Summary:
The Skarn Project mineralization differs significantly from that of the La Colorada Vein Mine; it is more geologically complex, and its distribution is more dispersed. In general, localized higher-grade zones are interspersed among bulk lower-grade zones, as is typical of skarn deposits. No definitive structural boundaries are currently modelled. The deposit remains open in most directions and is subject to ongoing exploration and delineation drill programs. It is assumed for the PEA mining inventory that all necessary permits and approvals would be obtained after satisfying all relevant criteria and regulations in each jurisdiction.
Deposit Types
The Skarn Project deposit is a typical Mexican porphyry-related skarn system associated with andesitic, dacitic and rhyodacitic intrusive bodies and dykes in contact with limestone and siltstone. Significant economic mineralization occurs in light garnet skarn, light and dark pyroxene skarn and in the collapse breccias dominated by skarn fragments. Garnet and pyroxene skarn zones contain zinc, lead, copper, and silver mineralization. The presence of late andradite garnet and pyroxenes (johansenite and hedenbergite) are distinctive in the calcic skarn found in the Skarn Project and exemplify a zinc-dominant skarn deposit.
Skarn Mineral Paragenesis
Multiple phases and superposition of hydrothermal events suggest that magmatic and hydrothermal activity has continued over time and may have produced possible deeper telescoping systems. Early-phase magmatic activity commenced with the emplacement of a copper-molybdenum porphyry, continued with the formation of a skarn carbon replacement deposits(CRD) deposit, and concluded with the development of epithermal veins, mantos, and hydrothermal breccias.
The skarn mineral paragenetic sequence is defined by the formation of magnetite, hematite I, pyrite, chalcopyrite and molybdenite at the end of the early porphyry phase and continued to the beginning of the late epithermal phase. Sphalerite, galena, tennantite, tetrahedrite and argentite and related clays and iron oxides are exclusively related to the later epithermal phase. (ASPAR November 2019-January 2020.)
Polymetallic Skarn and CRD Mineralization
Faults, primarily formed during the D2 deformation event, acted as a conduit for the intrusion of several porphyries and their associated hydrothermal fluids. To date, two intrusive centres with similar characteristics have been identified: the main intrusive centre located between the 901 and 903 zones of the Skarn Project; and a second centre north of the Skarn Project’s 902 zone.
Emplacement of the intrusions fractured the sedimentary rocks and increasing porosity and permeability of the limestone host rocks which were metamorphosed where in contact with the hot fluids. Additional mineralogic changes resulted from cooling and infiltration of groundwater in the porous areas. Recrystallization and mineralization occur predominantly around intrusive bodies and is associated with intrusive sills, dykes or offshoots, and tends to follow the sedimentary bedding.
The evolution of the skarn system begins with contact metamorphism that in its early phase gives rise to the occurrence of marble, calc-silicate hornfels and recrystallized limestone. Metasomatism progresses to a prograde phase forming garnet, pyroxene and wollastonite and later develops into a retrograde phasethat consists of two stages:
• An early stage associated with epidote, amphiboles, illvanite, vesuvianite, magnetite, hematite and muscovite;
• This is followed by a late stage associated with chlorite, quartz, calcite, rhodonite, and sulphides containing molybdenum, copper, zinc, lead, silver and manganese.
The final stage of skarn development is a distal phase consisting of CRD replacement bodies and veinlets of carbonate with sulphides. CRD deposits are high temperature polymetallic deposits formed in proximity to porphyry systems. Carbonate host rocks are replaced by massive sulphides with mineralogy that varies according to the distance from the intrusion. CRD bodies typically occur in the limestone near vein contacts or above the skarn.
Mineral and Metal Zonation
Skarn minerals include garnets, pyroxenes, pyroxenoids and amphiboles. Almandine and andradite are the most common types of garnets with rare occurrences of grossularite and uvarovite garnet. Well developed zonation of garnet species occurs within the skarn units as a function of proximity to the porphyry intrusion. Zones close to the intrusion are characterized by predominantly dark brown to red almandine garnets with lesser light-coloured pyroxenes. With increasing distance from the causative intrusion, garnet content decreases and the garnet species transitions to a distal zone characterized by dominant dark pyroxenes with lesser green to yellow andradite garnets. In many distal areas, no garnets are visible and the skarn is composed entirely of dark pyroxenes. The zonation follows a general NW-SE trend in the main mineralized zone with a secondary NE-SW trend in the extreme northeast.
Mineralization is associated with sulphides closely associated with skarn development and the onset of hydrothermal alteration of early skarn minerals. Sulphide textures vary from disseminated and patchy to semimassive and massive. Zinc, in the form of sphalerite, is typically yellowish-brown to reddish brown, and is accompanied by galena, pyrite, chalcopyrite and magnetite. CRD mineralization consists of massive to semimassive sulphides.
Zinc, lead, copped, silver and, to a lesser extent, gold grades are strongly associated with host skarn lithology and are inversely proportional to proximity to the intrusion (i.e., grades are lower in close proximity). The CRD and pyroxene skarn units away from the intrusion, are therefore associated with the highest grades while the dark brown to red garnet skarn is associated with the lowest grades. The marble and porphyry units host minor mineralization; with in the limestone mineralization occurs primarily as veins or veinlets.
Metal zonation is present in the system: copper is highest in the intrusive and immediately proximal to the causative intrusion. Molybdenite grade is proportional to copper grade in the porphyry, and iron content follows copper in the proximal dark brown to red garnet skarn. Zinc-lead-silver mineralization occurs in the transition zone between almandine and andradite garnets and grades to lead-zinc-silver towards the distal zone where pyroxenes and CRD predominate and are associated with the highest grades.
Skarn Morphology
The Skarn Project is comprised of several zones of mineralization located between 700 m and 1,900 m below surface, extending some 1,800m in a NE-SW direction and 650m in a NW-SE direction. Skarn geometry is dependent on the shape of the causative intrusion, the composition and orientation of the stratigraphy, and the lithological contacts that generate permeability in the host rock. Economic skarn mineralization is well developed in the retrograde stages in skarn layers ranging from a few centimetres to tens and hundreds of meters thick. The skarn system is currently defined by three zones of economic mineralization with corresponding zone numbers. These are termed the West Zone (902 zone), Central Zone (901 zone), and East Zone (903 zone).