T242 A Metal Rhythm: Understanding Cyclical Processes in Mineralization

Wednesday, 25 September 2019
8:00 AM - 12:00 PM
Room 126ABC, North Building

THE BONANZA GOLD ORES OF THE ALLEGHANY DISTRICT, CALIFORNIA

Paper number: T242-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

The Alleghany district contains the richest gold ores in the state of California. Very small ore shoots, locally termed "pockets" contain an average of 3 kg of gold per ton, far more than the more well-known Mother Lode ores. One 20-30 m section of a vein in the largest Alleghany deposit, the legendary Sixteen-to-One mine, is said to have yielded 45,000 troy ounces of gold during the 1920s (which would be valued at the equivalent of $62 million US at today's prices which exceed $1400 /per troy ounce).The Alleghany gold occurs in 114 Ma-old quartz veins (Bohlke & Kistler, 1986, Econ. Geol.) that crosscut suspect terrane consisting of retrograded upper-greenschist- to lower-amphibolite facies metamorphic rocks, such as quartzite, amphibolite, granite, and serpentinite with minor harzburgite (Coveney, 1981, Econ. Geol.). High-grade ores are exclusively associated with serpentinite wall rocks and usually contain abundant coarse-grained and brecciated arsenopyrite.

The ores formed from low-salinity hydrothermal fluids of uncertain origin at temperatures between ~200 and 300 degrees Celsius at 670 - 2000 bars of pressure (Coveney, 1981). The significance of the association of gold with arsenopyrite, dawsonite daughter minerals, and the CO2-rich fluids, and the cause of the one-to-one relationship between the gold ores and the serpentinites remain controversial, but it seems likely that the ore constituents were carried by different mechanisms than most gold ores (i.e., possibly not by chloride or sulfide complexes.) and that they were likely precipitated by chemical reduction.

The prospects of future mining in the district are uncertain. However, it should be emphasized that the Alleghany ores are vastly richer than Carlin-type ores, whose tenors range from only one to a maximum of about 30 g of gold per ton and thus generate huge amounts of waste. Given that the price of gold now hovers at ~$1400 per troy ounce and that there are legitimate concerns about the impact of large-scale mining on the environment, it may be time for a close look at the feasibility of surgically mining classic gold-quartz veins like those of Alleghany and other places, as a supplement to mining giant low-grade deposits such as Carlin.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-332040

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