REGIONAL STRUCTURAL CONTROL AND ALTERATION OF VOLCANICS HOSTING THE SAN NICOLAS MASSIVE SULFIDE DEPOSIT, ZACATECAS, MEXICO

San Nicolás is a Late Jurassic stratiform Zn-Cu-Au-Ag rich massive sulfide world class deposit in central part of México with a total resource of 120 Mt of ore grading 6.5 % Zn equivalent, discovered by Teck in 1997. The main sulfide body is a northwesterly trending elongate mound of massive sulfides 900 m long, over 200 m wide and up to 300 m thick, with a well-developed stringer zone and alteration system, directly beneath the mound.

The mineralization is controlled by a growing-fault at the NE side, with a plunge to the SW with 45 degres and a rhyolitic dome at the SW, both of them conformed a channel along of which sulfides were poured. The host-rock succession consists of variably altered mafics in the footwall and a rhyolite-tuff sequence in the hanging wall, with up to 900 m. thick, with an age range from 148.9 Ma to 117.94 Ma. The sulfide deposit and the enclosing volcanic sequence were metamorphosed under low greenschist conditions. The present form of the ore body could be the result of folding of ores and altered rocks.

The NW structures belong to the graben-horst system San Luis Potosi-Zacatecas, that has been working since the late Jurassic to the Neogene. The NE structures are part of the basin and range faults of Neogene age. All these structures have been recognized on the Landsat 5-7 images. Were analized 6 drill holes that cross the principal ore body and the hanging and foot walls, with more than 3000 m. with whole rock major geochemical analysis by ICP-ES and 43 trace elements by ICP MS; more than 100 thin sections were observed.

The widest alteration is a sericite-pyrite-rich zone in the dome of rhyolitic composition, strong calcareous alteration at the NW part of the body, at the hanging wall; strong calcareous alteration, intense pyritic alteration and intense k-feldespar alteration on the footwall. The various alteration facies can be discriminated geochemically in the Large's Alteration BoxPlot, it is possible also to identify common trend lines associated with particular styles of hydrothermal alteration.