THE SPECTRUM OF VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSITS IN ANCIENT SUBMARINE VOLCANIC BELTS: EXAMPLES FROM AUSTRALIA

Volcanic-hosted massive sulfide (VHMS) deposits in ancient, submarine volcanic belts of Australia include mound, lens and sheet-style Zn-rich polymetallic deposits, massive and disseminated pyritic Cu-Au deposits and disseminated, stratabound Au-only deposits.

Mound, lens and sheet-style polymetallic zinc-rich deposits such as Rosebery, Hellyer, Que River, Gossan Hill, and Thalanga are considered to have formed in relatively deep water environments. These deposits probably formed either on the seafloor (e.g. Hellyer, Que River), or by replacement of porous volcaniclastic units directly below the seafloor (e.g. Rosebery, Gossan Hill).

Massive and disseminated, pyritic Cu-Au deposits, such as those in the Mt Lyell field and at Highway-Reward, formed by sub-seafloor replacement and are associated with only minor zinc-lead massive sulfide ore. These deposits are commonly located in felsic volcanic centres dominated by shallow porphyritic intrusions (eg. Highway-Reward). The Cu-Au ore lenses may be stratabound (e.g. Mt Lyell) or cross-cutting pipes (e.g. Highway-Reward).

Syn-volcanic gold-rich deposits, with high gold/base metal ratios are less common than the Cu-Au and Zn-rich VHMS ore types. Henty formed by the replacement of particular volcanic units deposited in a relatively shallow-water.

Overall, there exists a spectrum of deposits from the classic VHMS deposit (Hellyer), to hybrids between VHMS-porphyry Cu (Mt Lyell) and VHMS-epithermal (Henty) end members. Zn-rich, polymetallic deposits form either on, or just below, the seafloor, whereas the Cu-Au and Au-only deposits form sub-seafloor by replacement of specific volcanic lithologies.

The styles of VHMS mineralisation in modern submarine arc are similar to many ancient VHMS deposits. Based on exploration and research in modern settings, geologists are now observing the direct analogues, and spectrum, of the polymetallic VHMS deposits currently being exploited in ancient back arc volcanic belts throughout Australia.