Australian Zn-Pb-Ag Ore-Forming Systems: A Review and Analysis

Zn-Pb-Ag mineral deposits are the products of hydrothermal ore-forming systems, which are restricted in time and space. In Australia, these deposits formed during three main periods at ~2.95, 1.69 to 1.58, and 0.50 to 0.35 Ga. The 1.69 to 1.58 Ga event, which accounts for over 65 percent of Australia’s Zn, was triggered by accretion and rifting along the southern margin of Rodinia. Over 93 percent of Australia’s Zn-Pb-Ag resources were produced by four ore-forming system types: Mount Isa (56 % of Zn), Broken Hill (19%), volcanic-hosted massive sulfide (VHMS; 12%), and Mississippi Valley (8%). Moreover, just 4 percent of Australia’s land mass produced over 80 percent of its Zn. The four main types of ore-forming systems can be divided into two “clans, ” based on fluid composition, temperature, and redox state. The Broken Hill- and VHMS–type deposits formed from high-temperature (>200°C) reduced fluids, whereas the Mount Isa- and Mississippi Valley-type deposits formed from low-temperature (<200°C), H2S-poor, and/or oxidized fluids. The tectonic setting and composition of the basins that host the ore-forming sys-tems determine these fluid compositions and, therefore, the mineralization style. Basins that produce higher temperature fluids form in active tectonic environments, generally rifts, where high heat flow produced by magmatism drives convective fluid circulation. These basins are dominated by im-