Mineralogical and Geochemical Features of «Hot Stones» Based on the Example of the Khlebopek Placer Deposit (Bodaybinsky District, Irkutsk Oblast, Russia)

Abstract

Introduction: «Hot stones», frequently detected in the waste dumps of placer deposits after gold washing using metal detectors, are a nuisance for prospectors seeking nuggets, as they do not contain «visible» gold. However, their study is of significant scientific and practical interest, enhancing our understanding of gold geochemistry, its concentration processes in placers, and potential exploration criteria for both primary and placer gold deposits.

Methodology: the mineralogical and geochemical features of «hot stones» from the Khlebopek placer deposit were studied using optical and electron microscopy, atomic emission spectroscopy (ICP-OES), and X-ray diffraction analysis.

Results and Discussion: «Hot stones» are heterogeneous in specific gravity, elemental, and mineral composition, despite producing identical signals on gold-tuned detectors. The signal arises from the presence of both magnetic and conductive minerals within the stones, which interact with the electromagnetic field generated by the detector coil. Their combined action forms a complex structure of conductors, magnetic minerals, and piezoelectrics. «Hot stones» contain mineral forms of gold and silver, where noble elements exist not only in zero-valent states but also in oxidized forms, such as tellurides and stibnites. The association of «hot stones» with micro- and nanoforms of gold aligns with the electrical precipitation model. A unique feature of these stones is their exceptionally high content of nickel minerals.

Conclusions: the notion that «hot stones» primarily consist of magnetic iron minerals is incorrect. Their mineral composition is far more diverse. «Hot stones», containing noble elements in oxidized forms with micro- and nanoparticles, serve as the source that begins to form zero-valent gold upon entering the hypergene zone. Gold-bearing pyrite-pyrrhotite ores and their associated magnetic anomalies, easily detectable by UAV-based magnetometric surveys, can serve as regional exploration criteria for both lode and placer gold.