Alteration mineralogy in detachment zones: Insights from Swansea, Arizona

Extreme tectonic extension of southwestern North America during the Oligocene–Miocene Epochs resulted in the formation of several major detachment structures (Davis et al., 1980; Spencer et al., 1995), including the Buckskin-Rawhide detachment fault in western Arizona. In the vicinity of Swansea, Arizona (Fig. 1), the fault juxtaposes Precambrian–Tertiary plutonic and metamorphic lower plate rocks against upper plate Precambrian, Paleozoic, Mesozoic, and Tertiary sedimentary, metasedimentary, volcanic, and plutonic rocks. The gently east-northeast–dipping Buckskin-Rawhide detachment fault is corrugated so that upper plate rocks are folded into synforms and exposed in east-northeast–trending valleys between low mountain ranges of lower plate basement (Fig. 2). Locally, the detachment fault is offset by normal faults that strike northwest, north, and northeast. In the Buckskin Mountains, and in the Swansea area in particular, K-metasomatism, massive carbonate replacements, and Cu-Fe mineralization are associated with Buckskin-Rawhide detachment fault and upper plate rocks (Spencer and Reynolds, 1986a, 1986b; Spencer and Welty, 1989).

Alteration and mineralization commonly accompany detachment faulting (Wilkins et al., 1989) and hydrothermal fluids seem to play an intimate role in the faulting process ( Reynolds and Lister, 1987). Lower plate rocks in detachment zones are usually chloritized and brecciated in the vicinity of the fault by deep, hot, reducing fluids (Kerrich, 1988; Halfkenny et al., 1989). Upper plate rocks are commonly K-metasomatized, probably by shallow, oxidizing, warm meteoric fluids (Brooks, 1986; Chapin and Lindley, 1986; Kerrich, 1988; Roddy et al., 1988). Mineralization along detachment faults may occur when metal-rich brines present in upper plate rocks contact the lower plate of the fault, which contains an amount of stored heat significant to drive hydrothermal convection and fluid mixing (Spencer and Welty, 1986; Kerrich, 1988; McKibben et al., 1988; Halfkenny et al., 1989; Wilkins et al., 1989).

Excellent exposures of extremely altered and structurally complex rocks in the Swansea area provide a fantastic opportunity to study the geochemical, mineralogical, and temporal relationships of detachment-related alteration. The occurrence of widespread secondary carbonate at Swansea is striking and unique, possibly indicat ing an advanced stage of meta-somatic alteration. In this study we investigated the mineralogy, geochemistry, and context of altered rocks in the Swansea area to understand the mineralogical effects of metasomatic fluids on primary rocks, fluid conduits responsible for metasomatic alteration, and the timing of structural deformation and mineralogical alteration in detachment zones. Of particular interest at Swansea is the origin of the secondary carbonates and their relationship to fundamental detachment-related K-metasomatism processes.

We use remote sensing analyses in addition to fieldmappingtoidentifythespatialdistributionof altered rocks in the Swansea area. Remote sensing was applied to studies of K-metasomatism by Beratan et al. (1997) and Beratan (1999): in those studies, the authors applied visible-near infrared (VNIR) data (λ = 0.5–3 µm) to identify the extent of alteration from the occurrence of red hematite. In this study, we utilize both VNIR and thermal infrared (λ = 6–30 µm) spectral data to map alteration and host-rock mineralogy in the Swansea area. Thermal infrared spectroscopy is a powerful mineralogical tool because essentially all minerals and mineraloids contain vibrational spectral absorptions in the thermal spectral range. Therefore, instead of mapping only hematite as a proxy for alteration, we are able to map the occurrence of clay minerals and carbonates, which are components of the alteration assemblage in the Swansea area. We utilize a spectral unmixing routine to determine rock mineralogy from thermal infrared laboratory spectra of field samples. These data are used in combination with more traditional techniques to identify the nature and extent of alteration mineralogy in the Swansea area.

