Gold behavior during endogenic and supergene alterations of sulfides in magnesian skarns

The paper presents materials on the genesis of gold deposits of the magnesian-skarn association. It is demonstrated that sulfides are precipitated at these deposits late in the course of the mineral-forming process and often contain visible and fine gold. Post-sulfide mineral-forming processes resulted in the widespread development of hydroxisulfides: tochilinite and valleriite in high-Mg rocks and borate ores affected by serpentinization, brucitizatin, and szaibelyitization. The newly formed hydrosulfides inherit gold from the replaced sulfides. The endogenic or supergene decomposition of tochilinite and valleriite in endogenic and supergene environments stimulates the dissolution of the fine-grained gold and its remobilization, first, by hydrothermal solutions and, subsequently, by meteoric waters. The possibility is discussed of the later regeneration of gold as a consequence of electrochemical processes or at geochemical barriers. The deposition of “newly formed” gold in weathering crusts and placers is discussed, along with the significance of this process for assaying the potential of the weathering crusts and placers. It is emphasized that a significant role in this process is played by cryogenic processes, which can increase gold concentrations in naturally occurring solutions and facilitate its later regeneration. The data presented in this paper are compared with data on gold and PGE deposits of other genetic types, which are hosted in ultramafic rocks and carbonatites, i.e., rocks petrochemically similar to magnesian skarns. It is demonstrated that the occurrence of hydroxisulfides in the ores is a significant geochemical and technological problem during the exploration for sulfide ores and their mining and processing. The magnesian skarn ores of the deposits discussed in this publication were determined to be a significant source of both primary and placer gold and, perhaps, PGE also. The materials presented in the paper characterize the behavior of gold in the endogenic and supergene processes at magnesian skarn deposits.