Summary:
The Cactus (Cactus East and Cactus West) and Parks/Salyer Projects occur in the desert region of the Basin and Range province of Arizona (AZ). These combined deposits are part of a large porphyry copper system. Major host rocks are Precambrian Oracle Granite and Laramide monzonite porphyry and quartz monzonite porphyry. The porphyries intruded the older rocks and form mixed breccias; monolithic breccias and occur as large masses, poorly defined dyke-like masses; and thin well-defined but discontinuous dykes. Structurally the deposit is complex with intense fracturing, faulting, and both pre-mineral and post-mineral brecciation. It is bounded on the east and west sides by normal faults.
Chalcocite and covellite are the only supergene sulfides recognized. The chalcocite blanket in the mineralized zone is irregular in thickness, grade, and continuity. The thickness of leached capping varies from less than 100 ft (30 m) to over 650 ft (198 m), with the thicker intercepts on the north side. Substantial quantities of oxidized copper minerals are found erratically distributed through the capping. Chrysocolla, brochantite, and malachite are the most common oxidized copper minerals. In upper portions of the capping, chrysocolla predominates, while brochantite and malachite are predominant in the lower portions. The dominant hypogene alteration assemblages in the deposit are phyllic and potassic. The major hypogene sulfide minerals in the deposit are pyrite, chalcopyrite, and molybdenite. Hypogene sulfides occur as disseminated grains, veins, and vug fillings.
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. 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.
Structurally both Cactus East and Cactus West deposits are complex with intense fracturing, faulting, and brecciation. Pre-mineral brecciation is related to the intrusion of the Laramide porphyries and occurs primarily in the west deposit which had a central core of pre-mineral brecciation that was a control for hypogene mineralization. Angular vugs are a diagnostic feature of the pre-mineral breccia. They occur between fragments in the breccia and vary in size from 0.2 in (0.5 cm) to 2.0 in (5.0cm). Post-mineral brecciation is ubiquitous in both deposits and has affected the rocks in a number of ways, depending on rock composition, degree and type of alteration, and relative location in the mineralized deposits. Manifestations of this period of brecciation include shattering, crushing and granulation, mixing of rock types, and the presence of linear breccia structures containing crushed sulfides. Mineralized fractures in the west deposit generally strike E-NE while post-mineral fractures strike N-NW.
In the West mineralized deposit the faults are often variable in strike and dip and are usually difficult to trace along strike. The prevailing strike direction is N60°E to E-W. Slickensides on some of the faults indicate that horizontal components of displacement are relatively common. Generally, the lack of predictable lithologic contacts to act as markers makes the direction and magnitude of displacement difficult to estimate. Total displacement on most of the faults is thought to be less than 100 ft (30 m). Both pre-mineral and postmineral movement is often present.
Besides being terminated at depth by the Basement fault, both deposits are bounded by normal faults that drop postmineral conglomerate into contact with the mineralized rocks. The west deposit is in a horst block formed with the Sacaton fault forming the east side which strikes N20°W and the West fault trending N45°W on the west side. The Sacaton Fault dips 60° to the east and has a displacement of up to 1,500 ft (457 m). The east deposit is the displaced portion of the west deposit in the hanging wall of the Sacaton fault.
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. 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 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 East and West deposits. Some, and probably all, of the original blanket formed prior to movement on the Sacaton and West faults. Substantial quantities of oxidized copper minerals are found erratically distributed through the capping of both deposits. In the East deposit, 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 the West deposit, but generally the oxides occur over a greater horizontal and vertical range, and the copper has likely been transported from the point of oxidation.
The Parks/Salyer deposit has undergone at least two periods of oxidation and leaching. A large suite of transported iron oxide is present, along with remnant copper oxide minerals left behind after the initial leaching and oxidizing events. Oxidized copper occurs erratically within the leach capping; most commonly observed near the lower contact between the leached zone and the enrichment. Minerals observed include hematite, limonite, goethite, jarosite, manganese oxides, chrysocolla, malachite, brochantite, azurite, atacamite, native copper, tenorite, and cuprite.