Summary:
ELG Deposits
Gold mineralization at ELG occurs in association with a skarn body that was developed along a 2 km long corridor following the northeast contact of the ELG granodiorite stock. The skarn zone occurs at the marble stratigraphic level of the Morelos Formation is in contact with hornfelsed sedimentary rocks of the Mezcala Formation. At El Limón skarn mineralization is also structurally controlled by NE-SW and WNW-ESE trending faults and fractures zones. Skarn alteration and mineralization at ELG are fairly typical of calcic gold-skarn systems. Zones of coarse, massive, garnetdominant skarn appear within and along the stock margin, with fine-grained pyroxene-dominant skarn more common at greater distances from the contact with the stock. Significant gold mineralization at ELG is associated with the skarn, preferentially occurring in pyroxene-rich exoskarn but also hosted in garnet-rich endoskarn that has been affected by retrograde alteration.
Pre and post mineralization dykes and sills are found to crosscut the hornfels and marble, along the structural trends mentioned above, and spatially associated with the skarn formation.
El Limón Pit
The main El Limón intrusion consists of an approximately peanut shaped stock of granodiorite composition, which is approximately 6 km long by 2.5 km wide and has a general elongation of N45W. Usually, the skarn is developed along the contacts with this stock, although the important bodies are controlled by major northwest and northeast structures coincident with the Cuautla Formation position and the intrusive contacts. The contact of the intrusion at El Limón, although irregular, is generally quite steep and almost perpendicular to bedding. At El Limón the skarn zone is divided in two structural domains limited by La Flaca Fault, a steeply dipping northeast trending fault zone. Skarn north of the La Flaca Fault is exposed on surface, trends north–northwest for approximately 700 m and dips 40º to 70º to the southwest. Typically, gold mineralization occurs within the main skarn body that developed at the marble–hornfels boundary. Irregular mineralized lenses of skarn also occur in the hanging wall hornfels. Centimeter scale fractures with skarn development are common in the hanging wall hornfels. Skarn south of the La Flaca fault extends southeast for about 800 m. The strike of the skarn is generally north northeast and dips gently to moderately northwest, and is primarily demarcated by drilling. In the contact zone between El Limón Norte and El Limón Sur, high grade mineralized skarn is developed, controlled by steeply dipping faults and fractures part of the La Flaca structural system. High-grade mineralization is also localized in the intersection of La Flaca fault zone and the west northwest structural trend (Ropecon Fault).
El Limón Sur
The El Limón Sur skarn occurs approximately 1 km south of the main El Limón skarn deposit and crops out on a steep ridge extending down the mountain towards the Balsas River. The El Limón Sur area is underlain by a similar stratigraphic succession as the southeastern portion of the El Limón deposit. In general, marbleized and hornfelsed sedimentary rocks are in contact with the El Limón granodiorite intrusive and pre/syn mineralization FP dykes. Pyroxene-garnet skarn occurs along the contact between hornfels or marble and granodiorite. There are two main areas of near-surface gold mineralization at El Limón Sur that are separated by a zone of mostly barren granodiorite. The northernmost mineralized area is developed in the contact between hornfels and granodiorite, trending northnorthwest for about 100 m and dips 50º to the southwest with widths ranging from 15 m to 40 m. The mineralization is characterized by retrograde-altered exoskarn containing sulfides and local argillic alteration. The southern mineralized area is smaller in extent but wider and dominated by endoskarn along with hydrothermal breccias hosted in the granodiorite and locally in contact with marbles and granodiorite. The hydrothermal breccias are developed within skarn and often display thin laminations and size-graded layering. In both areas, the skarn and the mineralization are controlled by a northeast trending structure hosting an FP dyke with endoskarn alteration along its margins. Best skarn development is located at the intersection of northeast and west-northwest structural trends. The mineralized zones are strongly oxidized in the near surface.
Guajes East
The Guajes East skarn zone is developed in the same lithologies on the opposite side of the same intrusion that is present at El Limón. Drilling indicates the skarn development at Guajes East is 300 m wide, up to 90 m thick, and is continuous along at least 600 m of the northwest edge of the intrusion.
At Guajes East, the intrusion underlies the sedimentary rocks and dips about 30° to the west, sub-parallel to bedding. There are also a number of shallow-dipping intrusive sills at Guajes that crosscut the skarn and although they are occasionally mineralized at or near their contacts, for the most part, the sills are non-mineralized. As of the end of March 2018, the Guajes East zone has been mined out.
Guajes West
A block of granodiorite that has been strongly altered to kaolinite, sericite, pyrite and carbonate with some brecciated and silicified portions, forms the hanging wall of the Amarilla fault, which can be traced along a distance of more than 2.5 km from the Balsas River to the Guajes West area. The fault, which strikes N30-40E and dips from 40º to 60º to the northwest, occurs 20 m to 50 m above the mineralization. Mineralization at Guajes West does not crop out and was discovered based on the El Limón geological model.
Mineralization
Gold and silver mineralization at El Limón and Guajes extends over 1,700 m along strike with widths up to 90 m. Mineralization at El Limón has been intercepted to a depth of 470 m from surface and intercepted at Guajes to a depth of 300 m from surface. The deepest mineralization known to date was intercepted to a depth of 1,100 m from the surface underneath the southern portion of El Limón skarn.
Gold and silver occurs most often with early sulphide mineralization but also with late carbonate, quartz, and adularia. Native gold most commonly occurs in close association with bismuth and bismuth tellurides but also occurs with chalcopyrite and as inclusions in arsenopyrite. The gold associated with bismuth tellurides is extremely fine-grained, in the range of a few micrometers to some tens of micrometers.
The dominant sulphides are pyrrhotite and pyrite with lesser but locally abundant amounts of chalcopyrite and arsenopyrite occurring in veinlets and open-space fillings. Petrographic studies indicate that pyrrhotite commonly has been partially replaced by a mixture of pyrite-marcasite, although the earliest pyrite is replaced by pyrrhotite.
Minor amounts of sphalerite and molybdenite are also present. Sphalerite tends to occur with, or as inclusions in, chalcopyrite. Molybdenite, although spatially closely associated with sulphides, usually is free in gangue and occurs as small laths and bent lamellae in the 20–50 µm size range. Coarse-grained stibnite along surface cavities has been found along some holes drilled in the east portion of the El Limón skarn.
Sub-Sill and ELD
The Sub-Sill and El Limón Deep (ELD) mineralization is associated with the ELG UG mining resources and reserves.
Mineralization at the Sub-Sill and at ELD deposits is primarily gold associated with bismuth minerals and variable contents of silver and copper. Gold occurs in low and high sulfidized pyrrhotite rich skarns, while silver and copper mineralization is primarily determined by the degree of sulfidation of the host skarn.