THE ENVIRONMENT OF ORE FORMATION IN LATE PHANEROZOIC IRON-OXIDE-RICH MINERAL DEPOSITS OF THE WESTERN USA, AND EVIDENCE FOR A NON-MAGMATIC HYDROTHERMAL ORIGIN (Invited Presentation)

Magnetite-rich ore deposits of USA Cordillera range in age Permian to Cenozoic and are associated with epizonal calc-alkaline plutons. Ores are found near plutonic contacts, both in plutons or adjacent meta-sedimentary or -volcanic wallrocks at <4-5 km crustal depth. The largest and most abundant deposits are associated with late Jurassic arc intrusions in a belt running from northwest Nevada to southwest Arizona. Igneous rocks span a broad range of composition from gabbro to granite.

Ores span the IO to IOCG continuum and range from magnetite-rich (±hematite) deposits with minor apatite and sulfide, to sulfide-rich deposits mined for copper ± gold (e.g., Pumpkin Hollow, NV; Lights Creek, CA).

Deposits are associated with widespread hydrothermal alteration, ranging from local high temperature Ca-pyroxene ± garnet (“skarn”) in metasedimentary rocks to hornblende and scapolite in intrusions proximal to ore, to regional sodic-calcic and sodic alteration affecting 10’s to >100 cubic km of rock as well as lesser amounts of associated potassic alteration, sericitic alteration, and local advanced argillic alteration that are confined to shallow depths (<3 km). Sodic-calcic alteration is closely associated with ores, in general, and is characterized by Na-rich plagioclase, local scapolite, and various Ca ± Mg silicate including actinolite, diopside, epidote, titanite, as well as calcite and chlorite. Most sodic-calcic alteration zones have been strongly depleted in Fe and K (<1 to >5 wt.% each), and therefore are potential sources of Fe deposited in adjacent magnetite-rich ores. For example, >15 Gt Fe are mobilized from the Humboldt gabbro and the local Buena Vista IO deposits contain ~2 Gt Fe.

Strontium, oxygen, and hydrogen isotopic studies of sodic-calcic alteration zones and small magnetite-rich alteration zones in the Yerington batholith indicate that fluids producing the alteration were not magmatic and rather are non-magmatic in accordance the presence of evaporites in the host rocks and the 35-40 wt% salinity of fluid inclusions. Mass balance estimates suggest >2 Bt of Fe were removed from sodic-calcic alteration zones in the Yerington batholith, and this Fe could be the source of the ~0.5 Bt of Fe deposited in the Pumpkin Hollow IOCG deposit at the batholith contact.

Geology, isotopic tracers, and hydrothermal alteration gain and loss estimates support a hydrothermal origin of western USA IO deposits in which sedimentary brine fluids extract iron as a result sodic-calcic wallrock alteration and transport it to ore zones.