GEOLOGY AND GEOCHEMISTRY OF THE MAMMOTH BRECCIA PIPE, COPPER CREEK MINING DISTRICT, SOUTHEASTERN ARIZONA

The Copper Creek mining district is located in southeastern Arizona, approximately 70 km northeast of Tucson. The district hosts more than 500 mineralized breccia pipes. In addition, buried porphyry-style, stockwork copper mineralization and a distal lead-silver vein are also present within the district. The breccia pipes are confined to the Copper Creek Granodiorite and the Glory Hole volcanic rocks, both of which contain extensive, steeply-dipping ENE-striking fractures that developed during Laramide orogeny.

The unexposed Mammoth Breccia Pipe was discovered by drilling and has a vertical extent of 915 m and a maximum width of 180 m. The breccia pipe consists of angular clasts of granodiorite cemented by quartz, chalcopyrite, bornite, anhydrite, and calcite. New ⁴⁰Ar/³⁹Ar geochronology from two biotite samples suggest a minimum age of 62.1 – 60.8 Ma for the Copper Creek Granodiorite, which was previously dated at 69.7 ± 2.7 and 65.8 ± 1.6 Ma by K-Ar techniques. New ⁴⁰Ar/³⁹Ar geochronology on six separates of hydrothermal white mica yields an age of 60.0 ± 0.9 Ma., which is interpreted to be the age of associated copper mineralization. Previously, other copper mineralization within the district had been dated at 56.9 ± 0.9 Ma by Re-Os techniques, indicating at least 4 – 5 m.y. of ore formation.

Fluid inclusion studies suggest that boiling, or unmixing, occurred during copper mineralization. A parent fluid with a salinity of ~ 10 wt. % NaCl equiv. unmixed into a dilute aqueous vapor (1 – 2.8 wt. % NaCl equiv.) and a hypersaline brine (33.4 – 35.1 wt. % NaCl equiv.). Final homogenization temperatures of the brine-bearing inclusions were as great as 375°C.

Sulfur isotope geothermometry of two cogenetic hydrothermal phases, anhydrite and chalcopyrite, gives a temperature of mineralization of 469 ± 25°C. Calculated oxygen and hydrogen isotope values for fluids in equilibrium with quartz and white mica range from 10.2 to 13.4‰ and -60 to -39‰, respectively, suggesting a dominantly magmatic component within the hydrothermal fluids responsible for mineralization.