Shoshonitic magmatism and the formation of the Northparkes porphyry Cu-Au deposits, New South Wales

Abstract

Four economic porphyry Cu-Au deposits occur at Northparkes, New South Wales: Endeavour 22 (E22),E26, E27 and E48. Mineralisation is centred on thin, pipe-like Late Ordovician to Early Silurian quartz monzonite porphyry complexes. Nine intrusive phases have been recognised, with a common sequence of emplacement recognised in all deposits. Pre-mineralisation intrusions include coarsegrained, equigranular monzodiorite and equigranular to weakly porphyritic biotite-quartz monzonite (U-Pb SHRIMP age 444.2 plus or minus 4.7 Ma). Three variably felsic quartz monzonite porphyry phases comprise the mineralised intrusive complexes: (i) volumetrically minor early and late mineralisation biotite-phyric quartz monzonite porphyry dykes; (ii) abundant synmineralisation K-feldspar-phyric quartz monzonite porphyry intrusions; and (iii) less abundant syn- to late mineralisation augite-biotite K-feldspar-phyric quartz monzonite porphyry intrusions. Post-mineralisation basaltic trachyandesite dykes and augitephyric monzonite porphyry dykes are also present (U-Pb SHRIMP age 436.7 plus or minus 3.3 Ma), as are younger mafic dykes. The regional volcanic and intrusive rocks define systematic geochemical trends consistent with high-temperature magmatic fractionation of basaltic trachyandesite through trachyte. However, the trace-element compositions and REE patterns of the mineralising intrusions cannot be explained by crystal fractionation alone because there is a return to more mafic compositions in the waning stages of intrusive activity. The intrusive complexes are interpreted to have formed due to the emplacement of mafic alkaline melts into the base of a crystallising, zoned, monzodiorite to monzonite magma chamber. Shallow crustal fault ruptures above the magma chamber probably caused instantaneous depressurisation and simultaneous egress of quartz monzonite porphyry and exsolved aqueous fluid into dilatant zones. Localised fracturing and additional volatile exsolution from the quartz monzonite melt is thought to have led to the formation of the quartz monzonite porphyry complexes and associated Cu-Au-bearing stockwork veins and related orthoclase alteration. Volatile-rich aqueous fluid partitioned LREE preferentially to MREE and HREE resulting in the development of distinctive U-shaped REE patterns of the ore-related intrusions.