MODERN EXPLORATION AT AURCANA CORP'S SHAFTER SILVER MINE, PRESIDIO COUNTY, TEXAS

The Shafter District produced approximately 35 million ounces of silver from high-grade, partially oxidized ores from 1880 to 1942. Although unusually silica rich, Shafter appears to be typical of Carbonate Replacement Deposits (CRDs) in the 2200 km long Mexican CRD Belt. Comparison with other CRDs indicates that only part of the Shafter system has been found, so Aurcana has begun exploration on two fronts: finding extensions to the known orebody, and seeking orebodies in previously untested environments.

The main Shafter orebody is a WSW-ENE elongate tabular replacement “manto”, composed of fine-grained silica with acanthite and minor galena, 2.5 km long, 75 m to 150 m wide, and up to 12 m thick. The manto is developed largely in limestones of the Permian Mina Grande Fm. immediately beneath shales of the Cretaceous Presidio Fm. This contact acted as a barrier to ore-fluids, but locally mineralization and alteration cross it. Mineralization occurs in limestones deeper in the section, but has not been systematically sought. Mineralization shows control by WSW-ENE and roughly N-S faults with up to tens of meters of pre, intra, and post (?)-mineral offset. Tertiary volcanic rocks cover the northern and eastern flanks of the district and the mineralized zone is cut by highly-altered WSW-ENE and WNW-ESE intermediate to felsic dikes. Relationships between the district’s igneous rocks and mineralization are poorly understood.

Historically, exploration mapping, drilling, and geophysical surveys (including IP, Resistivity and CSAMT) were focused on the near-mine area with considerable success. However, significant potential remains for district-scale discoveries guided by modern geological understanding of CRDs. The principal challenges to district-scale exploration are “seeing through” alluvial cover or nonreactive capping rocks and entrenched geological interpretation. Mapping and selective sampling of fracture fillings in altered capping rocks is underway and previously successful geophysical techniques are being expanded into favorable areas. More importantly, critical reevaluation of district geology has already led to drilling targets along principal ore feeders; following feeders into untested stratigraphic units; and seeking mineralization related to potential igneous source rocks.