Geological setting of gold-silver mineralization in the La India mining district, Nicaragua

Abstract

La India gold mining district covers a 50 km² area of fault-fill gold-silver mineralized quartz-adularia veins on the western margin of a Tertiary volcanic arc in western Nicaragua. Historic mining records and modern mineral exploration data, which to-date has defined a mineral endowment of over 2.3 Moz gold, provide a wealth of geological information. This paper draws on these observations and data to describe and classify the gold-silver mineralisation at La India, identify the geological controls, and interpret the timing of mineralisation within the regional tectonic setting. The district-scale gold-silver mineralisation at La India occurs in two adjacent geological settings with distinct mineralization characteristics and exploration potential: (1) an upland area of strongly faulted felsic to andesitic volcanics where the historic mine workings are located, and (2) an adjacent downthrown graben, the Sebaco Graben, where a thick sequence of andesite is preserved overlying the felsic volcanic sequence. Gold mineralisation is classified as rift margin-type low-sulphidation epithermal gold-silver fault and fracture-fill vein mineralization. In the historic mining area erosion has exposed the top of the high-grade epithermal zone. Minimal erosion in the Sebaco Graben means that the epithermal system is fully preserved at depth, with localised hydrothermal sinter outcrops, sporadic low-grade mineralised veins and a phreatic breccia pipe exposed at surface. Apart from the one phreatic breccia, the gold-silver mineralisation occurs in quartz veins and breccias that filled brittle faults and associated fractures and fissures which developed in an extensional tectonic setting. The structures containing the gold-silver mineralisation were formed as normal and trans-tensional faults with orientations consistent with southwest-directed extension: (1) a predominant northwest to north-northwest set parallel to the subducting plate; (2) secondary but locally extensive east-west, and (3) tertiary shorter and narrower northeast and north-striking veins. A district-scale north-northwest orientated through-going structure linking the major gold-silver deposits in the historic mining area is interpreted as a deep crustal conduit for the gold-silver bearing hydrothermal fluids. Other, as yet unidentified basement feeder structures may have fed mineralised corridors in the east and west of the district. The gold-silver mineralisation is best developed where structures pass through competent felsic volcanics and welded tuffs in the historic mine area, and also in the overlying andesite flows in the Sebaco Graben. Gold-silver mineralisation is less well developed where the structure passes through less competent unwelded tuffs and volcanic agglomerates. Gold-silver mineralisation is interpreted as occurring shortly before or at 8-10 Ma at the end of a long period of slab-rollback induced extension and arc volcanism. Post-mineralisation block faulting split the La India district into the upthrown, and subsequently eroded historic mine area where epithermal mineralisation is largely exposed at surface, and the well-preserved downthrown blocks such as the Sebaco graben where much of the gold-silver mineralisation is still hidden several hundred metres below surface.