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Geology

The Bunker Hill District, a ~16-kilometer-long area that includes our project area, lies on the west side of the Galiuro Mountains, in the heart of the southwestern U.S. porphyry copper province. 

In this district, there is a significant intersection of important copper belts: a northwest to southeast belt that includes Miami-Globe, Resolution, Ray, and Bisbee mine sites and a west-southwest to east-northeast belt that includes Lakeshore, Silver Bell, San Manuel-Kalamazoo, Safford, and Morenci mine sites. 

The district is centered on the Copper Creek granodiorite—the central of three Laramide granodiorite intrusions forming a northwest-trending cluster. This granodiorite was emplaced into Precambrian and Paleozoic sedimentary rocks, Late Precambrian diabase, and Cretaceous Glory Hole volcanics approximately 63 million years ago. The stock adjacent to the Glory Hole volcanics has been intruded by a sequence of narrow Laramide granodiorite, monzogranite, and quartz diorite porphyry plugs and dykes. Post-mineral, mid-Tertiary Galiruo Volcanics cover all rocks on the east and northeast. To the southwest, the district is bounded by a northwest striking range front-fault that down drops Tertiary Gila Conglomerate against the Laramide and older rocks.

The exploration area is marked by more than 400 hydrothermal breccia bodies ranging from a few feet to several hundred meters across.

Copper Creek contains multiple styles of Laramide copper, molybdenum, silver, and gold deposits, characterized by high primary copper grades. Early development of the district dating back to 1863 focused on the exposed copper-rich (>1% Cu) breccia bodies and peripheral silver-lead-zinc veins. However, the 1960s and 1970s saw deeper drilling programs discover porphyry-style sheeted and stockwork vein mineralization at depths between 365 and 1,220-meters. This was particularly apparent in the American Eagle and Keel areas, located beneath a small portion of the near-surface breccia cluster.

The breccias are varied in style and texture. They are monomictic to polymictic and consist of angular to subangular fragments from pebble to boulder in size. In most breccias, clasts are cemented by varying combinations of quartz, chlorite, carbonate, tourmaline, and locally magnetite, as well as sulfides including pyrite, chalcopyrite, bornite, covellite, chalcocite, and molybdenite. The breccias contain little clastic rock-flour matrix, suggesting only limited milling has occurred. , Clasts and wall rock surrounding the breccias are moderate to intensely quartz-sericite +/- kaolinite altered. Relative levels of copper, molybdenum, gold, and silver vary considerably from breccia to breccia. Molybdenite Re-Os from the breccias yield mineralization ages between 62 and 61 million years.

Breccia-hosted mineralization has been recognized at vertical depths of more than 975 meters. The largest breccia in the current resource, the Mammoth pipe, is “blind” but was drill intersected a mere 30 meters below the surface. The Mammoth breccia transitions downward into the Keel zone, interpreted as a magmatic cupola containing high-grade bornite-molybdenite mineralization. Over 90 percent of the surface 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. These are now recognized as forming an “early-halo type” porphyry system. In these deposits, copper minerals are disseminated in veins and vein halos characterized by biotite-muscovite K-feldspar alteration rather than the quartz stockwork veins common in many porphyry systems which may include sinuous A-type sugary quartz or straight-edged B-type quartz veins containing sulfides in a central suture. Well-known early-halo- mineralization styles are described from Butte, Montana, and parts of Chuquicamata and Los Pelambres, Chile. Early halo mineralization represents porphyry-style mineralization emplaced at deep crustal depths.

The American Eagle and Keel porphyry-style historic resources are connected and form a single copper-mineralized body at least 2 kilometers along strike and extending below 1km below surface.

The mineralized body that links American Eagle and Keel zones are controlled by a broad dome-shaped zone of common, subhorizontal to steep early halo quartz-sulfide veins with halos of biotite, muscovite, and abundant copper sulphides. The American Eagle 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, upgrading the same volume of rock. A near-vertical set of EDM veins extend 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 by depth, with pyrite-dominant mineralization near the surface transitioning into a chalcopyrite-dominant rock in the better-mineralized zones and with increasing bornite at depth.

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