Summary:
The Cerro del Gallo (CdG) copper-gold-silver deposit is considered to be a member of a distinct subclass of “reduced” porphyry-style copper-gold mineralization as first proposed by Rowins (2000b). 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, 2000b). CdG 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 CdG copper-gold-silver deposit also has characteristics supporting an intrusionrelated gold system (IRGS) model as proposed by Thompson et al. (1999), Rowins (2000b), Champion (2005), and 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 intrusive rocks 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 CdG Project has several epithermal veining systems, one of which, the Ave de Gracia, transects CdG. 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).
Deposit Dimensions
The CdG deposit is approximately 600 m by 300 m in size. Gold mineralization generally follows the intrusive contact and forms a concentric halo up to 75 m in width. Copper mineralization commonly follows the gold mineralization but extends into the intrusive and hornfels (HF) units. Silver mineralization is less controlled by the intrusive contact and a northwesterly striking trend.
Mineralization
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. These are similar to the style of mineralization that makes up the Guanajuato Mining District.
Skarns with associated occurrences of gold, copper, zinc, and lead occur around the periphery of the intrusive. Skarn mineralization is scattered and has only relatively small tonnage potential.
At CdG, mineralization is hosted in both felsic intrusive and volcanic tuff wall-rock. Mineralization is disseminated and vein or fracture controlled and extends from 200 m to 400 m outward from the mineralizing intrusive complex.
The strongest gold mineralization at CdG is associated with intense quartz stockwork development 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 vein density.
Sulfide comprises 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 CdG mineral system. Native gold occurs in quartz veins and as inclusions in pyrite, chalcopyrite and bismuthinite. Gold grains range in size from generally 0.5-4 micrometers in diameter to rarely 10-20 micrometers (Mason, 2005a; Mason, 2006a; Townend, 2006). Electronprobe microanalysis on a limited number of native gold grains indicates gold has a fineness of 860-880 with silver comprising the rest (Mason, 2006b, Mason, 2007).
The majority of silver at CdG 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 locally occurs as small grains of native silver or ruby silver as either pyrargyrite or proustite.
Within the CdG mineral system, 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 were reported (Mason, 2006b; Townend, 2006); however, bornite is not volumetrically significant.
Minor secondary copper minerals including malachite and azurite have formed by weathering of Cu-bearing sulfide minerals 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 and occurs as coarse discrete grains often associated with chalcopyrite.
Alteration
Hydrothermal alteration is zoned concentrically around the main intrusive complex and intensity decreases outwards. Overprinting hydrothermal events result in complex alteration patterns that are difficult to map as discrete areas. Within and outward from the main intrusive there is a general absence of primary textures due to high percentages of silica. Potassic alteration is the dominant style of alteration in the core of the hydrothermal system which is centered on the Main Intrusive.
The tuffaceous wall rocks in contact with the intrusive system have experienced pervasive hydrothermal alteration to form fine grained replacement assemblages of albite, Kfeldspar, biotite, sericite, quartz, sphene, rutile and pyrite (Mason, 2005b; Mason, 2007). Much of the mineralization is fracture controlled and related to K-feldspar, biotite, pyrrhotite, and chalcopyrite, but locally forms thicker granular textured veins dominated by quartz with minor biotite, chlorite, calcite, pyrite, pyrrhotite, chalcopyrite, molybdenite and bismuthinite (Mason 2005b).
Hydrothermal propylitic alteration overprints potassic alteration within both the felsic intrusives and surrounding wall-rocks and extends laterally beyond the zone of potassic alteration surrounding CdG.