Laterization and its control to gold occurrence in Laowanchang gold deposit, Guizhou Province, Southwest of China

Abstract

Laowanchang gold deposit, located in Qinglong County, Guizhou Province, Southwest China, is the first-discovered laterite type gold deposit in Guizhou Province. Although most scholars are inclined to agree that ore-forming substance was closely correlated with “Dachangceng layer”, it still remained controversial. Former researchers did not find existences of visible Au grains in limonite, clay minerals or pyrite under SEM observation; therefore, they considered Au occurrence mainly as micro-disseminated form. In this study, mineral composition, chemical components and chemical index of alteration of laterites indicated that three chemical weathering processes, including argillaceous, bauxitic and ferric process can be observed in Laowanchang laterization. Geochemically, such laterization can be attributed to early laterization stage. Components of K 2O, Na 2O, and CaO were highly leached during weathering indicating a strong chemical alteration. Concentrations of ore-forming elements (Au, As and Sb) varied greatly along laterite profiles. Concentrations of Au were usually in the range of 40 × 10 − 9 –4551 × 10 − 9 ; As in 0.1–0.9% with the highest value of 1.8%; Sb < 0.1%. Concentrations of Au, As and Sb in laterites were obviously higher than in silicalites of Dachanceng layer. During laterization, enrichment of Au and As were obviously related with enrichment of Fe. Au and As enriched in relatively reductive locations, while Sb in the relatively oxidized circumstances. Rare earth element (REE) contents were higher in laterite profiles with LREE/HREE ratios usually less than 10. REE had very similar distribution patterns in laterites, significantly differing from those of Dachanceng layer. Features of trace element pairs (including large ionic lithophile elements and incompatible elements) and ore-forming element pairs diagramed great differently between laterites and Dachanceng layer. These phenomena supported that laterites would not directly be developed from Dachanceng layer rocks, but mainly be related with Quaternary sediments. Ore-forming substances of Laowanchang gold deposit also connected less with Dachanceng layer. Au occurrences in laterites varied under TEM-EDX observations. Gold existed either as granular natural gold, or adsorbed by goethite and As-bearing goethite, or by siliceous matter and quartz, sometimes by illites. Reduction of Au 3+ to Au 0+ by Fe 2+ played an important role in enrichment of Au during laterization.