Endeavour 42 deposit, Lake Cowal, New South Wales: A structurally controlled gold deposit

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Endeavour 42 is a structurally controlled Au deposit with similarities to adularia‐sericite deposits. It is the largest of four gold prospects discovered in the Late Ordovician Lake Cowal volcanic complex, adjacent to the Gilmore Fault Zone, in central New South Wales, Australia. The Lake Cowal volcanic complex consists of calc‐alkaline to shoshonitic volcanic rocks and related sedimentary rocks that were deposited in a relatively deep‐water environment. The volcanic and sedimentary rocks of the Lake Cowal volcanic complex were intruded by diorite and granodiorite. Low‐grade porphyry Cu (0.2–0.35% Cu) mineralisation is developed in parts of the granodiorite intrusion. The gold deposits are developed north of the porphyry Cu mineralisation and occur within a north‐south corridor adjacent to a north‐south‐oriented body of diorite. The Endeavour 42 deposit is hosted by three volcanic units and a diorite. The stratigraphic units at Endeavour 42, consistently strike 215° and dip 50°NW, and comprise an upper unit dominated by redeposited pyroclastic debris and a lower conglomerate unit with clasts of reworked volcanic rocks. Separating these units is a sequence of trachyandesite lava and hyaloclastite breccias. Laminated mudstone and siltstone throughout the sequence are indicative of a relatively deep‐water, below wave‐base, environment. Porphyritic dykes, which are typically associated with zones of faulting, cross‐cut both the volcano‐sedimentary sequence and the diorite. The major fault orientations are 290° and 340°, forming subparallel conjugate fault sets. Both sets of faults are mineralised, contain deformed porphyritic dykes and are associated with sericitic alteration. Endeavour 42 is a sulfide‐poor gold deposit with free native Au and Au associated with pyrite and sphalerite. Minor galena, pyrrhotite and chalcopyrite are also observed. Irregular pyrite veinlets and carbonate‐sulfide veinlets occur in the upper unit of re‐deposited pyroclastic debris. Auriferous veins are parallel‐sided dilatant veins with quartz‐sulfide‐carbonate‐adularia. These veins display a consistent strike of 305° and a dip of 35°SW. Alteration and mineralisation were influenced by host‐rock composition and rheology. A pervasive alteration assemblage of chlorite‐carbonate‐hematite‐epidote is developed throughout the Lake Cowal volcanic complex. This is overprinted by sericite‐silica‐carbonate alteration around fault zones and dykes, with patchy and pervasive alteration of this type developed in the lava sequence and upper volcani‐clastic unit, reflecting permeability and probable alteration zoning. In the lower clastic unit, the diorite and, in parts of the lava sequence, a chlorite‐carbonate‐pyrite assemblage partially overprints sericite‐silica alteration, suggesting an evolving fluid composition, changing physico‐chemical conditions or a different alteration fluid. Age dating of the intrusive phases and sericitic alteration associated with mineralisation at Endeavour 42 yields ages of 465.76 ± 1 and 438.6 ± 0.5 Ma, respectively, suggesting that mineralisation post‐dates the Lake Cowal intrusive event and is related to intrusion of magma during the 440 Ma mineralising event, an important period in the eastern Lachlan Fold Belt.