GEOCHEMISTRY OF ARCHEAN SHOSHONITIC LAMPROPHYRES FROM THE YILGARN BLOCK, WESTERN-AUSTRALIA - AU ABUNDANCE AND ASSOCIATION WITH GOLD MINERALIZATION

Spatial and temporal associations between Archean mesothermal gold deposits, shoshonitic minor intrusions (e.g. lamprophyre dikes), and crustal-scale fault systems are well recognized features of some Archean terranes. It has been proposed that the association may be due to a combination of genetic factors, including intrinsic Au enrichment of shoshonitic magmas, and tectono-structural factors arising from crustal-scale orogenic activity in the Late Archean. To determine the nature of the association in the highly mineralized Archean Yilgarn Block, the major, trace and precious metal geochemistry of a suite of 49 lamprophyres and related microdiorite porphyries, covering a range of alteration states and proximities to gold mineralization, were investigated. The lamprophyres exhibit rock fabrics indicative of partial to extensive metamorphic recrystallization, range from primitive to more evolved compositions (MgO approximately 9 to <5 wt%) and have geochemical signatures typical of Phanerozoic subduction-related magmas. Variable mobile lithophile element (K, Rb, Ba, Sr) concentrations and anomalously high deltaO-18 signatures of the lamprophyres reflect their interaction with hydrothermal +/- metamorphic fluids. Lamprophyres emplaced in proximity to gold deposits are commonly affected by carbonation, have enhanced S and Au contents and have Au/Pd ratios that exceed primitive mantle values by up to several orders of magnitude. In contrast, lamprophyres emplaced in locations remote from gold mineralization tend to be depleted in S and Au and have low Au/Pd ratios. High Au contents were mostly acquired by interaction with Au-mineralizing fluids, whereas very low Au contents are the result of fluid leaching in lamprophyres remote from gold deposits. However, some lamprophyres of high F content display small intrinsic enrichments in Au of almost-equal-to 2 to 3 times typical igneous rock abundances. The F, S and CO2 contents of the Yilgarn lamprophyres can effectively discriminate mineralized lamprophyres from non-mineralized samples. This study shows that shoshonitic lamprophyres are unlikely to have contributed significant Au or other components to Yilgarn mesothermal gold deposits.