GEOLOGY AND MINERALIZATION
The Esperanza Gold Project is located within the physiographic province of the Sierra Madre del Sur, an orogenic belt that extends for around 1100kms NW-SE from the South Pacific coast of Mexico. The Esperanza Gold Project is located at the NW corner of the Mixteco Terrene, which emerges in the central part of the Sierra Madre del Sur, between the Caltepec and Teloloapan faults which represent its limits east and west, respectively. The basement of this terrene consists of deformed metamorphic rocks, represented mainly by migmatites, metasediments, metagranitoids and eclogitized ophiolites, which were grouped into the Acatlan complex metamorphic complex of Grenvillian to Early Triassic age.
Mineralization at the Esperanza Gold Project is associated with the intrusion of a stock of Granodiorite composition into the carbonate rocks of Guerrero-Morelos Platform, specifically the rocks of Xochicalco Formation. Spatially related to the intrusive contact with the carbonate rocks are varying degrees of skarn and marble development. The granodioritic stock, locally referred to as a feldspar porphyry, in hand sample presents a porphyritic texture with the development of phenocrysts of plagioclase (~35% in abundance) and potasic feldspar (~30% in abundance) that can reach up to 3cm in size, phenocrysts of quartz (~25%), and euhedral biotite (~10%) in a fine matrix consisting of very fine quartz-plagioclase. In thin section the plagioclase phenocrysts shows moderate alteration to sericite-clays as well as moderate silicification.
Intruded by the granodioritic stock are the limestone of the Xochicalco Formation of Aptian age (early Cretaceous), that have beds of varying thickness from very thin to medium. The color varies from dark grey to black according to the carbonaceous content or organic matter. Another feature of this formation is the abundance of chert bands. Distal to the intrusive contact the Xochicalco Formation is relatively fresh or unchanged usually having a dark grey colouring due to carbonaceous material, is fine-grained, with moderate calcite veinlet’s as stockwork (<1 mm thick) iron oxides in fractures, and occasional breccia texture that might be related to basin collapse.
Primary mineralization consists of gold, and to a lesser extent silver, associated with the skarn zones spatially related to the intrusive. As noted in the paragenetic mineral sequence the sulfides, iron oxides, and gold are directly associated with retrograde activity. Although sulfides are not commonly observed the abundance of iron oxide indicates that their presence was considerable prior to becoming oxidized. Gold values are often associated with jasperoid that appears to have been post-retrograde. Jasperiod can occur along fractures, in veins, and narrow lenses within the limestone or marble. Jasperoid outcrops from 1m to greater than 30m in thickness have been mapped, although core intercepts generally show that much narrower zones, less than 5m, generally exist. Gold assays in jasperoid have produced grades greater than 12g/t but not all jasperoid contains appreciable gold values, although
they are generally strong anomalous (>100ppb). The greater thicknesses of jasperoid observed at the surface, versus what is found in drill core, may indicate that the more pervasive silica flooding represents the top of the hydrothermal system during jasperoid development.
Intense argillic and/or potassic alteration (clays) and epidote development is common within the intrusive near the skarn contact. Although locally anomalous gold may be associated with this zone, the values are generally less than 0.5 g Au/t and thus far appear to be of little economic importance.