Regional geological setting of Guanajuato silver-gold deposits and quantitative petrophysical assessment of their hydrothermal alteration, Guanajuato, México
Por:
Vassallo L.F.
Publicada:
1 oct 2018
Resumen:
Geological setting of one of the world famous silver-gold deposits type is presented. The silver-gold vein system occurs in the southeastern portion of the Sierra de Guanajuato, which is a 100 km long, N45°W-trending mountain range in Central Mexico. The rocks exposed in the Sierra de Guanajuato and its surroundings can be divided into four broad stratigraphic successions: (1) the “Basal complex” which includes Mesozoic rocks and an early Eocene granitic batholith, (2) a thick (>1000 m) red bed sequence resting unconformably atop the basal complex, which was deposited in the lapse between 49 and 36 Ma, (3) a thick, faulted and tilted, Paleogene subaerial volcanic succession, and (4) Immediately north of Sierra de Guanajuato are exposed a widespread, flat-lying, sand and gravel deposits accumulated in a continental basin. These sediments contain fossil vertebrate faunas with ages between late Clarendonian and Blancan. Southeast of El Bajío fault system sand and gravel deposits, partially covered by basaltic andesites crop out. A similar relation between gravel deposits and mafic lava flows is seen at the top of Cerro El Cubilete, which is one of the highest point in Sierra de Guanajuato. Succession (1) experienced several shortening events during the Cretaceous and Paleocene, whereas successions (2) thru (4) were extended in several deformation pulses during the Cenozoic. Wall rock alteration changes in some petrophysical parameters were studied in the Veta Madre vein in the central part of the Guanajuato mining district. Hydrothermally altered rocks were characterized by their variations in the following petrophysical characteristics: (1) density, (2) effective porosity, (3) rapid volumetric saturation, and (4) constant saturation. Among the petrophysical characteristics was observed that is important for exploration at Guanajuato: 1. The rock density tends to fall near the structures that present ore bodies at depth, and having little change when no hydrothermal alteration is present.2. The effective porosity (Pefec) of rocks suitable for mineralization increase their values.3. The rapid volumetric conditioned saturation (A) plays a big role when the hydrothermal fluids pass near the rocks with high values and produces mineralization.4. The constant saturation (1/B), their values tend to increase in areas with mineralization. Samples of host rocks collected above known spikes such as Las Torres and Sirena exhibit hydrothermal alteration with the following petrophysical characteristics: density tends to decrease whereas porosity, rapid volumetric and constant saturation increase. The use of these petrophysical parameters may be a low-cost technique to predict the presence of undiscovered orebodies in the Guanajuato mining district and elsewhere where similar mineralization occurs. © 2018 Elsevier B.V.
Filiaciones:
Vassallo L.F.:
Universidad Nacional Autónoma de México, Centro de Geociencias, Querétaro, Mexico
|