Summary:
The Cordero deposit does not neatly fit into a specific category or class of conventional deposit model, due in part to the variability of mineralization type, style, and age across the large size of the project area. Observations from surface mapping and core logging in the resource area alone are consistent with overlapping mineralizing events of different ages. Of the deposit types that have been described and named in the technical literature, those most relevant to Cordero are:
- extensional intermediate sulphidation epithermal systems with many characteristics of the Real de Angeles in Zacatecas and many others across Mexico and the United States;
- carbonate-hosted Pb, Zn (Ag, Cu, Au) manto-style replacement (skarn) and crosscutting chimney-style sulphide mineralization associated with felsic to intermediate intrusive igneous rock and associated breccias formed from differing mechanisms like those at the Santa Eulalia mining district in north central Chihuahua and many others across Mexico and in the United States.
The Cordero property lies along the transition from the deeply incised dominantly volcanic Sierra Madre Occidental Province in the west to the more subdued topography related to the sediments of the eastern Mexican Basin and Range Province. The Cordero deposit is underlain by a pronounced north-northwest (NNW) trending fold and thrust marine sedimentary sequence.
The currently defined Cordero main resource pit is located along a central northeast (NE) trending magmatic-hydrothermal belt, namely the Cordero Porphyry Belt, with dimensions of approximately 2.4 km x 1.4 km. The deposit geology is comprised of mineralized Cenozoic facies, a felsic intrusive complex with a variety of breccias. The country rock is a Cretaceous, thin to medium-bedded, half-carbonate sequence consisting of interbedded calcareous mudstone, limestone, calcareous siltstone, and calcite sandstone.
Faulting is prevalent in the Cordero main deposit area, with evidence of early NNW-trending extensional faulting parallel to the sedimentary stratigraphy as well as late NE-trending transcurrent faulting across the stratigraphy. Two major NNWtrending, steeply SW-dipping extensional faults define the north and south boundaries of the resource pit. The Cordero Main structural corridor emplacement is complex in a transtensional NE-directed stress field with a series of steep NNW-dipping, strike-slip normal faults. A major sinistral (or left lateral) releasing bend has resulted in left lateral slip on the NNW-dipping Cordero Fault and Josefina Fault.
The bedding of the sedimentary package is generally dipping southeast (SE) with moderate to shallow angles. The veining system is predominantly NE-trending and steeply NNW-dipping, consistent with the faulting system. Rock mass fabric (fractures) within the sedimentary and volcanic packages are either parallel or orthogonal to the bedding, or in line with the NE-tending faulting/veining systems.
The Ag-Au-Pb-Zn content at Cordero is in sulphide minerals, with pyrite, sphalerite and galena accounting for the significant majority of metal content; lesser amounts of the metals of potential economic interest are contained in arsenopyrite, chalcopyrite, freibergite, argentite, rare pyrrhotite, and in the sulphosalts tetrahedrite and tennantite.
The primary gangue minerals are Ca-Fe-Mg carbonates and rhodochrosite in Mn-carbonates, adularia, quartz, barite, calcite, sericite, fluorite and chalcedony.
The rocks were altered as hydrothermal fluids percolated through interconnected faults, fractures, stockwork and along permeable lithologic contacts. The principal type of chemical alteration was caused by fluids that removed certain minerals and replaced them with their potassium-bearing cousins: adularia (a potassium-bearing alumino-silicate), potassium feldspars (orthoclase or sanidine), illite (a potassium-bearing clay), and the potassium-bearing micas: muscovite, biotite and phengite. Potassium-rich alteration is widespread throughout the main Cordero magmatic hydrothermal belt and accounts, in part, for the strong coincidence between the potassium spectral band on the radiometric geophysical survey and the intensity of Ag-Au-Pb-Zn mineralization.
Other alteration minerals include chlorite, chalcedony (a micro-crystalline form of reprecipitated silica) and buddingtonite, an ammonium mineral sourced from sedimentary rocks and associated with epithermal deposits.
The other type of alteration observed at Cordero is due to weathering and the percolation of oxygen-rich waters through the near-surface permeable layer. The common alteration minerals at shallow depths are jarosite, (iron hydroxy sulphate) goethite (iron oxyhydroxide), hematite (iron oxide), kaolinite and smectite (swelling clays) and gypsum (hydrated calcium sulphate).
Mineralized fluids from deep intrusions moved up faults and fractures at dilational jogs along a releasing bend, percolating out into the surrounding wallrock. In places, particularly where aperture suddenly increases at lithologic contacts, at dilational jogs or at fault intersections, the fracture density increases forming wider damage (or structural) zones that are better mineralized with wide alteration halos. Some of these zones host veins and vein-breccias in more favourable fluid corridors. Mineralizing fluids were able to penetrate the host rocks away from open fractures, travelling through thin cracks and through the connected permeability of the intrusive rock and the porosity of the sedimentary host rock. The disseminate-style low-grade mineralization extends several hundred meters from the major faults and fault intersections, developing stockwork mineralization with small veinlets crisscrossing to form an inter-connected mineralization network.
In high-grade zones that are dominated by veins and vein-breccias their associated alteration halos and metal grades are continuous in directions parallel to the steeply NW-dipping NE-SW-trending faults.