Thermodynamic modelling of gold deposition in graphite- and iron-bearing metasedimentary wallrocks of lode-gold deposits

Abstract

Graphitic and iron-rich rocks commonly are considered to be efficient sites for deposition of gold in lode-gold systems, and are given special attention in exploration programs. In this work, thermodynamic modelling methods are used to clarify the mechanisms and efficiency of deposition of native gold in graphite- and iron-bearing wallrocks of lode-gold deposits under greenschist facies conditions. In all of the models, a fully equilibrated ``lode-gold-type' mineralising fluid is initially generated by reaction of a simplified CO2-H2S- NaCl-Au-bearing fluid with granodiorite at middle greenschist facies conditions (P=300MPa, T=420°C), followed by reaction of the fluid with a specified rock composition at lower greenschist facies conditions (P=200MPa, T=320°C) reflecting uprise of the mineralising fluid to a higher crustal level. Effects of variable graphite and iron on gold deposition are evaluated by calculating mineral alteration assemblages (including native gold) where graphite and iron are allowed to vary across specified ranges, and effects of increasing amounts of mineralising fluid are evaluated by allowing fluid/rock (F/R) ratio to range widely. The results reveal that no gold is deposited where graphite and iron are absent from the wallrock, only a small amount of graphite is required in a graphite-bearing rock to achieve maximum deposition of native gold, only a moderate amount of Fe is required in an Fe-bearing rock to achieve maximum deposition of native gold, and Fe-bearing rocks deposit gold much more efficiently than graphite-bearing rocks. These results confirm the known importance of redox conditions for deposition of native gold, but elevate the importance of sulphidation reactions over graphite oxidation reactions in efficiency of gold deposition from lode-gold mineralising fluids.