Alteration Mineralogy in Detachment Zones: Insights from Swansea, Arizona

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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.

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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.

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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, by gouge or micro- breccia. The existence of kilometer-scale folds Figure 2. A conceptual diagram showing corrugations in the low-angle Buckskin-Rawhide with east-northeast–trending axes in the lower detachment fault. (A) Despite an overall northwest-southeast extension direction, local dips plate is evident in folded compositional layer- are northwest-southeast due to the corrugations. The Buckskin Mountains are composed ing and folded foliation. The synformal upper of lower plate basement rocks, while the valleys are composed of folded upper plate rocks plate rocks of the Swansea syncline occupy a (B). Upper plate rocks in the Swansea area are only crudely synformal, unlike those in the northeast-trending valley between antiformal conceptual model. ridges of basement. The Buckskin-Rawhide detachment fault has a regional northeast dip, but locally dips toward the hinges of the synformal BACKGROUND (~200 m) landslide breccia composed primarily corrugations in the fault, resulting in a complex of clasts of pre-Tertiary rocks (Tbxl). Above the map pattern (Fig. 4). Northwest-, north-, and Stratigraphy breccia is a thin limestone bed (0.3–5 m) and northeast-striking faults and folds that deform a laterally discontinuous bed of pebbly sand- the northeast-trending antiforms and synforms A diverse package of volcanic, sedimentary, stone to sandy conglomerate (which is capped appear younger than the Buckskin-Rawhide and complexly deformed plutonic and metasedi- by a thin limestone bed, unit Tlm). A sequence detachment fault in some localities, but are older mentary rocks makes up the upper plate of the of maﬁ c lavas and associated volcaniclastic than it in others, suggesting that most of the Buckskin-Rawhide detachment fault in the sediments (Tb) overlies the limestone. Another deformation occurred near the same time. Buckskin Mountains (Spencer and Reynolds, landslide breccia overlies the volcanic suite 1989). The Tertiary strata include a range of vol- (Tbxu); clasts of this breccia include a range of Detachment-Related K-Metasomatism canic, volcaniclastic, and sedimentary units rep- lithologies, but are dominated by clasts of Tb. resenting subaerial sedimentation during Mio- The highest Miocene strata include ~200 m of K-metasomatism is common in detachment cene extension. Detailed stratigraphic work by siltstone, sandstone, tuff, and carbonates (Tsc). zones, suggesting an intimate relationship with Spencer and Reynolds (1986) has been adopted Pliocene weakly consolidated silty sand is pres- the detachment faulting process (Chapin and here, in a simpliﬁ ed form (Fig. 3). In the Swan- ent throughout the area (Tbf). Lindley, 1986). During K-metasomatism, the

sea area, the basal Tertiary strata consist of inter- Clast provenance within the Tertiary sequence overall K2O content of affected rocks is drasti-

bedded tuff, limestone, sandstone, and siltstone indicates that deposition was related to tec- cally increased, with concomitant loss in Na2O. (unit Ttls). These rocks are overlain by a thick tonic extension and the consequential unroof- Primary minerals and mesostatic phases are

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replaced by assemblages of K-feldspar (adularia) + hematite ± clay minerals, ± quartz, although primary textures are usually preserved (Chapin 1000 m and Lindley, 1986; Hollocher et al., 1994; Ennis et al., 2000). K-metasomatism in detachment Tsc: zones may be the result of low-temperature Conglomeratic diagenetic processes or high-temperature hydro- sandstone thermal processes. The vast size of the alteration zone in some cases argues for diagenetic processes. However, it is possible that in some cases, high-temperature fl uids, which extract K+ from propylitic alteration at depth, migrate Tbxu: along the fault, lubricating it during extension Upper breccia unit (Kerrich, 1988). In the Harcuvar Mountains, 800 m adjacent to the Buckskin Mountains, oxidized basin brines metasomatized upper plate tuff and mafi c lava fl ows into secondary K-feldspar– hematite–quartz mineralogy (Roddy et al., Tb: 1988). K-metasomatism was contemporaneous Mafic-intermediate lavas with detachment faulting and related to, but (and interbedded prior to, mineralization in the Harcuvar Moun- limestone and sandstone) tains (Roddy et al., 1988).

MINERALOGY OF THE SWANSEA 600 m Tlm: AREA FROM REMOTE SENSING DATA Limestone, sandy limestone, and sandstone Remote sensing data provide a synoptic view Miocene of the mineralogy of the ﬁ eld site. We used Landsat thematic mapper and thermal infrared multispectral scanner (TIMS) data. The Land- Tbxl: sat data set provides VNIR spectral information. Lower breccia unit thickness For the purpose of this study, we primarily used

approximate the VNIR data to provide morphologic context and to map phyllosilicate abundances. The 400 m TIMS data measure emitted thermal infrared radiation in 6 channels from (λ = ) 8–13 μm, and can be used to map silicate and carbonate rocks (e.g., Kealy and Hook, 1993; Hook et al., 1994; Rivard et al., 1993; Ramsey et al., 1999). The TIMS surface emissivity data of the Ttls: Swansea area show the major mineralogical Interbedded tuff, variation in the scene (Fig. 5). In this band com- conglomerate, bination (bands 5, 3, and 1 are red, green, and sandstone, silstone, and blue, respectively), quartz-rich materials are red, limestone felsic materials are pink, clay-rich materials are 200 m blue-purple, carbonates are green, and maﬁ c materials are light blue. The most quartz-rich areas in this image (reddest) correspond to Mesozoic quartzites and to old Quaternary surfaces that have accumulated pavements made Paleozoic of quartz-rich clasts. The pink areas correspond metacarbonates, and to felsic plutonic and volcanic rocks, and sedi- Precambrian-Tertiary ments composed of felsic materials. Dark blue granitoids and areas correspond to volcanic materials that have mylonites been altered to clay minerals and feldspars. 0 m Green areas represent Paleozoic metacarbon-

pC-Tertiary ates, Tertiary limestones, and massive hydrothermal carbonate. Vegetation has low spectral Figure 3. A simpliﬁ ed stratigraphic column of Tertiary rocks in the Swansea contrast and an overall similar spectral character area (after Spencer and Reynolds, 1989). to carbonates in these data.

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113o 52’ 55’’ W 113o 51’ 55’’ W 113o 50’ 55’’ W

34 o 10’ 09’’ N 10’ 09’’ N 09’’ 10’ o Figure 6 X 34

km 0 0.5 1

113o 52’ 55’’ W 113o 51’ 55’’ W 113o 50’ 55’’ W Geologic Units: Mine tailings Tb Miocene basalt-basaltic andesite (and interbedded sandstone) Qa Pleistocene-Holocene alluvium (undifferentiated) Tbxl Miocene breccia (lower unit) (includes Tlm)

Tbf Pliocene basin fill Ttls Miocene tuff, limestone, and sandstone Pzc Paleozoic meta-carbonates Tc Miocene massive secondary carbonate (mostly Martin Frm.) Tsc Miocene sandstone and conglomerate XTi Precambrian-Tertiary intrusive rocks

Tbxu Miocene breccia (upper unit) mc mylonitic crystalline rocks

Contact Fault

XXX Dike Fault (location approximate or inferred)

Figure 4. A geologic map modiﬁ ed from Spencer and Reynolds (1986). Map units: mine tailings; Qa—Quaternary alluvium (Pleistocene– Holocene); Tbf—Pliocene basin ﬁ ll; Tc—massive hydrothermal carbonate replacements; Tsc—Miocene sandstone and conglomerate; Tbxu—Miocene upper breccia unit; Tb—Miocene basalt-basaltic andesite and interbedded sandstones; Tbxl—Miocene lower breccia unit (includes unit Tlm, Miocene limestone); Ttls—Miocene tuff, limestone, and sandstone; Pzc—Paleozoic carbonates (mostly Martin Forma- tion); XTi—Precambrian or Tertiary intrusive rocks; mc—mylonitic crystalline rocks, Precambrian–Tertiary.

