Summary:
Deposit Types
The mineral deposit type at the FCP site is a Laramide-age porphyry copper deposit consisting of a large core of copper sulfide mineralization underlying a zone of copper oxide mineralization. The central portion of the deposit is overlain by approximately 400 feet of flat-lying conglomerate and alluvial material that contains a fine-grained silt and clay interbed. The oxide and sulfide zones are separated from one another by a transition zone of mixed oxide-supergene sulfide ranging from 0 to 55 feet in thickness. The depth and grade of the sulfide zone renders it currently uneconomic to mine by conventional mining methods. The impermeability of the rocks and insoluble mineralization of the sulfide zone renders it uneconomic for ISCR methods.
Approximately 71% of the oxide mineralization is hosted by a Precambrian quartz monzonite and 26% by Tertiary granodiorite porphyry. The remaining igneous rocks associated with the deposit are Precambrian diabase and Tertiary andesite, latite, dacite, basalt, and aplite. The deposit occurs in a structural horst block, which is bounded on the east and west by grabens and is controlled by normal faults trending north to northwest.
Mineralized Zones
The mineralized zones consist of an iron-enriched leached cap, an oxide zone, and an underlying sulfide zone. In most instances, the transition from the copper silicates and oxides to the sulfide zone is quite abrupt. A majority of the copper oxide mineralization is located along fracture surfaces, but chrysocolla and copper-bearing clay minerals also replace feldspar minerals in the granodiorite porphyry and quartz monzonite. A barren or very low-grade zone, dominated by iron oxide and clay minerals, caps some portions of the top of bedrock especially in the western area. The mineralization on the eastern periphery of the deposit is typical of most Arizona porphyry copper deposits. The thickness of the oxide zone ranges from 40 feet to 1,000 feet and has an average thickness of 400 feet. The top of the oxide zone begins at or near the bedrock surface that underlies 400-425 feet of alluvial and basin-fill material. The lateral extent of mineralization in plan is approximately 3,500 feet across in an east-west direction and from 1,500 feet to over 3,000 feet across in a north-south direction.
Type, Character and Distribution of Mineralization
The main type of mineralization is oxide with underlying sulfide that are often separated by a thin transition zone of partially oxidized supergene sulfides. The underlying hypogene sulfide zone, because of its depth, low permeability, and relatively non-soluble mineralogy, is not favorable to develop by ISCR methods.
Mineralization in the oxide zone consists of chrysocolla, “copper wad,” tenorite, cuprite, native copper, and trace azurite, and brochantite. Oxide copper occurs as chrysocolla in veins and fracture fillings, while the remainder occurs as copper-bearing clays in fracture fillings and former plagioclase sites. The fracture-controlled mineralogy within the Florence deposit indicates that copper is not adsorbed onto the clay surfaces, but rather the copper resides in the octahedral site of the clays. The “copper wad” appears to be an amorphous mix of manganese, iron, and copper oxides that occurs as dendrites, spots, and irregular coatings on fracture surfaces. Cuprite occurs locally smeared out along goethite/hematite-coated fracture surfaces; the chalcotrichite variety of cuprite is also present on fractures or vugs, sometimes intergrown with native copper crystals.
The main hypogene sulfide minerals are chalcopyrite, pyrite, and molybdenite with minor chalcocite and covellite. Supergene chalcocite coats pyrite and chalcopyrite and dusts fracture surfaces. The supergene chalcocite blanket is very thin and irregular (zero to 50 feet) and is often partially oxidized. In most instances, the transition from the copper silicates and oxides to the sulfide zone is quite abrupt.
In general, the grade of oxide mineralization is very similar to that of the primary sulfide mineralization. The overall grade of the oxide and sulfide mineralization is approximately 0.36% TCu and 0.27% TCu, respectively.
Mineralization and Alteration
Hydrothermal alteration accompanied the intrusion and cooling of the Tertiary granodiorite porphyry stocks and dikes into the Precambrian quartz monzonite. Alteration in the granodiorite porphyry is primarily veinlet-controlled, whereas alteration in the quartz monzonite encompasses all three styles; pervasive, selectively pervasive, and veinletcontrolled. Potassic alteration (quartz-orthoclase-biotite-sericite) is the dominant alteration assemblage. Salmon-colored, secondary orthoclase replaces primary orthoclase phenocrysts, rims quartz ± biotite veins, and occurs as pervasive orthoclase flooding. Shreddy, secondary brown biotite replaces plagioclase and matrix feldspars, and occurs in biotite-sulfide veinlets.
A sericitic (quartz-sericite-pyrite) alteration zone surrounds the potassic zone and is especially evident in the deep portions of the sulfide mineralization. Fine-grained sericite selectively replaces plagioclase, orthoclase, and biotite, and forms thin alteration selvages along quartz ±sulfide veins. Propylitic (calcite-chlorite-epidote) alteration is visible in mafic dike rocks and is reported in exploration holes fringing the deposit.
The most noticeable feature in the oxide mineralized material zone is a late-stage argillic alteration assemblage consisting of montmorillonite - kaolinite ± illite ± halloysite. The conversion of sericite to clay minerals in plagioclase phenocrysts and along fracture surfaces is selectively pervasive. X-ray diffraction analyses indicated the clay is primarily a mixture of calcium-montmorillonite and kaolinite. These clay-altered plagioclase sites were favorable loci for remobilized copper generated from natural in-situ leaching.
PTF Geology, Mineralization, and Alteration
The PTF is in the northwest portion of the Florence deposit, in the hanging wall of the Sidewinder fault as shown in the plan view in Figure 7-2. The area is typical of the local deposit geology (including alteration and mineralization types) as confirmed by five Conoco and Magma HQ-diameter core holes, two sets of PQ and HQ-diameter core hole pairs drilled by FCP in 2011, and 24 rotary/reverse circulation boreholes drilled by FCP in 2017 and 2018. East-West and North-South profiles through the mid point of the PTF are shown in Figure 7-6 and Figure 7-7, respectively. Dikes of Tertiary granodiorite intrude the Precambrian quartz monzonite, with thin, scattered dikes of Precambrian diabase and post-mineralization Tertiary andesite. Because the PTF area is on the periphery of the deposit, the proportion of intrusive Tertiary granodiorite porphyry relative to the host rock (approximately 10%), is substantially lower than is found in the central portion.