Comparative studies of iron-oxide mineralization: Great Basin

Abstract

The study of iron-oxide-rich mineralization in the Great Basin demonstrates that these occurrences are widespread and are associated with a wide range of host rocks and associated igneous suites. Two well-exposed areas containing similar types of iron-oxide±(Cu-Co-REE) mineralization and voluminous sodium-rich hydrothermal alteration are hosted by contrasting coeval, mid-Jurassic igneous suites and demonstrate a complex, but similar hydrothermal system evolution. One area is hosted by mafic plutonic and volcanic rocks of the Humboldt mafic complex in west-central Nevada. The second area is hosted by felsic granitoids and volcanic rocks in the Cortez Mountains in north-central Nevada. Alteration and iron-oxide±(Cu-Co-REE) mineralization in both systems is synmagmatic and consists of high-temperature alteration assemblages composed of either scapolite or albite/oligoclase-rich sodic-calcic assemblages. Lower temperature alteration assemblages are composed of sodic and hydrolytic assemblages in addition to potassic alteration in the Cortez Mountains. Mass-balance studies show that all of the components observed within the hydrothermal system are compatible with derivation from an external fluid or from the associated igneous host rocks. Sodium is added in both systems and is approximately 5 times greater than the amount that could be sourced from a plausible magmatic fluid. Components such as iron, phosphorous, copper, cobalt, nickel, barium, and REE are leached from intensely altered portions within each system and are partially trapped up section at the deposit scale. Other elements such as manganese, zinc, and lead that are also mobilized during deep alteration are not trapped within the exposed portions of either hydrothermal system and are presumably lost to the hydrothermal system. Comparisons of mass-transfer in iron-oxide-rich hydrothermal systems with better-known igneous-related hydrothermal. systems demonstrates that mass-transfer systematically among systems and is strongly controlled by the composition of the hydrothermal fluid and the size of the heat source. Thus systems dominated by saline fluids show the largest amounts of mass-transfer. Mass-transfer estimates show that iron-oxide hydrothermal systems are second only to marine hydrothermal systems in terms of total amounts of materials transferred.