Multiscale petrotectonic control of gold mineralization in the El Bagre-Nechí District (Antioquia, Colombia): a review of previous model

Abstract

Existing data and new observations indicate that gold mineralization with grades in the order of tens of grams per tonne in quartz veinlets and hypogene sulphides in the El Bagre and Nechí districts is closely related to the Carboniferous granitic plutonism of the El Carmen stock and its analogue in the Nechí area. The ore bodies, tend to be preferentially located in flexures where the dilatant character of the controlling fractures increases. They are arranged in echelon patterns along NNW-trending kilometric corridors, preferentially subparallel to the contacts of the intrusive with its metamorphic country rock. This arrangement favours the influence of thermal volumetric contraction during late magmatic emplacement as a complementary mechanism controlling the distribution of mineralizing fluids. Since most of the mineralized bodies develop a mantiform geometry sub-parallel to the intrusive-host contact, they define a spatial relationship that is difficult to reconcile with an exclusively subvertical transcurrent environment attributed to the Otú Fault. Thus, a substantial involvement of petrotectonic control combined with differential thermal contraction-expansion of the granitic host rocks is suggested to dominate over the pre-existing structural model of fracture-related emplacement of the vetiform bodies in a transcurrent context. Therefore, the traditional relationship proposed for this mineralization with the activity of the Otú fault transcurrent corridor, associated with the Andean deformation, needs to be reconsidered as it produces temporal inconsistencies. Despite the limited surface expression and the thick edaphic cover, this alternative model favours the location of unexplored sectors with high potential to define remaining primary gold targets.