The distribution of gold in volcanic rocks of eastern Iceland

Abstract

Gold contents are reported for basaltic lavas and dykes from the Tertiary succession of eastern Iceland. In unsaturated (olivine-normative) tholeiites Au decreases in concentration with degree of differentiation. However, in saturated (quartz-normative) and more differentiated varieties, Au shows a high degree of positive correlation with incompatible elements such as Y and Zr. This behaviour suggests that in the latter rocks Au is not held in early crystallized phases but is enriched in the residual melt so that it follows the trend of the incompatible elements. Thus basaltic andesites and silica-rich icelandites contain twice as much Au (x̄ = 4.1 ng/g−1) as basalts. This general trend appears to follow an erratic but concomitant enrichment of S in the rocks. Oxidation and zeolite alteration have led to mobilization and to enrichment of Au in volcaniclastic units and in the porous top of a lava flow. It is suggested that in older volcanic piles of Icelandic type the most suitable source rocks for Au mineralization could be in the more evolved quartz-normative tholeiites (ferrobasalts), and basaltic andesites and icelandites, particularly dykes and volcaniclastic units. Loosely-held Au is readily mobilized from volcanic glass and sulfides due to weathering, burial metamorphism and hydrothermal alteration. Therefore only the freshest, glassy rocks should be expected to retain their primary Au concentrations.