Summary:
Deposit Types
The Cactus and Parks/Salyer deposits are 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). Both Cactus and Parks/Salyer have typical characteristics of a porphyry copper deposit which Berger et al. (2008) define as follows:
• A deposit 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 0.6 mi (1 km) to 2.5 mi (4 km) depth are genetically related to magma reservoirs emplaced into the shallow crust 3.5 mi (6 km) to over 5 mi (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 dykes.
• 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). Minor amounts of quartz are also found, with sericite and clay replacing plagioclase phenocrysts in the porphyries and granite. Quartz-sulfide veinlets are associated with the phyllic assemblage and comprise up to 1% of the rock by volume. 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. Since phyllic and supergene alteration are superimposed upon, and largely destroy, potassic alteration, it is uncertain how much of the quartz, sericite, and clay are part of the original potassic suite. Supergene alteration associated with the process of secondary enrichment of sulfides has modified the suite of hypogene alteration minerals.
Similar, if not identical, alteration assemblages can be found in Parks/Salyer. Both assemblages include hypogene and supergene alteration overprint. Hypogene alteration assemblages include both potassic and phyllic. Alteration minerals occurring in the potassic altered rock include secondary K-feldspar, magnetite, biotite, chlorite, quartz, sericite, and clay. Such zones are typically low grade. Secondary biotite, magnetite and chlorite characterize the potassic assemblage. Phyllic assemblages are noted to include strong secondary silicification, bleaching, quartz, sericite, pyrite, and clays. The secondary silica replacement appears as fine-grained replacement of the groundmass, intergrowing between the sericite and other clays. Alteration halos surrounding quartz-sericite and sulfide veins are common within these phyllic alteration zones. Phyllic zones are higher in grade compared to the potassic zones.
The major hypogene sulfide minerals at Cactus 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 CW, disseminated grains usually comprise less than 50% of the hypogene sulfides, but in CE, where granite breccia is the main rock type, disseminated grains account for over 50% of the sulfides.
The major hypogene sulfide minerals at Parks/Salyer are pyrite, chalcopyrite, and molybdenite. Trace amounts of bornite and sphalerite have been observed within the upper sections of the hypogene and lower edges of the supergene mineralization. Hypogene sulfides occur as disseminated grains, veins/veinlets, and patchy blebs. Disseminated sulfides are abundant in the brecciated rocks, monzonite porphyry, and in the granite. Sulfides are also present within quartz veins and veinlets throughout the deposit.
The total sulfide content for both CW and CE 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% Cu as chalcopyrite. Outside the zone the copper grade gradually drops off to less than 0.10% Cu. 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 as part of the pyrite shell. Molybdenite occurs in quartz veins and smears on fractures. The molybdenum content averages approximately 0.010% for CW and 0.025% for CE. Similarly, within the Parks/Salyer, molybdenite occurs in quartz veins, as smears on fractures, as well as in disseminated crystals in the groundmass.
The major supergene sulfide mineral at the Cactus deposits 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 major supergene sulfide minerals at Parks/Salyer are chalcocite, covellite, and pyrite. Digenite is also present in smaller quantities. The intensity of the secondary enrichment is greatest at the upper portion of the enriched zone, decreasing gradually towards the base. In the upper portions chalcocite and covellite completely replace chalcopyrite and partially replace pyrite. Near the base of the zone, chalcopyrite is partially replaced, and pyrite is rimmed by chalcocite. Covellite is discontinuous and often is seen as replacing blebs and grains of pyrite. The enrichment factor varies from 3:1 to 5:1. The most important control for supergene enrichment is the grade of primary mineralization which is controlled by a NE trending structural zone containing a higher density of quartz/sulfide veining.
The Cactus deposits have undergone two periods of oxidation and leaching. The first period resulted in the formation of what was probably a uniform high grade chalcocite blanket that was continuous through the CE and CW deposits. The original blanket likely formed prior to movement on the Sacaton and West faults. Substantial quantities of oxidized copper minerals are found erratically distributed through the capping of CW and CE. In CE the oxide minerals usually occur just above chalcocite mineralization and are thought to have resulted from in-place oxidation of chalcocite along zones of deep oxidation. Copper grades over 1.0% are common. In-place oxidation is also found in CW, but generally the oxides occur over a greater horizontal and vertical range, and the copper has likely been transported from the point of oxidation.