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

have resulted in development of assemblages of quartz, feldspars, and biotite ± aluminosilicates, muscovite, garnet and amphibole. Heterogeneous deformation was subsequently imposed on both the metasedimentary rocks and granitoids, resulting in variably developed foliation and formation of steeply-dipping mylonite zones up to several hundred meters wide. There are two main mylonite zones in the Hillgrove area, termed the Hillgrove Fault and Chandler Fault (Fig. 2), but other narrower zones are recognized. Intrusion of the Bakers Creek Diorite Complex into the Girrakool Beds was approximately coeval with the emplacement of the HillgroveAdamellite. The complex contains plutons with a range of compositions from quartz diorite to granodiorite. Contacts between the Bakers Creek Diorite Complex and the host metasedimentary rocks range from sharp to intercalated. A subsequent deformation event was accompanied by regional prehnite-pumpellyite to greenschist facies metamorphism. This was followed by the emplacement of several suites of late Permian to early Triassic (~240-255 Ma) I-type granitoids (Shaw and Flood 1981). Generation of several clusters of gold deposits coincided temporally with this latter deformation and magmatic episode. These hydrothermal systems range from Au-only to Sb-rich, commonly with significant As and locally associated W or Hg (Ashley and Craw 2004). In the Hillgrove area (Fig. 2), the hydrothermal ore veins transect the Hillgrove Adamellite and Bakers Creek Diorite Complex and developed coevally with the emplacement of shoshonitic lamprophyre dikes which commonly occupy the same structures. The lamprophyres have geochemical and temporal affinities (~245-255 Ma) with mafic granitoid (monzonite) members of the high-K Moonbi Plutonic Suite, the nearest intrusions of which though crop out about 15 km from Hillgrove. The Hillgrove area contains over 200 individual ore deposits (Fig. 2), most of which comprise steeply-dipping mineralized veins within fracture systems cross-cutting the Hillgrove Adamellite, Bakers Creek Diorite Complex and the metasedimentary rocks which host them (Ashley and Craw 2004). The largest individual vein systems are up to 1.3 km long, up to several meters wide, and extend to a depth of at least 900 meters. Veins pinch and swell, with high-grade ore shoots commonly occupying dilational structures, which occur at various scales forming pipe- and lens- shaped hydrothermal breccia masses (altered angular wall-rock clasts cemented by hydrothermal minerals). Individual veins and breccia masses show evidence of multiple generations of mineral precipitation, commonly with late, open- space infillings of quartz, stibnite (Sb 2 S 3 ) and carbonate. Wall-rock alteration occurs in all host rock types and the lamprophyre dikes, and consists of distal chlorite-ankerite-albite and proximal ankerite- sericite-quartz-pyrite-arsenopyrite-rutile assemblages (Ashley et al. 1994). Uncorrected fluid inclusion filling temperatures in quartz from pyrite-arsenopyrite-gold mineralization zones range from 195 to 250°C, and for stibnite-gold-bearing veins from 100 to 195°C (Comsti and Taylor 1984). Low-salinity hydrothermal fluids (4.8 to 1.8 wt.% NaCl equivalent) are inferred for the mineralization. THE MOLE GRANITE AND ITS HYDROTHERMAL ORE VEINS The Mole Granite is one of the large leucoadamellite plutons in the northern New England Batholith (Fig. 1). It was intruded into Permo-Carboniferous sediments and Permian volcanics and has an outcrop area of 650 km 2 (Kleeman et al. 1997). The granite has several roof pendants, the largest of which is the metapelitic Torrington roof pendant, the altitude and central location of which in the Mole Granite suggests the granite roof collapsed (Fig. 3). Due to the roof of the pluton dipping shallowly outward at 7-15° it has been estimated that the pluton is only partially unroofed, and that there is another 1200 km 2 of the pluton still buried under the surrounding host sediments and volcanics. It has also been estimated that the pluton was emplaced at a depth of about 4 km, the inferred lithostatic pressure at that depth being consistent with the pressure established on coexisting brine and vapor inclusions in one of the mineralized lodes (Audétat et al. 2000b). The intrusion has been dated by conventional U-Pb techniques on magmatic zircon and monazite at 247.6 Ma (earliest Triassic) (Schaltegger et al. 2000). It produced a thermal metamorphic aureole up to 10 km wide. Gravity-modeling and heat-flow measurements indicate the Mole Granite forms a sill-likemass1-4kmthick (Audétat et al. 2000b).The exposed part of the intrusion is very homogenous in its mineralogy and geochemistry, suggesting that most of the magma intruded in Snelling ◀ Radiohalos as an exploration pathfinder ▶ 2018 ICC 570 Figure 2. Geological map of the Hillgrove district (marked in Fig. 1) showing the location of the major Au-Sb vein systems and large hydrothermal alteration zones (after Ashley and Craw 2004). The sampled locations are indicated with the respective sample numbers.