A diverse package of volcanic, sedimentary, and complexly deformed plutonic and metasedi-mentary rocks makes up the upper plate of the Buckskin-Rawhide detachment fault in the Buckskin Mountains (Spencer and Reynolds, 1989). The Tertiary strata include a range of volcanic, volcaniclastic, and sedimentary units representing subaerial sedimentation during Miocene extension. Detailed stratigraphic work by Spencer and Reynolds (1986) has been adopted here, in a simplified form (Fig. 3). In the Swan-sea area, the basal Tertiary strata consist of inter-bedded tuff, limestone, sandstone, and siltstone (unit Ttls). These rocks are overlain by a thick (∼200 m) landslide breccia composed primarily of clasts of pre-Tertiary rocks (Tbxl). Above the breccia is a thin limestone bed (0.3–5 m) and a laterally discontinuous bed of pebbly sandstone to sandy conglomerate (which is capped by a thin limestone bed, unit Tlm). A sequence of mafic lavas and associated volcaniclastic sediments (Tb) overlies the limestone. Another landslide breccia overlies the volcanic suite (Tbxu); clasts of this breccia include a range of lithologies, but are dominated by clasts of Tb. The highest Miocene strata include ∼200 m of siltstone, sandstone, tuff, and carbonates (Tsc). Pliocene weakly consolidated silty sand is present throughout the area (Tbf).

Clast provenance within the Tertiary sequence indicates that deposition was related to tectonic extension and the consequential unroofing of progressively older rocks (Spencer and Reynolds, 1989). The stratigraphy is generally 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, punctuated with massive debris flows related to over-steepened topography (Spencer and Reynolds, 1989). Pulses of volcanism are evident in lava and tuff deposits, and contribute a large volume of volcaniclastic sediments. Tectonically driven slope instability resulted in the massive debris flows that deposited breccia intermittently into a lacustrine-fluvial setting.

The structural geology of the Buckskin Mountains was comprehensively summarized by Spencer and Reynolds (1989). The Buckskin-Rawhide detachment fault is a low-angle extensional fault with >55 km of offset (Reynolds and Spencer, 1985). Mylonitic lineation in the lower plate trends 040°–050° and is considered to be subparallel to extension direction. Top-to-the-northeast sense of shear is indicated by asymmetric feldspar tails and S-C fabrics observed in 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 with east-northeast–trending axes in the lower plate is evident in folded compositional layering and folded foliation. The synformal upper plate rocks of the Swansea syncline occupy a northeast-trending valley between antiformal ridges of basement. The Buckskin-Rawhide detachment fault has a regional northeast dip, but locally dips toward the hinges of the synformal corrugations in the fault, resulting in a complex map pattern (Fig. 4). Northwest-, north-, and northeast-striking faults and folds that deform the northeast-trending antiforms and synforms appear younger than the Buckskin-Rawhide detachment fault in some localities, but are older than it in others, suggesting that most of the deformation occurred near the same time.

K-metasomatism is common in detachment zones, suggesting an intimate relationship with the detachment faulting process (Chapin and Lindley, 1986). During K-metasomatism, the overall K₂O content of affected rocks is drasti-cally increased, with concomitant loss in Na₂O. Primary minerals and mesostatic phases are replaced by assemblages of K-feldspar ( adularia) + hematite ± clay minerals, ± quartz, although primary textures are usually preserved (Chapin and Lindley, 1986; Hollocher et al., 1994; Ennis et al., 2000). K-metasomatism in detachment zones may be the result of low-temperature diagenetic processes or high-temperature hydrothermal 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 fluids, which extract K⁺ from propylitic alteration at depth, migrate along the fault, lubricating it during extension (Kerrich, 1988). In the Harcuvar Mountains, adjacent to the Buckskin Mountains, oxidized basin brines metasomatized upper plate tuff and mafic lava flows into secondary K-feldspar–hematite–quartz mineralogy (Roddy et al., 1988). K-metasomatism was contemporaneous with detachment faulting and related to, but prior to, mineralization in the Harcuvar Mountains (Roddy et al., 1988).

