Vertical zoning and continuity in Fe oxide(-Cu-Au-Ag-Co-U-P- REE) (or 'IOCG') systems: Cordilleran insights

Recent studies of Cordilleran Fe oxide(-Cu-Au- Ag-Co-U-P-REE) (or 'IOCG') systems demonstrate that they formed in the upper 8 km of terrestrial crust. Fe ± Cu mineralization extends from near or at the contemporary surface to depths of 1-4 km whereas related Na-Ca alteration (including metal leaching) spans paleodepths from 1 to 8 km. Many well exposed systems show systematic vertical zoning in Fe(-Cu) mineralogy from near-surface Hm-rich assemblages (including some syngenetic deposits) through Mt-dominated, sulfide- bearing zones (including Mt-rich, silicate-poor skarns in carbonate rocks) to sulfide-poor Mt(-Act/Cpx±Ap) mineralization (i.e., Kiruna or 'IOA' type), and ultimately with depth into Fe- and base metal-depleted intense Na- Ca alteration. This vertical zoning matches early syntheses of the IOCG clan, but differs from recent suggestions that IOA, Mt, and Hm styles necessarily represent distinct types of systems. Although there is considerable variation in the geochemically defined IOCG family, Cordilleran evidence demonstrates that these several styles typically form in regular, predictable patterns as part of zoned hydrothermal systems. Differences reflect depth, local host rocks, structural style, available S (for Cu precipitation), and composition of contemporaneous igneous rocks. These insights can apply directly to district- to regional-scale exploration.