COMPOSITIONS OF APATITE-FORMING HYDROTHERMAL FLUIDS AT THE HUMBOLDT IRON-OXIDE APATITE DEPOSIT, NEVADA

The Humboldt Fe-oxide-apatite (±Cu-Co) deposit in west-central Nevada is a voluminous metasomatic system associated with a mafic igneous complex formed during Jurassic back-arc extension. This system is inferred to have formed via circulation of amagmatic brines, driven by magmatic heat. To characterize the nature of the mineralizing hydrothermal fluid in the Humboldt system, we conduct a detailed petrographic and geochemical investigation of apatite-hosted fluid inclusions from Humboldt. Petrography and microthermometry of fluid inclusion assemblages show three distinct groups of inclusions present in the Humboldt system: 1) primary, liquid+vapor inclusions of ~14 wt% NaCl eq.; 2) primary, liquid+vapor+halite inclusions of ~38 wt% NaCl eq.; and 3) secondary, liquid+vapor+halite inclusions of ~37 wt% NaCl eq. Liquid+vapor inclusions homogenized to the liquid between 100-135°C. Halite-bearing inclusions also showed vapor disappearance between 100-135°C, but homogenized by halite melting between 280-300°C. No CO₂ was detected in any of the inclusions. Laser ablation ICP-MS analyses of all three inclusion types showed that the apatite-forming fluid had consistently low [K], [Cs] and [Li], versus anomalously high [Mg]. These results suggest that the apatite-forming fluids at Humboldt are unlike typical magmatic-hydrothermal fluids, and appear broadly similar to Mg-rich basinal brines (e.g., perhaps derived from evaporated seawater). These results are consistent with a model invoking fluids of amagmatic origin at the Humboldt system. However, at present our results cannot exclude the possibility that the fluid composition may reflect significant fluid-rock reaction prior to apatite precipitation.