Geochemistry of the Rosebery ore deposit

The Rosebery ore deposit is located in the Cambrian Mt. Read Volcanic Belt at the west coast of Tasmania. Two tabular Zn-Pb-Cu (-Ag-Au) orebodies of the deposit are hosted next to each other in a 45¬¨‚àû east dipping sedimentary horizon and belong to the massive sulphide deposits which were generated within pyroclastics during a period of reduced volcanism. In the Rosebery area the calc-alkaline Mt. Read Volcanics are divided into rhyolitic pyroclastics and overlying andesitic to rhyolitic lavas. The ash flow tuff in the footwall of the ore deposit is uniformly welded. The volcanic rocks in the hanging wall consist mostly of a mixed sequence of unwelded and welded pyroclastics. In the mine area the two pyroclastic units are separated by the 70-150 m wide ore-bearing sedimentary horizon which includes siltstone and associated reworked tuff and tuff. The footwall and to a minor degree the hanging wall have been subjected to hydrothermal alteration. In the field, this alteration is marked by plagioclase-free quartz-sericite (-chlorite) schists which exhibit, two different facies: siliceous augen schists and thinly cleaved K- feldspar bearing schists. Geochemically the collected rock samples fall into two groups: altered and unaltered volcanics. This, classification is made possible by cluster analysis despite. the large chemical variation within each. group. In addition, an almost perfect separation into these two groups can be achieved by discriminant analysis when the chemistry of the schiss is compared to that-of the unaltered pyroclastics. On a univariate basis geochemical differences existfor a large number of elements. A clear pattern of enrichment (SiO\\(_2\\), K\\(_2\\)O, Rb; S) and depletion (Al\\(_2\\)O\\(_3\\), Na\\(_2\\)O, TiO\\(_2\\), CaO, Sr, Zr, Y, Nb) can be contour-plotted for a smaller - number of elements which show a statistically significant difference between altered and unaltered rocks. Both geological and geochemical alteration extend for ‚Äöv¢¬ß 2 km in N-S direction and have a maximal. E-W width of about 700 m. However the geochemical approach is more successful in detecting 'the hanging wall alteration and in defining the hydrothermal channel in the footwall. Based on underground geology and geochemistry, there Is evidence for a separate hydrothermal vent for each of the two orebodies apart from the alteration halo on the surface which belongs entirely to the vent below the larger southern orebody. In the immediate footwall of the northern orebody a central area of > 300 m across is totally devoid of feldspars, as below the southern orebody. This aureole of intense alteration is largely composed of siliceous augen schists with an increased amount of disseminated mineralization which is clearly reflected in the chemical distribution patterns of the ore forming metals including sulphur. Based on constant rock volume the element mobilization pattern in this part differs slightly from that of the surface alteration. Most importantly CaO is not depleted, SiO\\(_2\\) shows a stronger enrichment and the decrease in Zr and TiO\\(_2\\) is more pronounced. A good spatial relationship exists between the sulphide enrichment in the footwall schists and the maximum ore grade in the overlying orebody. This footwall zone is surrounded, by moderately altered pyroclastics containing albitic feldspars which range from clear euhedra to relics. Additional exploration methods based on REE distribution and whole rock oxygen isotope analyses and the composition of phyllosilicates produce less satisfactory results than major and conventional trace element chemistry. Some of the original features of hydrothermal oxygen isotope and REE chemistry and mineral compositions have been partly obliterated due to re-equilibration during the Devonian Tabberabberan orogeny. Based on contoured metal zonations and also drill core logging, the northern orebody can be shown to be essentially divided into a Cu-Fe rich bottom part and a Pb-Zn rich top half. This distribution is complicated by the occurrence of smaller amounts of Pb-Zn ore in the bottom part and Cu-Fe ore in the top half. This zonation can be best explained by the deposition of a blanket of galena-sphalerite ore during the initial stage of hydrothermal activity and subsequent introduction and/or replacement by chalcopyrite-pyrite ore by hydrothermal solutions at an increased temperature. Boundary conditions for the hydrothermal fluids can be established based on the sulphide mineral assemblage in the ore and the silicate mineral assemblage in the alteration zone.