Geochronological and geochemical evidence for two ore-forming events in the Northparkes district NSW

The Northparkes district hosts economic Cu-Au mineralisation associated with discrete pencil porphyry systems emplaced in the Late Ordovician (∼439 Ma; Lickfold et al., 2003, 2007, Crawford, 2007a). Geochemical and geochronological data from the recently discovered Two-Thirty Prospect ~25 km northwest of Parkes, NSW indicates that two ore-forming events occurred within the Northparkes district. The Two-Thirty Prospect is hosted by the moderately east dipping Goonumbla Volcanic Complex on the western-limb of the Milpose syncline (Fig. 1). Mineralisation at the Two-Thirty Cu-Mo-(Au) prospect is hosted by a magmatic-hydrothermal breccia related to the 448±4.4 Ma intrusion of the Two-Thirty porphyry, one of five dated porphyry intrusions that make up the Two-Thirty intrusive complex.

Whole-rock geochemical data indicate that the Two-Thirty porphyry is calc-alkaline, distinguishing it from the alkalic porphyry deposits typical of the Northparkes district. Both Ce4+/Ce3+ and Eu/Eu* ratios from whole rock and zircon trace element analysis indicate that the Two-Thirty porphyry has a geochemical signature consistent with signatures for fertile, oxidised, medium tonnage, calc-alkaline porphyires from Northern Chile and the Central Asian Orogenic Belt (Ballard, 2002, Loucks, 2014; Shen et al., 2015).

The discovery of the Two-Thirty prospect has implications for the exploration in the Northparkes district and Macquarie Arc. It is the only significant occurrence of magmatic–hydrothermal breccia-hosted mineralisation discovered within the Macquarie Arc to date, and is older than mineralised porphyry deposits at Northparkes and Cadia, and younger than the deposits at Cowal, Copper Hill, Marsden and Ridgeway.