Abstract

Gold- and silver-bearing veins occur in three northwest-trending fault systems, the Sierra, Veta Madre, and La Luz. Orebodies in the Sierra and Veta Madre were arbitrarily divided into two groups, those that occur above the 2,100-meter level called "Upper" ore and those that occur primarily between the 1,700- and 2,200-meter levels called "Lower" ore. One ore shoot occurs below the 1,700-m level and is called "Deep" ore. Depending on the elevation of the surface, either "Upper" or "Lower" ores outcropped, were easily discovered, and are now largely mined out. It was recognized that three abandoned mines located in the southeast part of the Veta Madre had produced from "Upper" ore. In 1968, deep drilling below the bottom of these old mines discovered new bodies of "Lower" ore at depth.Evidence is presented to show that the vein material did not have its source in hydrothermal vein material from nearby granites (Wandke and Martinez, 1928), from lateral secretion by meteoric waters through Tertiary lavas (Taylor, 1971), or from solutions emanating from the mantle. The source is believed to be the Mesozoic sedimentary rocks that underlie the area. Alkaline ground water in these rocks was heated to temperatures above 350 degrees C during the period of volcanism in the middle Oligocene. The hot solutions leached vein-forming materials from the surrounding rocks and escaped along faults formed by crustal subsidence following volcanism. The solutions moved upward along the faults, and the disposition of vein material during the upper Oligocene was caused primarily by a decrease in temperature estimated to be approximately 10 degrees C/100 m.The veins were formed in at least three stages. In the first stage, white massive quartz, calcite, and adularia with minor quantities of gold and silver were deposited. The alteration of the wall rock took place at this time. In the second stage, gray chalcedonic quartz containing economic quantities of gold and silver was introduced into the fault vein system. The "Upper", "Lower", and "Deep" orebodies formed during the second stage contain essentially the same minerals deposited in the same sequence but in different proportions. It is suggested that the three ore types had a common source and were formed penecontemporaneously with the upward decrease in temperature causing the different ore types to be localized within fairly specific elevations.

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