Arizona Sonoran Copper Company Inc. principal asset is a 100% interest in the former Sacaton mine, now called the Cactus Mine, which it acquired from the American Smelting and Refining Company Multi-state Custodial Trust (“ASARCO Trust”) in July 2020.
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Summary:
The Cactus deposit is a portion of a large porphyry copper system that has been dismembered and displaced by Tertiary extensional faulting. Porphyry copper deposits form in areas of shallow magmatism within subduction-related tectonic environments (Berger et al., 2008). Cactus has typical characteristics of a porphyry copper deposit which Berger et al. (2008) define as follows:
- One wherein copper-bearing sulfides are localized in a network of fracture-controlled stockwork veinlets and as disseminated grains in the adjacent altered rock matrix.
- Alteration and mineralization at 1 km to 4 km depth are genetically related to magma reservoirs emplaced into the shallow crust (6 km to over 8 km), predominantly intermediate to silicic in composition, in magmatic arcs above subduction zones.
- Intrusive rock complexes that are emplaced immediately before porphyry deposit formation and that host the deposits are predominantly in the form of upright-vertical cylindrical stocks and/or complexes of dikes.
- Zones of phyllic-argillic and marginal propylitic alteration overlap or surround a potassic alteration assemblage.
- Copper may also be introduced during overprinting phyllic-argillic alteration events.
Alteration and Mineralization
The dominant hypogene alteration assemblages in the deposit are phyllic and potassic. Phyllic alteration is characterized by quartz, sericite, and clay, but quartz and sericite predominate. Secondary silica in the porphyries occurs as a fine-grained replacement of the groundmass (intergrown with sericite and clay).
Alteration minerals occurring in rocks of the potassic assemblage include varying quantities of biotite, chlorite, quartz, sericite, and clay, with traces of secondary K-feldspar, calcite, and anhydrite. Secondary biotite and chlorite characterize the potassic assemblage. Supergene alteration associated with the process of secondary enrichment of sulfides has modified the suite of hypogene alteration minerals. In Cactus West, effects of this supergene overprint are not always assessable due to post-enrichment oxidation and leaching penetrating the chalcocite blanket into the primary sulfide zone.
The major hypogene sulfide minerals at Sacaton are pyrite, chalcopyrite, and molybdenite. Traces of bornite and sphalerite have been observed in concentrate samples. Hypogene sulfides occur as disseminated grains, veins, and vug fillings. Disseminated sulfides are more abundant in the granite and strongly brecciated rocks than in the porphyries and weakly brecciated rocks. In the West mineralized zone, disseminated grains usually comprise less than 50% of the hypogene sulfides, but in the East mineralized zone, where granite breccia is the main rock type, disseminated grains account for over 50% of the sulfides.
The total sulfide content for both mineralized zones is variable, ranging from approximately 1.0% to 4.0% by volume. Rock type and pre-mineral brecciation cannot be directly correlated to variations in total sulfide content. North and south of the mineralized zones the total sulfide content decreases similarly to the overall alteration intensity. Drilling and pit mapping have defined a core zone within which the grade of hypogene mineralization is at least 0.40% copper as chalcopyrite. Outside the zone the copper grade gradually drops off to less than 0.10% copper. The pyrite: chalcopyrite ratio varies from 1:1 to 3:1 within the core zone and increases to 10:1 or more outside of it. Molybdenite occurs in quartz veins and as smears on fractures. The molybdenum content averages approximately 0.010% for the West mineralized zone and 0.025% for the East mineralized zone.
The major supergene sulfide mineral at Cactus is chalcocite. Covellite and digenite are also present in much smaller quantities. The intensity of secondary enrichment is greatest at the top of the enriched zone and decreases gradually toward the base. In the upper portions of the enriched zone chalcocite completely replaces chalcopyrite and partially replaces pyrite. Toward the base of the zone chalcopyrite is partially replaced and pyrite is rimmed by thin coatings of chalcocite. The enrichment factor (the ratio of supergene copper grade to hypogene copper grade) varies from 3:1 to 5:1 for both mineralized zones. The most important control for supergene enrichment is the grade of primary mineralization. The bulk of economic supergene mineralization is underlain by primary sulfides averaging at least 0.40% copper.