The TIMS data can be used to map silicate and shows the carbonate abundance using a spectral The TIMS carbonate index map shows sev- carbonate minerals. TIMS bands are positioned index from TIMS data. To produce the index, eral important details. The highest values in the to take advantage of Si-O stretching absorptions band 4 was divided by band 6 and the resulting map correspond to surfaces that are dominated in silicate minerals, but carbonate minerals con- data were stretched using a simple linear contrast by carbonate, the most obvious of which are the tain a C-O vibrational absorption in the spectral enhancement. Artiﬁ cial color values have been Miocene limestone unit (Tlm) and Paleozoic region of TIMS bands 5 and 6, which allows rela- applied to the data so that “hot” colors are car- marble (Pzc) and the ﬂ oat derived from them. A tive carbonate abundance to be mapped. Figure 5 bonate rich and “cold” colors are carbonate poor. narrow band of high-index values occurs along

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XTi + Qa Tb Tbxu 10’ 09’’ N 09’’ 10’

o Tbxl 34 10’ 09’’ N 09’’ 10’ o PZc 34 Tb PZc

XTmc 00Kilometers 1 0 0.5 1 Kilometers

abundant carb no carb Low carbonate in unit Tbxl

Intermediate abundances in unit Tb 10’ 09’’ N 09’’ 10’ o

X 34 10’ 09’’ N 09’’ 10’ o 34

Abundant carbonate along BRDF trace

Abundant mica abundant clay no clay in unit Tbxl 10’ 09’’ N 09’’ 10’ o 10’ 09’’ N 09’’ 10’ 34 o 34

Abundant clay minerals in unit Tb

113o 52’ 55’’ W 113o 51’ 55’’ W 113o 50’ 55’’ W

Figure 5. Mineralogy of the Swansea ﬁ eld area from remote sensing data. Thermal infrared multispectral scanner (TIMS) emissivity data (top) show that the Swansea site is dominated by lithologies composed of feldspars (pink), clays (blue-purple), carbonates (green), and quartz (red). A TIMS spectral index delineates relative carbonate abundance (middle) showing high carbonate abundances in various limestone units and massive secondary carbonates located along the trace of the Buckskin-Rawhide detachment fault (BRDF). Intermediate carbonate abundances are observed within the altered basalt unit (Tb). A spectral index using Landsat thematic mapper data maps the abundance of phyllosilicates (bottom), which show high abundances of mica and clay minerals in the lower breccia unit (Tbxl) and altered basalt unit (Tb), respectively.

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the Buckskin-Rawhide detachment fault trace. proxy for metasomatism, although our mapping of alteration minerals, including the relationship This corresponds to a zone of massive hydro- includes primary mineralogy, secondary clay between alteration minerals and bedrock units thermal replacement and illustrates the pervasive occurrences, and carbonate mapping. and structures, as well as crosscutting relation- nature of this carbonate along the fault zone in ships between various alteration phases. the Swansea area. A key observation is the occur- FIELD MAPPING OF ALTERATION Mapping results indicate that near the rence of intermediate carbonate values within the Buckskin-Rawhide detachment fault (within sev- basalt surface unit (Tb). These values correspond Field mapping of alteration mineralogy was eral meters), all lithologies are replaced by mas- to patchy carbonate replacements observed in the carried out in the vicinity of the mafi c vol canic sive carbonates, but away from the fault, alteration fi eld and suggest that the carbonate is pervasive suite (Tb) (Fig. 6). This subset region was is primarily tied to host lithology. At map scale, throughout the unit. The carbonate is not spa- chosen because sample and remote sensing the basalt is everywhere altered to 20%–40% tially related to the Buckskin-Rawhide detach- analyses indicate that unit Tb is the most highly brown carbonate, although locally (decimeter to ment fault (i.e., there is not a gradual decline altered area of the Swansea site. Mapping was meter scale) the carbonate can be dominant or in carbonate abundance moving away from the performed to understand the geologic context sparse. The carbonate primarily occurs as small fault zone). The lower breccia unit (Tbx), which is just beneath the basalt, contains essentially no carbonate. Together, these observations show that carbonate occurrence is controlled by (1) the N structure of the Buckskin-Rawhide detachment fault and (2) the host-rock lithology (within primary carbonate rocks, but also as occurrences of secondary carbonates in the basalt unit, but not in the breccia unit). The ratio of Landsat bands 5/7 measures metal-OH stretching in phyllosilicates, and the intensity of this band ratio can be used as a proxy for clay abundances in altered rocks (Abrams et al., 1977). In the Swansea area, the Landsat 5/7 ratio image delineates areas rich in micas and clay minerals. The most phyllosilicate-rich areas are the Tertiary basalt unit, the upper landslide breccia, and part of the lower landslide breccia. In the basalt unit and the upper landslide breccia, which is composed of clasts of basalt and tuff, the phyllosilicate detection likely corresponds to secondary clay minerals present within metasomatized rocks. In the lower breccia, the phyllosilicate detection corresponds to a landslide subunit that is dominated by clasts of schist (Spencer and Reynolds, 1986). 250 m The remote sensing data indicate that the Swansea site is dominated by carbonate and intermediate to felsic silicate minerals. Surface Geologic Units Alteration Units compositions refl ect a combination of original Qa Alluvium, undifferentiated Massive carbonate host-rock mineralogy and alteration mineralogy, Tb Basalt-basaltic andesite Patchy carbonate replacements and mineralogical associations illustrate key spa- Jasper Tlm Limestone tial relationships between structures, rock units, Celadonite and mineral occurrences. The remote sensing Ts Sandstone observations show that the Buckskin-Rawhide Tbxl Breccia (lower unit) Chloritic breccia detachment fault zone is totally replaced by Pzc Meta-carbonates Structures carbonate throughout the Swansea area, and XTi Intrusive igneous rocks (undifferentiated) Basalt dike that away from the immediate vicinity of the mc Mylonitic crystalline rocks Fault (location approximate) fault zone, metasomatic alteration (as mapped Fault by clay mineral and carbonate occurrence) is controlled by rock type rather than structure or Figure 6. A fi eld geologic map of altered volcanic units present in the Swansea area. Figure lo- proximity to the Buckskin-Rawhide detachment cation is shown in Figure 4. Geologic units are the same as the geologic map in Figure 4, but fault. The infrared mineral mapping used here is the boundary between the lower breccia (Tbxl) and the basalt (Tb) is further broken down similar to VNIR oxide mapping by Beratan et al. here to include a sandstone unit (Ts) and a limestone unit (Tlm) beneath the basalt. Alteration (1997), who used airborne visible infrared imag- units: massive carbonate replacements (~80%–100% carbonate); patchy carbonate replace- ing spectrometer (AVIRIS) data in the Whipple ments (~20%–40%); patchy jasper replacements (~1%–5%); patchy celadonite replacements Mountains to map red hematite occurrence as a (~1%–5%), usually associated with calcite; chloritic breccia.

