Summary:
Deposit Types
The Copper Creek district contains mineralization styles broadly consistent with magmatic-hydrothermal systems, also known as porphyry systems. However, Copper Creek has some geologic characteristics that are uncommon or subordinate in other well-known porphyry copper deposits and differ from the well-known models (e.g., Lowell and Guilbert, 1970; Seedorff et al., 2005; Sillitoe, 2010), including the prevalence of mineralized EH-style veins (Proffett, 2009), batholith-hosted mineralization, and the abundance of breccia-hosted mineralization. Copper Creek is primarily a Cu-Mo system.
Generally, alteration zonation, typical for porphyry deposits elsewhere (e.g., Halley et al., 2015), is weakly developed in batholith-hosted systems with only subtle potassic alteration and poorly developed propylitic halos. Intense argillic or advanced argillic alteration is not widespread.
Distal polymetallic veins (Pb-Zn-Mn-Cu-Ag) are also known at Copper Creek and were the first to be mined in the district (Bluebird Mine and Bunker Hill), but these are not currently part of the resource. Supergene oxidation and secondary copper enrichment at Copper Creek is minor compared to other copper deposits in the Southwestern North American Copper province and restricted to the uppermost 20 to 40 m.
Mineralization
Copper Creek is a low-sulphide mineral system compared to other porphyry systems in Arizona. A strong, overall vertical and lateral sulphide mineral zoning pattern exists with pyrite near-surface, grading with depth to pyrite>chalcopyrite, chalcopyrite>pyrite, then chalcopyrite>bornite, bornite> chalcopyrite, and finally bornite>molybdenite.
Mineralization occurs in three fundamental styles:
• High-grade mineralization is related to chalcopyrite and pyrite cementing breccias. This style predominates in breccias near present-day surface. Molybdenum and gold grades vary between breccias with the most-consistent grades observed at Childs Aldwinkle.
• The Keel Zone represents a magmatic cupola. Mineralization in the upper part of the Keel Zone consists of coarse chalcopyrite and lesser pyrite and fine-grained disseminated sulphides associated with discontinuous thin quartz veins, potassium feldspar alteration, and anhydrite. The deeper part of Keel is characterized by miarolitic cavities filled with potassium feldspar and later bornite and chalcopyrite as well as molybdenite. This style can have highgrade mineralization.
• American Eagle mineralization is dominated by EH-style veins. Mineralization is somewhat lower-grade compared to the previous styles but forms a greater volume. Most of the copper occurs above the monzogranitic layer at depth, but molybdenum is also relatively high below that host rock compositional boundary.
In general, copper correlates with silver. Gold (Au) analyses are limited, but where data are available, gold grades are low. However, gold is locally significant at Childs Aldwinkle.
Crosscutting relationships indicate that EH-style mineralization occurred early in the development of the system but after the intrusion of early granodiorite porphyries (gdp1 and gdp2). Mineralization at Keel is intimately associated with the intrusion of gdp2, and current interpretations suggest overprints the Keel mineralization. In general, breccia-style mineralization post-dates EH veins, but in some areas (e.g., Copper Giant area) multiple overprinting breccia systems may be present.
The overall sulphide and alteration zonation within breccias is consistent with a slight tilt to the east. Mineralization at Holly, Glory Hole, and Old Reliable breccias is dominated by pyrite>chalcopyrite; these are hosted by GHv, which overlie the TKgd. The Copper Prince, Copper Giant and Childs Aldwinkle breccias, which are located farther to the northeast, have comparatively less pyrite and crosscut the Copper Creek Granodiorite, suggesting a deeper level of exhumation in the footwall of southwest dipping extensional faults.
Breccia bodies likely have a magmatic root zone but with the exception of Keel and Mammoth, breccia bodies lack the drill density at depth to confidently locate their likely source of magmatic fluids and metals.
Oxidation affects the near-surface mineralization to a limited degree. Complete oxidation is confined to the uppermost 20 m or less, whereas mixed sulphide and oxide mineralization is limited to the uppermost 30 to 40 m of the transitional zone. Secondary sulphide enrichment (chalcocite) is most prevalent at Old Reliable, whereas localized occurrences are present at Childs Aldwinkle and Copper Prince. The limited oxidation and supergene enrichment, together with the structural preservation, distinguishes Copper Creek from other copper deposits in the southwestern U.S., such as the Ray or San Manuel-Kalamazoo deposits.
Veining
Mineralization in the Copper Creek district is closely related to EH veins, which form vertical sheeted vein zones and domal-sub-horizontal zones, with less defined A, B, and D-vein sets. The former is more common south of the CCZ in the American Eagle mineralized zone, while the latter is common north of the CCZ in the Copper Giant mineralized zone.
Early introduction of copper occurred in the sheeted EH veinlets with centimetre-scale envelopes of sericite>biotite ± potassium K-feldspar and widespread disseminated chalcopyrite. Copper grades exceed 0.5% Cu where steep east-northeast-striking EH vein sets cut a shallowly dipping set. EH veins are cut by and reopened by D-type quartzsericite ± tourmaline-specular hematite, which contain variable amounts of pyrite, chalcopyrite, bornite, molybdenite, tennantite, and lesser galena. Distal veins include polymetallic Pb-zinc (Zn)-manganese (Mn)-Cu-Ag veins at the historical Bluebird and Bunker Hill mines, located, respectively, some 1.5 km northeast and 2.2 km south from the Mammoth.