Supergene gold mineralization from exploited placer deposits at Dziwiszów in the Sudetes (NE Bohemian Massif, SW Poland)

Abstract

Representative ore samples collected from outcrops in the exploited Dziwiszów placer gold deposit in Lower Silesia offer the opportunity to study supergene transformations which probably occurred over historic periods. The ore samples are alloys of gold and silver, with trace amounts of Hg and Pd. There are three distinct sub-types of detrital gold: (1) flaky, (2) nugget-like; and (3) irregular/craggy grains, respectively. All samples record the features of supergene processes such as dissolution textures, as well as the formation and aggregation of authigenic nano- and microparticles of native gold. Surface authigenic morphotypes include nano-particles of semi-spherical or blebby native gold, as well as overgrowths of crystalline phases of platy gold and micro-aggregates of bacteriomorphic Au, which provide evidence for microbial gold biomineralization. The growth history of individual native gold particles is documented by nano- to micro-textural investigations of sample grains ranging from isolated semi-spherical nanoparticles through agglomerates to irregularly shaped platy micro-nuggets of gold. The presence of nano- to microphase native gold embedded in fine-grained assemblages of clayish masses and the absence of any evidence for abrasion of individual grains suggest the authigenic origin of blebby native gold particles and overgrowths of platy native gold grains. Hence, we propose that these particles were formed in situ. Chemical transfer may be related to both the precipitation of Au from a colloidal solution, as well as to its adsorption by clay minerals and surface precipitation on particles of the mineral substrate.