Summary:
Deposit Types
All mineralization at Cozamin occurs in veins, and fracture-controlled systems of veinlets. Currently mined mineralization at Cozamin is best described as an intermediate sulphidation system. The copper-rich intermediate sulphidation mineralization is an early phase that is enveloped, overprinted or brecciated by zinc-rich intermediate sulphidation mineralization. The copper veins are inferred to be higher temperature, have significantly fewer vugs and can be massive pyrrhotite-pyrite-chalcopyrite with little gangue. Zinc-rich veins also tend to be sulphide rich, like the copper-rich ones, but with slightly more gangue. Well-banded quartz, or quartzcarbonate veins are inferred to be lower temperature and best classified as low sulphidation. They often have open space filling textures with quartz druse vug linings and typically gold and silver rich with lesser base metals and are generally not being mined today, but were historically important.
This transition from intermediate sulphidation copper-dominant mineralization to intermediate sulphidation zinc-dominant mineralization is thought to be the result of an evolving, telescoped hydrothermal system. Blocks or fragments of massive chalcopyrite-pyrite-pyrrhotite mineralization enveloped by zinc-dominant mineralization are observed in drill core and in mine workings. This telescoping system is closely associated with the district’s largest center of rhyolite flow domes which may be the shallow expression of a hidden, inferred buried felsic stock.
Mineralization
Cozamin’s dominant mineralized vein systems include the MNV and the MNFWZ. On surface, the MNV was mapped for 5.5 km across the property. It strikes approximately EW and dips on average at 60° to the N. There are several shafts that provide access to the historical workings at Cozamin. The largest historical mined area is San Roberto with a strike length of 1.4 km, and the second largest mined area is San Rafael mine with a strike length of 0.5 km. Mineralization peripheral to these workings was the principal target of Capstone’s early exploration programs at Cozamin. The MNV system average thickness is five meters. It has been drill tested to an approximate depth of 1,500m, remaining open at depth. The MNFWZ is not exposed at surface, however based on drill testing to an approximate depth of 1,450m, it strikes approximately 145° over a length of more than 2.2 km and dips on average 54° to the NE. Thickness of mineralization in the MNFWZ areas most prospective for exploration varies from 0.5m to 15m true width. MNFWZ mineralization remains open locally up-dip, down-dip, and along strike to the east and west.
The MNV system occupies a system of anastomosing faults. The mineralized bodies within the Mala Noche Fault System appear to be strongest where the individual faults coalesce into a single fault zone. Results from exploration and mine development to date indicate that some of the strongest mineralization in the San Roberto mine on the MNV system plunges to the west at approximately -50° within the vein. Post mineralization offsets of the MNV are minimal and occur along high angle, normal faults that strike northeast. The MNFWZ comprises multiple veins in close spatial association with rhyolite dikes and locally cross-cut the intrusions themselves. The strongest mineralization at the MNFWZ plunges to the northwest at approximately -10° within the vein. The relative age of the copper mineralization ranges from contemporaneous with to perhaps slightly post the rhyolite magmatism. Similar to the MNV, post mineralization offsets at the MNFWZ are minimal and occur along high angle normal faults.
Mineralization in the MNV at Cozamin appears to have been episodic. Intermediate sulphidation pyrite-pyrrhotite-chalcopyrite dominant mineralization is enveloped, overprinted or brecciated by younger sphalerite dominant intermediate sulphidation epithermal alteration and mineralization in a telescoped, intrusive related hydrothermal system. Well-banded quartz, or quartz-carbonate veins, best classified as low sulphidation are also observed but are generally volumetrically insignificant in the area of the mine. These veins have open space filling textures with quartz druse vug linings. The MNV in the San Roberto mine workings shows contained sulphides to occur as disseminations, bands and masses. The San Roberto area hosts both the copper dominant and zinc dominant epithermal events, whereas the San Rafael area is only associated with the zinc dominant epithermal event. Both events are also present at MNFWZ. Conclusions about mineralization styles are based on observations in drill core and the exposure of the coppersilver phase of mineralization in mine workings, however a large portion of the upper parts of the mine are not accessible.
Pyrite is the dominant vein sulphide and typically comprises approximately 15% of the MNV in the San Roberto mine. It occurs as fine disseminations and veinlets, coarse crystalline replacements, and pseudomorphs of epithermal textured carbonate minerals and possible barite. Arsenopyrite locally occurs as minor, microscopic inclusions in pyrite. Pyrite content in the MNFWZ veins is typically greater than 20%.
Pyrrhotite is the second most common sulphide mineral but is present only in the intermediate and deeper levels of the San Roberto mine, and the up-dip portion of the MNFWZ. It occurs as replacement masses, pseudomorphs of platy masses and acicular replacements probably after amphibole. Pyrrhotite commonly occurs as an envelope to, or intermixed with, strong chalcopyrite mineralization. Pyrrhotite ranges from monoclinic to hexagonal, or a combination of these polytypes.
Chalcopyrite is the only copper sulphide recognized visually at Cozamin. Like pyrrhotite, it is more common at the intermediate and deeper levels of the mine. It occurs as disseminations, veinlets and replacement masses. These masses appear to be fractured and brecciated at intermediate levels in the mine. Mineralization at the MNFWZ in the copper dominant veins is chalcopyrite dominant in contrast to the polymetallic nature of the main MNV.
Sphalerite is the most common economic sulphide in the zinc-dominant areas of MNFWZ, such as Vein 10-southeast, and in MNV’s San Roberto-Zinc zone and San Rafael. Most of the sphalerite is marmatitic. It occurs as disseminations and coarse crystalline masses and is commonly marginal to the chalcopyrite-dominant portion of the vein. Sphalerite is locally present in the MNFWZ copper dominant veins, shifting to the dominant sulphide in the zinc dominant veins.
Franklinite, a zinc oxide in the spinel group of minerals, accounts for some of the zinc mineralization in the MNV. Recovery of zinc is lower in areas of franklinite mineralization.
Galena is less common than sphalerite but is generally associated with it. Where it is abundant, it occurs as coarse crystalline replacement masses. Both coarse and fine crystalline masses of galena are argentiferous.
Argentite is the most common silver mineral. It has been identified microscopically occurring as inclusions in chalcopyrite and pyrite. Assays indicate that silver is also probably present in sphalerite and galena. Bismuth and silver selenides occur as inclusions predominantly in chalcopyrite and pyrite.
At MNV and MNFWZ, moderate propylitic wall rock alteration is generally limited to 3 m into the hangingwall and footwall. The main gangue minerals are quartz and calcite, and in some cases rhodochrosite, gypsum, barite, or ilvaite. The quartz occurs as coarse-grained druse crystalline masses, and cross-cutting quartz veinlets.