Geology and Mineralization
The Las Minas area is located at the intersection between the Sierra Madre Oriental volcanic province and the Trans-Mexican Volcanic Belt. The region is locally underlain by a Cretaceous carbonate platform which has been intruded by mid-Tertiary (and younger) sills and dikes. The cretaceous rocks have been covered by up to 200 metres of pyroclastic flows, mostly related to the Trans-Mexican Volcanic Belt, which lies on the southern flank of the region. The whole is underlain by a proterozoic crystalline basement of generally high metamorphic grade.
The package of cretaceous carbonate rocks and plutonic intrusives is the horizon of economic interest. Up to 1000m of its stratigraphy is intermittently exposed by the erosion of deep canyons between the high volcanic plains and the low-lying coastal strip. Many of these canyons mark the presence of structures within the carbonate platform, some of which can be traced for tens of kilometres. The carbonate rocks are relatively pure and only weakly deformed, with locally extensive marble development up to 500m in thickness and skarn of up to 200m when in contact with large volumes of plutonic rock. The most prominent and economically important intrusive lithology in the Las Minas area is a highly evolved, medium grained dioritic phase composed of calcium-rich plagioclase, generally 10 to 15% hornblende +/- biotite and various Fe-Ti oxides. On the Mexican Gold claims this dioritic rock forms a thick sill underlying the entire claim area, a stock complex over 2km in diameter, and numerous smaller dikes and sills. All of these types of intrusive features have Au-Cu-Ag mineralized skarn and other mineralization styles and commodities associated with them. The historic mines and showings on the Mexican Gold claims are found, regardless of commodity affiliation, over 800m of stratigraphy. The system is interpreted as a mid-crustal, high temperature environment.
The predominant Au-Cu-Ag-magnetite skarn is described in detail in the Las Minas Deposits section. Most of the larger historic mines targeted this deposit type. It occurs both on sill and dike contacts where it is respectively thicker or richer. The sill-contact mineralization is centred around the intersection line of steep structures within the carbonate and the contact. Another environment (Santa Cruz, San Antonio) appears as lateral ‘flares’ in prepared ground attached to the interpreted stock complex.
Zn-Pb-Ag skarn mineralization has been sampled on dike contacts in several locations and in one historic mine with over 200m of workings. These showings do not point toward any definitive metals zoning, either vertically or laterally, in the claim area.
One of the larger and better known historic operations (known as La Miqueta) mined multiple narrow, steep, Au-bearing quartz veins within a silicified biotite-carbonate schist developed in limestone just above the major sill contact. Several smaller workings mined quartz veins in narrow structures within the dioritic sill itself proximal to its upper surface, where recent sample assaying up to 54.1g/t and 62.2 g/t Au over 1m have been taken. This deposit type is not yet well-understood, and surface work in the area was only initiated in 2017.