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(1–5 cm) irregular patches and irregular veins breccia unit (Tbxl) has a bulk chemistry that is out on rock powders, both as random mounts 1–10 cm wide. Jasper occurs as irregular patchy roughly similar to granitic materials, refl ecting and, in some cases, on oriented clay mineral replacements and is present throughout the basalt their granitic and schist protoliths. The basal samples of the <0.2 μm particle size separates unit, although it is most strongly concentrated in section of the lower breccia shows fi eld evidence (see Moore and Reynolds, 1997). Two thermal the central region of the unit. At map scale, the for incorporation of soft carbonate sediment infrared techniques were used: a qualitative jasper only composes a small fraction (<1%) of into the base of a debris fl ow that emplaced the analysis of the bulk mineralogy of rocks and the basalt. Celadonite occurs within the basalt breccia, probably indicating that the landslide crystal chemistry of individual phases from at the 1%–5% level primarily as lining on white entered a shallow alkaline lake during emplace- whole-rock spectra (Fig. 7) (Michalski, 2005), calcite replacements. Carbonate, jasper, and ment. This emplacement environment explains and a linear spectral unmixing technique to celadonite replacements are not observed in other the elevated CaO content (13.8%) and high quantify bulk-rock mineralogy from rock spec- rock units, with the exception of the upper brec- K/Na ratio (2.5) in the basal section of the lower tra (Fig. 8) (e.g., Ramsey and Christensen, 1998; cia, which is composed of clasts of the basalt. breccia; the basalt portion of the fl ow may have Hamilton et al., 1997; Feely and Christensen, Some of the carbonate in the basalt is of been metasomatized by alkaline, briny lake 1999; Ruff, 1998). The spectral unmixing tech- primary origin; a bed of cross-laminated lacus- water after emplacement. Irregular patches nique uses an input library of known spectra to trine sandy carbonate was identifi ed within the and clasts of carbonate in the breccia matrix model the measured spectrum of a rock using a basalt, indicating that an alkaline lake formed in thin section suggest that the high CaO val- least squares minimization algorithm. at least once between eruptions. This unit is ues are probably indicative of both entrained The bulk mineralogy of rocks determined by laterally discontinuous and rapidly grades car bonates in the fl ow matrix and metasomatic infrared spectroscopy and XRD refl ects primary from a thin (10–30 cm) bed to a thick (~3 m) alteration. Samples from the Tertiary basalt (Tb) mineralogy of various host rocks and various massive bed of carbonate, which we interpret and upper breccia (Tbxu) units show evidence degrees of secondary alteration (Table 1). The as tufa related to Miocene hot springs. Other of extreme K-metasomatism. The K/Na ratios, most important mineralogical results come areas where more massive replacements of car- which are close to 1 in the other rock units, are from the basaltic and breccia units. Near the bonate are present within the basalt could also 104–526 in the basalt unit and 31–44 in the fault (sample 02048), the basalt is composed

be related to Miocene hot springs. upper breccia. Total K2O in these units (Tb and of mostly calcite, with minor abundances of Mylonitic crystalline rocks beneath the Tbxu) ranges from 8.8 to 11.8 wt%. High abun- K-feldspar and clay minerals. Away from the Buckskin-Rawhide detachment fault contain dances of CaO are observed in the basalt unit, Buckskin-Rawhide detachment fault, the aver- chloritic breccia, which is common in the vicin- and are concomitant with high loss on ignition age composition of unit Tb is ~40% calcite, ity of detachment faults (Kerrich, 1988; Half- (LOI) values and relatively low silica. Upper 40% K-feldspar, 15% clay minerals, and 5% kenny et al., 1989). In the Copper Penny area, plate granites (Jxg) have bulk chemistries sug- combined silica and hematite. Qualitative chloritic breccia is composed of highly brec- gestive of only weak alteration processes. Bulk analysis indicates that the clay mineral com- ciated granitic basement rocks and secondary silica, alumina, and alkalies are roughly similar ponent of unit Tb is illite. There is no evidence chlorite. Detachment-related chloritic breccia to unaltered granite. The K/Na ratios of the two from spectroscopy or XRD for primary miner- probably forms from hot, reducing fl uids at granitic samples studied are not indicative of als such as amphibole, pyroxene, plagioclase, or depth (Kerrich, 1988), and therefore we consider intense meta somatism (K/Na = 1.2, 1.9). Upper olivine in any of the basalt samples. The lower the chloritic breccia in the Swansea area to be a plate meta carbonates (Pzu) are rich in MgO breccia unit (Tbxl) is composed approximately unique alteration assemblage that is unrelated to (26.3–34.3 wt%), refl ecting their dolomitic of K-feldspar, intermediate plagioclase (oligo- the alteration of the upper plate rock units. character. However, while MgO appears to clase and andesine), silica (quartz and opal- have been extremely mobile in the basalt, MgO CT), musco vite, clay minerals, and traces of BULK-ROCK GEOCHEMISTRY was not removed in any signifi cant proportion carbonates and oxides. The upper breccia unit from the carbonate unit. Lower plate mylonitic (Tbxu) is dominated by K-feldspar, silica, and Rock samples were collected from a repre- rocks have bulk chemistries roughly similar to calcite. The lower plate breccia unit appears sentative suite of units in the Swansea–Copper unaltered granitic materials. Elevated Fe and Mg unmetasomatized because it is composed of pri- Penny area to understand the mineralogy and in the lower plate mylonite are related to chlorite- mary granitic-schistose materials. The basaltic geochemistry through stratigraphic and hori- rich veins and matrix in the chloritic breccia. unit (Tb) and the upper breccia (Tbxu) are both zontal spaces. Bulk-rock geochemistry was Sample 03–017, collected near the Swansea intensely altered (Fig. 9). Other rock units in the obtained by X-ray fl uorescence at the Geo- mine, shows evidence of Na-metasomatism area, such as the lower plate mylonitic granite analytical Laboratory at Washington State Uni- (K/Na = 0.11), and may represent a unique geo- (mc), upper plate granite (TXi), and various versity. Major element chemical analyses for 23 chemical setting related to mineralization. Sec- sedimentary rocks, are relatively unaltered, rock samples are reported in Table 1. ondary carbonates sampled from the fault zone composed of typical primary phases. Bulk chemistry of rock units is controlled by show evidence for both carbonate and silicate the primary lithology and degree of alteration. components, but have no evidence for detectable DETAILED ALTERATION Overall alteration trends include a net decrease Na. Ore deposits sampled from the Swansea and MINERALOGY

in SiO2, MgO, FeO*, and Na2O, and net Copper Penny mining areas are silica rich; the

increases in K2O and CaO. A basal limestone Swansea ore is extremely Fe rich. Analysis of detailed textural, chemical, and bed from unit Tlm has bulk chemistry indicative mineralogical relationships is critical to under- of both carbonate and silicate components (81.6 BULK-ROCK MINERALOGY standing the processes and timing of events

wt% CaO and 14.8 wt% SiO2). While there is through which rocks were altered in this detach-

some detectable K2O, there is no detectable Bulk-rock mineralogy was analyzed by opti- ment zone. Here we discuss the detailed altera-

Na2O, suggesting that the silicate component of cal petrography, X-ray diffraction (XRD), and tion mineralogy of the key rock units in the this rock has been K-metasomatized. The lower thermal infrared spectroscopy. XRD was carried Swansea area.