Remote sensing data provide a synoptic view of the mineralogy of the field site. We used Landsat thematic mapper and thermal infrared multispectral scanner (TIMS) data. The Land-sat data set provides VNIR spectral information. For the purpose of this study, we primarily used the VNIR data to provide morphologic context and to map phyllosilicate abundances. The 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 Swansea area show the major mineralogical variation in the scene (Fig. 5). In this band combination (bands 5, 3, and 1 are red, green, and blue, respectively), quartz-rich materials are red, felsic materials are pink, clay-rich materials are blue-purple, carbonates are green, and mafic 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 of quartz-rich clasts. The pink areas correspond to felsic plutonic and volcanic rocks, and sediments composed of felsic materials. Dark blue areas correspond to volcanic materials that have been altered to clay minerals and feldspars. Green areas represent Paleozoic metacarbonates, Tertiary limestones, and massive hydrothermal carbonate. Vegetation has low spectral contrast and an overall similar spectral character to carbonates in these data.

The TIMS data can be used to map silicate and carbonate minerals. TIMS bands are positioned to take advantage of Si-O stretching absorptions in silicate minerals, but carbonate minerals contain a C-O vibrational absorption in the spectral region of TIMS bands 5 and 6, which allows relative carbonate abundance to be mapped. Figure 5 shows the carbonate abundance using a spectral index from TIMS data. To produce the index, band 4 was divided by band 6 and the resulting data were stretched using a simple linear contrast enhancement. Artificial color values have been applied to the data so that “hot” colors are carbonate rich and “cold” colors are carbonate poor.

The TIMS carbonate index map shows several important details. The highest values in the map correspond to surfaces that are dominated by carbonate, the most obvious of which are the Miocene limestone unit (Tlm) and Paleozoic marble (Pzc) and the float derived from them. A narrow band of high-index values occurs along the Buckskin-Rawhide detachment fault trace. This corresponds to a zone of massive hydrothermal replacement and illustrates the pervasive nature of this carbonate along the fault zone in the Swansea area. A key observation is the occurrence of intermediate carbonate values within the basalt surface unit (Tb). These values correspond to patchy carbonate replacements observed in the field and suggest that the carbonate is pervasive throughout the unit. The carbonate is not spatially related to the Buckskin-Rawhide detachment fault (i.e., there is not a gradual decline in carbonate abundance moving away from the 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 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).

The remote sensing data indicate that the Swansea site is dominated by carbonate and intermediate to felsic silicate minerals. Surface compositions reflect a combination of original host-rock mineralogy and alteration mineralogy, and mineralogical associations illustrate key spatial relationships between structures, rock units, and mineral occurrences. The remote sensing observations show that the Buckskin-Rawhide detachment fault zone is totally replaced by carbonate throughout the Swansea area, and that away from the immediate vicinity of the fault zone, metasomatic alteration (as mapped by clay mineral and carbonate occurrence) is controlled by rock type rather than structure or proximity to the Buckskin-Rawhide detachment fault. The infrared mineral mapping used here is similar to VNIR oxide mapping by Beratan et al. (1997), who used airborne visible infrared imaging spectrometer (AVIRIS) data in the Whipple Mountains to map red hematite occurrence as a proxy for metasomatism, although our mapping includes primary mineralogy, secondary clay occurrences, and carbonate mapping.

Field mapping of alteration mineralogy was carried out in the vicinity of the mafic vol canic suite (Tb) (Fig. 6). This subset region was chosen because sample and remote sensing analyses indicate that unit Tb is the most highly altered area of the Swansea site. Mapping was performed to understand the geologic context of alteration minerals, including the relationship between alteration minerals and bedrock units and structures, as well as crosscutting relationships between various alteration phases.

