Chapter 17. Institute for Mineral Resources, Department of Geosciences, University of Arizona, 1040 East Fourth Street, Tucson, Arizona

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1 2010 Society of Economic Geologists, Inc. Special Publication 15, pp Chapter 17 Characterization and Reconstruction of Multiple Copper-Bearing Hydrothermal Systems in the Tea Cup Porphyry System, Pinal County, Arizona PHILLIP A. NICKERSON, MARK D. BARTON, AND ERIC SEEDORFF Institute for Mineral Resources, Department of Geosciences, University of Arizona, 1040 East Fourth Street, Tucson, Arizona Abstract This study exploits a cross-sectional view of the Laramide magmatic arc in the northern Tortilla Mountains, central Arizona, that was created by tilting during severe Tertiary extension of the Basin and Range province. Building upon earlier work, we combine the results of geologic mapping of rock types, structures, and hydrothermal alteration styles, with a palinspastic reconstruction, to provide a system-wide understanding of the evolution of the composite magmatic and hydrothermal Tea Cup porphyry system. Geologic mapping revealed the presence of at least three, and possibly four, mineralizing hydrothermal systems in the study area that are associated with widespread potassic, sericitic, greisen, sodic (-calcic), and propylitic alteration. The alteration envelops both porphyry copper and porphyry molybdenum (-copper) mineralization. Two areas flanking compositionally distinct units of the composite Tea Cup pluton are characterized by intense potassic and sericitic alteration. Intense alteration and mineralization akin to iron oxide-copper-gold systems was recognized in several areas. The U-Pb dating of zircons from porphyry dikes suggests that hydrothermal activity in the study area was short lived (~73 72 Ma). Subsequently, between ~25 and 15 Ma, the Tea Cup porphyry system was tilted ~90 to the east and extended by >200 percent due to movement on five superimposed sets of nearly planar normal faults. Each fault set was initiated with dips of ~60 to 70, but modern dips range from 70 to 15 overturned from the youngest to the oldest set. Tertiary normal faulting resulted in the exposure of pieces of the porphyry system from paleodepths of >10 km. Palinspastic reconstruction of a ~30-km-long cross section reveals that the Tea Cup pluton formed by sequential intrusion of at least four compositionally distinct units. Each major unit generated its own hydrothermal system. The most intense alteration in each hydrothermal system formed above the cupolas of each major phase of the pluton. Potassic alteration dominates the core of each system, whereas feldspar-destructive acid alteration overlaps with the potassic alteration but also extends to higher levels within each system. Deep sodic (-calcic) alteration overlain by iron oxide-rich chlorite-sericite-pyrite alteration flanks these central systems and generally extends 2 to 4 km away from the center of the hydrothermal systems. Greisen-style alteration was recognized 1 to 2 km beneath the potassic alteration in one porphyry copper system but overlaps and extends above the exposed porphyry molybdenum (-copper) system. Propylitic alteration occurs in a distal position and surrounds the other alteration styles. The alteration mapping, combined with the palinspastic reconstruction, revealed two covered exploration targets centered on intense potassic alteration, demonstrating that palinspastic reconstruction represents a powerful exploration technique in a district with more than 100 years of exploration history. Introduction PORPHYRY copper deposits in the Laramide (ca Ma) province of southwestern North America have received considerable attention at the scale of individual orebodies (Titley and Hicks, 1966; Titley, 1982a). Recent recognition of alteration assemblages formed on the distal flanks and roots of the systems (Seedorff et al., 2008) suggests, however, that a system-scale understanding of their evolution still remains to be developed. One fact