Morphogenesis and composition of native gold in the Chelopech volcanic-hosted Au–Cu epithermal deposit, Srednogorie zone, Bulgaria

Abstract

Native gold is an important economic component of the complex ores of the Chelopech high-sulphidation volcanic-hosted epithermal Au–Cu deposit (Bulgaria). The ore consists of pyrite, chalcedonic silica, chalcopyrite, enargite, luzonite, tennantite, bornite, sphalerite, galena, and numerous other sulphide, arsenide and telluride minerals. Gold is paragenetically associated with most of the arsenic-bearing and base metal sulphide minerals. The chemistry and morphology of gold grains, which were separated from disintegrated ores, were systematically studied by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The gold is characterised by high fineness (~950) and a mean composition of 94.1 wt% Au, 5.27 wt% Ag, 0.53 wt% Cu and 0.10 wt% Fe. Gold grains show variable morphology: subhedral flakes, irregular grains, euhedral isometric crystals, elongated rods, wires and fine-fibrous crystals, {111} twins, fine dendrite-like formations, spongy gold, and polycrystalline grains. The densest faces {111} are morphologically the most important. Morphometric measurements show a pronounced flatness of the gold particles. The mean value of the Corey factor (a measure of flatness from 0 to 1 = spherical) is 0.14 and the mean length to width value is 1.64. It is suggested that crystal growth of gold took place in small voids, fine cracks and intergranular space, the geometry of which have controlled the highly variable grain shapes. No direct correlation between the size and composition of gold grains exists, but larger grains tend to be of higher fineness.