Summary:
Deposit Types
Mineralization on the Tepal property is characteristic of porphyry copper-gold-molydbenum systems. This is interpreted based on its porphyritic host rocks, its alteration, its copper-gold with minor molybdenum (and silver) signature, and its quartz-dominated stockwork veinlets and disseminations associated with sulphide mineralization.
Porphyry copper ± molybdenum ± gold deposits in Mexico are abundant and dominantly occur within a belt on the western side of the country (Valencia-Moreno et al., 2007). The majority are in northwest Mexico, such as the giant Cananea and La Caridad deposits in Sonora. However, multiple porphyry deposits occur in the south, and these, in general, appear to be more copper-gold dominated, with island-arc related signatures (Valencia-Moreno et al., 2007). An example of a porphyry-style deposit nearby to Tepal is La Verde in Michoacán.
Mineralization
Mineralization on the property consists of zones of stockwork quartz veinlets, sulphide veinlets and disseminated sulphide mineralization that are hosted within intrusive rocks, volcanic rocks and breccias. These sulphide-bearing zones contain significant concentrations of copper and gold, and to a lesser extent, molybdenum and silver. The mineralization is hosted in three distinct deposits: the North Zone and South Zone with relatively high-grade copper and gold, and the Tizate Zone with relatively lower-grade copper and gold, but higher-grade molybdenum.
Morphologically, two of the zones, the North Zone and the Tizate Zone, are crudely tabular with shallow to moderate dips. Both have rough dimensions of approximately 1,100 by 600 m and thicknesses of up to 200 m. The South Zone has a smaller footprint, 600 by 500 m, but a greater vertical extent of up to 400 m and dipping steeply to the south.
There is an oxide horizon and a narrow transition layer present above the sulphide mineralization in all deposits on the Tepal property. The depth of oxidation ranges from 20 to 40 m on the hilltops and 0 to 20 m in the drainages. Minerals in the oxidized zone include malachite, chalcocite, azurite, tenorite and minor chrysocolla. Shonk (1994) indicated that a very thin supergene-enriched layer exists locally at the base of the oxide horizon and consists of chalcocite and covellite coatings on sulphide grains and local areas of poddy, massive chalcocite. While minor chalcocite has been noted in drill core, drill hole assays do not indicate any leaching of copper from the oxide horizon or any local copper enrichment zones at the oxide-sulphide interface. The transition zone may be up to 15 m thick; however, it is usually significantly less than this and in some cases is absent altogether. The transition is identified by the overlapping presence of iron oxides and sulphide mineralization.
Primary sulphide mineralization consists dominantly of chalcopyrite and pyrite, with locally significant bornite and molybdenite. The highest consistent grades of copper and gold mineralization are associated with low pyrite/chalcopyrite ratios and increasing bornite. Local areas of very high-grade gold are associated with thicker veins that cross-cut Tizate, and contain pyrrhotite, sphalerite, galena, silver sulphides, as well as chalcopyrite and pyrite.
Micron-sized native gold is usually associated with the chalcopyrite, either as grains attached to the surface or fracture fillings within copper sulphides (Duesing, 1973), although free grains can also occur. Copper mineralization typically occurs as disseminated chalcopyrite and bornite within veinlets and disseminated within altered porphyritic groundmass (Shonk, 1994). Molybdenum mineralization occurs as molybdenite.
Several different generations of veinlets are associated with copper-gold mineralization, and future work will refine this paragenesis. These include early granular dark grey quartz veinlets with sulphides and locally magnetite, thin chlorite-sulphide veinlets, light grey to white quartz-sulphide veinlets, sulphide-only veinlets and sulphide veinlets with chlorite-biotite-muscovite halos. Late-stage unmineralized calcite-dolomite-ankerite veinlets and gypsum-only veinlets cross-cut the mineralized veinlets. The earlier veinlet group of granular dark grey quartz with fine-grained sulphides, as well as granular subhedral to euhedral quartz in the groundmass with fine-grained disseminated sulphides, is the assemblage most associated with copper and gold mineralization.
Intensity of mineralization is strongly related to the density of veining (Shonk 1994). In the North Zone and South Zone, copper and gold values are relatively synchronous. Silver and molybdenum values are also somewhat elevated in the core areas, but distribution is more erratic and is not always coincident with gold or copper values.
In Tizate, copper and gold values are on average, lower than averages in the North Zone and South Zone. Grade distribution, however, is very even. Both the silver and molybdenum values are more significant at Tizate than in the other deposits, and they show greater coincidence with gold and copper, particularly with respect to molybdenum.
Alteration
Alteration in and around the deposits shows features that are typically associated with copper-gold porphyry systems (e.g., alteration facies of potassic and inner propylitic zones within the core with peripheral propylitic halos, and retrograde alteration facies of phyllic and argillic alteration). The type and intensity of these alteration facies varies between the deposits, likely due to a function of depth in the mineralizing system. The overall geometries and thicknesses of these alteration zones are not well defined and future work remains to be done.
Surface alteration from mapping and short-wave infrared spectral analysis shows the North Zone to have a broad propylitic alteration halo (chlorite-epidote) with inner mixed propylitic-sericite (chlorite-epidote-white micas) alteration. More proximal to the deposit is just sericitic alteration (white mica) as well as discrete local zones of argillic-sericitic (mixed clays and white micas) and advanced argillic (alunite-pyrophyllite±vuggy quartz) alteration.
The South Zone at surface has a similar broad propylitic alteration halo as the North Zone; however, the inner propylitic-sericite alteration is much smaller, and the alteration is elongated to the west with a larger argillic-sericitic alteration zone. To the northwest of the South Zone is an area of hot advanced argillic alteration (pyrophyllite-white micas) within the wide argillic zone. This advanced argillic alteration zone could be the roots of lithocap related to the South Zone, or it could be related to a different source altogether.
The Tizate Zone has an alteration halo quite different than the North Zone and South Zone. It sits in broad argillic (clay) alteration with east-northeast to northeast-striking elongated sericitic alteration zones with only some very small discrete advanced argillic alteration areas. Further north of Tizate is a zone of potential greisen and sodic-calcic (albitic) alteration, potentially associated with the large batholith to the north.
In the south of the property, south of the east-northeast-striking fault, there is a block of dominantly sediments, with little to no alteration, except for local skarn-like alteration reported in limestone packages (Shonk, 1994). Likewise, there are relatively unaltered rocks in the far north of the property. Little is known about the alteration in the east of the property due to shallow cover, or to the far west due to lack of mapping and sampling.