Geologic and geochemical study of the Picacho gold mine, California: gold in a low-angle normal fault environment |
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| Authors: | Steven Losh Dan Purvance Ross Sherlock E Craig Jowett |
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| Institution: | (1) Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA;(2) Glamis Gold Inc, Picacho Mine, 1891 Rail Avenue, Yuma, AZ 85365, USA;(3) SRK Consulting, Suite 800, 580 Hornby Street, Vancouver, BC, Canada, V6C 3B6;(4) Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, ON, Canada, N2L 3G1 |
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| Abstract: | The Picacho gold deposit, located in southeasternmost California, is a low-grade gold deposit in a nearly flat-lying denudational
fault of regional extent and probable Oligocene age. The deposit is hosted by intensely fractured and faulted Mesozoic leucogranite
and by chloritic augen gneiss and schist, and is overlain unconformably and in fault contact by unmineralized late Oligocene
Quechan volcanic rocks. The deposit is structurally characterized by normal and normal-oblique faults of low to high dip at
shallow depths in the mine, merging downward with a synchronous, low-dipping ore-stage extensional fault system (the Chocolate
Mountains/Gatuna Fault) of probable Oligocene age in deeper portions of the deposit. The fault system was infiltrated during
much of its active life by hot, dilute, highly exchanged meteoric water having temperatures of 170°–210° C, salinity <2 wt%
NaCl equivalent and calculated δ18Ofluid between −2.6‰ and 5.2‰. This main-stage fluid precipitated quartz, pyrite, and specular hematite, accompanied by silicification
and sericitization. Auriferous ore-stage pyrite was precipitated late in the fault evolution probably by mixing of reducing
ore fluid with relatively oxidized main-stage fluid during regional Oligocene extension on the Chocolate Mountains/Gatuna
Fault. The Picacho deposit is characterized by a gold–arsenic–antimony geochemical signature consistent with bisulfide complexing
of gold in reducing fluid, in contrast with typical denudation fault-hosted base-metal-rich deposits associated with high-salinity
fluids elsewhere in the southwestern United States. The deposit is overprinted by Miocene normal faults having a wide range
of dips. These postore faults are associated with red earthy hematite precipitation, pyrite oxidation, and supergene enrichment
of gold.
Editorial handling: L. Meinert |
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| Keywords: | Low-angle normal fault Detachment fault Gold Picacho California |
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