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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 1.02 .59 79.02 11 6 12 76 77 14.81 16.97 19 35604 237864 234316 233241 3-018 03-013 02-051 03-016 02-045 3-018 03-013 03-001 788 3784330 3784731 3783933 3784308 3784307 3784938 y basalt (Tb), Tertiary breccia—upper (Tbxu), Tertiary breccia—lower (Tbxl), mylonitic crystalline ary limestone (Tlm), and Tertiary hydrothermal carbonate (Tc). XRD minerals are calcite, K-feldspar hyllosilicates (phyll), and all others (other). (musc), dolomite (dol), and wollastonite (wo). Thermal infrared (TIR) minerals are alkali feldspars—K-feldspar and 11 4 12 15 15 21 15 14 17 16 22 12 17 47 24 21 15 14 17 5 2 25 1 2 4 6 2 1 1 64 54 25 1 2 64 4 6 2 1 1 27 24 21 31 16 34 10 27 24 21 31 12 51 7 43 22 42 27 16 50 TABLE 1. CHEMICAL AND MINERALOGICAL DATA FOR SAMPLES FROM SWANSEA, ARIZONA : UTM locations are given in easting (E) and northing (N) for zone 12 (NAD 1927). XRD—X-ray diffraction. Rock units are Tertiar 15.97 9.11 15.71 14.49 12.97 14.75 15.44 13.18 12.41 11.91 12.22 13.83 15.28 15.71 16.98 9.31 11.51 1.04 12.79 0.55 1.38 1.21 0.16 0.14 0.17 0.03 0.04 0.25 0.08 0.16 0.40 0.14 0.02 0.01 0.26 0.20 0.16 0.18 0.15 0.19 0.08 0.27 3 49.71 38.19 50.55 73.15 71.03 69.72 75.41 72.28 77.27 53.98 62.50 58.31 68.03 66.66 67.97 51.29 45.56 32.17 39.80 7.94 5.30 1.04 0.54 1.05 0.02 0.05 0.03 0.06 0.22 0.45 1.00 0.41 0.03 0.48 0.45 0.52 0.57 1.00 0.83 0.63 0.96 5 2 2 O 0.08 0.01 0.10 0.02 0.00 3.35 6.87 2.42 3.63 0.00 3.48 0.22 0.28 2.02 3.67 3.26 4.40 0.10 0.07 0.04 0.09 O 2 O O O 11.80 0.05 0.04 0.59 0.09 5.38 0.78 4.69 4.44 0.01 4.78 9.42 8.76 5.10 4.15 4.44 3.19 11.39 7.39 9.64 5.40 11.79 11.31 O 2 2 2 Note SiO Na FeO 5.36 6.66 6.10 1.76 2.29 0.13 0.88 25.58 0.95 1.33 1.73 6.17 2.32 2.54 2.87 4.43 3.43 3.82 2.82 7.29 5.80 5.24 6.45 K wo x x wo Unit Tb Tb Tb Tb Tb Tb Tb Tbxu Tbxu Tbxl Tbxl Tbxl mc mc mc JXg JXg Ore Ore Pzc Pzc Tlm Tc Tlm Pzc Pzc Ore Ore JXg JXg mc mc mc Tbxl Tbxl Tbxl Unit Tbxu Tb Tb Tbxu Tb Tb Sample 02-047 02-048 02-049 03-002 03-017 03-015 03-008 0 03-009 03-010 03-006 02-046 03-012 02-053 02-050 02-052 03-007 03-011 UTM E 234621 UTM 235072 234937 235315 235477 236376UTM N 235028 3784021 3783837 235881 3783974 235650 3783738 3783843 3785566 234143 3785188 3784705 234608 3784632 3783800 3784287 234362 3784490 3783767 3784960233146 235543 3784191 3784417 238051 3784 237674 236372 237986 233480 2 TiO CaO 14.87 36.44 13.44 13.33 22.18 43.34 29.74 20.24 11.80 13.82 3.17 3.74 2.83 1.46 2.86 1.05 0.83 0.26 0.66 55.56 64.31 81 64.31 55.56 1.46 2.86 1.05 0.83 0.26 0.66 13.82 3.17 3.74 2.83 11.80 20.24 29.74 0.33 43.34 1.03 22.18 14.87 0.42 36.44 13.44 13.33 0.66 0.33 0.79 0.62 0.01 0.04 0.05 0.02 0.00 0.52 0.21 0.50 0.09 0.02 0.33 0.77 1.23 1.01 MnO CaO MgO 0.68 2.47 26.26 0.81 1.08 0.67 33.42 0.29 0.64 1.33 0.38 0.79 0.03 0.44 0.70 1.35 1.50 1.54 1.18 0.68 0.24 0.58 0.52 MgO P tot 99 100 100 100 basement (mc), Jurassic or Precambrian granite (JXg), mineralized rocks from the mines (Ore), Paleozoic carbonates (Pzc), Terti 101 99 101 102 101 99 101 103 100 (kspar), non-albite plagioclase (plag), albite (al), hematite (hem), clay minerals (clay), quartz (qtz), muscovite or sericite 101 96 100 98 101 100 albite (alk), non-albite plagioclase (plag), quartz and opal (silica), total carbonate (carb), magnetite + hematite (oxides), p K/Na 146 526 116 112 158 180 104 44 31 2.5 1.1 1.4 0.7 1.6 0.11 1.9 1.2 0.00 1.4 2.5 0.00 0.00 0.00 0.00 146 104 526 2.5 0.00 1.4 44 180 116 31 158 1.9 1.2 0.00 112 K/Na 1.6 0.11 2.5 1.1 1.4 0.7 XRD x x x calcite x x x x kspar x x x x x plag x x x x x x Al TIR (%) (%) x x x x x al x x x x x x x x hem x x clay x x x x x x dol x x TIR alk 46 4 38 39 49 15 56 plag plag silica silica oxide 1 oxide 25 phyll 24 9 7 8 13 35 8 8 18 10 2 5 11 16 4 other 44 5 2 8 49 7 3 10 4 12 7 3 1 54 26 41 48 15 58 39 16 qtz x x x musc x x x x carb 19 carb 83 13 38 47 74 31

