Geologic reconstruction of paleosurfaces in the Sombrerete, Colorada, and Fresnillo districts, Zacatecas State, Mexico

Grupo Catorce, S.A. de C.V., Mexico City, Mexico

Recent mapping has identified the paleosurfaces contemporaneous with the epithermal polymetallic vein systems of Sombrerete, Colorada, and Fresnillo, Zacatecas. The characteristics considered indicative of an environment close to fossil erosion levels and found in these systems include stratiform deposits of cryptocrystalline or opaline silica, intrusive or stratiform breccia bodies related to hydrothermal explosions, pervasive fine-grained silicification of volcanic or clastic rocks associated with the erosional surfaces, tabular zones of argillic alteration in the footwall of the jasperoid bodies, and the occurrence of mercury mineralization in veinlets or breccia bodies.Ag-Pb-Zn ore zones generally occur within the Mesozoic basement rocks, whereas the paleosurfaces are located within the overlying Tertiary volcanic pile. Premineralization volcanic rocks exhibit compositions which range from rhyolite to andesitc whereas the postmineralization volcanic units are mainly rhyolitic. The similar stratigraphic position of the paleosurfaces and the K/Ar dates from the three districts indicate that the hydrothermal systems may have operated between 35 and 30 m.y. ago. This age interval might represent the most important metallogenic epoch of ore-forming systems in the belt which occupies the eastern margin of the Sierra Madre Occidental in the state of Zacatecas; it coincides with a hiatus in volcanism.Stratigraphic reconstruction and fluid inclusion data predict hydrostatic or hydrodynamic fluid pressures, and depth from the palcosurface to mineralization. The Pabellon vein in the Sombrerete district lacks evidence for fluid boiling and the reconstructed depth to the top of mineralization is at least 1,000 m. However, for hydrothermal systems that operated along the liquid-vapor boiling curve (the Santo Nifio vein in Fresnillo and the No Conocida vein in Colorada), depths to the top of mineralization of less than 500 m are indicated. Higher temperature and salinity fluids coincide with ore zones at depth, and fluids of progressively lower temperature and salinity occur in the high-level environment near the paleosurfaces. These trends most likely result from fluid mixing and dilution of ore-forming fluids circulating in the deeper parts of the systems, with meteoric waters of near-surface derivation.Combined geologic and fluid inclusion data become useful exploration and development tools. On a districtwide scale the identification of the paleosurface defines the highest level of a fossil hydrothermal system and can help to evaluate from the top downward the sectors of the district where economic potential remains preserved at depth. On a regional scale, the geologic level which corresponds to the hiatus of volcanism within the Tertiary volcanic pile defines an excellent guide for new mineralized systems along the eastern margin of the Sierra Madre Occidental. Recognition of the palcosurface and its expressions can aid in locating exploration targets in terranes not yet exposed at depth by erosion.

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