Constitutional features, and some exploration implications, of three zinc-bearing stratiform skarns of eastern Australia.

Abstract

Stratiform calc-silicate masses containing noteworthy ZnS occur in Australia at Ban Ban, Queensland, and at Brindabella and Willi Willi, New South Wales. Those of Ban Ban are close, but not obviously related, to an intrusive contact; those of Brindabella and Willi Willi have no apparent spatial relationship with such a contact. Detailed study of the Ban Ban and Brindabella occurrences show them to be very similar both in mineralogy and in chemistry; they constitute sharp Fe-Ca-Mn-Zn-S anomalies with respect to their enclosing rocks. Zn is not related to Fe3+/Al in andradite/grossular but does have a tie with whole-rock and garnet MnO. The Willi Willi occurrence is also similar and illustrates spectacularly the incidence of skarn mineralogy in a virtually unmodified, unmetamorphosed environment of organic carbonate sedimentation. It is considered that these three sulphide-bearing 'skarns' were formed as syngenetic, exhalative calc-silicate-rich aprons that have subsequently undergone diagenetic/metamorphic grain growth; that the association of sulphides and carbonates and the high Fe3+ content of the main minerals (garnet, epidote, magnetite) reflect original deposition in a well-aerated environment of carbonate sedimentation, rather than later metasomatic replacement of carbonate; that the low-grade Zn of the Brindabella occurrence is very similar to the low-grade periphery of the otherwise high-grade Ban Ban occurrence; and, hence, that as with other types of stratiform ore, systematic documentation of constitutional variation in space in metal-bearing skarns should lead to more accurate recognition of peripheral zones and, thus, to improved evaluation of drill-core data.-R.A.H. Show more