Occurrence and Geological Significance of Layered Stratiform Intrusion in the Yilgarn Block, Western Australia

Abstract

I WISH to announce the presence in the Yilgarn Block¹, Western Australia, of a major stratiform basic igneous complex with many features similar to the well documented Bushveldt², Skaergaard³ and Stillwater layered⁴ intrusions. There is a stratiform basic sill 64 km south-east of Mount Magnet within the complexly deformed Archaean greenstones of the Yilgarn Block⁵. The sill, here provisionally designated the Challa Intrusion, crops out over an area in excess of 400 km², with a discontinuous strike length of more than 200 km in a north-westerly direction from Challa through Barrambie to Yarrabubba, 30 km south of Meekatharra (Fig. 1). The separate occurrences of vanadium-rich titaniferous magnetites that outcrop along the whole of the sill from Youanmi in the south through Wymandoo, Windimurra, Challa, Windsor and Barrambie to Yarrabubba in the north, have not been recognized before as part of the one deposit. The sill consists of alternating layers of gabbros, anorthosites, pyroxenites and magnetite layers and lenses; the principal rock type is a medium to coarse grained gabbro with an average grain size of approximately 4 mm.