The Nevoria Gold Skarn Deposit, Southern Cross Greenstone Belt, Western Australia: II. Pressure-Temperature-Time Path and Relationship to Postorogenic Granites

摘要

The Nevoria deposit (production 11.88 t Au, 0.411 Ag) is one of more than 20 gold skarns mined in the 3.1 to 3.0 Ga Southern Cross greenstone belt. The belt constitutes part of the continental foreland of the 2.7 to 2.6 Ga Norseman-Wiluna fold belt, the youngest Archean orogen of the Yilgarn craton. The Nevoria skarns are confined to iron formations at the limb of a regional F_1 anticline bent around the Ghooli orthogneiss dome. The refolding (F_2) took place at 2775 to 2724 Ma during batholith emplacement and contact metamorphism at estimated P-T conditions of 610 deg +- 50 deg C and 400 ± 100 MPa (14-km burial depth). This structural setting differs from that of the mezozonal lode gold deposits in the adjacent fold belt, which are controlled by fault-vein arrays in crustal-scale shear zones. The Nevoria orebodies (6-7 g/t Au), composed of pyrrhotite-rich hedenbergite-actinolite and almandine-hornblende skarns, are dated by a concordant U-Pb age (2635,7 +- 1.2 Ma) and by a less precise Pb-Pb errorchron age (2630 +- 13 Ma, MSWD = 5.9, n = 7), both defined by allan-ite-bearing almandine and by cogenetic scheelite. These ages demonstrate that the skarns formed 90 m.y. after amphibolite-facies metamorphism in the Southern Cross greenstone belt and at least 20 m.y. after transpres-sional faulting related to late-orogenic deformation in the Norseman-Wiluna fold belt.The peak fluid temperature during skarn formation is estimated at 550 deg to 600 deg C, based on the Fe-Mg exchange thermometry of almandine-biotite and almandine-hornblende pairs. Calcic-potassic mineral assemblages in skarn hosted by amphibolite constrain the pressure to 300 to 400 MPa, based on the reaction of biotite + cummingtonite + anorthite to almandine + grossular + biotite. The pressure estimate confirms that the Nevoria deposit formed at considerable depth (11-14 km) in a midcrustal environment. Substantial uplift did not take place until 2565 +- 25 Ma, when biotite in the associated granite closed to Rb-Sr diffusion at 300 deg C ambient temperature.