that has hindered a system-scale understanding so far is the complex normal faulting that dismembered most Laramide porphyry systems after their emplacement. Faulting commonly separates originally adjacent parts of porphyry systems by kilometers and juxtaposes unrelated parts of the systems (Proffett, 1977; Stavast et al., 2008). System-scale understanding of dismembered copper porphyry systems requires careful attention to postmineral structural geology. The effects of extension can be removed through palinspastic reconstruction of the systems whereby Corresponding author: , nickersp@ .arizona.edu the dismembered pieces of the hydrothermal systems are put back into context. The exposure of different paleodepths in the multiple fault blocks then becomes an aid to understanding timing and processes of copper porphyry formation. For example, root zones beneath mineralization, which would most likely never be drilled in an upright system, can be exhumed and, therefore, examined in footwall blocks of normal faults (Seedorff et al., 2008). Exploration in the Basin and Range province can utilize the fact that extension often preserves and hides pieces of mineral deposits, leaving vectors toward mineralization to the structurally aware geologist. The structural complexities in highly faulted areas of the Basin and Range province have been studied extensively (e.g., Crittenden et al., 1980; Dickinson, 1991; Davis et al., 2004) and have produced long-standing controversies that center on the geometry, timing, and magnitude of normal faulting (Wernicke, 1981; Lister and Davis, 1989; Miller et al., 1999; Maher, 2008). Economic geologists working in extended porphyry systems provided some of the first insights into this debate (Lowell, 1968; Proffett, 1977) and continue to refine the 299

2 300 NICKERSON ET AL. understanding of this issue utilizing knowledge gained from drill hole data and geophysical methods, which are not commonly available to structural geologists (Dilles and Gans, 1995; Wilkins and Heidrick, 1995; Stavast et al., 2008). This study describes the rock types, structure, and hydro - thermal alteration and provides a palinspastic reconstruction of the Laramide Tea Cup porphyry system in the northern Tortilla Mountains, Pinal County, Arizona. The results are based on new field work, combined with earlier mapping by Schmidt (1971), Cornwall and Krieger (1975a, b), Bradfish (1979), Richard and Spencer (1997), and Barton et al. (2005). The new mapping for this study and the work of Barton et al. (2005) paid particular attention to the characterization of hydrothermal alteration, the recognition of internal variations in the igneous rocks, and the structural geology of the area. These features, in combination with the general geologic patterns reported in earlier work, are critical to interpreting the structural evolution of the study area and thereby reconstructing the magmatic and related hydrothermal systems at Tea Cup. Geologic Setting The Tea Cup porphyry system is located in the northern Tortilla Mountains of east-central Arizona, the heart of the porphyry copper belt of southwestern North America (Fig. 1), and within an area that has been the focus of long-standing investigation (e.g., Ransome, 1903; Cornwall, 1982; Maher, 2008). Major nearby porphyry copper districts include Ray (Phillips et al., 1974), Globe-Miami (Peterson, 1962), Superior (Hammer and Peterson, 1968; Manske and Paul, 2002), Poston Butte (Nason et al., 1982), Christmas (Koski and Cook, 1982), Mission-Pima (Barter and Kelly, 1982), and Sierrita-Esperanza (West and Aiken, 1982). Exploration for porphyry copper deposits in the region has ebbed and flowed in tandem with copper prices (Lowell, 1978; Paul and Manske, 2005), with times of intense exploration mainly in the late 19th and middle 20th centuries. The discovery of the Resolution deposit near Superior in the mid- 1990s has renewed the interest of both junior and major mining companies in the region. FIG. 1. Geologic map of south-central Arizona, showing the study area, nearby porphyry copper deposits, the Catalina core complex, and the Tortilla Mountains (geology from Reynolds, 1988) /98/000/ $

