Abstract of Granites & Coper Gold Metallogenesis in the Australian Proterozoic

Record 2001/12

Lesley A.I. Wyborn

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• Introduction

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Introduction

These notes focus on some basics of granites, their various types, and the relationship between Proterozoic granites and Au + base metal deposits. AGSO and the State and Territory Geological Surveys have built up a large whole-rock geochemistry granite database as part of their regional mapping programs and some specialised projects (eg. Champion & MacKenzie, 1994). Recently, all data on Proterozoic granites and felsic volcanics were complied as part of an industry funded project to look at the metallogenic potential of Australian Proterozoic granites (Wyborn et al., 1998a, b). Using GIS techniques, the project investigated spatial relationships between specific granite types, host rock compositions and hydrothermal Au, Cu, Zn, P b, Sn, W and Mo mineralisation. Mineralogical, geochemical (~7500 analyses), and age data of Proterozoic granites and felsic volcanics, as well as the age and mineralogical composition of sediments within 5 kms of granite boundaries, were collated for 20 provinces. This project highlighted significant spatial relationships between specific granite and mineral deposit types.

In most Proterozoic orogenic belts of Australia, regardless of age, granites and their comagmatic felsic volcanic occupy at least 20% by area and in some provinces, 40%. Because of the large aereal extent and inferred subsurface volume of granite bodies, defining the field characteristics, chemical composition, and processes of crystallisation for any given major granitic event is essential in understanding the metallogenic p otential of any province. Even if the metals do not come from the granite itself, intrusion of hot magmas can cause major perturbations in the local geothermal environment. As well, the heat generated by radioactive decay has been considered important in raising the local geothermal gradient long after the emplacement of the gra nite (eg. Solomon & Heinrich, 1992; Sandiford & Hand, 1998).