HIGH SULFIDATION GOLD-COPPER MINERALIZATION AT PEAK HILL, NSW, AUSTRALIA: DEFORMATION OF AN UNUSUAL EPITHERMAL DEPOSIT

High sulfidation gold-copper mineralization at Peak Hill occurs in a zone of hydrothermal alteration hosted by Ordovician intermediate volcanic and sedimentary rocks. Alteration facies are zoned with an extensive pyrophyllite core grading out through kaolinite-alunite, to sericite alteration. An illite-smectite assemblage envelopes the advanced argillic and phyllic facies, and passes out to distal chlorite-epidote alteration. Disseminated gold-copper mineralization is concentrated around the margin of the pyrophyllite core, where gold occurs as inclusions in pyrite. Copper is present as enargite, tennantite, and chalcopyrite.

At least three periods of deformation are recognized at Peak Hill, though the presently observed structures did not control the mineralization. Ore minerals and hydrothermal alteration minerals and zoning are preserved, but have been extensively deformed and recrystallized. The Peak Hill shear zone developed in the zone of hydrothermal alteration and produced the intense S1 foliation. S2 shear bands and an associated crenulation cleavage are also developed in the alteration zone and overprint S1. Sinuous S1 trends are related to late open cross-folds with S3 axial surfaces. Late faults and discontinuous shear zones cut all deformation fabrics at high angles.

The Peak Hill Au-Cu deposit is a different style of high sulfidation deposit, distinguished by abundant pyrophyllite in the core of the alteration system. This contrasts with the presence of a core of vuggy quartz in typical high sulfidation epithermal deposits such as Lepanto and the Nansatsu deposits. The ore zones at Peak Hill occur on the margin of the advanced argillic alteration facies, so sampling the core of the system during early exploration appeared negative. The presence of pyrophyllite suggests that hydrothermal fluid temperatures were high (>300°C) and that the deposit formed at a depth ³1.5km (under hydrostatic fluid pressures). Formation at this depth ensured preservation of the Peak Hill mineralization and alteration facies during subsequent deformation and uplift. This style of deposit is likely to be recessive due to the abundance of pyrophyllite and sericite, suggesting there may be other concealed deposits in deformed and eroded terrains prospective for magmatic-associated Au-Cu mineralization.