3 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 301 The metamorphic basement of the northern Tortilla Mountains is represented by the Mesoproterozoic Pinal Schist (ca. 1.7 Ga). These crystalline rocks were intruded by the Madera Diorite at 1.6 Ga and the anorogenic Ruin Granite at 1.4 Ga (Fig. 2). Subsequently, the basement was beveled and unconformably overlain by approximately 1 km of dominantly siliciclastic sedimentary rocks of the Proterozoic Apache Group and Troy Quartzite. Near the time of deposition of the Troy Quartzite, the siliciclastic sedimentary sequence and the underlying crystalline rocks were intruded by diabase sheets, sills, and dikes, dated at ~1.1 Ga (Shride, 1967; Wrucke, 1989). The diabase sheets are most abundant in the upper 1 km of the Ruin Granite and in the overlying sedimentary rocks of the Apache Group (Howard, 1991). The consistent orientation and depth of emplacement of the sheets and sills across much of Arizona and parts of California allows them to be used as structural markers in the crystalline rock (Howard, 1991; Barton et al., 2005; Maher, 2008). The Proterozoic strata of the Troy Quartzite and Apache Group are disconformably overlain by approximately 1 to 1.5 km of Paleozoic strata, mainly carbonate rocks. During the Laramide, regional contraction (Davis, 1979) produced basement-cored uplifts similar to those of the central Rocky Mountains, and a large magmatic arc was built on the western margin of the North American plate (Drewes, 1976; Dickinson, 1989). The arc formed between about 84 and 61 Ma and shows an overall progression to more felsic compositions over time (Cornwall, 1982). Contraction clearly postdates early, relatively mafic magmatism (Willden, 1964). Porphyry copper formation is related to somewhat younger (74 61 Ma) magmatism of intermediate to silicic composition (Titley, 1982b), which, in the cases of at least certain deposits, postdates reverse faulting (Seedorff et al., 2005a). Arc magmatism produced numerous intrusions and at least ten porphyry systems in the surrounding area (Maher, 2008). A period of tectonic quiescence and erosion characterized the post-laramide period until ~25 Ma (Dickinson, 1991; Howard and Foster, 1996), when extension dismembered and tilted the Laramide porphyry systems and surrounding host rocks (Barton et al., 2005). In the study area, ~90 eastward tilting occurred along multiple sets of ~north-south striking normal faults. Synextensional, late Oligocene-early Miocene conglomerates and alluvial fan deposits comprise the oldest post-laramide rocks in the study area. Younger middle Mio - cene sedimentary and igneous rocks associated with a rhyolite dome field appear to overlie the Whitetail Conglomerate in an angular unconformity (Dickinson, 1995; Richard and Spencer, 1997). Postextension, newly formed basins were filled with Tertiary gravels and widespread pediment surfaces were formed on older rocks. Today, the landscape is undergoing incision as the pediment surfaces are exhumed by downcutting along the Gila River (Richard and Spencer, 1997). Geologic Units A wide range of units, from the Proterozoic to the Quaternary, are exposed in the study area (Fig. 2). Proterozoic and Paleozoic rocks Mesoproterozic igneous and siliciclastic sedimentary rocks constitute the basement throughout the study area and are FIG. 2. Geologic map of the study area. The map is based on new mapping and previous work by by Schmidt (1971), Cornwall and Krieger (1975a, b), Bradfish (1979), Richard and Spencer (1997), and Barton et al. (2005) /98/000/ $

