Geology
The geology at the Property is dominated by an approximately 400 m-thick succession of intermediate to felsic tuffs and andesitic to dacitic lava flows with local horizons of volcanic conglomerate. The volcanic succession correlates with the mid-Tertiary volcanic rocks exposed in the Tomochi area. Tectonic features such as such as faults, cleavage and fractures are more prominent in the intermediate volcanic units. Dominant structures at district levels include: (1) NE-SW and NW-SE oriented strike-slip faults, with the former set controlling mineralization at Los Letreros and probably those anomalous sectors at Agua Salada Creek, La India and La Flor sectors; (2) NNE-SSW to NNW-SSE oriented structures, and (3) WSW-ENE oriented normal faults and semi-circular features at the NW part of the Property. In general these structures are attenuated or are concealed by younger felsic units. Regional structures, characterized as normal faults (SGM, 2001a), have dominant NW-SE strike.Mineralization
Mineralization on the Property, exposed and exploited at Los Letreros Mine, is hosted within a 30 to 50-metre wide fault-controlled zone that contains multiple northeast-southwest striking, sub-vertical, polymetallic sulphide veins within mid-Tertiary andesitic tuffs. Thus far three sub-parallel veins have been identified within this zone.
Passing from the southeast to the northwest across the zone, the veins are designated Veins A, B and C. Vein A is a Au-Ag-Pb-Zn vein exploited from the El Union underground workings and the surface workings of San Agustin. Vein B is a Ag-Cu-Pb-Zn vein located 10 to 15 metres northwest of Vein A. It was accessed underground from the El Zinc workings. Vein C is a Ag-Cu rich vein, located 25 to 30 metres northwest of Vein B and was accessed from the Siete Leguas and Lumbrera workings.
Mineralization at Colosio-Los Letreros is hosted by a structurally-controlled, sub-vertical system of sulphide-bearing veins with a dominant northeasterly strike, within a package of volcanic rocks. On the basis of surface and underground observations it is inferred that the width of the mineralized system varies from 20 to 50 m. Three sulphide-vein bearing structures have been recognized (named Veins A, B and C), each up to few metres wide, consisting of thin stringer veins of massive sulphides (sphalerite, galena, pyrite, silver sulphides) hosted in strongly tectonized/altered host rock. The mineralized system can be traced along strike for many hundreds of metres and its continuity downwards remains to be established. Volcanic host rocks proximal to the polymetallic mineralized systems - in the range of few tens of metres wide - exhibit the effects of propylitic alteration (chlorite-clays-calcite±epidote) with sulphide disseminations (0.5 to 1%). These altered zones locally contain significant enrichments of silver and base metals and may provide a wider, and previously untested, target for future exploration.
A 0.4-metre long chip sample from the La Union workings, graded 870 g/t Ag, 8.8% Pb, and 1.89% Zn. At the El Zinc workings, a 0.4 m chip sample collected across the B-Vein structure returned 168 g/t Ag, 0.2% Cu, 2.75% Pb, 8.75% Zn.
The San Agustín workings are interpreted to be the surface expression of the Vein A structure at the Los Letreros Mine. Here, a 1 metre x 1 metre panel sample returned 7308 g/t Ag, 0.36 g/t Au 6.78% Cu, 5.76% Pb and 12.8% Zn.
The mineralized structures at the Los Letreros Mine typify shallow and structurally controlled polymetallic systems on which brittle structures such as faults and fractures served as conduits for mineralized hydrothermal solutions. According to the classification scheme proposed by Camprubí and Albinson (2006) for epithermal deposits in Mexico, the mineralized system at Los Letreros can be ascribed to intermediate sulphidation epithermal systems rich in base metals. These mineral deposits tend to be associated with andesitic-dacitic stratovolcanoes and may extend many hundreds of metres in their vertical dimension.
The interpretation of the formation of the mineralization at Los Letreros in the upper parts of a stratovolcano has important implications for future exploration. Mineral deposits related to intrusions and stratovolcanoes commonly occur at all depths of formation -- shallow, intermediate and deep. As metal-rich magmatic-hydrothermal fluids pass upwards from a deep-seated heat source towards the paleosurface, they form a variety of deposit styles; porphyry Cu-Au and skarn deposits at deeper levels, vein and epithermal Au-Ag deposits at intermediate to shallow levels. Given this spectrum of styles of mineralization within volcanic-related hydrothermal systems, there is a good potential for discovery of additional epithermal-style mineralization at greater depths at the Colosio Property. While there is no guarantee that such mineralization exists on the Property, the shallow level of formation of the Los Letreros veins suggests that the rocks at deeper levels below this mineralization provide a prospective exploration target.
