Deformation, magmatism, and sulfide mineralization in the Archean Golden Mile fault zone, Kalgoorlie gold camp, Western Australia

The Golden Mile fault zone is a key controlling structure to the estimated 75 Moz gold endowment of the Kalgoorlie gold camp in the Yilgarn craton of Western Australia. The earliest structures in the fault are F (sub 1) folds that developed during D (sub 1) recumbent-fold and thrust deformation (<2685 + or - 4 Ma). These F (sub 1) folds are overprinted by a pervasive NW- to NNW-striking S (sub 2) cleavage related to sinistral shearing beginning with 2680 + or - 3 Ma D (sub 2a) sinistral strike-slip and culminating with ca. 2660 Ma D (sub 2c) sinistral-reverse movement. The majority of deformation in the fault zone correlates to ca. 2675 Ma D (sub 2b) deformation, which is characterized by sinistral-normal kinematic indicators. Late, ca. 2650-2640 Ma D (sub 3) dextral-reverse kinematic indicators overprint the earlier D (sub 2) structures. Pyrrhotite-chalcopyrite-pyrite-sphalerite-galena assemblages were emplaced throughout the D (sub 2) event within NE-trending D2a tensile fractures, NW- to NNW-striking D (sub 2b) normal faults and associated breccias, and NW- to NNW-striking D (sub 2c) low-angle veins, with the latter D (sub 2b) and D (sub 2c) structures correlating to the Fimiston and Oroya mineralization types, respectively. All D (sub 2a-) , D (sub 2b-) , and D (sub 2c) -related sulfides in the Golden Mile fault zone show similarly restricted delta 34S ( approximately 1.0-4.5 ppm) and elevated Delta 33S ( approximately 2.0-3.0 ppm) values that reflect strong local sulfur contribution from shales of the Lower Black Flag Group and host-rock buffering of hydrothermal fluids related to the Fimiston and Oroya mineralization events. This host-rock buffering decreased fluid fO (sub 2) , favoring the development of pyrrhotite-pyrite stable sulfide assemblages and causing respective decreases and increases in fluid Au-Te and Pb-Bi-Sb concentrations. At the camp scale, the Golden Mile fault zone exerted a primary control on the distribution of porphyry dikes and gold deposits; however, magma and hydrothermal fluid circulation was favored in adjacent, higher-order structural sites due to the fault zone's incompetent rheology and tendency for ductile deformation and diffuse fluid flow. Other Archean examples such as Au deposits of the Larder Lake-Cadillac deformation zone in the Superior craton illustrate that this type of diffuse fluid flow in large-scale crustal fault zones can result in disseminated economic mineralization. However, this study highlights that host-rock effects on fluid chemistry in large-scale crustal fault zones exercises a strong control on a fluid's propensity to form ore. The results of this study emphasize that both the rheology and chemistry of rocks within and adjacent to large-scale deformation zones act as important controls on the formation of gold ore in Archean terranes.