4 302 NICKERSON ET AL. the primary host of Laramide hydrothermal alteration and mineralization. The oldest of these is the ca. 1.4 Ga Ruin Granite (Dickinson, 1991) usually recognized by 1- to 5-cmlong K-feldspar crystals but is locally represented by an equigranular, fine-grained phase (Richard and Spencer, 1997). The Ruin Granite is overlain by sedimentary rocks of the Apache Group comprised of siltstones and sandstones of the Pioneer Formation, the Dripping Spring Quartzite, Mescal Limestone, and a cap of basaltic flows. The younger Troy Quartzite crops out in the eastern one-third of the study area (grouped with the Apache Group in Fig. 2), as described by Cornwall and Krieger (1975b). The ca. 1.1 Ga Sierra Ancha Diabase crops out as sills, flat sheets, and dikes. The unit commonly displays a strong subophitic, diabasic texture. Mineralogically, the diabase consists of plagioclase laths in a black groundmass of pyroxene and magnetite. In the study area, the sheets and sills dip vertically and in the region served as reactive hosts for hydrothermal alteration and mineralization (Force, 1998). The Paleozoic section in the study area reaches a thickness of approximately 1 to 1.5 km. These mostly carbonate rocks are restricted in the eastern one-third of the study area (Fig. 2) and consist of the Cambrian Bolsa Quartzite and Abrigo Formation, the Devonian Martin Limestone, the Mississippian Escabrosa Limestone, and the carbonate-dominated Pennsylvanian to Permian Naco Group (Cornwall and Krieger, 1975b). Laramide igneous rocks Laramide volcanic and plutonic rocks are widely distributed in the northern Tortilla Mountains and adjacent areas. The intrusive complexes, including the Tea Cup pluton, localized and generated hydrothermal systems and associated mineralization. Williamson Canyon Volcanics: This unit crops out west of the Gila River in the southeastern portion of the study area (Fig. 2). It consists of porphyritic, basaltic andesitic volcanic breccias, with abundant xenoliths of Troy Quartzite and unidentified rock fragments (Willden, 1964; Koski and Cook, 1982). The basaltic andesite was extruded as mafic eruptions during the early phases of construction of the Laramide arc. Tea Cup pluton: In its principal exposures, the pluton (Fig. 2) is crudely zoned from its outermost marginal phase of minor biotite-hornblende quartz monzodiorite to main interior phases of hornblende-biotite granodiorite, biotite granite, and westernmost biotite-muscovite ± garnet granodiorite (Barton et al., 2005). The biotite-hornblende quartz monzodiorite is relatively fine grained (1 4 mm), with 15 to 30 percent mafic minerals consisting of hornblende greater than biotite; it makes up a small mass on the southeastern edge of the pluton and is the oldest of the intrusive phases (Fig. 2). The hornblende-biotite granodiorite has varied grain size (2 8 mm), contains 10 to 15 percent mafic minerals with biotite subequal to hornblende, and is mainly equigranular to seriate, with sparse larger (5 15 mm) feldspar and quartz crystals in its more porphyritic phases. This unit comprises the eastern part of the Tea Cup pluton and forms its prominent cupola (Fig. 2). The biotite granite occupies the central part of the pluton and also crops out southwest of the main pluton. The unit is comprised of equigranular, mediumgrained leucocratic granitoids with 40 percent 3- to 10-mmdiam anhedral quartz, 25 percent pink subhedral 3-to 4-mmdiam K-feldspar intergrown with 30 to 40 percent subhedral white plagioclase in 2- to 4-mm-diam grains, and 2 to 3 percent 1-mm-diam biotite (Richard and Spencer, 1997). The biotite-muscovite ± garnet granodiorite is an equigranular, medium- to medium-fine grained, locally aplitic unit. It consists of 30 percent quartz in 2- to 4-mm-diam anhedral grains, 60 to 70 percent feldspar, mostly plagioclase, in 2-to 6-mmdiam, anhedral grains, and 5 to 7 percent mica, with a variable muscovite to biotite ratio (Richard and Spencer, 1997). Porphyry dikes: Several varieties of Laramide dikes occur in the study area (Table 1). Most dikes are strongly porphyritic with varied contents of quartz, feldspar, hornblende, and mica (biotite ± rare muscovite) phenocrysts. Porphyritic dikes are concentrated in two swarms, one east of the Tea Cup pluton and another centered on Box-O Wash (Fig. 2). Several dike types are found in both