Base Metal Mineralization at Mineral Hill Central Western New South Wales

Abstract

Secondary silver, lead, and copper mineralization at Mineral Hill in the central west of New South Wales has been intermittently worked since its discovery in 1908. Production, mainly from the Iodide mine, has amounted to 350,000 oz silver, 2,350 oz gold, 2,700 tons of lead, and minor copper. The bulk of these ores occurs in a large, broadly curved quartz-iron oxide body extending to a depth of at least 350 feet. This quartz body has been injected into shallow-water Siluro-Devonian sediments adjacent to their anticlinially folded contact with an interbedded acid-volcanic and pyroclastic sequence containing limestone, spicular chert, and possibly tuffaceous pelite horizons. Residual argentiferous galena-rich pockets have occasionally been encountered in the main workings. Recent diamond drilling, however, has intersected predominantly disseminated copper rich ores within the volcanics. Although drilling did not fully test this mineralization, a copper body containing moderate reserves and grades has been outlined at shallow depths. The mineralogy of the copper- and lead-rich associations is quite different, but it is considered that Mineral Hill primary imnerahzation can be correlated with the nearby Melrose base metal deposits and, consequently, with Cobar-type ore. A feature of both the Melrose and Mineral Hill ores is the wide variety of pyrite forms. (framboidal, circular, comminuted, gel-pyrite, growth-zoned euhedral disseminations, and veins) tor which several different origins (including colloidal) have been proposed. The origin of the base rnetals could be related to the quartz-feldspar porphyry or granite which crop out several miles from Mineral Hill. Since the copper-rich ore is confined to the volcanics in the apical region of folding, however, a volcanic exhalative origin, with subsequent remobilization during tectonism, is more feasible. The significance of metal ratios and lead isotope values in relation to a volcanic source is briefly discussed (as are the implications of an origin of this nature on future prospecting). Whatever its source, ore formation in its present form probably took place under mesothermaHo hypothermal conditions with the ore minerals undergoing deformation and recrystaization