Abstracts Earth and Environmental Sciences

Geology and mineralisation of the Cape Horn - Lyell Comstock area, Mt Lyell

by GR Green

Stratigraphically the disseminated chalcopyrite-pyrite ores of Cape Horn occur in ignimbrites (?) whereas altered lithic tuffs higher in the sequence of the Mt. Read Volcanics are the host rocks for the chalcopyrite-pyrite-bornite mineralization of Lyell Comstock. Chalcopyrite-sphalerite-galena-deposits in replacement (?) chert provide a mineralogical transition between the copper ores and the banded exhalative pyrite-galena-sphalerite lenses of the Tasman Shaft. On the basis of ore and gangue mineralogy and texture correlations are tentatively proposed between the West Lyell and Cape Horn, and between the Lyell Comstock and North Lyell-Crown Lyell deposits. This stratigraphic correlation is in sympathy with the distribution of trace elements in' the sulphides. Geochemical studies indicate that at least some of the Cape Horn ore represents an addition to, rather than a replacement of, the host rock. Copper mineralization occurs in areas of sericitization and silicification locally representing the most intense facies of alteration. Clastic sediments overlying the ores contain feldspars in all stages of alteration and detrital sulphides. These are interbedded with Middle Cambrian limestones, providing the most convincing evidence to date of the age of the hydrothermal alteration and sulphide deposition.