Early Proterozoic metamorphism at The Granites gold mine, Northern Territory; implications for the timing of fluid production in high-temperature, low-pressure terranes

Abstract

Gold mineralization at The Granites, Northern Territory, Australia, occurs in a sequence of amphibolite facies pelitic and semipelitic schists forming part of the Early Proterozoic Granites-Tanami block. Peak metamorphism at The Granites at approximately 600 degrees C and approximately 3.5 kbars occurred during the development of near-vertical foliations and associated stretching fabrics and, in keeping with many Australian Proterozoic metamorphic belts, was followed by isobaric cooling, reflecting the extremely transient nature of the thermal perturbation. Cummingtonite and hornblende schists occur in alteration assemblages surrounding mineralized calcite veins in the pelitic and semipelitic schists, as well as in the footwall and host gold lodes, and are associated with high-temperature metasomatic addition of Ca and depletion of K. The mobilization of the significant volumes of fluid necessary for metasomatism and associated mineralization during convergent deformation is modeled as a consequence of the lithospheric-scale deformation geometry appropriate for the generation of high-temperature, low-pressure terranes (e.g., Loosveld and Etheridge, 1990; Sandiford and Powell, 1991). We show that if crustal thickening is accompanied by appreciable mantle lithospheric thinning, in excess of 50 percent devolatilization of the low crust may occur during prograde metamorphism and active crustal thickening.