MINERALOGY AND GEOCHEMISTRY OF FENITIZED ALKALINE ULTRABASIC SILLS OF THE GIFFORD CREEK COMPLEX, GASCOYNE PROVINCE, WESTERN AUSTRALIA

JOANNA M. PEARSON
Key Centre for Mineral Deposits, University of Western Australia, Nedlands 6907, Australia

WAYNE R. TAYLOR
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Abstract

The Proterozoic Gifford Creek complex in Western Australia is a high-level alkaline complex hosted in 1.8 Ga granitic basement. Magmatism occurred in two episodes. During the first phase at 1.68 Ga, with which this paper is concerned, a swarm of ultrabasic sills (Lyons River sills) was emplaced. This was accompanied by the development of an extensive belt of fenite comparable in extent to that surrounding a major alkaline intrusive complex. However, no large bodies of alkaline rock are known from the area, suggesting that the source of fluids is an unexposed alkaline intrusive body. The later episode of magmatism at 1.3 Ga was carbonatitic in character. Among the Lyons River minor intrusive bodies, two forms are recognized: a swarm of ultrabasic sills in which primary igneous textures are preserved, and a series of intensely deformed ultrabasic lenses. The degree of metasomatism in the fenite belt varies from pervasive K-feldspathization of the country rock in the outermost part of the aureole to intense growth of alkali amphibole and aegirine in the innermost aureole. Primary minerals in the ultrabasic sills have been replaced by alkali amphibole and aegirine, but based on the interpretation of pseudomorphs, the sills originally consisted of olivine macrocrysts set within a groundmass of mica, perovskite, titanian magnetite, and carbonate. The preserved igneous textures, e.g., cm-scale layering and gravity settling of macrocrysts, are similar to those described from the Benfontein kimberlite sills in South Africa. The deformed ultrabasic lenses have been emplaced plastically, at subsolidus temperatures, into overlying sediments possibly in response to movement on a fault. No primary textures remain, and the rock is now comprised of deformed phlogopite and potassian magnesio-arfvedsonite set in a carbonate matrix. Major and trace element geochemistry of the Lyons River sills show strong affinities to carbonate-rich ultrabasic rocks such as the Benfontein sills, Igwisi Hills lavas or some examples of aillikite. Stable isotope compositions of matrix carbonates are remarkably similar to those of the Benfontein sills: 13C -5.5‰, 18O +10‰, indicating little disturbance during alkali metasomatism. Coexisting carbonates preserve compositions indicative of temperatures of fenitization above 450 C.