Las Minas Mineral Deposits
El Dorado - Juan Bran
The El Dorado deposit is a consistent, single, flat lying sheet of Au-Cu-(Ag)-magnetite skarn developed on the sub-horizontal contact between the carbonate platform and the regionally extensive dioritic sill described above. The zone is lenticular in cross-section E to W, and has been drilled over an area of >650 by >420m. There is very little variation in the mineral distribution, textures and zoning at El Dorado; only the thickness (4 to 60m, generally 12-30m) and grade varies. The mineralization is almost entirely confined to prograde garnet-(pyroxene) exoskarn, with ore minerals chalcopyrite +/- bornite replacing magnetite or interstitial to coarse garnet grains. Magnetite occurs massive and as well-defined, sub-planar, contact-conformable, centimetric bands in massive garnet or less commonly as randomly curving banding in diopside. Pyrite is a rare, post mineral phase; pyrrhotite and sulfarsenides are not present. Above the very sharp exoskarn front, the carbonate is invariably marble, with only proximal (<10m) and weak calc-silicate alteration. A common feature directly above the deposit is irregularly distributed late to post-mineral -stage hydrothermal breccia, which penetrates intermineral dioritic and endoskarn dikes and sills as well as marble. The breccia is generally matrix-supported in brown to green to red fe-carbonate + hematite, and shows multiple stages of activation, and some dissolution textures when in marble. It very rarely contains anomalous Au values.
A lesser amount of mineralization occurs in endoskarn, as fracture-controlled, remobilized sulphides +/- magnetite, often associated with weak development of hydrous retrograde minerals (epidote-chlorite-vesuvianite). Strong retrograde mineral assemblages, especially with hematite, is destructive of mineralization both in exoskarn and endoskarn. Endoskarn grades downward into the underlying intrusive rock, where alteration including chlorite-saussurite-pyrite and magnetite destruction may persist for 10 to 40m.
The Santa Cruz deposit is a series of dipping, stacked lenses 8-40m thick, located 100 to 150m above the SE portion of the El Dorado. These mineralized lenses occur within a greater package of exo and endoskarn in a corridor approximately 185m wide by 120m thick by 120m deep. Between the two deposits is marble or inter-mineral endoskarn dikes and sills, much of it lying in the intermittent hyrothermal breccia zone. Santa Cruz is exposed on a steep slope along its entire west side, and continues across a small canyon in to the Nopaltepec target. Much of the character of the mineralization at Santa Cruz is very similar to El Dorado, though the former features notable sulphide redistribution (both destruction and re-concentration), likely through the action of multiple interpenetrating dikes and sills of calcic dioite and endoskarn. There is also further suite of silica-poor minerals (olivine-augite-serpentine) proximal to a sheeted dike complex (or stock) which marks the south end of the marble / skarn / mineralization. This vertically-extensive volume of intrusive rock is virtually identical in chemistry, mineralogy, texture and alteration schemes to the dioritic sill underlying the region, and shows mineralized skarn developed on its north contact.
Further mineralization has been intersected at Santa Cruz at drilling depths >150m, and there is evidence on surface of mineralized magnetite skarn above the present drilling envelope. Both of these targets remain to be tested.