dike swarms. The dikes have a preferred orientation striking 070, with steep dips, and are structural markers. Tertiary sedimentary and volcanic rocks Variably dipping late Oligocene to Miocene sedimentary rocks (Fig. 2) of the Whitetail, Cloudburst, and San Manuel Formations are found across the study area. Locally, thin tuff beds occur within the sedimentary sequences. They constitute evidence for synextensional sedimentation and provide a key element in reconstructing the history and timing of faulting. Ripsey and Hackberry Wash: The largest exposures of Tertiary rocks in the study area occur near Ripsey Wash and Hackberry Wash. In Ripsey Wash, the Tertiary rocks are part of the San Manuel Formation, which is comprised of gray to buff strata of alluvial fan and braided plain facies, with local megabreccia bodies and interstratified silicic tuffs (Dickinson, 1991). Exposures of this unit dip as steeply as 45 east in Ripsey Wash (Fig. 2). Potassium-argon dating of the silicic tuffs yielded ages of 20.3 and 17.5 Ma in this section (Dickinson, 1991). Hackberry Wash contains exposures of the San Manuel Formation and the older Cloudburst Formation. The Cloudburst Formation is lithologically similar to the San Manuel Formation but is reddish in color. Exposures of the Cloudburst Formation have been tilted as much as 100 to the east in Hackberry Wash (Maher et al., 2004). A K-Ar date of a tuff near the basal contact of the Cloudburst Formation yielded an age of 25.4 Ma just west of Hackberry Wash (Dickinson, 1991). Red Hills area and Donnelly Wash: A small exposure of Whitetail Conglomerate was mapped near the Red Hills prospect. It is a dark reddish brown, poorly sorted to massive conglomerate that probably correlates with the Hackberry Member of the Cloudburst Formation (Dickinson, 1991; Maher et al., 2004). Cobbles and rare boulders are comprised of granitoids, schist, and less abundant felsic volcanic or hypabyssal rocks. An avalanche breccia is located in and near Donnelly Wash (Fig. 2). The unit is comprised of a monolithologic breccia consisting of unsorted angular clasts of mostly Ruin Granite (Richard and Spencer, 1997). Similar deposits have been mapped in the eastern part of the study area by Krieger /98/000/ $

5 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 303 TABLE 1. Laramide Dikes Located in the Study Area Relative age (1= oldest, Name Description 7 = youngest) Location Reference Hornblende 1- to 4-mm phenocrysts include andesine, hornblende, 1 South and east of the Tea Cornwall rhyodacite and sparse quartz and makeup 1 to 30% of the rock; Cup pluton, and throughout and Krieger porphyry groundmass is fine grained, locally trachytic in texture the Box-O Wash dike swarm (1975a) and consists of plagioclase, hornblende, magnetite, K-feldspar, quartz, sphene, and apatite Rhyodacite Phenocrysts 1 to 4 mm long make up 10 to 30% of the 2 East of the Tea Cup pluton Cornwall porphyry rock; both phenocrysts and groundmass consist of and Krieger andesine, biotite, hornblende, magnetite, and sparse quartz (1975a) Large quartz Dikes have distinctive prominent quartz phenocrysts as well as 3 Widespread in the east of Cornwall rhyodacite plagioclase, biotite, and hornblende phenocrysts; groundmass is the Tea Cup pluton, and to and Krieger porphyry anhedral-granular and consists of K-feldspar, all phenocryst the north Sonora and Kearny (1975a) minerals, and accessory apatite, allanite, zircon, and sphene quadrangles; less abundant to the west in the Box-O Wash dike swarm Red Hills dikes Phenocrysts of 2- to 12-mm quartz (3 8%), plagioclase 4 Most highly concentrated Richard and (15 25%), up to 3 cm K-feldspar (2 5%), and 1- to near the Red Hills prospect Spencer 3- mm biotite (1 2%); dikes grade from those with and continue to the east in (1997) prominent quartz to those with prominent plagioclase the Box-O Wash dike swarm Intermediate Compositionally variable dikes ranging from crystal poor 5 Located mostly north of the Richard and composition light gray felsite with 1 to 3% 1- to 