Changes in the mineral composition of rocks in the series “root source – weathering crust – placer” using the example of the Khlebopek gold deposit (Bodaybinsky district, Irkutsk region)

Abstract

Introduction: The practice of gold mining operations shows that a significant part of metal is left in tails. There are two main reasons for that. First of all, a considerable mass of gold and other accompanying metals are found in the rocks in the form of compounds with oxygen, sulphur, arsenic, iodine, and other elements. Second, the particles of these compounds are found in the form of nano and micro-dimensions, which excludes or significantly hinders their extraction using traditional methods. However, if needed, it is possible to track the transformation paths of noble elements from the source of initial position to placers through intermediate areas, combining production and financial capabilities of production organisations and modern hardware research methods. The Khlebopek placer gold deposit, located in the Bodaibo district of the Irkutsk region, can serve as the most successful example of considering this series of transformations.

The geological structure of the Bodaibo district has been sufficiently described in many works. In particular, the mineralogical and geochemical features of the gold deposits in the district and the typomorphic features of ore gold, pyrites [1‒4], etc., were described in detail. Fewer works have been devoted to studying the specific features of changes in the mineral composition of placer deposits in the district [5‒6]. In particular, only one work was dedicated to the Khlebopek deposit [6]. The Khlebopek placer deposit is geographically located about 30 km to the south of the giant ore deposit Sukhoy Log. The Khlebopek stream is a stream of water of about 15 km long. Its tributaries are no longer than 2–2.5 km. They drain strata of carbonaceous shales of Vendian-Riphean age (carbonaceous siltstones, phyllites, sandstones with thin carbonate sublayers), which were subject to regional metamorphism between 520‒570 million years.

Research methodology: The elemental compositions of bedrock, weathering crusts, placers and gale-lixiviation dumps were studied using X-ray fluorescent units: ARL Perform’x (Switzerland); Schimadzu (Japan); Vega 400 (Israel). The presence of significant contents of gold and platinoids (more than 1‒5 g/t) was confirmed (and verified by an assay test and mass spectrometry) for dozens of studied samples selected from the larger part of the site. The presence of colloid particles of gold in the water of the tributaries and the stream was proved by Raman spectroscopy (RamanStation 400F).

Discussion of results: Using electron microscopy and X-ray diffractometry, we studied samples of crystalline schists, sand-clay stream deposits of tributaries of the Khlebopek stream, and the morphology of gold at the placer deposit of the same name. The detailed structure, elemental composition, and composition of the crystalline phases of the corresponding rocks were examined. We came to the conclusion on the leading role of chemical processes that contribute to the release of mineral phases of gold from rocks and their further transformation into colloidal forms and, subsequently, into coarse nugget-type gold.

Conclusion: processes of transformation of mineral forms of gold in the hypergenesis zone, contributing to their transition to colloidal forms and the deposition of metals in zero valence form.