Secondary gold mineralization in the Amani Placer Gold Deposit, Tanzania

Abstract

The Amani Placer Gold Deposit (APGD) is located in a braided river valley in southwestern Tanzania and is characterised by large gold nuggets with anomalously fine macroscopic textural features as well as significant levels of silver depletion (i.e., high fineness gold) in the nugget rim. The study site thus represents an ideal natural laboratory in which to explore the controls on the low-temperature biogeochemical cycle of gold. Using X-ray Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) geochemical data, this study evaluated the sediment and pore-water chemistry of the river sediment profile, and using Scanning Electron Microscopy (SEM) coupled these results with a high resolution morphological and chemical investigation of the ambient gold nuggets collected from the auriferous gravel horizon. Down-profile metal distributions indicate a steep redox gradient and observed oximorphic mottling textures along with slumped river terraces, reveal that the redox conditions fluctuate as a function of seasonality and episodic fluvial incision events, respectively. Fluctuating physico-chemical conditions during early diagenesis result in changes to the stability of ambient gold bearing complexes (e.g., Au(S2O3)23− in oxidising condition and Au(HS)2− in reducing conditions), thereby leading to nanophase gold precipitation on clay minerals, organic matter, and importantly on pre-existing gold nugget surfaces. The diagenetic biogeochemical processes operating in the APGD thus formed supergene-enriched gold nuggets characterized by delicate textures and low Ag (<6 wt% Ag) rims. We predict that fluctuating diagenetic conditions may impact gold nugget chemistry at other placer gold deposits, and that the chemical signature arising from these processes may be measurable in paleoplacer deposits such as the supergiant Witwatersrand Au deposit.