Geologic models of sediment-buffered hydrothermal vents: A case study of the Red Dog Zn-Pb-Ag orebody, western Brooks Range, Alaska.

摘要

The Red Dog Zn-Pb-Ag orebody is a supergiant sedimentary exhalative (SEDEX) deposit hosted by Carboniferous strata in the western Brooks Range. SEDEX deposits are diverse stratabound concentrations of Zn-Pb {dollar}pm{dollar} Cu sulfides that result from the discharge of hydrothermal solutions at or near the sea (or lake) floor.; The original relationship between the Red Dog orebody and host-rocks has been obscured by post-mineralization deformation events that occurred during the Cretaceous Brooks Range Orogeny. Mineralization facies, fault strain kinematics, and plan and section analysis were used to reconstruct the orebody. The reconstructed 2400m by 400m vent field contains four principal vents: Hilltop, Main, West and Aqqaluk. Stable isotope, microthermometric and Raman analyses suggest that Red Dog formed by normal geologic processes that occur during basin development. These include development of a stable and focused hydrogeologic system, evolution of Zn-Pb-Ba-rich formation waters, production of abundant reduced sulfur at the ore-forming site, and a mechanism to preserve sulfide mineralization near the sea floor. Hydrogeologic and reaction path models were developed to further constrain the Red Dog ore-forming process.; Mineralization began in the Late Mississippian when the Arctic Alaska Basin underwent a period of basin resurgence that altered the regional hydrogeologic system. Faults penetrated the basement, which resulted in the discharge of hot {dollar}({lcub}sim{rcub}300spcirc{dollar}C), Zn-Pb-Ba-bearing formation waters ({dollar}ge{dollar}12 eq. wt.% NaCl) that mixed with CH{dollar}sb4{dollar}-rich, low salinity fluids ({dollar}<{dollar}3.5 eq. wt.% NaCl). The low salinity fluids are interpreted to be shale waters derived from the Paleozoic sequence. The hot formation waters resulted in thermogenic reduction of SO{dollar}sb4sp{lcub}2-{rcub}{dollar} transported in the shale waters as the two fluids were discharged toward the sea floor. Early and main stage ore-fluids were discharged into unlithified host-sediments near the sea floor, but by the end of the main stage, the hydrogeology of the vent system changed and fluids were focused along fractures and discharged at the sea floor. The mineralizing event at Red Dog is characterized by a continuum between early epigenetic and syngenetic sulfide mineralization. Upper Pennsylvanian strata do not host Zn-Pb concentrations, thus marking the end of the Red Dog ore-forming event. However, barite within Upper Pennsylvanian strata does suggest local hydrothermal brine discharge during the late Pennsylvanian.