STRUCTURAL SETTING AND METAL ZONING OF THE AG-AU VEINS OF THE REAL DE MINAS DE ZACUALPAN DISTRICT, CENTRAL MEXICO

The Real de Minas de Zacualpan District is located 100 km southwest of Mexico City, and centered within an area of the oldest known mining activity in North America, pre-dating the arrival of Hernan Cortez in 1519.

The 423 km²Zacualpan District is an 18 km wide structural corridor of dextral transpressional shear, hosted in the Guerrero Terrane, in which andesitic volcanic rocks of the Vila de Ayala Formation act as a rigid block resulting in brittle deformation. Felsic intrusive rocks of the Tilzapotla Formation, located midway between the Zacualpan and Taxco Districts, appear to have acted as a point heat source which has focused the flow of the hydrothermal solutions. The age of the veins for the Taxco District range from 30 - 32 Ma, and by analogy the Zacualpan veins are presumed similar, with the age of the Tilzapotla Formation at 32 Ma.

The Zacualpan District is geochemically zoned and tilted with Mesothermal Au-Cu-As veins occurring in the central and southwest portions, and with Epithermal Ag-Pb-Sb Veins in the northeast. Factors controlling geochemistry of the veins are: 1) higher gold contents and ratios in proximity to felsic intrusives of the Tilzapola Formation, 2) higher gold values in local extensional structures, such as dilational jog and pull-apart structures, 3) depth of emplacement, and 4) topography.

Important structural features include: 1) a series of northeast dipping dextral structures which often host veins that are tens of kilometers long, which in turn seem to spawn and control the distribution of the secondary structures, 2) north-northwest striking dilational jog features, that have a vertical sense of displacement and are bounded by the northeast dipping dextral structures, which give the overall appearance of a horst-looking structure, 3) splays from the northeast dipping dextral veins that are generally local in extent, 4) reverse structures that are southeast dipping that appear to form in local zones of constriction for the northeast dipping dextral structures and 5) an enigmatic series of north to northeast striking veins located in the central portion of the district.

A dynamic model based upon incremental movement for the northeast dipping dextral structures currently appears to provide a comprehensive explanation for structural development of the Zacualpan District.