Geology and Setting of the Miguela A-Zone, Guarayos Greenstone Belt, Eastern Bolivia (original) (raw)

2000, VMS deposits of Latain America

The Miguela A-Zone volcanogenic massive sulphide (VMS) deposit is a new discovery. It is located at the western border of the Pre-Cambrian shield of eastern Bolivia, hosted in a Proterozoic greenstone belt and metamorphosed to amphibolite facies. The mineralization occurs in a 400-meter thick felsic unit within a thicker succession of tholeiitic mafic volcanic rocks. The main rock types are meta-rhyolite and quartz-muscovite-rich meta-sediments close to the VMS mineralization, and silica and iron-rich mudstone and siltstone, magnetite or pyhrrotite-rich banded iron-formation on top and lateral to the known VMS mineralization. This litho-facies is informally known as the “La Pastora” formation and can be traced for at least 28 kilometers of strike length. This unit hosts all important geochemical and geophysical anomalies and gold and base metal showings of the area. The mineralization occurs in three lenses, is confined to two different stratigraphic levels, and is related to a system of synvolcanic growth faults. The texture of the mineralization is massive, semi-massive and finely laminated. The most important minerals are pyrite and chalcopyrite with lesser amounts of sphalerite and galena. Magnetite and pyrrhotite are rare in the A-Zone, but abundant in the northern El Bagre and southern El Porvenir zones. The lowermost A-3 lens is underlain and laterally surrounded by a zone of hydrothermal alteration, intense pyritization and sodium depletion. The main A-1 lens is the best investigated mineralized structure. It is tabular in shape, dips 45 degrees northwest, exhibits a true thickness between 12 and 23 meters and carries a quartz-rich layer on top with a significant enrichment of barium. It is continuous for 450 meters in strike, 350 metres down-dip and remains open along strike and to depth. It contains an Inferred Resource of 1.62 Million tonnes grading 3.76% Cu, 1.26 g/t Au, 11.3 g/t Ag and 0.33% and a geological resource potential of 4.25 million tonnes. This size compares well with the dimensions of the deposits of other VMS districts and places it in the median size category of known VMS deposits in Canada and Australia. The A-Zone is deeply weathered due to the intensive lateritization of the regolith since the Miocene. A 45-meter thick supergene chalcocite and a 40-meter thick oxide zone overlie the primary sulphide mineralization. The oxide zone is enriched in gold and depleted in base metals and silver, whereas copper is strongly enriched in the supergene zone. Routine geochemical and geophysical surveys, commonly used in massive sulphide exploration in Canada and elsewhere, have proved effective in the deeply laterized Bolivian Shield. Exploration on the Miguela and El Porvenir concessions have identified several targets with geochemical and geophysical signatures comparable to the A-Zone, which remain to be drill tested. Further exploration of the remainder of the Guarayos Greenstone Belt is a priority. Given the exploration success in a short period of time the Guarayos Greenstone Belt has excellent potential to host world class deposits.