Resumo

MENGELLE-LOPEZ, Jorge Jaime et al. The La Esperanza volcanosedimentary sequence (Lower Cretaceous) north of Guanajuato, Mexico: importance in the exploration of volcanogenic massive sulfides. Bol. Soc. Geol. Mex [online]. 2013, vol.65, n.3, pp.511-525. ISSN 1405-3322.

In the vicinity of the city of Guanajuato, a marine volcanosedimentary sequence crops out hosting two volcanogenic massive sulfide (VMS) occurrences: Los Mexicanos with Fe−Cu−Zn, and Guapillos with Fe−Cu−Pb−Zn±Au−Ag. This sequence, from base to top, consists of: (a) submarine vitreous tuffs with clasts of andesitic composition, (b) interbedded andesitic lavas, laminar limestones, conglomeratic sandstones with andesite clasts and black shales, (c) lava flows of andesitic, dacitic and rhyolitic composition, and (d) black shales with interbedded rhyolitic pyroclastic deposits. This sequence is intruded by subvolcanic bodies of granitic composition. The submarine volcanic rocks are calc-alkaline, while the trace element signature (including REE) of andesites, suggests that these rocks formed in a back-arc basin associated to an oceanic island arc. This setting favoured the development of submarine volcanism. Moreover, the development of submarine hydrothermal systems was triggered by felsic volcanism of explosive and submarine nature, probably associated to submarine calderas. The processes of formation of VMS deposits in the sequence in the Sierra de Guanajuato are associated to dacitic-riolitic volcanism.

Palavras-chave : VMS deposits; calc-alkaline; tholeiitic; back-arc basin; hydrothermal.