GEOCHEMISTRY AND AGE OF METAMORPHOSED FELSIC IGNEOUS ROCKS WITH A-TYPE AFFINITIES IN THE WILLYAMA SUPERGROUP, OLARY BLOCK, SOUTH AUSTRALIA,AND IMPLICATIONS FOR MINERAL EXPLORATION

Leucocratic quartzofeldspathic gneisses form a significant proportion of the lower part of the Palaeoproterozoic Willyama Supergroup sequenc e in the Olary Block, South Australia and have correlatives in the adj acent Broken Hill Block. Field and geochemical data demonstrate that t hese rocks were originally rhyolitic volcanics and granite, with A-typ e affinities consistent with magma production during intracratonic rif ting, supporting tectonic models proposed for the Willyama Supergroup in the Broken Hill Block. Although the rocks have characteristic high- field-strength element enrichment, many have undergone extensive perva sive pre- or syn-metamorphic sodic alteration and are typically rich i n albite. Sensitive high resolution ion microprobe (SHRIMP) U-Pb zirco n data tightly constrain the depositional and early intrusive history. Zircons from an interpreted metavolcanic rock containing relict quart z phenocrysts yield an age of 1699 +/- 10 Ma, whereas a metagranitoid sample has an age of 1703 +/- 6 Ma. These results are compatible with recent geochronological data on felsic metavolcanic rocks from the Bro ken Hill Block (Page and Laing, 1992) and are indicative of widespread magmatism during deposition of the Willyama Supergroup, Nd signatures for the two Olary Block samples imply the presence of a significant c omponent from a depleted mantle source. The A-type metavolcanic rocks are locally associated with small iron formations, some of which grade into stratiform barite-rich horizons, Although potentially favourable for sediment-hosted exhalative PbZn mineralisation, the Fe- and Ba-ri ch units, along with transgressive vein and breccia occurrences of Fe oxides +/- quartz +/- pyrite cutting both the metavolcanic and metagra nitoid rocks, may be more prospective for epigenetic Cu-Au mineralisat ion related to later metamorphic and/or magmatic events. Partial melti ng of the A-type suite during high grade regional metamorphism at simi lar to 1600 +/- 20 Ma led to the formation of local volumes of sodic g ranitoids and pegmatites containing U-Th-Ti-REE-F mineralisation.