Summary:
Mineralization at the Cordero Project is polymetallic (Pb, Zn, Ag, Au, Cu) and occurs in a large CRD +/- skarn. The oldest mineralization at Cordero is replacement calc-silicate skarn with Zn-Cu and lesser Pb- Ag, considered spatially and genetically related to vertically extensive Eocene-age quartz monzonite intrusion(s) at Sanson recently dated at ~ 38Ma (U-Pb zircons at 38.02 +/- 0.53 Ma), an age close to the molybdenite mineralization that crosscuts it at ~ 38Ma (La Ceniza Re-Os on molybdenite at 38.50 +/- 0.16 Ma). Alteration envelopes composed of adularia, sanidine, K-feldspar, white micas to high-grade silver-rich mineralization at Pozo de Plata and elsewhere within the current resource pit places the alteration associated with high grade silver-base metal mineralization at ~36 to 38 Ma (adularia 40Ar/39Ar isochron age of 37.56 ± 0.04 Ma (2s, MSWD = 1.44) from Pozo de Plata and alkali feldspar, sanidine, white micas returned age dates ~36 to 38 Ma. These results suggests that mineralization taken from widely spaced locations within the current resource pit are temporally and likely genetically related.
The Cordero deposit massive sulfides formed at contacts of reactive wall rock with rhyodacite laccolith/sill complex that transition to veinlet/disseminate within these high-level intrusions. Alteration associated with mineralization is typically phyllic (+/- adularia) in faults/fractures discordant (crosscutting stratigraphy) as well as concordant (parallel to stratigraphy) in bedding parallel faults, some along fold axes. Replacement style Zn-Cu mineralization in calc-silicate skarn is dominant at the northeast end of the current resource pit with cross-cutting Zn-Pb and quartz Mo+/-Cu veinlet mineralization.
Regionally, Cordero lies in an area where sedimentary rocks of the Eastern Basin and Range geological province meet the volcanic rocks of the Sierra Madre Occidental province. The tectonic and magmatic history of the Sierra Madre Occidental (Tertiary Volcanic Province) is thought to extend into parts of eastern and southern Chihuahua as far south as Cordero where the landscape is dominated by Oligocene- Miocene basaltic-andesites, Oligocene ignimbrites, and Eocene volcanic and intrusive rocks (Ferrari et al., 2007). There are three major southwest to northeast magmatichydrothermal belts that crosscut the Cordero property subparallel to major transcurrent faults in the area. Other faults in the area include reverse (compressional), extensional and normal faults.
The relationship at Cordero between structural, stratigraphic, magmatic, and geochemical characteristics is complex. The focus of drilling in the current resource area in the past decade has been along the central Cordero magmatichydrothermal belt comprised of high-K felsic to intermediate igneous rocks and related breccias, locally forming resistant silicified structural domes bisected by a series of sub-parallel transcurrent mineralized structural corridors (e.g., Cordero, Parcionera, Josefina and Todo Santos) to name a few. The Cordero structural corridor has uniquely been exploited by a sheeted dyke complex that can be followed for at least 3 km from Pozo de Plata in the southwest to La Boquilla in the northeast and beyond. Several NNW-trending reverse faults have severely deformed the sediments and several parallel NW-trending normal faults (e.g., Mega and Southwest faults) have offset the sedimentary and igneous rock package down to the southwest in a stair-step fashion.
Metal tenor, mineralization style and associated alteration changes from La Ceniza and Sanson in the northeast where replacement style Zn-Cu (Ag-Pb) calc silicate skarn cut by quartz molybdenite- (chalcopyrite) stockwork has recently been defined in several deep drill holes including C23-760 to a downhole depth of 1700.9 m. In sharp contrast the Pozo de Plata breccia complex in the southwest is dominated by veinlet, disseminate, and open-space vein breccia silverlead-(zinc) mineralization where gold grades are higher lacking calc-silicate skarn.
Historical small-scale mining was focused on NE-trending Ag-Pb-Zn mineralized structural corridors comprised of vein, vein breccia, stockwork, and mill breccias that bisect earlier intrusions and associated calc-silicate skarn alteration/mineralization. At the Pozo de Plata breccia complex higher gold grades are associated with the interface between galena-pyrite in electrum and spatially associated with silver tellurides. Favoured mineralization sites include a variety of breccias derived from differing mechanisms including contact breccia, intrusive breccia, mill breccia, mud/phreatic breccia, fault breccia and sedimentary collapse breccia as well as mineralization in disseminate, vein selvage, and open space breccia cement.
The precious and base metal mineralization is spatially associated with sulphide minerals such as pyrite, argentiferous galena (the main silver-bearing phase), sphalerite (both iron-rich and iron-poor), and chalcopyrite as well as pyrargyrite, hessite, tetrahedrite, rare electrum and PGM's. Weathering has created a near-surface oxide layer, locally up to 40 m in thickness, where sulphide minerals are generally absent and precious metals including silver and gold are elevated in grade.
Cordero has characteristics of contrasting paleo-levels juxtaposing different temperatures of emplacement. The southwest part of the resource pit presents as intermediate temperature of formation (e.g., sulfosalt dominant) in shales and calcareous siltstones with similarities to some extensional (E-type) intermediate sulphidation epithermal systems. The majority of Cordero presents as both intermediate temperature Ag-Au-Pb > Zn (open-space breccia, sulfosalt-dominant to high-temperature Zn- Pb-Ag+/-Cu+/-Au+/- Mo CRD-skarn, a magmatic-hydrothermal system directly related to the emplacement of quartz monzonite and associated intrusions. This deposit type comprises many economically important deposits located throughout the Cordillera of North and South America and are attractive exploration targets. Northeast Cordero mineralization is characterized by extensive Zn-Cu calc-silicate skarn forming annular metamorphic aureoles around exposed and buried quartz monzonite intrusions as well as Pb-Zn +/- Ag mineralization along fluid escape structures (e.g., Cordero and associated faults). In contrast, southwest Cordero is characterized by massive sulphide Pb-Zn (Ag-Cu-Au) replacements (mantos) forming parallel to favorable stratigraphy at rhyodacite sill contacts and sulphides in crosscutting veinlet/vein breccia/stockwork networks at high angles to sill contacts.
Structurally controlled and replacement-style mineralization is exposed at surface in the current resource pit and extends for 3.3 km along strike, and to a known depth of 1700 m below surface. Mineralization styles include vein, vein breccia (open-space fill), stockwork, massive sulphide and disseminate in rhyodacite associated with phyllic alteration forming along several parallel NE-trending structural corridors (e.g., Cordero, Parcionera and Josefina faults). High grade intervals form over several 10’s of meters, contained in a lower to moderate grade interval up to 100 meters. Massive sulphide and sulphide-cemented open-space breccia is common at the interface of rhyodacite, and shale/siltstone contacts forming high grade lenses along sill or laccolith contacts and can extend for several 10’s of meters. Cordero mineralization has been defined along strike for 3.3 km to a known depth of 1700.9m primarily hosted in skarn horizons in favorable stratigraphy.