Alteration mineralogy in detachment zones: Insights from Swansea, Arizona Joseph R. Michalski Geophysics and Planetary Geosciences Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA Stephen J. Reynolds School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA Paul B. Niles NASA Johnson Space Center, Houston, Texas, USA Thomas G. Sharp Philip R. Christensen School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA ABSTRACT tion minerals observed in the fi eld and from Buckskin Mountains, and in the Swansea area in remote sensing data shows that alteration particular, K-metasomatism, massive carbonate Rocks in detachment zones are commonly was driven by reactivity of host rocks and replacements, and Cu-Fe mineralization are asso- enriched in K2O, thought to originate from host-rock permeability; normal faults and ciated with Buckskin-Rawhide detachment fault K-metasomatism by basin brine associated fractures associated with detachment faulting and upper plate rocks (Spencer and Reynolds, with tectonically controlled basins in semi- were not signifi cant conduits of hydrothermal 1986a, 1986b; Spencer and Welty, 1989). arid settings. We used infrared spectroscopic fl uids. These results illustrate well the spatial Alteration and mineralization commonly and remote sensing techniques to investi- relationships between alteration minerals and accompany detachment faulting (Wilkins et al., gate the geologic and mineralogical con- fl uid conduits in detachment zones, which are 1989) and hydrothermal fl uids seem to play an text of K-metasomatism associated with the usually studied only by chemical analyses. intimate role in the faulting process ( Reynolds Buckskin-Rawhide detachment fault near and Lister, 1987). Lower plate rocks in detach- Swansea, Arizona, where spectacular altera- Keywords: detachment fault, K-metasomatism, ment zones are usually chloritized and brec- tion and exceptional exposures are observed. infrared, emission spectroscopy, Arizona. ciated in the vicinity of the fault by deep, hot, The goals are to (1) determine the miner alogy reducing fl uids (Kerrich, 1988; Halfkenny associated with K2O enrichment in this area, INTRODUCTION et al., 1989). Upper plate rocks are commonly (2) defi ne the lithologic and structural controls K-metasomatized, probably by shallow, oxidiz- on alteration in this region, and (3) construct Extreme tectonic extension of southwestern ing, warm meteoric fl uids (Brooks, 1986; Chapin a general model for alteration in detachments North America during the Oligocene–Miocene and Lindley, 1986; Kerrich, 1988; Roddy et al., zones, context of K2O enrichment, and rela- Epochs resulted in the formation of several 1988). Mineralization along detachment faults tion to detachment-related ore deposits. In major detachment structures (Davis et al., 1980; may occur when metal-rich brines present in the Swansea area, Miocene volcanic rocks Spencer et al., 1995), including the Buckskin- upper plate rocks contact the lower plate of the were completely and pervasively altered in Rawhide detachment fault in western Arizona. fault, which contains an amount of stored heat an early stage of K-metasomatism to ferru- In the vicinity of Swansea, Arizona (Fig. 1), the signifi cant to drive hydrothermal convection and ginous illite, K-feldspar, and hematite, and fault juxtaposes Precambrian–Tertiary plutonic fl uid mixing (Spencer and Welty, 1986; Kerrich, later replaced by calcite, celadonite, hematite, and metamorphic lower plate rocks against upper 1988; McKibben et al., 1988; Halfkenny et al., and jasper. The mineralogy of these altered plate Precambrian, Paleozoic, Mesozoic, and 1989; Wilkins et al., 1989). rocks and their geologic context suggest ini- Tertiary sedimentary, metasedimentary, volcanic, Excellent exposures of extremely altered tial K-metasomatism by warm, alkaline sur- and plutonic rocks. The gently east-northeast – and structurally complex rocks in the Swansea face water and/or groundwater related to a dipping Buckskin-Rawhide detachment fault is area provide a fantastic opportunity to study Miocene lacustrine environment. We pro- corrugated so that upper plate rocks are folded the geochemical, mineralogical, and tempo- pose that the carbonate overprint occurred into synforms and exposed in east-northeast– ral relationships of detachment-related altera- due to increased fl uid temperatures as the trending valleys between low mountain ranges tion. The occurrence of widespread secondary K-metasomatized rocks moved down the of lower plate basement (Fig. 2). Locally, the carbonate at Swansea is striking and unique, detachment fault in an environment of high detachment fault is offset by normal faults that possibly indicat ing an advanced stage of meta- heat fl ow. The spatial distribution of altera- strike northwest, north, and northeast. In the somatic alteration. In this study we investigated Geosphere; August 2007; v. 3; no. 4; p. 184–198; doi: 10.1130/GES00080.1; 12 fi gures; 2 tables. 