fossils are mainly temporary fodinichnia (feeding traces) structures and comprise mainly Teichichnus and Planolites, with scattered Rhizocorallium and Zoophycus type burrows. The trace fossils have a restricted faunal diversity, and may be assigned to the Cruziana ichnofacies. This ichnofacies is normally understood as characterising the region between daily wave base and storm wave base, in low to moderate energy regimes. The mineralisation at Mount Chalmers occurs in two main ore lenses, the Main Lode and the West Lode, and one minor lens, the South Lode. The massive sulphide mineralisation occurs on the flank of a rhyolite dome. The massive sulphide mineralisation consists of massive, layered and fragmental sulphides. Underlying the massive sulphide mineralisation is an extensive well developed network stringer veins. Footwall alteration at Mount Chalmers is dominated by silica along with extensive zones of chlorite alteration. Dolomite, sericite and kaolinite alterations are spatially restricted to high angle normal faults. Textural, petrographic and microthermometric investigations of primary fluid inclusions yielded homogenisation temperatures of 160-268¬¨‚àûC and salinities of 5-8 NaCl equiv. wt %. Semi-quantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Mt. Chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz also indicates a significant base metal concentration in these fluids. Cation composition and higher salinities relative to seawater suggests that a magmatic input of ore metals during seawater leaching of the footwall volcanic pile was a distinct possibility. The ˜í¬•\\(^{34}\\)S values for pyrite from Mount Chalmers range from -17.6 to -1.6 ‚ÄövÑ‚àû, with a median value of-5.7 ‚ÄövÑ‚àû Chalcopyrite has a narrower range of ˜í¬•\\(^{34}\\)S values compared to that for pyrite Both sphalerite and galena have restricted ranges in their ˜í¬•\\(^{34}\\)S values compared to pyrite and chalcopyrite. Barite has a broad range in ˜í¬•\\(^{34}\\)S values and has a skewed distribution. In combination with the fluid inclusion evidence, the favoured model to explain the ˜í¬•\\(^{34}\\)S values in sulphides and barite was hydrothermal fluid that was probably dominated by evolved seawater, but one that had a minor, but significant input of sulphur from a magmatic source. The departures from the
ormal\" range of ˜í¬•\\(^{34}\\)S values to values as low as -17.1 ‚ÄövÑ‚àû can be explained by the hydrothermal fluid interacting with biogenic sulphur within microniches within the sediments as the fluids passed through the volcano-sedimentary pile. Mount Chalmers has a very narrow range of 6 180 values (+9.1 to +9.8 ‚ÄövÑ‚àû). Modelled w/r ratios for both open and closed systems indicate that the Mount Chalmers hydrothermal fluid was dominated by seawater with the possibility of the minor input of magmatic fluid. Calculated high water fluxes over a short time period (5000 years) indicates that high water/rock ratios (‚Äöv¢‚Ä¢1) prevailed throughout most of the life the Mount Chalmers hydrothermal system. Palaeontological evidence shows that the Berserker beds were deposited in a shallow-submarine environment (‚Äöv¢¬ß300 m). Contact relationships between the volcanics and sedimentary facies reveals that peperitic facies are widespread. Silicic and mafic magmas have intruded at shallow levels into volcano-sedimentary pile. The Mount Chalmers VHMS mineralisation is spatially and temporally associated with the intrusion of a rhyolitic lava dome(s) into the volcano-sedimentary pile. Studies to date on modern and ancient VMS deposits have indicated that a minimum water depth between 1000 to 1500 m is required to prevent boiling of hydrothermal fluids and therefore concentrate metal deposition on the seafloor. However these minimum water depths are in strong disagreement with the minimum water depth as suggested by the palaeontological and fluid inclusion evidence for the Mt. Chalmers VHMS deposit indicating that is possible for a hydrothermal system to form an exhalative VHMS in a shallow-marine environment. Fluid inclusion and isotopic evidence suggests that there was a minor but significant input of metals vapour and S from a magmatic source."