The Proceedings of the Eighth International Conference on Creationism (2018)

equal amounts of plagioclase and K-feldspar), 18% granodiorite, 1% diorite, 1% quartz-bearing monzonite, and <0.2% gabbro. The plutons have been grouped into five or six suites based on their geochemical, mineralogical and isotopic characteristics, and their claimed emplacement ages (O’Neil et al. 1977; Hensel et al. 1985). Two of these suites have S-type characteristics (that is, sedimentary protoliths), the Hillgrove and Bundarra Suites (Fig. 1), whereas the remainder are I-type (that is, igneous protoliths), the Moonbi, Uralla and Clarence River Suites. Leucoadamellite plutons that have been difficult to classify are grouped separately. The country rocks into which the New England Batholith was intruded are part of the New England Fold Belt (Leitch 1975). The sedimentary and metasedimentary rocks are divided into western and eastern sequences separated by a major fault zone. The 12-13 km thick western sequence consists of gently-folded, fossiliferous Devonian-early Permian volcanogenic shales, mudstones, limestones, sandstones, and greywackes with occasional cherts, conglomerates and interbedded lavas (Packham 1969). The eastern sequence consists of similar age but strongly deformed, tightly- folded and regionally metamorphosed, volcanogenic greywackes, siltstones and mudstones, interbedded with locally abundant basaltic lavas and cherts. Most of the plutons in the New England Batholith are barren of any economic metal deposits. However, several host, or are related to, hydrothermal ore veins of economic significance. The Hillgrove Granite (Fig. 1) hosts Au-Sb-W ore veins which have been discontinuously exploited since 1857 (Ashley and Craw 2004). Similarly, the Mole Granite (Fig. 1) hosts the Torrington Sn-W-topaz ore veins which have been exploited since 1881 (Audétat et al. 2000a, b). THE HILLGROVE GRANITE AND ITS HYDROTHERMAL ORE VEINS Au-Sb-W mineralization in the Hillgrove area occurs in veins, breccias and the immediately adjacent altered wall rocks (Ashley and Craw 2004). It is hosted by the deformed Paleozoic metasedimentary Girrakool Beds, and the late Carboniferous (~300 Ma) Hillgrove Granite, an S-type adamellite (or monzogranite as strictly defined), of the Hillgrove Plutonic Suite and the early Permian Bakers Creek Diorite Complex which intruded them (Fig. 2). The mineralized structures are commonly associated with lamprophyre dikes (Ashley et al. 1994). The Girrakool Beds are possibly Carboniferous and comprise quartzofeldspathic greywacke, siltstone and siliceous (cherty) shale with a felsic volcaniclastic provenance. After their deposition, these rocks were strongly deformed, folded and metamorphosed to low grade, resulting in generally steeply-dipping beds and a strong cleavage parallel to the axial surfaces of meter-scale folds. The Hillgrove Plutonic Suite (Fig. 1) was then emplaced into the Girrakool Beds in the late Carboniferous. The major pluton of the suite is the Hillgrove Adamellite, a massive to foliated S-type biotite monzogranite, locally with megacrystic K-feldspar, and accompanied by related feldspar-quartz porphyry dikes. Intrusive contacts with the enclosing metasedimentary rocks are sharp and commonly undeformed. However, contact metamorphic effects are apparent in the host metasedimentary rocks up to several hundred meters from the intrusion and Snelling ◀ Radiohalos as an exploration pathfinder ▶ 2018 ICC 569 Figure 1. The distribution of the granite plutons, plutonic suites and leucoadamellites in the New England Batholith, eastern Australia (after Shaw and Flood 1981). The sampled plutons are indicated with the respective sample numbers.