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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 25 20 20 Tlm calcite 15 25 15 12 Si-O, Si-O-M, and M-O Si-O-M, Si-O, in silicates ) ) -1 -1 m) 12 m) μ μ 10 Tbxl plagioclase muscovite quartz pectra of pure minerals. The sample spectra minerals. pectra of pure mple 03–049) shows evidence for only calcite, mple 03–049) shows evidence for ioclase, quartz, and phyllosilicates. al absorptions in silicate minerals; C-O, vibra- Wavelength ( Wavelength Wavenumber (cm Wavenumber Wavelength ( Wavelength Wavenumber (cm Wavenumber 10 8 Si-O in silicates C-O in calcite 7 , or other cations). Tertiary limestone (Tlm) is dominated by calcite Tertiary cations). other , or 8 2+ Si-O in silicates , Mg 2+ , Fe 6 3+

, Fe

Emissivity 3+ Emissivity 20 20 15 25 15 25 PZc dolomite calcite 12 12 ) Si-O, Si-O-M, and M-O Si-O-M, Si-O, in silicates -1 ) Tb illite K-feldspar calcite -1 m) μ m) μ 10 10 Wavelength ( Wavelength Wavenumber (cm Wavenumber Wavelength ( Wavelength Wavenumber (cm Wavenumber 8 8 Si-O in silicates Si-O in silicates C-O in dolomite 7 7

C-O in calcite

6 6

Emissivity Emissivity Figure 7. Thermal infrared emission spectra of samples of several geologic units in the Swansea area, compared with reference s with reference compared emission spectra of samples several geologic units in the Swansea area, Thermal infrared 7. Figure labeled: Si-O, vibration are absorption groups Various in those rocks. contain absorptions characteristic of the phases present Al tional absorptions in carbonates; and M-O-Si, vibrational silicates (M = with various minor silicates. Paleozoic metacarbonates (Pzc) show evidence for dolomite and minor silicates. The basalt (Tb; sa silicates. dolomite and minor silicates. Paleozoic metacarbonates (Pzc) show evidence for with various minor plag K-feldspar, unit (Tbxl) shows evidence for breccia The lower is observed. pyroxene and K-feldspar; no plagioclase or clay,

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Wavelength (μm) Tertiary Basalt 7 8 10 12 15 20 25 The basalt unit (Tb) contains a silicate suite of alteration minerals and patchy replacements of carbonate everywhere that it exists. Despite being totally replaced by secondary mineralogy, the silicate portion of the basalt retains a volcanic plagioclase-porphyritic texture in thin section. Plagioclase phenocrysts

are totally replaced by K-feldspar (Or97) (Table 2), which commonly occurs as microcrystalline aggregates. The groundmass has Spectrum of Tbxl (02-046)

Emissivity been totally replaced by a dark red-brown, Spectral model fit mottled material, which appears to be pre- 24% alkali feldspar dominantly composed of Fe-bearing clay 11% plagioclase feldspar minerals. Based on chemistry measured with 15% silica (quartz + opal-CT) the electron microprobe, the formula of this 41% total phyllosilicates (muscovite + Al-clays) clay is calculated as (assuming a single clay 10% other minerals (carbonate, sulfate, oxide) 3+ mineral phase) (K0.72Ca0.03)(Al1.12Mg0.37Fe0.47)

(Al0.29Si3.71)O10(OH)2, which is characteristic of dioctahedral clay with chemistry interme- Wavenumber (cm-1) diate between illite and glauconite. Refl ected light microscopy shows that fi nely dissemi- Figure 8. A thermal infrared emissivity spectrum of a sample of lower breccia unit Tbxl nated hematite is present within the ground- and a model-fi t spectrum. Mineral abundances derived from linear spectral deconvolu- mass in low abundances (<5%), and in cases tion are shown. where these grains are not easily avoided with the microprobe, they could contribute to the interpreted Fe content of the clay minerals. In some cases amphibole pseudomorphs are observed in the groundmass, although no Mineralogy K+/Na+ amphiboles are observed in the rock. 0 40 80% 0 1 10 100 1000 All of the basalt samples show signifi -

Tbxu Clay + cant carbonate replacement; the carbonate K-fel

+ m usually occurs as patchy replacements and

dsp Plag ica veins that obliterated primary textures. Dis- ar rupted pieces of rock with basaltic texture Tb + are chaoti cally distributed throughout the Ca rbo car bonate. This and embayment relationships na te Normal range at the boundary between the carbonate and K-feldspar–groundmass texture suggest that Tlm K O = 0.59 / Na O = 0 2 2 the carbonate replacement occurred after the K-metasomatism (Fig. 10). In addition, some K-feldspar is partially replaced by car bonate along twin planes or as microcrystalline + Tbxl patchy replacements within feldspar crystals. Petrographically, the carbonate appears either translucent or with a mottled brown color. It is difﬁ cult to tell optically whether the carbonate is calcite with an oxide stain, siderite, or Pzc + both, but XRD and thermal infrared analyses of the bulk-rock samples and samples of the carb secondary carbonate alone all suggest that mc calcite is the dominant carbonate. Electron + microprobe analysis of the carbonate indicates that it is Ca rich (Table 2). Some of the carbonate replacements have an exterior rim of Figure 9. The relationship between mineralogy, K/Na ratios, and stratigraphy. K-meta- green-yellow clay. Electron microprobe chem- somatism was strongly localized in the basalt unit (Tb) and debris-ﬂ ow breccia composed of istry of the outer green clay suggests a chemi- 3+ volcanic materials (Tbxu). Due to lower reactivity and permeability, the lower breccia unit cal formula of (K0.72Ca0.03)(Al0.90Mg0.39Fe0.68)

is not K-metasomatized. See Figure 4 for rock units. (Al0.27Si3.73)O10(OH)2.

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TABLE 2. CHEMISTRY OF MINERALS IN BASALT Green clays are also intergrown within the FROM MICROPROBE ANALYSES carbonates, toward the interior of carbonate K-feldspar Calcite Matrix clay Hematite (wt%) (wt%) (wt%) (wt%) replacements. These green clays are more Fe rich: (K )(Al Mg Fe3+ )(Al Si )O (OH) . SiO2 65.8 0.1 50.1 2.2 0.86 0.41 0.55 0.96 0.08 3.92 10 2

Al2O3 17.2 0.0 16.1 0.6 Based on these chemical measurements, the MgO 0.0 0.0 3.3 0.0 interior green clay is similar to celadonite. Fe O 0.0 0.2 8.4 93.8 2 3 Detailed XRD analysis was performed on CaO 0.1 56.3 0.4 0.2 μ Na O 0.2 0.0 0.0 0.0 oriented clay mounts of the <2 m (average 2 diameter) size fraction of pulverized basalt K2O 14.0 0.1 7.7 0.4 MnO 0.0 1.1 0.0 0.2 material to determine the structure and crystal Total 97.30 57.78 86.12 97.44 chemistry of the matrix clay (the brown mottled clay replacing maﬁ c matrix materials). Dif- fraction data were collected using a Siemens diffractometer with a Cu-Kα radiation source from 2° to 35° 2 θ in 0.2° steps. The sample was run both in air-dried and ethylene glycol– treated preparations to determine if expand- able (smectite) layers are present. The results suggest that the dominant clay mineral in the basalt is an Fe-rich dioctahedral clay with lim- ited expandability, similar to illite, although more Fe rich than typical illite (Fig. 11). The XRD analysis supports an interpretation of ferruginous illite present in the basalt. To summarize, the basalt unit appears to have been totally replaced by secondary minerals, which formed in two phases. An initial phase of metasomatism converted all of the original minerals to secondary K-feldspar, Fe-rich illite, and hematite, with minor quartz, while preserv- ing the original texture of the rock. A second phase of alteration disrupted the original texture and resulted in abundant calcite intergrown with minor celadonite and minor hematite.

