MULTIPLE ORIGINS OF GAHNITE (ZINCIAN SPINEL) ASSOCIATED WITH BROKEN HILL-TYPE LEAD-ZINC-SILVER MINERALIZATION IN THE PROTEROZOIC CURNAMONA PROVINCE, AUSTRALIA

The Broken Hill Domain of the Curnamona Province is host to the most extensive (> 250 km) gahnite-bearing rocks in the world where they serve as exploration guides to Broken Hill-type (BHT) mineralization (i.e., stratiform/stratabound Zn-Pb-Ag deposits hosted in amphibolite/granulite terrains that contain elevated Mn-Ca-Fe and depleted S contents). In the giant Broken Hill deposit, blue quartz-gahnite lode is spatially associated with sulfides. Away from Broken Hill, blue quartz-gahnite rocks are generally medium- to coarse-grained, narrow (1-2 m wide) and poorly laminated. Most examples of blue quartz-gahnite lode are generally concordant to other rock types in which they are hosted but it can also crosscut massive sulfides, metasedimentary gneisses and quartz-garnetite. Although the structural and paragenetic relationship between blue quartz-gahnite lode and BHT mineralization suggests that the precursors to gahnite-bearing rocks formed prior to the main deformational event (D1), the spatial relationship of some gahnite-quartz rocks to D2 and D3 folds indicates that these rocks had a protracted history of development. In addition to occurring in quartz-gahnite rocks, gahnite is also observed in garnetite, quartz garnetite, banded iron formation, sillimanite-garnet gneiss, pegmatite, amphibolite, and quartz veins. Most gahnite in the Broken Hill deposit is stabilized by reactions in the system Zn-Fe-Al-Si-S-O with the Zn and Fe component of gahnite being originally derived from exhalative/inhalative hydrothermal alteration of pelagic sediments (e.g., Al). It also formed by deposition from a metamorphic hydrothermal solution. Zincian spinel in and adjacent to the Broken Hill deposit contains between 5 and 15 mole % spinel sensu stricto, whereas that associated with rocks in and associated with minor BHT deposits exhibits a broader range of spinel sensu stricto compositions (0 to 20 mole %). Data show that gahnite formed at various times in the metamorphic history by different mechanisms and that gahnite composition can be used to discriminate between BHT deposits and sulfide-free occurrences.