The Geology of the Argyle (AK1) Diamond Deposit,Western Australia

Abstract

The Argyle (AK1) pipe in the Kimberley region of Western Australia is the world’s largest source of natural diamonds, and it has produced more than 835 million carats since mining began in 1983. The ~1.2 Ga olivine lamproite pipe lies within the Paleoproterozoic Halls Creek orogen at the southeastern margin of the Kimberley craton, and it was emplaced during a period of extension associated with large-scale, NNE-trending strikeslip faulting of the orogen. AK1 is composed of four coalesced, steep-sided diatremes aligned along an NNE-trending fault and tapering to narrow feeder zones at depth. The body is infilled by volcaniclastic olivine lamproite and cut by late olivine lamproite dikes. Two distinct groups of volcaniclastic rocks are present. The dominant type comprises mainly quartz-rich lapilli tuffs and coarse ash tuffs formed by numerous phreatomagmatic eruptions, when olivine lamproite magma was erupted through water-rich sands and silts of the Carr Boyd Group. In contrast, latestage olivine lamproite lapilli tuffs, devoid of accidental quartz grains, fill the center of the northern diatreme. Following emplacement, the body was tilted 30° to the north and extended north-south by the sinistral northnorthwest–south-southeast Gap fault system and east-northeast–west-southwest by the dextral Razor Ridge fault. The Razor Ridge fault offsets the most southerly diatreme (southern tail) from the main part of AK1. Reversal of structural offsets affecting AK1 reveals a geometry which closely resembles that of other ultramafic diatremes. Variations in diamond grade and quality within AK1 indicate that at least two distinct magmas with unique diamond contents have been emplaced at Argyle.