Mapping results indicate that near the Buckskin-Rawhide detachment fault (within several meters), all lithologies are replaced by massive carbonates, but away from the fault, alteration is primarily tied to host lithology. At map scale, the basalt is everywhere altered to 20%–40% brown carbonate, although locally (decimeter to meter scale) the carbonate can be dominant or sparse. The carbonate primarily occurs as small (1–5 cm) irregular patches and irregular veins 1–10 cm wide. Jasper occurs as irregular patchy replacements and is present throughout the basalt unit, although it is most strongly concentrated in the central region of the unit. At map scale, the jasper only composes a small fraction (<1%) of the basalt. Celadonite occurs within the basalt at the 1%–5% level primarily as lining on white calcite replacements. Carbonate, jasper, and celadonite replacements are not observed in other rock units, with the exception of the upper breccia, which is composed of clasts of the basalt.

Some of the carbonate in the basalt is of primary origin; a bed of cross-laminated lacustrine sandy carbonate was identified within the basalt, indicating that an alkaline lake formed at least once between eruptions. This unit is laterally discontinuous and rapidly grades from a thin (10–30 cm) bed to a thick (∼3 m) massive bed of carbonate, which we interpret as tufa related to Miocene hot springs. Other areas where more massive replacements of carbonate are present within the basalt could also be related to Miocene hot springs.

Mylonitic crystalline rocks beneath the Buckskin-Rawhide detachment fault contain chloritic breccia, which is common in the vicinity of detachment faults (Kerrich, 1988; Halfkenny et al., 1989). In the Copper Penny area, chloritic breccia is composed of highly brecciated granitic basement rocks and secondary chlorite. Detachment-related chloritic breccia probably forms from hot, reducing fluids at depth (Kerrich, 1988), and therefore we consider the chloritic breccia in the Swansea area to be a unique alteration assemblage that is unrelated to the alteration of the upper plate rock units.

Rock samples were collected from a representative suite of units in the Swansea–Copper Penny area to understand the mineralogy and geochemistry through stratigraphic and horizontal spaces. Bulkrock geochemistry was obtained by X-ray fluorescence at the Geo-analytical Laboratory at Washington State University. Major element chemical analyses for 23 rock samples are reported in ⁰¹.

Bulk chemistry of rock units is controlled by the primary lithology and degree of alteration. Overall alteration trends include a net decrease in SiO₂, MgO, FeO*, and Na₂O, and net increases in K₂O and CaO. A basal limestone bed from unit Tlm has bulk chemistry indicative of both carbonate and silicate components (81.6 wt% CaO and 14.8 wt% SiO₂). While there is some detectable K₂O, there is no detectable Na₂O, suggesting that the silicate component of this rock has been K-metasomatized. The lower breccia unit (Tbxl) has a bulk chemistry that is roughly similar to granitic materials, reflecting their granitic and schist protoliths. The basal section of the lower breccia shows field evidence for incorporation of soft carbonate sediment into the base of a debris flow that emplaced the breccia, probably indicating that the landslide entered a shallow alkaline lake during emplacement. This emplacement environment explains the elevated CaO content (13.8%) and high K/Na ratio (2.5) in the basal section of the lower breccia; the basalt portion of the flow may have been metasomatized by alkaline, briny lake water after emplacement. Irregular patches and clasts of carbonate in the breccia matrix in thin section suggest that the high CaO values are probably indicative of both entrained carbonates in the flow matrix and metasomatic alteration. Samples from the Tertiary basalt (Tb) and upper breccia (Tbxu) units show evidence of extreme K-metasomatism. The K/Na ratios, which are close to 1 in the other rock units, are 104–526 in the basalt unit and 31–44 in the upper breccia. Total K₂O in these units (Tb and Tbxu) ranges from 8.8 to 11.8 wt%. High abundances of CaO are observed in the basalt unit, and are concomitant with high loss on ignition (LOI) values and relatively low silica. Upper plate granites (Jxg) have bulk chemistries suggestive of only weak alteration processes. Bulk silica, alumina, and alkalies are roughly similar to unaltered granite. The K/Na ratios of the two granitic samples studied are not indicative of intense meta somatism (K/Na = 1.2, 1.9). Upper plate meta carbonates (Pzu) are rich in MgO (26.3–34.3 wt%), reflecting their dolomitic character. However, while MgO appears to have been extremely mobile in the basalt, MgO was not removed in any significant proportion from the carbonate unit. Lower plate mylonitic rocks have bulk chemistries roughly similar to unaltered granitic materials. Elevated Fe and Mg in the lower plate mylonite are related to chlorite-rich veins and matrix in the chloritic breccia. Sample 03–017, collected near the Swansea mine, shows evidence of Nametasomatism (K/Na = 0.11), and may represent a unique geochemical setting related to mineralization. Secondary carbonates sampled from the fault zone show evidence for both carbonate and silicate components, but have no evidence for detectable Na. Ore deposits sampled from the Swansea and Copper Penny mining areas are silica rich; the Swansea ore is extremely Fe rich.

