The GRE46 Epithermal Gold Deposit, Cowal, New South Wales: Geology, Mineralization, Alteration, and Ore Genesis

Abstract

The 2.74 Moz structurally controlled epithermal GRE46 gold deposit is located on the western margin of the Cowal Igneous Complex in central New South Wales, Australia. At the regional scale, the Cowal Igneous Complex is located toward the southern end of the poorly exposed Junee-Narromine volcanic belt, the westernmost of four remnant volcanic belts that together constitute the Ordovician to early Silurian Macquarie arc. The GRE46 deposit is located at the northern end of a 4.5-km-long structural corridor that is defined by several prominent arc-parallel structures. This structural corridor also contains the E40, E41, and E42 gold deposits, and has a collective pre-mining gold endowment of over 14 Moz. The GRE46 deposit is hosted in a sequence of calc-alkalic to shoshonitic subaqueous volcanic and volcaniclastic rocks, interpreted to have formed in an intra-oceanic magmatic arc environment. The host stratigraphy is dominated by reworked volcanic deposits and nonvolcanic sedimentary deposits that include mud- to sandstones, pebble to cobble conglomerates and polymictic volcanic breccias and debris flows. Lesser primary volcanic rocks consist of coherent andesite to dacite flows with common hyaloclastite and peperite textures, and diorite to granodiorite dikes and sills. Gold mineralization at GRE46 occurs primarily in association with millimeter- to centimeter-scale quartz-carbonate-pyrite veins, with minor chalcopyrite, galena, sphalerite, and rare telluride minerals. Multiple styles of mineralized veins are present, including composite and banded dilatant veins, shear veins, stringer veins, and high-grade quartz-sulfide breccia veins. The host rocks were variably hydrothermally altered, with the style and intensity of alteration influenced by proximity to fluid-flow controlling structures and protolith compositions. The highest gold grades are closely associated with pervasive quartz + white mica + ankerite + pyrite alteration that overprinted chlorite + albite + calcite ± magnetite alteration. At the deposit scale, gold mineralization was strongly influenced by the pre-existing structural architecture, leading to heterogeneous hydrothermal fluid flow in zones of enhanced permeability that were created due to competency contrasts in the volcano-sedimentary stratigraphic package. GRE46 can be classified as an intrusion-related epithermal style of gold mineralization. The deposit has several characteristics, including the ore and gangue mineralogy, style and textures of associated veining, and the alteration assemblage, which are broadly consistent with both the intermediate sulfidation and the carbonate-base metal epithermal models.