Summary:
The Media Luna deposit is located on the south side of the Balsas River, ~7 km south south-west of the ELG Mine Complex.
Systematic drilling has identified a gold-copper-silver mineralized skarn with approximate dimensions of 1.4 km x 1.2 km and ranging from 4 m to greater than 70 m in thickness. The mineralization occurs in most parts around 500 m below surface but crops out at surface at the north side of the Media Luna ridge. Skarn alteration and associated mineralization is open on the east, southeast, southwest, and northwest margins of the area.
The surface geology of the Media Luna area is dominated by Morelos Formation limestone, which is intruded by numerous porphyry dykes and sills. The contact between the Morelos Formation and the El Limón granodiorite stock is exposed in the northern sector of the area, and Mezcala Formation sediments are present in the southern portion of the area. Morelos Formation limestone is typically converted to grey to white marble along with the intrusive contacts, often accompanied by clay and iron-oxide. The Mezcala Formation is locally converted to biotite–hornfels where cut by dykes and sills. The mineralization hosting skarn occurs at the contact of the El Limón granodiorite with the Morelos limestone. The skarn, like the wall rocks, is intruded by numerous porphyritic dykes and sills.
Media Luna is a magnesian skarn and has formed where rocks are more dolomitic, whereas ELG is a calcic skarn. Reflecting the protolith, a calcic skarn contains garnet, pyroxene, and pyrrhotite, whereas a magnesian skarn contains Mg-rich minerals like olivine, periclase, chondrodite, phlogopite, ludwigite, vesuvianite, talc, serpentine, and magnetite. This difference in mineralogy has several important implications. First, the magnesian protolith enables the formation of Mg-rich minerals and forces the iron that would have gone into garnet and pyroxene in a calcic skarn system, to precipitate as magnetite. In a calcic skarn, the excess iron will precipitate as pyrrhotite in a reduced system or pyrite in a more oxidized system. Second, precipitation of magnetite at relatively high temperature means that the available sulfur that would have gone into pyrrhotite formation can precipitate Cu as chalcopyrite from fluids that are present in the skarn environment. This explains why, although the gold contents of ELG and Media Luna are similar, Media Luna has significant Cu, whereas ELG does not.
Across the entire Media Luna deposit, gold is geochemically strongly correlated with bismuth and tellurium. Gold commonly occurs as native Au as well as gold-rich electrum. Chalcopyrite is the principal Cu mineral in the deposit. The Zn–Fe–S system is represented by sphalerite, sulfosalt, galena and Ag-Fe–S rich minerals, such as argentopyrite.
Sulphide minerals including pyrrhotite, chalcopyrite and sphalerite, as well as native metals and bismuth minerals, are intergrown with retrograde amphibole and are thought to have formed shortly after or at the same time as amphibole. There are areas of early pyrrhotite that have patches of ragged and porous-looking pyrite; some even show lamellae of marcasite forming in pyrrhotite. Late pyrite appears to be associated with darker colored chlorite, typically yellowbrown to brownish-green. Chlorite commonly occurs along with garnet or pyroxene grain boundaries or as cross-cutting veinlets with calcite. Late pyrrhotite and pyrite appear to have formed under a reduced state of oxidation.
In general, elevated gold grades (Au-As, Au-Bi) are found in the hanging wall of the skarn package, and copper mineralization dominates along the footwall. These domains commonly overlap near the major dykes. This zonation is more evident on the central-south portion of the deposit. Zn rich domains are constrained to the northern edge of ML and associated with dykes. These dykes are currently interpreted as the main feeders of the mineralization at the deposit scale.
Generally, the skarn contacts are very sharp, with a transition zone of only a few centimeters. Minerals at these locations are very fine-grained and almost impossible to identify in hand samples. Moving across the contact from veined marble to skarn, the mineralogy is dominated by phlogopite and magnetite for 2 to 10 cm before visible pyroxene and garnet appear.