184 For permission to copy, contact [email protected] © 2007 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/4/184/857288/i1553-040X-3-4-184.pdf by guest on 24 September 2021 Alteration mineralogy in detachment zones 114°0′0″W 113°52′30″W 113°45′0″W dip BRDF BRDF (Inferred) Normal fault Black Mesa 34°15′0″N Planet/ Mineral Hill 34°15′0″N Bill Williams River Buckskin Mountains Figure 4 Swansea Copper Penny ′ ″ 34°7 30 N s 34°7′30″N n Mountains untain Buckski skin Mo uck B Cactus Plain (Dunes) 0 2.5 5 km 114°0′0″W 113°52′30″W 113°45′0″W Figure 1. Advanced spaceborne thermal emission and refl ection radiometer (ASTER) image showing the geographic context of the Swansea and Copper Penny mine areas in the Buckskin Mountains of western Arizona. The fault trace of the Buckskin-Rawhide detachment fault (BRDF) is shown where mapped or inferred. The low angle of the detachment fault plane results in a complex map pattern and the preser- vation of several klippen. Several major northwest-southeast–striking normal faults are shown. the mineralogy, geochemistry, and context of altered rocks in the Swansea area. Remote sens- vibrational spectral absorptions in the thermal altered rocks in the Swansea area to understand ing was applied to studies of K-metasomatism spectral range. Therefore, instead of mapping the mineralogical effects of metasomatic fl uids by Beratan et al. (1997) and Beratan (1999): in only hematite as a proxy for alteration, we are on primary rocks, fl uid conduits responsible those studies, the authors applied visible-near able to map the occurrence of clay minerals and for metasomatic alteration, and the timing of infrared (VNIR) data (λ = 0.5–3 μm) to identify carbonates, which are components of the altera- structural deformation and mineralogical altera- the extent of alteration from the occurrence of tion assemblage in the Swansea area. We utilize tion in detachment zones. Of particular interest red hematite. In this study, we utilize both VNIR a spectral unmixing routine to determine rock at Swansea is the origin of the secondary car- and thermal infrared (λ = 6–30 μm) spectral mineralogy from thermal infrared laboratory bonates and their relationship to fundamental data to map alteration and host-rock mineralogy spectra of fi eld samples. These data are used in detachment-related K-metasomatism processes. in the Swansea area. Thermal infrared spectros- combination with more traditional techniques to We use remote sensing analyses in addition to copy is a powerful mineralogical tool because identify the nature and extent of alteration min- fi eld mapping to identify the spatial distribution of essentially all minerals and mineraloids contain eralogy in the Swansea area. Geosphere, August 2007 185 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/4/184/857288/i1553-040X-3-4-184.pdf by guest on 24 September 2021 Michalski et al. ing of progressively older rocks (Spencer and A Reynolds, 1989). The stratigraphy is gener- ally correlative with units in adjacent basins, although the basal units are somewhat unique to the local area, suggesting a restricted basin at that time (Spencer and Reynolds, 1989). In general, the stratigraphic package records the progressive development and widening of a tectonic basin related to extension, punctu- ated with massive debris fl ows related to over- steepened topography (Spencer and Reynolds, lt lte e 1989). Pulses of volcanism are evident in lava n FaulaneFau Fault and tuff deposits, and contribute a large volume p pla plan of volcaniclastic sediments. Tectonically driven slope instability resulted in the massive debris fl ows that deposited breccia intermittently into a lacustrine-fl uvial setting. B Structure Swansea synform The structural geology of the Buckskin ins nta Mountains was comprehensively summarized ou in M by Spencer and Reynolds (1989). The Buckskin- ksk U Buc D Rawhide detachment fault is a low-angle exten- sional fault with >55 km of offset (Reynolds and Spencer, 1985). Mylonitic lineation in the lower plate trends 040º–050° and is considered to be Tertiary strata Detachment subparallel to extension direction. Top-to-the- fault northeast sense of shear is indicated by asym- metric feldspar tails and S-C fabrics observed in mylonitic basement outcrop. Within meters of the detachment fault, mylonitic fabrics are overprinted by chloritic breccia, and at the fault,