Geology, mineralogy, geochemistry, and geothermometry of Kelley Limestone jasperoids, Magdalena mining district, New Mexico

Abstract

Jasperoid, a microcrystalline silica rock, occurs along the crest of the Magdalena Mountains and along the west-facing dip slope. It is the focus of this study because in similar settings in Nevada it is associated with economically important deposits of very fine grained gold. In the Magdalenas, jasperoid extensively replaces the Mississippian Kelly Limestone. Mineralogy displays strongly oxidized pyrite with internal zones of silver and copper sulfides; galena and sphalerite follow pyrite in the paragenesis. Geochemistry suggests that silicification of the limestones occurred in a single event. Gold ranges in concentration from 2 to 2,100 ppb, averaging 274 ppb; silver ranges from 2 to 200 ppm, averaging 22.3 ppm. Geothermometry indicates high temperatures consistent with the association of the silicification with proximal intrusive activity. Heat was advected through open space by the silicification event, and silica-rich fluids were carried into the karsted carbonate rocks of Kelly Limestone along faults and dikes. The fluids were prevented from reaching higher in the section by an impermeable seal of Pennsylvanian black shales and siltstones.The Magdalena mining district is located in the northern part of the Magdalena Mountains, 3-5 mi southeast of the town of Magdalena, Socorro County, NM. The mining district is accessible by four-wheel drive vehicle and foot trails from NM Highway 60. This study area covers about 3 mi2 and lies entirely within the boundaries of the Magdalena 7.5 minute quadrangle. Elevations in the map area range from 5,955 to 10,179 ft.Silicification of the upper part of the Mississippian Kelly Limestone in the northern Magdalena Mountains appears to be related to the thermal and hydrothermal processes that occurred during evolution of the 32.1 Ma Socorro caldera and 28.9 Ma Sawmill-Magdalena caldera which both overlapped the Magdalena mining district. However, it may be related to younger intrusions not exposed at the surface. In caldera settings, mineralization is often much later and related to intrusions into the ring-fracture zone which provides the structural pathway for mineralizing fluids. An example is the Mogollon mining district in southwestern NM, where the mineralization is some 10 Ma younger than the caldera.