Bulk-rock mineralogy was analyzed by optical petrography, X-ray diffraction (XRD), and thermal infrared spectroscopy. XRD was carried out on rock powders, both as random mounts and, in some cases, on oriented clay mineral samples of the <0.2 µm particle size separates (see Moore and Reynolds, 1997). Two thermal infrared techniques were used: a qualitative analysis of the bulk mineralogy of rocks and crystal chemistry of individual phases from whole-rock spectra (Fig. 7) (Michalski, 2005), and a linear spectral unmixing technique to quantify bulk-rock mineralogy from rock spectra (Fig. 8) (e.g., Ramsey and Christensen, 1998; Hamilton et al., 1997; Feely and Christensen, 1999; Ruff, 1998). The spectral unmixing technique uses an input library of known spectra to model the measured spectrum of a rock using a least squares minimization algorithm.

The bulk mineralogy of rocks determined by infrared spectroscopy and XRD reflects primary mineralogy of various host rocks and various degrees of secondary alteration ⁰¹. The most important mineralogical results come from the basaltic and breccia units. Near the fault (sample 02048), the basalt is composed of mostly calcite, with minor abundances of K-feldspar and clay minerals. Away from the Buckskin-Rawhide detachment fault, the average composition of unit Tb is ∼40% calcite, 40% K-feldspar, 15% clay minerals, and 5% combined silica and hematite. Qualitative analysis indicates that the clay mineral component of unit Tb is illite. There is no evidence from spectroscopy or XRD for primary minerals such as amphibole, pyroxene, plagioclase, or olivine in any of the basalt samples. The lower breccia unit (Tbxl) is composed approximately of K-feldspar, intermediate plagioclase (oligo-clase and andesine), silica (quartz and opal-CT), muscovite, clay minerals, and traces of carbonates and oxides. The upper breccia unit (Tbxu) is dominated by K-feldspar, silica, and calcite. The lower plate breccia unit appears unmetasomatized because it is composed of primary granitic-schistose materials. The basaltic unit (Tb) and the upper breccia (Tbxu) are both intensely altered (Fig. 9). Other rock units in the area, such as the lower plate mylonitic granite (mc), upper plate granite (TXi), and various sedimentary rocks, are relatively unaltered, composed of typical primary phases.

Analysis of detailed textural, chemical, and mineralogical relationships is critical to understanding the processes and timing of events through which rocks were altered in this detachment zone. Here we discuss the detailed alteration mineralogy of the key rock units in the Swansea area.

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) ⁰², which commonly occurs as microcrystalline aggregates. The groundmass has been totally replaced by a dark red-brown, mottled material, which appears to be predominantly composed of Fe-bearing clay minerals. Based on chemistry measured with the electron microprobe, the formula of this clay is calculated as (assuming a single clay mineral phase) (K_0.72Ca_0.03)(Al_1.12Mg_0.37Fe_0.47³⁺)(Al_0.29Si_3.71)O₁₀(OH)₂, which is characteristic of dioctahedral clay with chemistry intermediate between illite and glauconite. Reflected light microscopy shows that finely disseminated hematite is present within the ground-mass in low abundances (<5%), and in cases 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 amphiboles are observed in the rock.

