Starcore is engaged in exploring, extracting and processing gold and silver through its wholly-owned subsidiary, Compañia Minera Peña de Bernal, S.A. de C.V. (“Bernal”), which owns the San Martin mine in Queretaro, Mexico.
- subscription is required.
Summary:
The San Martin deposit is composed by a tabular, vein-like subvertical mineralised structure that becomes to a sub-horizontal mineralised structure or “manto-like” close to surface. This mineralised structure is recognized for over 2 km along strike, with thicknesses between 1.5 and 17 metres and 400 m of vertical extent or “favourable zone”. In general, the mineralisation is hosted in the contact of limestone-shale of Soyatal-Mexcala Formation and associated to a silicified rhyolitic dike.
The San Martín gold-silver district hosts classic, medium-grade gold-silver, epithermal vein deposits characterized by low sulphidation mineralization and adularia-sericite alteration. The San Martin veins are typical of most other epithermal silver-gold vein deposits in Mexico in that they are primarily hosted in the Upper Cretaceous black limestone and calcareous shales of the Soyatal-Mexcala Formation. Tertiary Lower Volcanic series of rhyolite flows, pyroclastics and epiclastics, overlain the sediments.
Mineralization at San Martín occurs in association with an epithermal low sulphidation, quartz-carbonate, fracture- filling vein hosted by a structure trending approximately N40°- 60°E, dipping to the 50° to 90° to the southeast.
The San Martin structure has been known in different stages of exploration and has adopted several names, San José, San José II, San Martín, Cuerpo 28, Cuerpo 29, Cuerpo 30 and Cuerpo 31. The structure itself is offset by a series of faults of northeast trending that divides the oreshoots. The structure behaves vertical at the San José and San Martin areas (Tronco) and becomes flatter from Cuerpo 28 to 31 (Mantos), and mineralization follows the planes of the folded rocks.
The San Martin vein itself has been known underground traced for 2 km along trend, with widths between 1.5 to 10 metres and averages approximately 4.0 m. A secondary mineralized vein is located, both in the footwall and hangingwall, of the San Martin vein, on the wetern limb of the local fold that contains the mineralization. This structure is the Santa Elena and represents a good target for exploration to the NE and SE of San Martin.
Mineralization
The mineralization of the San Martin deposit is given in the lithological contact of a massive shalelimestone package. The progressive deformation to the NW generated fracturing and later the continuous compressive deformation will cause stages or episodes of brecciation, with injection of hydrothermal fluids from an intrusive body identified at depth. This limestone sequence is over- thrusting the shales and at its contact a tabular-looking breccia has been generated which has hydrothermal origin. In San Jose and San Martin Areas the white breccia cemented by crystalline quartz and calcite has thicknesses of the order of 5m -20m. Later it presents a silicified zone with the presence of jasper and slight oxidation (halo of decrease in economic values), it has dimensions of 5m-10m, and later in its contact with the sterile limestones can be seen a tectonic or hydraulic gap.
The mineralization has a linear corridor of 3.1km and its general orientation is NNW-SSE, however, in greater detail the main orientation of the mineralized areas is the following: Santa Elena N305°, for the San Jose I area it is N060°, San Jose II is N-S, San Martin N050°, Areas 28, 29, 30, 31 and 32 have an N330° and Area 33 has an E-W orientation. The gap of the main thrust dips to the E but presents a gradual change in its inclination which is more vertical in the southern area of the mine (San Jose I, San Jose II, San Martin Areas) with inclinations of 60°-45°. However, for Areas 28, 29 the inclinations are between 35°-22° and for Areas 30-33 the inclination becomes smaller (<25°).
In the lower areas of the mining infrastructure, particularly in the areas of San Jose I and II, visibly the appearance of the breccia becomes different, for these areas the host rock has a lot to do with it, since the massive contact of the shales is in contact with large lenses of marl and was where the mineralization originated, acquiring aspects of banded clays with white to reddish colors, this area presents light-medium argillic alteration.
Another structural zone related to mineralization occurs in the areas detached from the lithological contact, which appear as duplex-type structural arrangements, which form thin veins of quartz- calcite.
From the San Martin Area to Area 33, there is a horizontal zone of mineralization that the first explorers call it “Manto”, which may correspond to a mineralized stack-type structure, which is made more noticeable by its almost horizontal inclination towards the north of the mine. This area corresponds to the high levels of the thrust and is controlled by a low-angle fault on the roof of the mineralization, frequently in contact with Tertiary volcanic rocks. For this area, it is common to observe areas of hydrothermal-type textures.
The areas that have greater prospecting and development at depth (San Martin, Area 28, Area 29) have something in common, and that is, in their basal areas, the stratigraphy becomes carbonaceous, for this reason the mineral that is located in those levels it appears carbonaceous.