Breccia Units

The primary focus here is on the lower breccia unit (Tbxl), which can be split into upper and basal units. The upper portion of the lower breccia unit (Tbxl) consists of clasts of feldspar- sericite-quartz schist in a dark, mottled matrix. 500 μm Feldspar is microcrystalline overall, although some large (100 μm diameter) crystals of plagioclase are observed. The fine-grained matrix appears to be mineralogically similar to the clasts, although it also contains minor carbonate carbonate and possibly clay minerals. XRD basalt and thermal infrared spectroscopy results indi- replacement texture cate that andesine and oligoclase are present in both the matrix and the clasts; this observation strongly suggests that K-metasomatism has not affected the lower breccia unit (Tbxl), at least in the upper section of the unit. Figure 10. A fi eld photo of the altered basalt unit (Tb) and a photomicrograph (crossed The basal section of the lower breccia has polars) of a sample of the rock. The fi eld photo shows patchy, light carbonate replacements petrographic relationships similar to those of the observed within what otherwise looks like an unaltered volcanic rock. In the photomicro- rest of the lower breccia, but the matrix is richer graph, the dark area with plagioclase-porphyritic texture is composed of metasomatic clay in carbonate, consistent with fi eld relationships minerals, hematite, and K-feldspar. Although K-metasomatism preserves the original basalt suggesting entrainment of lacustrine sediments texture, later carbonate replacement obliterates previous textures. during emplacement. The slightly higher K/Na

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plate rocks have been variably altered to illite, K-feldspar, calcite, hematite, celadonite, and 02-049 (< 2 μm) ethylene glycol solvated quartz. The observations described here provide 001 02-049 (< 2 μm) air dried - offset for clarity insight into (1) the context of alteration minerals in detachment zones, (2) the composition of hydrothermal ﬂ uids associated with detachment faulting, (3) ﬂ uid conduits and pathways, and (4) the relative timing of alteration and tectonism. 003 On the basis of textural evidence and crosscutting relationships, upper plate volcanic rocks in the Swansea area were affected by two epi-

Intensity sodes of alteration. During the fi rst episode the primary mineralogy of basaltic rocks (and land- 002 slide debris composed of them) was intensely c K-metasomatized. Although the primary rock textures were preserved, all of the plagioclase was altered to K-feldspar, and amphibole and mafi c groundmass in these rocks were totally altered to illite + hematite. The second episode involved dissolution of silicate metasomatic θ alteration minerals and precipitation of calcite, 2- hematite, and celadonite. The carbonate depo- Figure 11. X-ray diffraction data of oriented clay mounts of the <2 μm size fraction of sition clearly occurred after the fi rst episode of sample 02–049 (unit Tb; see Fig. 4). Both air-dried and ethylene glycol–treated samples K-metasomatism, although the actual difference are shown. The major refl ections correspond to {001}, {002}, and {003} planes in the clay. in time may have been small. The strong peak near 9° 2 θ corresponds to a d-spacing of 10.1 Å, consistent with illite. The While general models for detachment-related location of this peak is the same for both the ethylene glycol–treated (expanded) and air- alteration consider fl uids of magmatic, meta- dried (not expanded) samples, indicating that the clay is entirely illitic with no appreciable morphic, deep hydrothermal, and meteoric ori- amount of smectite layering. The {002} peak is somewhat depressed compared to typical gins, most recent research favors alteration by illite due to scattering by octahedral iron in the Swansea clay. Evidence for calcite, which is low-temperature meteoric fl uids in detachment present as a minor admixture, is marked c. terranes (Chapin and Lindley, 1986; Roddy et al., 1988; Beratan, 1999). We interpret the metasomatic alteration and carbonate replacement at Swansea as low-temperature phe- ratio of the lower breccia (e.g., sample 03–006; the metasomatic alteration. The breccia unit nomena related to alkaline lakes and shallow Table 1) hints that it may be slightly meta- contains clasts of granite and schist, and a sig- groundwater of a meteoric origin. Several lines somatized, but XRD suggests that oligoclase and nifi cant component of fi ne-grained matrix mate- of evidence point to a low-temperature metaso- andesine remain. It is plausible that metasoma- rial. The presence of low-permeability fi nes in matic origin for the secondary minerals present tized matrix material existed within an alkaline the breccia could have protected it from meta- in Tertiary rocks in the Swansea area. First, the lake at the surface and was incorporated within somatic fl uids in part. In contrast, basalt can geologic context of altered rock units indicates the breccia unit when it was emplaced. be a relatively permeable or impermeable rock that they were deposited in a sedimentary basin. A dike of basalt present within the lower brec- type, depending on the thickness of the unit, the The presence of siltstone, sandstone, and lime- cia shows similar evidence for K-metasomatism . internal fracturing, and presence of internal fl ow stone above, below, and within the basalt unit XRD of the dike material shows no evidence for horizons. Although it is diffi cult to know the all point to deposition of the lava in what was primary minerals; only K-feldspar, clay miner- original primary permeability of the basalt, it is an active fl uvio-lacustrine system. In particular, als, hematite, and carbonates remain. However, possible that it had greater inherent permeability the presence of tufa within the basalt is evidence the matrix material present in the breccia in the than the breccia unit. that lakes existed within and/or on the basalt immediate vicinity of the dike is only partially while it was still cooling. However, no pillows metasomatized. Matrix material away from the ALTERATION PROCESSES AND or hyaloclastites were observed; thus lava fl ow- dike is much less metasomatized. The obser- GEOLOGIC IMPLICATION ing into a standing body of water lacks support. vation of a highly metasomatized basalt dike Therefore, it may be more likely that the lake present within relatively unmetasomatized Well-exposed, highly altered rocks in the was shallow and transient. Second, the mineral- breccia is important. This may indicate that Swansea area provide insight into fundamen- ogy of alteration products indicates that the fl uid

K-metasomatism is controlled solely by reac- tal detachment-related alteration processes. was oxidizing and CO2 rich, which is consistent tivity of rocks; the dike is more reactive than the Remote sensing, fi eld, and laboratory analyses with near-surface sources. Third, the mineral- breccia unit. However, this explanation is not show not only the identity of alteration miner- izing fl uid was alkali rich, which is consistent suffi cient because the breccia unit also contains als, but also their abundance, distribution, and with surface waters that have leached ions from plagioclase feldspar, which is highly reactive context (Fig. 12). In the Swansea area, lower surface materials, potentially felsic pyroclastic to the metasomatizing solutions. Permeability plate rocks have been altered to a chlorite and rocks (White, 1984; Hoch et al., 1999). The must also play an important role in controlling quartz within and below the fault zone. Upper Fe-K–rich clays at Swansea are stabilized by

