The Tarcoola Goldfield was selected for study as an example of Au-dominated mineralisation associated with Hiltaba Suite granites. The study shows that the Au mineralisation is the same age as intermediate Hiltaba Suite magmatism, but that the granites in the goldfield form a suite that significantly predates the mineralisation, and have been given the name Paxton Suite.
The Paxton Suite comprises a dominant phase of coarse-grained hornblende-biotite-titanite granite, a lesser felsic fractionate series of medium- to fine-grained biotite granite dykes and sills, and minor medium-grained hornblende-biotite-titanite granodiorite. Each of these phases, dated by the SHRIMP U-Pb method on zircons, has an age of ~1715 Ma. Geochemistry and geochronology shows evidence for only one suite.
The granites are overlain by the Tarcoola Formation (Hein et al. 1994). A sample of immature pebbly arkose, taken from the Peela Conglomerate Member at the base of the Formation, contains two populations of zircons, with ages of ~1730 Ma and ~1715 Ma (Holm, this volume). Fanning (1990) dated zircons from two airfall tuffs in the middle parts of the Tarcoola Formation and obtained 1656 ± 7 Ma, giving the depositional age of that part of the Tarcoola Formation.
Both the Paxton Suite and the Tarcoola Formation were intruded by high-K amygdular microdiorite dykes of the Lady Jane Diorite, which crosscut, and are crosscut by, mineralised veins (Budd & Fraser submitted).
Sericites from altered and variably mineralised granites gave Ar-Ar ages of ~1580 Ma, as did magmatic hornblende from a sample of the Lady Jane Diorite (Budd & Fraser submitted). The magmatic nature of the hornblende, and the crosscutting relationships of the diorite dykes and mineralised veins show that ~1580 Ma is the age of intrusion of the diorites, alteration and mineralisation. The Lady Jane Diorite is probably part of the Hiltaba Suite. Fanning (1997) had previously dated a sample of altered granite at 1578 ± 7 Ma by SHRIMP. As mentioned above, this study found no evidence of more than one suite of felsic intrusives, which raises the possibility that the younger zircons are hydrothermal. However, a felsic component to Hiltaba Suite magmatism cannot be ruled out at Tarcoola.
A Pb-Pb isotope study of galenas, K-feldspars and whole-rock samples of granite gives a maximum age of Pb mineralisation of ~1645 Ma. It is permissive of the Paxton Suite being the source of Pb in the Tarcoola Goldfield, but it is pointed out here that parts of the Tarcoola Formation carry feldspar which is probably derived from the Paxton Suite or regional correlatives, and that these may also have been a source of Pb.
Nd isotopes from samples of mineralisation, Paxton Suite granites, Lady Jane Diorite, and the Tarcoola Formation are not definitive but an input to the ore signature by the Lady Jane Diorite is favoured.
Fluid inclusions are dominantly of low salinity and have homogenisation temperatures mostly between 200 C and 380?C (not pressure corrected). CO2 is a common component, and mixed CO2-aqueous inclusions have CO2 vapour + CO2 liquid + brine at standard temperature and pressure, indicating relatively high trapping density.
