A REDOX REGIME FOR INTRUSION-RELATED GOLD SYSTEMS

Granitoids responsible for generating hydrothermal ore deposits have a wide range of geochemical characteristics, most critical among them is the primary oxidation state of the parent magma. Redox state of granitoids, or more specifically fluid derivation from an oxidized or reduced magma, controls the metal tenor of the mineralization as well as other deposit aspects such as mineralogy, fluid chemistry, alteration style etc... Mineral exploration discoveries and geological studies on gold deposits over the past 20 years are sufficient to confidently recognize oxidized and reduced end-members of the intrusion-related gold systems classification.

Oxidized intrusion-related gold systems are directly associated with porphyry copper deposit environs, so their tectonic settings are within co-eval calc-alkaline arcs, and their roots. Gold is always associated with copper, and there is a continuum in copper:gold ratios across the class. Deposit features are similar to porphyry copper deposits. Type deposits include La Colosa and Boddington.

Reduced intrusion-related gold systems generally form far inboard from the arc, in pericratonic continental margin settings dominated by older miogeoclinal sediments or their metamorphosed equivalents. Gold is the dominant, and generally only metal of interest, but is associated with Bi, Te and As. The regional metallogeny may be dominated by tungsten. Type deposits include Fort Knox and Dublin Gulch.

In both redox types, there is diversity in the styles of mineralization, as the fluids take advantage of the wide-range settings available in, around and above the cooling pluton. Ore grades are generally low, about 1 ppm gold, but the systems have the ability to generate large deposits, hosting 100 to 500 tonnes of gold.

Therefore, redox is the controlling regime for the characterization and classification of intrusion-related gold systems. This builds on redox similar regimes established for magnetite vs ilmenite-series granitoids and related metallogeny, and that developed for gold skarn classification. Increasingly efficient bulk mining and low-grade recovery methods, supported by the increasing price of gold, ensure that more intrusion-related gold systems will be discovered and exploited.