ARCHITECTURE AND GENESIS OF HYDROTHERMAL SYSTEMS IN TILTED MID-CENOZOIC CALDERAS, NORTHERN GREAT BASIN, USA

Extensional faulting in the northern Great Basin has exposed several Mid-Cenozoic caldera-related hydrothermal systems in exceptional 3D detail. In north-central Nevada, Miocene normal faults expose a >5 km thick section through the 34 Ma Caetano caldera (CC), including >4 km of intracaldera rhyolitic tuff and overlying sediments. Magma resurgence and pluton emplacement drove a hydrothermal system that altered >120 km² of caldera fill to depths >1.5 km. Alteration was focused in early granite porphyry and nearby intracaldera tuff and lake sediments. Early, pervasive quartz-kaolinite-pyrite and minor residual quartz alteration grades outward to quartz-smectite-pyrite alteration. Hematite, quartz, and barite veins and breccias cut early alteration. O and H isotopic data indicate that hydrothermal fluids were dominantly ¹⁸O-exchanged meteoric water, probably from the caldera lake.

The 29 Ma Job Canyon caldera (JCC) in the Stillwater Range is tilted 70-90°, exposing a 10 km thick crustal section that includes ~2 km thick post-collapse intermediate lavas, >2 km thickness of intracaldera rhyolitic tuff, pre-caldera lavas, and the resurgent granodioritic IXL pluton. The upper 500 m of the pluton through the lower part of the post-collapse lavas are pervasively altered to vertically zoned propylitic, sericitic, and argillic assemblages. Sparse quartz±calcite and quartz+pyrite veins cut tuff. Hydrothermal fluid compositions from O and H isotopic data indicate derivation from moderately ¹⁸O-exchanged meteoric water that circulated to depths ≥6 km.

Major differences between hydrothermal systems include (1) fluid types: CC: low pH, T<~270°C, early H₂S dominant, late oxidized H₂SO₄ dominant; JCC: early neutral pH, T up to ~400°C; late lower pH. Both systems were relatively S poor; (2) CC: intense alteration from paleosurface to ≥1.5 km depth; JCC: alteration intensity decreases at depths ≤2 km; (3) CC: early fluid flow mostly lateral through H₂O-saturated tuff and sediments, late downward flow of oxidized fluids; JCC: lateral flow through tuff and volcaniclastic rocks with high-T upflow zone along a caldera margin. Quartz-alunite and porphyry–style alteration, veins, and mineralization are not exposed in either system, similar to most other Great Basin calderas that lack significant Au-Ag-Cu-Mo deposits.