T237 System-Scale Zonation of Ore Systems: Insights into 3D Architecture from Lateral and Deep Exposures Due to Mining and Structural Deformation

Tuesday, 24 October 2017
1:30 PM - 5:30 PM
Yakima 1

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

Paper number: T237-6
Program: Technical Sessions
Time: 2:50 PM – 3:05 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-300903

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.