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 dep osits, shoshonitic minor intrusions (e.g. lamprophyre dikes), and crus tal-scale fault systems are well recognized features of some Archean t erranes. It has been proposed that the association may be due to a com bination of genetic factors, including intrinsic Au enrichment of shos honitic magmas, and tectono-structural factors arising from crustal-sc ale orogenic activity in the Late Archean. To determine the nature of the association in the highly mineralized Archean Yilgarn Block, the m ajor, trace and precious metal geochemistry of a suite of 49 lamprophy res and related microdiorite porphyries, covering a range of alteratio n states and proximities to gold mineralization, were investigated. Th e lamprophyres exhibit rock fabrics indicative of partial to extensive metamorphic recrystallization, range from primitive to more evolved c ompositions (MgO approximately 9 to <5 wt%) and have geochemical signa tures typical of Phanerozoic subduction-related magmas. Variable mobil e lithophile element (K, Rb, Ba, Sr) concentrations and anomalously hi gh deltaO-18 signatures of the lamprophyres reflect their interaction with hydrothermal +/- metamorphic fluids. Lamprophyres emplaced in pro ximity to gold deposits are commonly affected by carbonation, have enh anced S and Au contents and have Au/Pd ratios that exceed primitive ma ntle values by up to several orders of magnitude. In contrast, lamprop hyres 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 v ery low Au contents are the result of fluid leaching in lamprophyres r emote from gold deposits. However, some lamprophyres of high F content display small intrinsic enrichments in Au of almost-equal-to 2 to 3 t imes typical igneous rock abundances. The F, S and CO2 contents of the Yilgarn lamprophyres can effectively discriminate mineralized lamprop hyres 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.