Geological setting and alteration characteristics of the Hillside mineralising system, Yorke Peninsula

Abstract

The Hillside Cu-Au deposit of the Yorke Peninsula, South Australia is attributable to the iron oxide-copper-gold (IOCG) family of deposits. It has clear genetic affinities with other IOCG deposits of the Gawler Craton and defines the southernmost occurrence of this style of mineralisation in the Olympic Cu-Au province. A broad study on the mineral paragenesis and geochemistry of the deposit was carried out on drill core provided by Rex Minerals, the holder of the Hillside tenement. The deposit is hosted by the Palaeoproterozoic metasedimentary and metavolcanic Wallaroo Formation and the highly brecciated Mesoproterozoic Arthurton Granite, a Hiltaba Suite equivalent. These breccias are expressions of the Pine Point Fault Zone, which is the dominant structural control on mineralisation. Coastal outcrops are dominated by the Arthurton Granite and Wallaroo Group basement with overlying Stansbury Basin and St Vincent Basin sedimentary sequences. The alteration mineralogy is divisible into four assemblages; a garnet dominated calc-silicate assemblage, an epidote-quartz vein assemblage, a K-feldspar-carbonate-quartz-chlorite-sericite assemblage related to the mineralisation, and later carbonate veins that also host minor mineralisation. Ore mineral paragenesis occurs in an overlapping sequence; the first mineral precipitated was magnetite which was later extensively martitised, with hematite followed by pyrite, then chalcopyrite and later bornite. A later phase of copper and uranium minerals occur with the carbonate veins. Iron oxide chemistry and sulphur isotopes from the sulphides support this paragenesis. Bulk rock composition was found to have a decrease in silica and an increase in iron oxide with progressive alteration, while alkali metals (Na, K) and aluminium were also lost from the rock and alkali earths (Mg, Ca) were gained during alteration. Trace element and rare earth element plots show no strong changes that correlate to progressive alteration. Temperature of formation as estimated by TitaniQ thermometry has two major peaks; 450°C-465°C and 495°C-500°C, likely corresponding with a primary and a recrystallised quartz stage. Chlorite thermometry is in agreement with several samples: however, others overestimate the temperature by an unacceptable amount. It is suggested that mineralisation is genetically linked to the intrusion of the Arthurton Granite and activation of the Pine Point Fault, which together supplied both the heat source and plumbing to allow the flow of metal bearing fluids, possibly of magmatic origin. Descriptions of the mineralisation exploited in the historic Hillside Mine are different in their ore mineralogy and likely correspond to a later hypogene or supergene enrichment phase.