Abstract

Low grade gold and associated erratic silver values are reported from organic-rich mudstones and sandstones of the Eocene Carrizo Formation and upper Wilcox Group in southern Medina County, Texas. The mineralization occurs over a small area centered around an abandoned shaft about three miles southeast of Yancey. Several drill holes, including rotary and core, have yielded reported scattered gold and silver values from the surface to a depth of over three hundred feet. Gold values are below 0.05 oz/ton, and silver values range up to several ounces per ton as determined by fire assay.

Mineralization and alteration are not apparent in the single small outcrop of ferruginous Carrizo sandstone containing the shaft. The nature of the mode of occurrence of the gold and silver mineralization is undetermined, and the full extent of the mineralized zone is yet to be defined. Highest gold values to date occur in organic-rich mudstones with associated pyrite or marcasite nodules.

The occurrence does not appear to be of placer origin, as are several documented localized gold occurrences in other Eocene sediments of the south Texas coastal plain. Gold and silver in trace quantities are described within contact zones of Cretaceous intrusions located to the north of the Balcones fault system in northern Medina County. Similar intrusions are abundant in the subsurface in this region, which lies near the southern boundary of the fault system. Low intensity geothermal activity along portions of the fault zone may have remobilized and transported gold and silver associated with subsurface igneous bodies and fixed them in their present location within the organic-rich host rocks. Another hypothesis which cannot be ruled out at this point is that the gold and silver may have been concentrated in the host rocks by very low temperature metal-rich saline fluids which migrated through this region during loading and dewatering of the thick Wilcox section.