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Geology, Alteration and Mineralization of the Modoc Hot Springs Gold Prospect, Imperial County, California

By
Frank L. Hillemeyer
Frank L. Hillemeyer
Fischer-Watt Gold Co., Inc., 114 Tucker, Suite #7, Kingman, Arizona 86401
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Michael D. Johnson
Michael D. Johnson
Fischer-Watt Gold Co., Inc., 340 Freeport Blvd., Suite #3, Sparks, Nevada 89431
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Richard R. Kern
Richard R. Kern
Homestake Mining Company, 155 Glendale, Suite #18, Sparks, Nevada 89431
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Published:
January 01, 1991

Abstract

The Modoc prospect is a Plio-Pleistocene epithermal gold-bearing hot springs system located in the northwestern Salton Trough. To date, Modoc offers the best preserved and developed fossil analogue of the currently active geothermal systems in the continental rift. A surficial hot springs origin for the system is evidenced by interbedded, nearly flat-lying chalcedonic sinter beds exhibiting a strong layered morphology, hummocky pool structures and well preserved fossil reed casts.

The primary structural control on the hot springs system is a major ENE-striking, southerly dipping, oblique-slip fault system separating Jurassic granodiorite on the north from Plio-Pleistocene continental sediments on the south. This “Truckhaven” fault system appears to be coincident with the edge of a large basin related to crustal thinning and progradation of the Colorado River Delta into the proto Gulf of California.

Gold mineralization occurs as electrum and native gold (8 to 1800 microns) and is associated with sheeted and banded chalcedony veins, silicified and adularized sandstone and conglomerate, and siliceous sinter beds. Gold and chalcedony precipitated simultaneously in numerous episodes and preliminary fluid inclusion data indicate temperatures ranging from 90 to 135°C. Gold mineralization is locally high grade (up to 1.4 ounces of gold per ton over a 20′ (6.1 m) width) and is found in highly anomalous concentrations over an area measuring 8,000′ (2,438 m) long by up to 2,000′ (610 m) wide. Correlation coefficient data indicate a strong gold, silver, copper affinity. The typical hot springs pathfinder elements As, Sb and Hg are present in unusually low concentrations.

The Modoc hot springs system is more akin to the active geothermal fields situated on the margins of the rift than those in the center of the Trough. It is strikingly similar to the high-silica Dunes geothermal system, which has no surface expression largely due to the development of hydrothermally induced caprocks. The rediscovery of the Modoc hot springs gold prospect leaves exploration geologists wondering how many other such occurrences underlay the seemingly barren sediments of the Imperial and Mexicali Valleys.

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Contents

Society of Economic Geologists Guidebook Series

The Diversity of Mineral and Energy Resources of Southern California

Michael A. McKibben
Michael A. McKibben
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Society of Economic Geologists
Volume
12
ISBN electronic:
9781934969656
Publication date:
January 01, 1991

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