Soil geochemical exploration of the Presidio mine, Shafter Mining District, Presidio County, Texas

The Shafter Mining District is principally notable for the Presidio mine, which was in almost continuous operation from 1883 to 1947, and yielded over 32.6 million ounces of silver and 8,400 ounces of gold valued at approximately {dollar}18,000,000. Numerous faults, mostly of normal displacement, help to control the location and shape of the ore bodies. The ore occurs mainly as replacement deposits (mantos) controlled by bedding planes and to some extent by fault-fissures in Permian age limestones. A few ore deposits formed along thrust planes and related low-angle shear zones. There are numerous steep veins and some replacement ore deposits in Cretaceous age limestone beds.; The main goal of this research was to determine if soil geochemical anomalies occur over the hidden silver-rich ore bodies of the Presidio mine, Shafter Mining District, Texas because no surface expression of these hidden ore bodies had been discovered. Two hundred and ten soil samples were taken at 4 meter spacing along 18 lines that were oriented north-south at an interval of 4 meters. The samples were leached sequentially with 10% (1.6N) cold nitric acid and concentrated (6N) hot hydrochloric acid. The leachates were analyzed for the major elements Ca, Mg, and Fe and the minor and trace elements Zn, Pb, and Ag by ICP-OES techniques. Soil anomalies were found for all elements analyzed revealing the presence of faults and fractures that communicated with the hidden ore bodies. The anomalies also reveal that the ore is not mineralogically uniform in composition, which is consistent with the mining records. Lead was generally a good pathfinder element for silver.