Summary:
The Cerro del Gallo copper-gold-silver deposit can be considered to be a member of a distinct subclass of “reduced” porphyry-style copper-gold mineralization as first proposed by Rowins (2000c). These reduced porphyry copper-gold deposits lack primary hematite, magnetite and sulfate minerals, but contain abundant hypogene pyrrhotite, commonly have carbonic ore fluids, and are associated with ilmenite-bearing, reduced I-type granitoids (Rowins 2000c). Cerro del Gallo displays all these features. In addition, there is a consistent pattern of higher temperature potassic alteration overprinted by lower temperature propylitic-style mineral alteration. Propylitic alteration boundaries are gradational and irregular, and more widespread than potassic alteration.
The Cerro del Gallo copper-gold-silver deposit also has characteristics supporting an intrusion related gold system (IRGS) model as proposed by Thompson et al 1999; Rowins 2000c; Champion 2005; Hart 2005. IRGS deposits are typically found in continental tectonic settings inboard of convergent plate boundaries, often where the regional metallogeny is characterized by tungsten-tin magmatic provinces. Felsic intrusives have an intermediate oxidation state between ilmenite and magnetite series with a gold-enriched metal assemblage derived from igneous fractionation, and a distinctive metallogenic signature of gold, bismuth, tin and tungsten. Hydrothermal fluids are carbonic, and pyrrhotite is common.
The Cerro del Gallo Project has several epithermal veining systems, one of which, the Ave de Gracia, transects Cerro del Gallo. None of the vein systems has had chemical determination for classification of sulfidation type, although there are several geological characteristics similar to low sulfidation epithermal deposits, all of them determined by geological mapping and geological description. Some of these characteristics are: a) sericite or illite ± adularia, chlorite alteration minerals; b) filling cavities/porosities vein type, banding and hydrothermal breccias; c) carbonate replacement textures; d) low content of bulk sulfur, low presence base metals (Pb, Zn).
In and around the Cerro del Gallo deposit there is also massive sulfide mineralization, however, these occurrences need further investigation. These rocks are composed of more than 60% sulfides, with variable quantities of pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite and galena and are normally strata bound unless remobilized. In the earlier logging programs these occurrences of massive sulfides were described as skarns.
The main copper-gold mineralizing event is considered to be intrusive related and referred to as a copper-gold porphyry system. Within the district there are also younger epithermal vein systems high in silver and lead.
At Cerro del Gallo mineralization is hosted in both felsic intrusive and volcanic tuff wall-rock. Mineralization is disseminated and vein or fracture controlled and extends from 200 meters to 400 meters outward from the mineralizing intrusive complex.
The strongest gold mineralization at Cerro del Gallo is associated with intense quartz stockwork veining and silicification within a wall-rock annulus forming the outer limits of a felsic stock with the system losing intensity outward with a decrease in stockwork and quartz veining density.
Sulfide make up less than 2% by volume of the mineralized rocks. Gold-copper mineralization is zoned concentrically around the felsic intrusive with higher grade gold mineralization proximal to and within an outer annulus of the intrusion. The highest copper grades are found outward from the gold zone. Zinc mineralization is locally anomalous outside the copper zone, metal zonation boundaries are gradational and there is an overlap in the gold-copper zone and the copper-zinc zone.
Pyrite is the dominant iron sulfide mineral with accessory pyrrhotite and marcasite dispersed throughout the mineralized system. Pyrite occurs in two forms; as euhedral to subhedral cubic crystals and crystal aggregates in veins and disseminations of primary origin, and also as secondary fine-grained patches after pyrrhotite (Mason 2005a). Pyrrhotite is most common in the outer copper zone. It is magnetic and in high enough concentration to form a donut shaped magnetic anomaly around the Cerro del Gallo mineral system.
Native gold occurs within vein quartz and inclusions within pyrite, chalcopyrite and bismuthinite. Gold grains range in size from generally 0.5-4 microns in diameter to rarely 10-20 microns (Mason 2005a; Mason 2006a; Townend 2006). Electron-probe microanalysis on a limited number of native gold grains indicates gold has a fineness of 860-880 with silver making up the rest (Mason 2006b, Mason 2007). In terms of geochemistry and all elements assayed, gold has the strongest correlation with bismuth.
The majority of silver at Cerro del Gallo is related to late structurally controlled epithermal veins that overprint the intrusive related copper-gold system. Within the veins, Perez (2018) identified tetrahedrite and electrum as the main silver minerals associated with galena and sphalerite. Silver can also occur as small grains of native silver or ruby silver as either pyrargyrite or proustite.
Chalcopyrite is the most common base metal sulfide phase and occurs in fracture veinlets often associated with intense silicification and as disseminations. Chalcopyrite is commonly closely associated with pyrite and marcasite, and rarely as inclusions in coarse arsenopyrite (Townend 2006). Traces of bornite have also been reported (Mason 2006b; Townend 2006), however this mineral is not volumetrically significant.
Minor secondary copper minerals including malachite and azurite have formed through weathering processes and are locally present in surface outcrops. Native copper, covellite and chalcocite are found deeper in the regolith profile, and their formation is attributed to supergene weathering processes.
Arsenopyrite is relatively common, occurring as coarse discrete grains often associated with chalcopyrite.