BASALT-RELATED FE-OXIDE(±CU-AG-CO-AU) SYSTEMS: THE HUMBOLDT MAFIC COMPLEX EXAMPLE, WESTERN US

Numerous Fe-oxide(±Cu-Ag-Co-Au) occurrences within the Humboldt mafic complex, western Nevada, are part of a belt of Jurassic Fe-oxide-rich mineralization associated with arc and back-arc-related mafic to felsic magmatism in western N. America. This and many other basalt-related systems worldwide share key features with Fe-oxide(-Cu-Au) deposits associated with intermediate and felsic rocks. A full understanding of this family of deposits must account for this compositional spectrum.

The Humboldt mafic is a large (~2500 km3) volcano-plutonic complex and consists of several gabbroic centers that feed overlying basalts and intrude an evaporite-bearing clastic and carbonate sequence. Roughly 2/3 of these rocks are pervasively altered to sodic-calcic and sodic assemblages. These zone from deep high-temperature Na-scap + hbl + titan ± mt assemblages (200 km3) centered on dike swarms to shallow, lower temperature alb + chl + carb ± hem ± Cu(Fe)-sulfide assemblages (450 km3). Fe±Cu mineralization is similarly zoned from deeper mt + apat + hbl + titan -bearing replacements, veins and breccias, to shallower hem(mt) ± py ± cpy ± bn veins and breccia bodies associated with alb and ± carb assemblages.

The Humboldt occurrences resemble other mafic-hosted global occurrences and together these deposits represent an end-member in the Fe-oxide(-Cu-Au) family of deposits. Geologic settings vary globally and include rift-basins, flood basalts, and volcanic arcs. Other mafic-associated examples include deposits in the eastern US, Siberia, and China, with modern analogues in the Salton Sea and Danakil Depression. Most districts share voluminous alkali-rich alteration (Na-Ca, Na, K) and enormous amounts of metal movement which may represent significant economic targets. Both barren and mineralized systems are known; however, many of the latter are poorly documented. Mafic-hosted hydrothermal systems contrast with intermediate- to felsic-hosted systems mainly in having common high-T scap alteration, higher mt to hem ratios, and abundant Co-bearing phases. With some exceptions, qtz, F-rich minerals, REE, and U are rarer in mafic-hosted systems. These differences follow from differences in igneous compositions. The mafic-hosted end-members represent the best evidence for derivation from an external brine source.