GeologyThe Copper Creek area, part of the Bunker Hill District, Pinal County, Arizona, lies on the west side of the Galiuro Mountains, 75 kilometers (45 miles) northeast of Tucson. The district lies in the heart of the southwestern U.S. porphyry copper province, at the intersection of important belts of deposits trending NW-SE (Miami-Globe, Resolution, Ray, Bisbee) and WSW-ENE (Lakeshore, Silver Bell, San Manuel-Kalamazoo, Safford, Morenci).
The district is centred on the Copper Creek granodiorite, the central of three Laramide granodiorite intrusions forming a northwest-oriented cluster. The Copper Creek granodiorite was emplaced approximately 62 million years ago into Precambrian and Paleozoic sedimentary rocks, Late Precambrian diabase, and Cretaceous Glory Hole volcanics. The Copper Creek stock and adjacent Glory Hole volcanics have been intruded by a sequence of Laramide granodiorite, monzogranite, and quartz diorite porphyry plugs and dykes. The district is marked by over 400 hydrothermal breccia bodies, ranging from a few feet to several hundred feet across, which (like the porphyry bodies) are concentrated in two northwest-trending belts (see map). Post-mineral, mid-Tertiary Galiuro Volcanics cover all these rocks on the east and northeast. To the southwest, the district is bounded by a northwest-trending range-front fault which downdrops Tertiary Gila Conglomerate against the Laramide and older rocks.
The Copper Creek area contains multiple styles of Laramide copper-molybdenum-silver +/- gold deposits characterized by relatively high primary copper grades. Early development of the district dated from 1863, and focused on the exposed copper-rich (>>1% Cu) breccia bodies and peripheral silver-lead-zinc veins. In the 1960s and 1970s deeper drilling discovered porphyry-style sheeted and stockwork vein mineralization (~0.8% Cu) at depths between 350 and +1,200 m (1,200 to +4,000 feet) in the American Eagle and Keel areas, beneath a small portion of the near-surface breccia cluster.
The breccias are clast- to matrix-supported and consist of angular to subrounded, pebble- to boulder-sized, commonly quartz-sericite altered fragments formed from the host wall rock. Matrices of most breccias are partially to completely filled with varying combinations of quartz, pyrite, chalcopyrite, bornite, molybdenite, tourmaline, specularite, and minor rock flour. The upper levels of some breccias also contain chalcocite. High-grade mineralization in the breccias typically occurs as prominent coarse masses and clots of copper minerals filling open spaces, or as sheeted veins along the edges of the pipes. Relative levels of copper, molybdenum, gold and silver vary considerably from breccia to breccia. Molybdenite Re-Os ages from the breccias yield mineralization ages between 62 and 57 million years.
Breccias are known to persist over 1000 meters vertically. They terminate abruptly upward into lower grade material; the Mammoth pipe, the largest breccia in the current resource, is "blind" a mere 32 meters below the surface. Where drilling density is sufficient, the bottoms of the breccia pipes neck downwards into bodies of granodiorite porphyry. Over 90 percent of the mapped breccia bodies have not been drilled.
The American Eagle and Keel porphyry zones were previously classified as "hybrid porphyry" or "sheeted vein type" deposits due to their unusual vein styles. Recent work by Redhawk geologists has led to the recognition that the Copper Creek porphyry ores are typical of the "early-halo type" of porphyry system (Proffett, J.M., 2009, Geology, v. 37, p. 675-678). In these deposits the dominant style of copper-bearing veins are early dark micaceous (EDM) veins or early potassic halos lining incipient fractures, rather than the A-type sugary quartz stockwork veins common in many porphyry systems. Well known early-halo type porphyry deposits include Butte, Montana, and Chuquicamata and Los Pelambres, Chile.
Recent drilling indicates the American Eagle and Keel porphyry-style resources are connected and form a single copper-mineralized body at least 1,500 meters long northwest-southeast, which is open in most directions. This mineralized body is controlled by a broad dome-shaped zone of common, subhorizontal to steep EDM quartz-sulfide veins with halos of biotite, sericite, and abundant copper sulphides. The EDM vein zone is hosted mostly in Copper Creek granodiorite but appears centred on a cluster of syn-mineral granodiorite porphyry bodies. Superimposed on the EDM vein zone are breccias and zones of intense quartz-sericite alteration, both of which tend to carry high-grade copper which upgrades the same volume of rock. A near-vertical set of EDM veins extends above the well mineralized dome-shaped EDM vein zone to the present surface, where outcropping veins are widespread and typically trend east-northeast. Sulfides are zoned with depth, with pyrite-dominant mineralization near the surface transitioning into chalcopyrite-dominant rock in the better mineralized zones, with increasing bornite at depth.