On the Source of Orogenic Gold

Mining & Minerals - Evaluations / Gold-Silver Exploration & Mining Projects

Andrew G. Tomkins reports that gold has historically been a key strategic commodity, and the study of gold deposit formation has long been investigated. Debate on the genesis of so-called orogenic gold deposits (more than 75% of gold recovered through history) continues largely because it is difficult to reliably identify the source of gold. In this paper, the author identifies a geochemical tracer in orogenic gold deposits, ethane (C2H6), which is found in carbonaceous metasedimentary rocks. Orogenic gold deposits dominantly form in metamorphic rocks in the mid-to shallow crust (5 to 15 km depth), at or above the brittle-ductile transition, in compressional settings that facilitate transfer of hot gold-bearing fluids from deeper levels.

The term orogenic is used because these deposits likely form in accretionary and collisional orogens. Transfer of weakly oxidized, low-salinity fluids to the sites of gold deposition is controlled by earthquake events allowing fluids to rapidly traverse large thicknesses of crust. This rapid rise takes the fluids out of equilibrium with their surroundings, promoting destabilization of the gold-carrying hydrosulfide complexes of Au(HS)2 and AuHS. The chemical cause of gold precipitation, facilitated by a temperature-pressure decrease, varies from place to place, and mechanisms such as fluid-rock reaction, boiling, fluid mixing, and chemisorption on surfaces of pyrite and arsenopyrite have been proposed. The author investigated the cause of gold deposition by sampling rocks where precipitation occurred, and structures through which fluids flowed. In contrast, the question where the gold comes from is more difficult, because the potential sources are distant to the sampled gold deposits.

Original URL: https://pubs.geoscienceworld.org/geology/article-lookup/41/12/1255