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Normal faults present throughout the area Transient lake surface K I Ca Ce H S Cl did not localize K-metasomatism or carbonate K-Meta Groundwater alteration. In the basalt unit, the normal faults Carb expose pervasively altered sections of the basalt, but there is no indication that alteration is more intense within the normal fault zones. Normal faulting postdates K-metasomatism and carbonate replacement. Ore deposits Mineralogical, remote sensing, geochemical, with burial/downfault Increasing temperature Increasing Chloritic breccia and ﬁ eld observations from Swansea contribute D along the detachment ee p re fault duced insights into fundamental detachment-related fluid Heat source: s alteration processes (Fig. 12). Similar to other Hot lower- te S detachment environments, upper plate rocks ra he p plate rocks ar sli Heat provided by zon at Swansea were severely chemically altered

Depth warm lower plate e by saline alkaline fl uids of meteoric origins. The pervasive nature of the alteration points to Temperature vigorous alteration environment with high water/ rock ratios. However, the alteration of upper plate Figure 12. An idealized model illustrating the structural relationships in detachment zones, rocks by convecting basin brines was strongly fl uid conduits, and the spatial relationships between K-metasomatism, carbonate replace- affected by permeability. At Swansea, the altera- ments, chloritic breccia, and ore deposits. The heat that drives hydrothermal activity comes tion was concurrent with detachment faulting, from the lower plate rocks, which have been extracted from depth during extremely rapid but prior to the formation of normal faults asso- extension. Higher slip rates therefore result in higher heat fl ow. At least two fl uid sources are ciated with it. Detailed mineralogy shows at least present—a shallow, oxidized, alkaline fl uid of meteoric origin, and a deeper, less oxidized, two separate metasomatic assemblages, prob- hotter fl uid migrating along the fault. K-metasomatism occurs at shallow levels in the crust, ably related to two alteration episodes or increas- perhaps even at the surface in transient alkaline lakes. Overprinting carbonate replace- ing heat fl ow due to burial or increased exten- ments occur at slightly higher temperatures, perhaps during progressive burial and move- sion rate. Most of the K-metasomatized volcanic ment downfault. Ore deposits occur at higher temperatures, where fl uids of different origins rocks at Swansea are not mineralized, although mix, carrying metals removed from altered upper place rocks. The inset shows inferred other rock units in the vicinity are mineralized. alteration zones based on observations at Swansea and previous work: K-feldspar (K), illite Simple mass-balance considerations provide (I), calcite (Ca), celadonite (Ce), hematite (H), silica (S), and chlorite (Cl). insight into the origin of alteration minerals and ore deposits at Swansea. The carbonate alteration associated with the metasomatized basalt

higher aK and higher pH, lending further sup- migrated along the several-meter-thick fault unit has increased the total CaO abundance of port that the metasomatic fl uids in the Swansea zone. Away from the fault zone, K-metasomatism the basalt to ~25%. Compared to an average area were at least slightly alkaline. Similar clay is pervasive in reactive rock units, such as the value for unaltered basalt of ~9%–10%, this minerals have been observed in modern alkaline basalt (Tb). The lack of K-metasomatism of increase requires a signifi cant infl ux of Ca to saline lakes (Jones and Weir, 1983). granitic materials in the lower debris-fl ow unit explain the alteration; the Ca required could Although secondary calcite occurs within could be due to much lower overall reactivity not have been liberated solely by replacement K-metasomatized rocks elsewhere (Roddy et al., of these materials compared to the basalt. How- of plagioclase. The Fe content of the basalt was 1988; Hollocher et al., 1994), the carbon- ever, these granitic materials contain intermedi- drastically reduced during K-metasomatism. ate alteration observed at Swansea is striking, ate plagioclase, which should have been altered Given the map area of the basalt in the Swan- widespread, and unusually abundant. While the to K-feldspar if exposed to the same solutions sea area (0.75 km2), conservative estimates of K-metasomatism is essentially isovolumetric that altered the basalt. The presence of unaltered average thickness (100 m) and previous density and preserves textures, the carbonate alteration plagioclase in the debris fl ow points toward (2800 kg/m3), and an estimate of FeO content disrupts original textures and fl oods all available permeability as a factor in controlling metaso- of unaltered basalt (10% FeO), the total change void space. A reasonable explanation for the ori- matizing fl uids. The debris fl ow contains coarse in Fe content of the basalt, i.e., the amount gin of the later-stage carbonate replacements is clasts of granite and schist, but also a large com- of liberated Fe, can be calculated. Alteration precipitation due to increased fl uid temperatures ponent of fi ne materials. Plagioclase occurs in of the basalt at Swansea liberated ~4 × 106 t as the upper plate rocks became progressively both the coarse and fi ne clastic components. of metallic Fe. The average Cu content of the buried and moved down-fault. Calcite also Hydrothermal fl uids may have fl owed through basalt is ~30 ppm. Using a conservative estimate occurs as a late-stage phenomenon in fractures the more permeable volcanic units more than of 100 ppm average Cu content of the basalt, in the adjacent Harcuvar Mountains (Roddy through the debris-fl ow unit, resulting in dis- ~14,700 t of Cu were removed from the basalt. et al., 1988), albeit in trace abundances. parate degrees of alteration. Leising et al. (1995) Historical records show that 12,026 t of Cu ore at The occurrence of detachment-related altera- discussed the importance of groundwater con- 2.43% average grade, or 292 t of Cu metal, were tion in the Swansea area was controlled by struc- vection in the process of K-metasomatism. mined from Swansea (Spencer and Welty, 1989). ture, reactivity of host rocks, and permeability. Lateral and vertical transport of hydrothermal These simplifi ed calculations show that com- Total replacement of all rocks within meters of solutions is predicted in heterogeneous strati- plete alteration of the basalt could have produced the Buckskin-Rawhide detachment fault zone graphic sections that have been structurally dis- the Cu and Fe present in mineral deposits at suggests that extremely reactive, hot fl uids rupted and dismembered. Swansea; a deep crustal source is not required.