All of the basalt samples show significant carbonate replacement; the carbonate usually occurs as patchy replacements and veins that obliterated primary textures. Disrupted pieces of rock with basaltic texture are chaoti cally distributed throughout the carbonate. This and embayment relationships at the boundary between the carbonate and K-feldspar–groundmass texture suggest that 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 patchy replacements within feldspar crystals. Petrographically, the carbonate appears either translucent or with a mottled brown color. It is difficult to tell optically whether the carbonate is calcite with an oxide stain, siderite, or both, but XRD and thermal infrared analyses of the bulk-rock samples and samples of the secondary carbonate alone all suggest that calcite is the dominant carbonate. Electron microprobe analysis of the carbonate indicates that it is Ca rich ⁰². Some of the carbonate replacements have an exterior rim of green-yellow clay. Electron microprobe chemistry of the outer green clay suggests a chemical formula of (K_0.72Ca_0.03)(Al_0.90Mg_0.39Fe_0.68³⁺) (Al_0.27Si_3.73)O₁₀(OH)₂.

Green clays are also intergrown within the carbonates, toward the interior of carbonate replacements. These green clays are more Fe rich: (K_0.86)(Al_0.41Mg_0.55Fe_0.96³⁺)(Al_0.08 Si_3.92)O₁₀(OH)₂.

Based on these chemical measurements, the interior green clay is similar to celadonite.

Detailed XRD analysis was performed on oriented clay mounts of the <2 µm (average diameter) size fraction of pulverized basalt material to determine the structure and crystal chemistry of the matrix clay (the brown mottled clay replacing mafic matrix materials). Diffraction 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 expandable (smectite) layers are present. The results suggest that the dominant clay mineral in the basalt is an Fe-rich dioctahedral clay with limited 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 preserving 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.

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. 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 and possibly clay minerals. XRD and thermal infrared spectroscopy results indicate 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.

The basal section of the lower breccia has petrographic relationships similar to those of the rest of the lower breccia, but the matrix is richer in carbonate, consistent with field relationships suggesting entrainment of lacustrine sediments during emplacement. The slightly higher K/Na ratio of the lower breccia (e.g., sample 03–006; ⁰¹) hints that it may be slightly metasomatized, but XRD suggests that oligoclase and andesine remain. It is plausible that metasomatized matrix material existed within an alkaline lake at the surface and was incorporated within the breccia unit when it was emplaced.

A dike of basalt present within the lower breccia shows similar evidence for K-metasomatism . XRD of the dike material shows no evidence for primary minerals; only K-feldspar, clay minerals, hematite, and carbonates remain. However, the matrix material present in the breccia in the immediate vicinity of the dike is only partially metasomatized. Matrix material away from the dike is much less metasomatized. The observation of a highly metasomatized basalt dike present within relatively unmetasomatized breccia is important. This may indicate that K-metasomatism is controlled solely by reactivity of rocks; the dike is more reactive than the breccia unit. However, this explanation is not sufficient because the breccia unit also contains plagioclase feldspar, which is highly reactive to the metasomatizing solutions. Permeability must also play an important role in controlling the metasomatic alteration. The breccia unit contains clasts of granite and schist, and a significant component of fine-grained matrix material. The presence of low-permeability fines in the breccia could have protected it from metasomatic fluids in part. In contrast, basalt can be a relatively permeable or impermeable rock type, depending on the thickness of the unit, the internal fracturing, and presence of internal flow horizons. Although it is difficult to know the original primary permeability of the basalt, it is possible that it had greater inherent permeability than the breccia unit.

Well-exposed, highly altered rocks in the Swansea area provide insight into fundamental detachment-related alteration processes. Remote sensing, field, and laboratory analyses show not only the identity of alteration minerals, but also their abundance, distribution, and context (Fig. 12). In the Swansea area, lower plate rocks have been altered to a chlorite and quartz within and below the fault zone. Upper plate rocks have been variably altered to illite, K-feldspar, calcite, hematite, celadonite, and quartz. The observations described here provide insight into (1) the context of alteration minerals in detachment zones, (2) the composition of hydrothermal fluids associated with detachment faulting, (3) fluid conduits and pathways, and (4) the relative timing of alteration and tectonism.

