CORDILLERAN IOCG SYSTEMS – INTEGRATING AND HONORING THE CUMULATIVE EVIDENCE (Invited Presentation)

The IOCG clan, with its alphabet soup of affiliated deposit types, continues to attract attention yet this group generally lacks system-scale geological characterization and robust comparisons with other system types. This contributes to diverse and often incompatible interpretations.

Why? It shouldn’t be the evidence. Many Cordilleran IOCG systems (in both North and South America) have the benefit of exceptional exposures, minimal overprinting, and well-understood contexts (often including contrasting deposit types). Among these, the Yerington, NV district through the fundamental work of John Dilles and colleagues provides an exceptional case study and a useful point of comparison with other Cordilleran examples.

What are the shared characteristics? – Distinctive Fe-oxide-dominated, low-sulfide mineralogy with accessory Cu, Au, Co, REE etc., and associated vertically zoned, voluminous sodic-calcic to K-silicate-hydrolytic alteration (in aluminous rocks; skarns in carbonate hosts). They form in the upper few km of the crust, they exhibit diverse forms (massive to breccias to veins to stratabound) with prominent but varied structural controls, and they generally formed in areas with evaporitic (saline) materials or environments. Magmatism is common (though not universal), and often contemporaneous though remarkably diverse in composition – from mafic to felsic. Geochemical data (& geology) demonstrate massive system-scale element mobility in the presence of ubiquitous brines. These data typically allow multiple interpretations, but in some cases indubitably require non-igneous components. Contrasting, often better understood systems may be present (e.g., PCDs at Yerington) and provide key comparisons.

Any viable interpretation of these systems should account for this breadth of evidence and, also, the many basic geochemical and petrological predictions that apply across system types. Too often narrow criteria are applied (other evidence as above being ignored) sometimes yielding seemingly compelling but typically non-unique interpretations. Generation of robust interpretations requires the integration of careful, detailed geological, petrological and geochemical evidence, as exemplified by John’s studies at Yerington.