Aspects of the geology of massive sulfide deposits from the Balcooma District, Northern Queensland and Rosebery, Tasmania : implications for ore genesis

Balcooma massive sulfide district The Balcooma and Dry River South volcanogenic massive sulfide deposits occur in the Cambro-Ordovician Balcooma metamorphic belt in northern Queensland. Structural and stratigraphic studies indicate: (1) the Balcooma deposit originally contained three stratigraphically separate sulfide bodies in a pelitic lens that have been folded and faulted to produce the present five bodies; (2) faulting has been more significant than folding at the Dry River South deposit; and (3) the two prospects occur at approximately the same stratigraphic level where significant volcaniclastic input ceased. Alteration studies indicate that chloritic alteration at Balcooma has involved silica, iron, magnesium, copper, zinc and sulfur addition, and sodium, calcium, potassium, strontium and rubidium loss. An outer zone of quartz-muscovite-pyrite alteration involved silica, base metals and sulfur addition, and sodium, calcium, and strontium loss at Balcooma. Alteration at Dry River. South involved similar changes to the Balcooma quartz-muscovite-pyrite alteration. Halos of weak sodium loss exist around both deposits. Metal zonation studies indicate that gold is associated with copper at both deposits. Mineragraphic studies confirm this association at Balcooma. Silver occurs as a trace metal in galena in lead-rich mineralization from both prospects, but in Balcooma copper-rich mineralization it occurs in chalcopyrite. Precious metals at Rosebery Metal zonation studies at Rosebery indicate that gold is strongly associated with zinc at the top of the massive sulfide mound, and silver is associated with lead. Electrum is usually associated with pyrite although in baritic zones it occurs with other . minerals. Tetrahedrite hosts most silver. Geological and geochemical controls on metal deposition Observation and geochemical modelling indicate that constant 100Zn/(Zn+Pb) ratios result from the saturation of mineralizing fluids in zinc and lead. This ratio is useful in distinguishing massive sulfide mineralization from other types of mineralization. Gold in volcanogenic massive sulfide deposits has two associations: (1) with zinc at the top of the mound (e.g. Rosebery), and (2) with copper at the base of the mound (e.g. Balcooma and Dry River South). The former association is caused by thio-transport of gold in high pH (>4.5), low temperature (<275°C) fluids and deposition by oxidation and dilution at the seawater interface. The latter association is caused by chloro-transport of gold in low pH (<4.5), high temperature (>275°C) fluids and deposition by pH increase and/or temperature decrease. High gold grades are favored at extreme pH conditions. Silver in massive sulfide deposits has three occurrences: (1) in chalcopyrite in copper-rich mineralization (e.g. Balcooma), (2) in galena in zinc-lead-rich mineralization (e.g. Dry River South and Balcooma), and (3) in tetrahedrite in zinc-lead-rich mineralization (e.g. Rosebery). The fluid temperature and the Bi/Sb ratio of the mineralizing fluid controls these associations. Electrum from deposits of the copper-gold association has a generally high and narrow range in fineness (700 to 900), whereas electrum from the zinc-gold association has a lower and wider range (300 to 800). These differences can be modelled by assuming gold chlorocomplexes in the copper-gold association and gold-thiocomplexing in the zinc-gold association.