On the basis of textural evidence and cross-cutting relationships, upper plate volcanic rocks in the Swansea area were affected by two episodes of alteration. During the first episode the primary mineralogy of basaltic rocks (and landslide debris composed of them) was intensely K-metasomatized. Although the primary rock textures were preserved, all of the plagioclase was altered to K-feldspar, and amphibole and mafic groundmass in these rocks were totally altered to illite + hematite. The second episode involved dissolution of silicate metasomatic alteration minerals and precipitation of calcite, hematite, and celadonite. The carbonate deposition clearly occurred after the first episode of K-metasomatism, although the actual difference in time may have been small.

While general models for detachment-related alteration consider fluids of magmatic, metamorphic, deep hydrothermal, and meteoric origins, most recent research favors alteration by low-temperature meteoric fluids in detachment terranes (Chapin and Lindley, 1986; Roddy et al., 1988; Beratan, 1999). We interpret the metasomatic alteration and carbonate replacement at Swansea as low-temperature phenomena related to alkaline lakes and shallow groundwater of a meteoric origin. Several lines of evidence point to a low-temperature metasomatic origin for the secondary minerals present in Tertiary rocks in the Swansea area. First, the geologic context of altered rock units indicates that they were deposited in a sedimentary basin. The presence of siltstone, sandstone, and limestone above, below, and within the basalt unit all point to deposition of the lava in what was an active fluviolacustrine system. In particular, the presence of tufa within the basalt is evidence that lakes existed within and/or on the basalt while it was still cooling. However, no pillows or hyaloclastites were observed; thus lava flowing into a standing body of water lacks support. Therefore, it may be more likely that the lake was shallow and transient. Second, the mineralogy of alteration products indicates that the fluid rich, which is consistent was oxidizing and CO₂ with near-surface sources. Third, the mineralizing fluid was alkali rich, which is consistent with surface waters that have leached ions from surface materials, potentially felsic pyroclastic rocks (White, 1984; Hoch et al., 1999). The Fe-K–rich clays at Swansea are stabilized by higher a_K and higher pH, lending further support that the metasomatic fluids in the Swansea area were at least slightly alkaline. Similar clay minerals have been observed in modern alkaline saline lakes (Jones and Weir, 1983).

Although secondary calcite occurs within K-metasomatized rocks elsewhere (Roddy et al., 1988; Hollocher et al., 1994), the carbonate alteration observed at Swansea is striking, widespread, and unusually abundant. While the K-metasomatism is essentially isovolumetric and preserves textures, the carbonate alteration disrupts original textures and floods all available void space. A reasonable explanation for the origin of the later-stage carbonate replacements is precipitation due to increased fluid temperatures as the upper plate rocks became progressively buried and moved down-fault. Calcite also occurs as a late-stage phenomenon in fractures in the adjacent Harcuvar Mountains (Roddy et al., 1988), albeit in trace abundances.

The occurrence of detachment-related alteration in the Swansea area was controlled by structure, reactivity of host rocks, and permeability. Total replacement of all rocks within meters of the Buckskin-Rawhide detachment fault zone suggests that extremely reactive, hot fluids migrated along the several-meter-thick fault zone. Away from the fault zone, K-metasomatism is pervasive in reactive rock units, such as the basalt (Tb). The lack of K-metasomatism of granitic materials in the lower debris-flow unit could be due to much lower overall reactivity of these materials compared to the basalt. However, these granitic materials contain intermediate plagioclase, which should have been altered to K-feldspar if exposed to the same solutions that altered the basalt. The presence of unaltered plagioclase in the debris flow points toward permeability as a factor in controlling metaso-matizing fluids. The debris flow contains coarse clasts of granite and schist, but also a large component of fine materials. Plagioclase occurs in both the coarse and fine clastic components. Hydrothermal fluids may have flowed through the more permeable volcanic units more than through the debris-flow unit, resulting in disparate degrees of alteration. Leising et al. (1995) discussed the importance of groundwater convection in the process of K-metasomatism. Lateral and vertical transport of hydrothermal solutions is predicted in heterogeneous stratigraphic sections that have been structurally disrupted and dismembered.

