The spilite-keratophyre association of West Tasmania and the ore deposits at Mt. Lyell, Rosebery and Hercules
Creators: name: Solomon, M
Publisher: name: University Of Tasmania
Year: 2023

description: Western Tasmania was the site of geosynclinal sedimentation during the Cambrian and the Late Precambrian. In the Early Cambrian there was a marked change from Precambrian ( ?) deposition of sandstones, mudstones and dolomite to a typical synorogenic greywacke sedimentation that continued into the Upper Cambrian. This change was accompanied by extrusion of spilitic and keratophyric volcanics. The basic volcanics are best exposed on King Island and include pillow lavas, breccias called isolated-pillow, broken-pillow, and broken-flow breccias and also tuffs similar to palagonite tuffs in Iceland and \aquagene tuffs\" in British Columbia. The pillows (which develop by a combination of surface tension and gravitational effects) and also the finely globular tuffs indicate an aqueous environment. The spilites contain three principal types: (a) picrite ( b) tholeiitic and (c) augite-rich. They are generally low in titania and rich in alumina. Differentiation is characterised by iron enrichment but follows neither alkaline tholeiitic or calc-alkaline trends. In some areas the volcanics show considerable alteration (with development of hydrogrossular chlorite quartz sericite and calcite) most of which is considered to be of deuteric origin. The spilites are associated with small amounts of keratophyre and quartz keratophyre. Within the Cambrian geosyncline around the margin of a geanticlinal ridge in the centre of Tasmania (the Tyennan Geanticline) there developed a volcanic arc (the Mt. Read Volcanic Arc) which commenced to form at much the same time as the spilites but continued to develop probably well up into the Cambrian. This arc is composed largely of quartz keratophyres both sodic and potassic which form massive flows and probably also intrusive bodies. Albiteandesites occur near Queenstown. Basal members are characteristically potassic and those near Rosebery (the Primrose Pyroclastics) show many features of ignimbrites or froth-flows. Lenses of banded tuff and mudstone in the Arc indicate aqueous deposition but at least some of the volcanic activity was probably subaerial. Entry of nuees ardentes into the sea or slumping off the flanks of volcanoes or submarine volcanic activity probably gave rise to density currents which swept into the flanking basin to deposit feldspar-rich fragmental rock. Variation diagrams show calc-alkaline affinities for the keratophyres. As in other spilite-keratophyre provinces the quartz keratophyres are far more abundant than other volcanics and differentiation from spilitic magma seems unlikely. Rocks associated with the spilites and keratophyres include later serpentinites and gabbros and sedimentary cherts forming a typical ophiolite suite. The Mt. Read Volcanics are intruded by small partly concordant sodic and potassic granites. It is tentatively concluded that the spilites and keratophyres are derived from independent magmas intruded during a phase of renewed sagging and stretching of the geosyncline. The intake of water and possibly other material into the magmas may have affected the subsequent crystallisation leading to formation of albite-diopside and albite-chlorite rocks or the magma may have been hydrous from inception. Some potassic quartz keratophyres contain thick veins of hematite and magnetite at several points along the Mt. Read Volcanic Arc and several of these occurrences have associated copper mineralization. Most of Tasmania's metallic ore deposits (mainly tin lead zinc and copper) occur in areas of Cambrian sedimentation and particularly where spilites or keratophyres are abundant. Some of the deposits are clearly related to Devonian granites but the two largest deposits Mt. Lyell (copper) and Rosebery-Hercules (zine-lead copper) occur in the Mt. Read Volcanic Arc and lack any obvious granitic source. Mt. Lyell ores are mainly pyrite-chalcopyrite and chalcopyrite-bornite-pyrite assemblages with very minor pyrite-sphalerite-galena lodes. They form lenses replacing cleaved and hydrothermally altered Mt. Read Volcanics adjacent to Ordovician Owen Conglomerate. They were deposited after alteration and cleavage development both related to the Devonian Tabberabberan Orogeny. The hydrothermal alteration resulted in quartz-sericite and quartz-chlorite \"schists\" with minor pyrophyllite and probably took place between 350 and 600¬¨‚àû at about 800 atmospheres. The temperature of ore deposition is unknown but may have risen to 700¬¨‚àû C. The NW cleavage and parallel thrust and transcurrent faults provided local structural controls but more important controls appear to be the Great Lyell Fault Zones which dates back to the Cambrian and the distribution of certain potassic keratophyres. Massive hematite bodies at the base of the Owen Conglomerate give a possible indication of pre-Ordovician mineralization. The Rosebery and Hercules ores form discrete pyrite-sphalerite-galena- chalcopyrite lenses replacing a fine-grained tuff that overlies the Primrose Volcanics. This tuff exerts a pronounced stratigraphic control and bedding and cleavage are also important on a smaller scale in controlling the orientation of individual lenses. The ores which are locally banded follow sericite-carbonate alteration and Devonian cleavage. Local structural controls are not obvious at Rosebery and Hercules but both mines (and other small deposits) lie on a major fault zone (the Rosebery Fault Zone) which probably was active in Cambrian times. The ores show chemical features typical of magmatic deposits and their final phase of deposition was clearly of Devonian age. However there is evidence of a pre-Ordovician phase of mineralization and the ores may be of volcanic origin deposited within the Mt. Read Volcanics soon after their eruption."