THE ROLE OF MAFIC SHOSHONITIC MAGMAS IN THE ORDOVICIAN PORPHYRY CU-AU DEPOSITS OF NORTH PARKES, NSW, AUSTRALIA

Four economic porphyry Cu-Au deposits, Endeavour 22 (E22), E26, E27 and E48, occur at North Parkes, NSW. They have a combined resource of 131.7 Mt @ 1.1 % Cu and 0.5 g/t Au. Mineralization is centered on thin, pipe-like, Late Ordovician quartz monzonite porphyry (QMP) complexes. Eight intrusive phases have been recognized. The pre-mineral intrusions are a coarse-grained, equigranular monzodiorite intrusion and an equigranular to weakly porphyritic biotite quartz monzonite (BQM). A series of three variably felsic QMP phases comprise the centre of the intrusive complexes; 1) volumetrically minor early- and late-mineral biotite phyric QMP dykes, 2) volumetrically dominant syn-mineral K-feldspar phyric QMP intrusions and 3) less abundant syn-to late-mineral augite-biotite K-feldspar phyric QMP intrusions. Post-mineral basaltic trachyandesite dykes and augite-phyric monzonite porphyry dykes are also present.

Geochemical characteristics of the regional volcanic and intrusive rocks define a systematic trend consistent with high-temperature magmatic fractionation of basaltic trachyandesite through trachyte. However, the REE patterns of QMP phases are not explainable by crystal fractionation effects alone. Also, there is a return to more mafic compositions in the waning stages of intrusive activity. The QMP complexes are interpreted to have formed in response to the emplacement of a series of mafic shoshonitic melts into the base of a crystallizing, zoned, monzodiorite to monzonite magma chamber. Episodic movement along a deep-seated, mantle-tapping structure (the Lachlan Transverse Zone?), could have triggered the emplacement of these mafic shoshonitic melts. Related movements on shallow-crustal fault systems above the magma chamber probably caused instantaneous depressurization and the repeated simultaneous egress of melt (QMP) and exsolved aqueous fluid into dilatant zones. Localized fracturing and additional volatile exsolution from the QMP melt is thought to have led to the formation of the narrow QMP complexes and associated Cu-Au-bearing stockwork veins and related orthoclase alteration. The volatile-rich aqueous fluid partitioned LREE preferentially to MREE, preferentially to HREE, resulting in the development of distinctive "u-shaped" REE patterns of the ore-related intrusions.