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CONCLUSIONS Brooks, W.E., 1986, Distribution of anomalously high spectroscopy of terrestrial materials [Ph.D. thesis]: K2O volcanic rocks in Arizona: Metasomatism at Tempe, Arizona State University, 215 p. the Picacho Peak detachment fault: Geology, v. 14, Moore, D.M., and Reynolds, R.C., Jr., 1997, X-ray diffrac- Results from Swansea, Arizona, provide insight p. 339–342, doi: 10.1130/0091-7613(1986)14<339: tion and the identifi cation and analysis of clay miner- into detachment-related alteration and miner- DOAHKV>2.0.CO;2. als: New York, Oxford University Press, 378 p. Chapin, C.E., and Lindley, J.I., 1986, Potassium metaso- Ramsey, M.S., and Christensen, P.R., 1998, Mineral abun- alization processes. Volcanic rocks at Swansea matism of igneous and sedimentary rocks in detach- dance determination: Quantitative deconvolution of ther- were pervasively and totally altered to K-feldspar, ment terranes and other sedimentary basins: Economic mal emission spectra: Journal of Geophysical Research, ferruginous illite, hematite, and jasper in an ini- implications, in Beatty, B., and Wilkinson, P.A.K., eds., v. 103, no. B1, p. 577–596, doi: 10.1029/97JB02784. Frontiers in geology and ore deposits of Arizona and Ramsey, M.S., Christensen, P.R., Lancaster, N., and Howard, tial stage of K-metasomatism, and subsequently the Southwest: Arizona Geological Society Digest, D.A., 1999, Identifi cation of sand sources and transport overprinted by calcite, hematite, and celadonite v. 16, p. 118–126. pathways at the Kelso Dunes, California, using thermal Davis, G.A., Anderson, J.L., Frost, E.G., and Shackelford, infrared remote sensing: Geological Society of Amer- replacement. These two episodes of alteration are T.J., 1980, Mylonitization and detachment faulting in ica Bulletin, v. 111, p. 646–662, doi: 10.1130/0016- interpreted to have occurred at shallow depths the Whipple-Buckskin-Rawhide Mountains terrane, 7606(1999)111<0646:IOSSAT>2.3.CO;2. by convecting basin brines. The intrinsic perme- southeastern California and western Arizona, in Crit- Reynolds, S.J., and Lister, G.S., 1987, Structural aspects tenden, M.D., et al., eds., Cordilleran metamorphic of fl uid-rock interactions in detachment zones: Geol- ability and reactivity of each rock type controlled core complexes: Geological Society of America Mem- ogy, v. 15, p. 362–366, doi: 10.1130/0091-7613(1987) the alteration pattern. K-metasomatism and car- oir 153, p. 79–129. 15<362:SAOFII>2.0.CO;2. bonate replacement happened prior to normal Ennis, D.J., Dunbar, N.W., Campbell, A.R., and Chapin, Reynolds, S.J., and Spencer, J.E., 1985, Evidence for C.E., 2000, The effects of K-metasomatism on the large-scale transport on the Bullard detachment fault, faulting associated with the detachment fault. mineralogy and geochemistry of silicic ignimbrites west-central Arizona: Geology, v. 13, p. 353–356, doi: Metals liberated from the alteration of volcanic near Socorro, New Mexico: Chemical Geology, v. 167, 10.1130/0091-7613(1985)13<353:EFLTOT>2.0.CO;2. p. 285–312, doi: 10.1016/S0009-2541(99)00223-5. Rivard, B.S., Petroy, B., and Miller, J.R., 1993, Measured rocks would have been transported in the brine Feely, K.C., and Christensen, P.R., 1999, Quantitative com- effect of desert varnish on the mid- infrared spectra of and could have produced the ore deposits inter- positional analysis using thermal emission spectros- weathered rocks as an aid to TIMS-imagery interpre- preted to have formed at greater depth, along the copy: Application to igneous and metamorphic rocks: tation: Geoscience and Remote Sensing, v. 31, no. 1, Journal of Geophysical Research, v. 104, no. E10, p. 284–291. detachment fault zone. These results demonstrate p. 24,195–24,210, doi: 10.1029/1999JE001034. Roddy, M.S., Reynolds, S.J., Smith, B.M., and Ruiz, J., the importance of K-metasomatism by meteoric Halfkenny, R.D., Kerrich, R., and Rehrig, W.A., 1989, Fluid 1988, K-metasomatism and detachment- related fl uids in detachment zones and the signifi cance of regimes and geochemical mass transport in the devel- mineralization, Harcuvar Mountains, Arizona: Geo- opment of mylonites and chloritic breccias at Copper logical Society of America Bulletin, v. 100, p. 1627– permeability differences in alteration of rocks by Penny, Buckskin Mountains, in Spencer, J.E., and 1639, doi: 10.1130/0016-7606(1988)100<1627: convecting groundwater in these environments. Reynolds, S.J., eds., Geology and mineral resources KMADRM>2.3.CO;2. of the Buckskin and Rawhide Mountains, west-central Ruff, S.W., 1998, Quantitative thermal emission spectros- Normal faults have long been considered impor- Arizona: Arizona Geological Survey Bulletin 198, copy applied to granitoid petrology [Ph.D. thesis]: tant fl uid conduits in detachment zones, but this p. 190–202. Tempe, Arizona State University, 234 p. may not be true in the upper crust, where meta- Hamilton, V.E., Christensen, P.R., and McSween, H.Y.J., Spencer, J.E., and Reynolds, S.J., 1986a, Geologic map of 1997, Determination of Martian meteorite lithologies the Swansea–Copper Penny area, central Buckskin somatic alteration occurs. Simple mass-balance and mineralogies using vibrational spectroscopy: Jour- Mountains, west-central Arizona: Tucson, Arizona relationships show that intense metasomatism of nal of Geophysical Research, v. 102, p. 25,593–25,603, Bureau of Geology and Mineral Technology. volcanic rocks can liberate signifi cant quantities doi: 10.1029/97JE01874. Spencer, J.E., and Reynolds, S.J., 1986b, Geologic map Hoch, A.R., Reddy, M.M., and Drever, J.I., 1999, Importance of the Planet–Mineral Hill area, northwest Buckskin of metals that could produce ore deposits from of mechanical disaggregation in chemical weathering in Mountains, west-central Arizona: Tucson, Arizona the same fl uid at greater depth, along the detach- a cold alpine environment, San Juan Mountains, Colo- Bureau of Geology and Mineral Technology. rado: Geological Society of America Bulletin, v. 111, Spencer, J.E., and Reynolds, S.J., 1989, Tertiary structure, ment fault. p. 304–314, doi: 10.1130/0016-7606(1999)111<0304: stratigraphy, and tectonics of the Buckskin Mountains, IOMDIC>2.3.CO;2. in Spencer, J.E., and Reynolds, S.J., eds., Geology and ACKNOWLEDGMENTS Hollocher, K., Spencer, J.E., and Ruiz, J., 1994, Compo- mineral resources of the Buckskin and Rawhide Moun- sitional changes in an ash-fl ow cooling unit during tains, west-central Arizona: Arizona Geological Survey K-metasomatism, west central Arizona: Economic Bulletin 198, p. 103–167. Funding for this project was received in part from Geology, v. 89, p. 877–888. Spencer, J.E., and Welty, J.W., 1986, Possible controls of Geological Society of America student grants to Hook, S.J., Karlstrom, K.E., Miller, C.F., and McCaffrey, base- and precious-metal mineralization associated Michalski, the J.H. Courtright Scholarship from the K.J.W., 1994, Mapping the Piute Mountains, Califor- with Tertiary detachment faults in the lower Colorado Arizona Geological Society, the Arizona State Univer- nia, with thermal infrared multispectral scanner (TIMS) River trough, Arizona and California: Geology, v. 14, sity graduate research support program, and the Plan- images: Journal of Geophysical Research, v. 99, p. 195–198, doi: 10.1130/0091-7613(1986)14<195: etary Imaging and Analysis Facility and Advanced no. B8, p. 15,605–15,622, doi: 10.1029/94JB00690. PCOBAP>2.0.CO;2. 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