A COUPLED HYDROGEOLOGIC DISSOLUTION AND REPRECIPITATION MODEL OF CARBONATE ROCKS ABOVE THE BUCKSKIN/RAWHIDE DETACHMENT FAULT, SWANSEA AREA, ARIZONA

In the Swansea, Arizona area, upper-plate carbonate rocks are widespread along the Buckskin-Rawhide detachment fault (BRD) where they commonly separate lower-plate crystalline rocks from syn-extensional basin sediments and hold important clues to the hydrogeology and mineralization histories of the area. Some of the carbonate rocks are mapped as sedimentary (Paleozoic) but, based on field relations and analysis of geochemical and remote sensing (TIMS) data (Michalski and others, 2007), much is mapped as a secondary deposit interpreted to have been derived from the leaching of overlying felsic volcanic rocks and downward percolation through lacustrine basin deposits of a low-temperature, alkaline, Ca-rich groundwater. However, I have found faint remnants of sedimentary bedforms, similar to those in the sedimentary rocks, throughout much of the “secondary” carbonate. Also, dissolution and collapse structures are widespread in the Paleozoic and “secondary” rocks. I reinterpret the “secondary” carbonate rocks as remnants after protracted dissolution of the sedimentary rocks, and the collapse structures record this dissolution history above the BRD. In this interpretation, an important question is, “Where did all the dissolved carbonate go?” Much of it precipitated as lacustrine limestone at about a dozen stratigraphic intervals in a km-thick sequence of interbedded volcanic rocks and landslide deposits in the directly adjacent basin. This revised interpretation hydrogeologically couples dissolution in uplands with limestone precipitation in adjacent basins, analogous to coupling in the detachment-fault terrain of the Mormon Mountains, Nevada. Dissolution probably occurred as a consequence of the corrosive effect of deeply derived upward-moving endogenic CO2-bearing fluids at a redox boundary with groundwater, and local ores at the Swansea and Copper Penny mines developed in that hydrogeologic regimen.