Structural control for the location of veins and felsic domes in the Zacatecas mining district, Mexico [Control estructural para el emplazamiento de vetas y domos félsicos en el distrito minero de Zacatecas, México]
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abstract
The Zacatecas mining district was considered one of the most important silver deposits from colonial times until the end of the XIX century; mining operations are currently attractive, as they have found new sites with profitable economic mineralization. The study area is located north of the city of Zacatecas, where marine Triassic sequences, volcano-sedimentary deposits of the Early Cretaceous, clastic and volcanic rocks of the Eocene crop out. The Mesozoic rocks were affected by at least two stages of compressive deformation associated with folding and reverse faulting, which caused imbrication of most of the sequence. These deformation events occurred mainly during the Cretaceous and early Tertiary and are more conspicuous in the volcano-sedimentary deposits. After the last compressive event of the Laramide orogeny, a transtensive deformation phase dissected the mountain into five blocks separated by La Cantera, Mala Noche; Veta Grande, Pánuco and Casa de Cerros faults. In this paper we propose a model for the emplacement of the veins and felsic domes of the Zacatecas mining district based on structural control by transtensional and extensional movement. The fault and vein systems have an echelon arrangement with development of reidel fractures and horsetail patterns at fault terminations. Movement along the faults is supported by kinematic indicators on the fault planes, where there are two overlapping sets of grooves: one horizontal (dextral) and one oblique. This suggests that a second event of normal faulting took place along the rupture planes generated during the first event (lateral). First stage faults and fractures provided conduits for mineralization in the principal veins and the ascent of felsic dikes and domes. The second fault reactivation event was normal-left oblique, which is evident in La Cantera fault and corresponds to the second stage of mineralization. The Bufa dome was emplaced in the dilation zone of this fault during the first stage of activity, and was later affected by hydrothermal events during extension, causing alteration (argilization and silicification) in the fault zone. The ore bodies are found mainly in dilation zones of the four main fault steps formed by transtension. The study area is located within Basin and Range Province, and is characterized by the presence of basins, half grabens, and horsts developed mainly during the Oligocene. Faults that dissected the Zacatecas range have an average orientation of N60°W, coinciding with the system known as San Luis-Tepehuanes.