Normal faults present throughout the area did not localize K-metasomatism or carbonate alteration. In the basalt unit, the normal faults 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.

Mineralogical, remote sensing, geochemical, and field observations from Swansea contribute insights into fundamental detachment-related alteration processes (Fig. 12). Similar to other detachment environments, upper plate rocks at Swansea were severely chemically altered by saline alkaline fluids of meteoric origins. The pervasive nature of the alteration points to vigorous alteration environment with high water/rock ratios. However, the alteration of upper plate rocks by convecting basin brines was strongly affected by permeability. At Swansea, the alteration was concurrent with detachment faulting, but prior to the formation of normal faults associated with it. Detailed mineralogy shows at least two separate metasomatic assemblages, probably related to two alteration episodes or increasing heat flow due to burial or increased extension rate. Most of the K-metasomatized volcanic rocks at Swansea are not mineralized, although other rock units in the vicinity are mineralized.

Simple mass-balance considerations provide insight into the origin of alteration minerals and ore deposits at Swansea. The carbonate alteration associated with the metasomatized basalt unit has increased the total CaO abundance of the basalt to ∼25%. Compared to an average value for unaltered basalt of ∼9%–10%, this increase requires a significant influx of Ca to explain the alteration; the Ca required could not have been liberated solely by replacement of plagioclase. The Fe content of the basalt was drastically reduced during K-metasomatism. Given the map area of the basalt in the Swan-sea area (0.75 km²), conservative estimates of average thickness (100 m) and previous density (2800 kg/m³), and an estimate of FeO content of unaltered basalt (10% FeO), the total change in Fe content of the basalt, i.e., the amount of liberated Fe, can be calculated. Alteration of the basalt at Swansea liberated ∼4 × 10⁶ t of metallic Fe. The average Cu content of the basalt is ∼30 ppm. Using a conservative estimate of 100 ppm average Cu content of the basalt, ∼14,700 t of Cu were removed from the basalt. Historical records show that 12,026 t of Cu ore at 2.43% average grade, or 292 t of Cu metal, were mined from Swansea (Spencer and Welty, 1989). These simplified calculations show that complete alteration of the basalt could have produced the Cu and Fe present in mineral deposits at Swansea; a deep crustal source is not required.

Results from Swansea, Arizona, provide insight into detachment-related alteration and mineralization processes. Volcanic rocks at Swansea were pervasively and totally altered to K-feldspar, ferruginous illite, hematite, and jasper in an initial stage of K-metasomatism, and subsequently overprinted by calcite, hematite, and celadonite replacement. These two episodes of alteration are interpreted to have occurred at shallow depths by convecting basin brines. The intrinsic permeability and reactivity of each rock type controlled the alteration pattern. K-metasomatism and carbonate replacement happened prior to normal faulting associated with the detachment fault. Metals liberated from the alteration of volcanic rocks would have been transported in the brine and could have produced the ore deposits interpreted to have formed at greater depth, along the detachment fault zone. These results demonstrate the importance of K-metasomatism by meteoric fluids in detachment zones and the significance of permeability differences in alteration of rocks by convecting groundwater in these environments. Normal faults have long been considered important fluid conduits in detachment zones, but this may not be true in the upper crust, where metasomatic alteration occurs. Simple mass-balance relationships show that intense metasomatism of volcanic rocks can liberate significant quantities of metals that could produce ore deposits from the same fluid at greater depth, along the detachment fault.

Funding for this project was received in part from Geological Society of America student grants to Michalski, the J.H. Courtright Scholarship from the Arizona Geological Society, the Arizona State University graduate research support program, and the Planetary Imaging and Analysis Facility and Advanced Training Institute (sponsor award 1230449). We thank Lynda Williams for support in the analysis of clay mineralogy of our samples.