Alteration mineralogy in detachment zones: Insights from Swansea, Arizona

Rocks in detachment zones are commonly enriched in K₂O, thought to originate from K-metasomatism by basin brine associated with tectonically controlled basins in semiarid settings. We used infrared spectroscopic and remote sensing techniques to investigate the geologic and mineralogical context of K-metasomatism associated with the Buckskin-Rawhide detachment fault near Swansea, Arizona, where spectacular alteration and exceptional exposures are observed. The goals are to (1) determine the miner alogy associated with K₂O enrichment in this area, (2) define the lithologic and structural controls on alteration in this region, and (3) construct a general model for alteration in detachments zones, context of K₂O enrichment, and relation to detachment-related ore deposits. In the Swansea area, Miocene volcanic rocks were completely and pervasively altered in an early stage of K-metasomatism to ferruginous illite, K-feldspar, and hematite, and later replaced by calcite, celadonite, hematite, and jasper. The mineralogy of these altered rocks and their geologic context suggest initial K-metasomatism by warm, alkaline surface water and/or groundwater related to a Miocene lacustrine environment. We propose that the carbonate overprint occurred due to increased fluid temperatures as the K-metasomatized rocks moved down the detachment fault in an environment of high heat flow. The spatial distribution of alteration minerals observed in the field and from remote sensing data shows that alteration was driven by reactivity of host rocks and host-rock permeability; normal faults and fractures associated with detachment faulting were not significant conduits of hydrothermal fluids. These results illustrate well the spatial relationships between alteration minerals and fluid conduits in detachment zones, which are usually studied only by chemical analyses.