2-mm plagioclase crystals Red Hills prospect in the Spencer dikes to crystal-rich dikes that contain 5 to 25%, 1- to 6- mm Box-O Wash dike swarm (1997) plagioclase and generally sparse 2- to 4-mm quartz phenocrysts; mafic crystals are 1 to 2 mm of biotite and hornblende; also includes equigranular, microcrystalline to fine-grained diorite to granodiorite dikes Muscovite- Phenocrysts 0.3 to 4 mm in diam of quartz, plagioclase, 6 Crops out the Box-O Wash Cornwall bearing quartz sanidine, biotite, magnetite and sparse muscovite make up dike swarm and through the and Krieger latite porphyry 10 to 40% of the rock; groundmass is anhedral-granular Tea Cup pluton; in both (1975a) to aphanitic and is comprised of intergrown K-feldspar, instances there appears to quartz, plagioclase, magnetite, apatite, and sericite be only one dike of this type Granite Very fine grained to aphanitic groundmass with 10 to 15% 7 A single large dike north of Richard and porphyry 4- to 10-mm-diam quartz crystals, 10% K-feldspar crystals the Red Hills prospect and Spencer up to 4 cm long, 40% 2- to 4-mm-diam plagioclase west of Box-O Wash (1997) crystals, and 2 to 3% biotite crystals (1977). In Donnelly Wash, Richard and Spencer (1997) considered these rocks to be younger than the Whitetail Conglomerate. However, Maher (2008) found evidence that similar deposits to the east represented the base of the correlative Cloudburst Formation. Located structurally above the rock avalanche breccia at Donnelly Wash are volcanic units that crop out extensively in the western one-third of the study area (Fig. 2). These include several varieties of tuff and basalt. K-Ar and Ar-Ar age dating suggest that these volcanic units range between 19 and 15 Ma in age (Richard and Spencer, 1997). Overlying the volcanic units is an unnamed conglomerate (Fig. 2). It consists of massive, crudely to moderately well bedded, cobble to boulder conglomerate and poorly sorted gravels. It also contains local synsedimentary volcanic deposits (Richard and Spencer, 1997). Hydrothermal Alteration and Mineralization At least three, and possibly four, hydrothermal systems containing distinctive combinations of potassic, sericitic, greisen, sodic, sodic (-calcic), and propylitic alteration formed in and surrounding the Tea Cup pluton (Figs. 3, 4; Table 2; alteration terminology is the same as in Seedorff et al., 2005b). All but one of these systems have porphyry copper-style alteration and mineralization and are localized on cupolas of the Tea Cup pluton. The other distinctive system is distal from the most intense potassic and sericitic alteration and contains locally intense alteration and mineralization akin to iron oxide-copper-gold (IOCG) systems (e.g., Barton and Johnson, 2000; Williams et al., 2005). Potassic and sodic (-calcic) alteration Two types of feldspar-stable alteration are prevalent in the study area. The core of the hydrothermal system is defined by porphyry copper-style potassic alteration, whereas sodic (-calcic) alteration similar to IOCG systems (Barton and Johnson, 2000; Williams et al., 2005) is developed in a distal position. Potassic alteration, which is typified by secondary biotite and/or K-feldspar plus quartz veins, formed in several distinct centers. The most intense potassic alteration in the study area /98/000/ $

6 304 NICKERSON ET AL. FIG. 3. Map illustrating the distribution of hydrothermal alteration in the Tea Cup area. is near the cupola of the hornblende-biotite granodiorite, immediately west of the Kelvin prospect (Figs. 2, 3), where sulfide-poor veins of quartz-k-feldspar, biotite, quartz, and magnetite-quartz are abundant (Fig. 4A). A second, distinct area of widespread potassic alteration is centered just east of Mount Grayback near the cupola of the biotite-muscovite ± garnet granodiorite (Fig. 4B), where it takes the form of quartz-k-feldspar-muscovite ± biotite ± sulfide veins associated with aplitic to pegmatitic phases of the two-mica granite. Finally, to the west, weak to moderate potassic alteration occurs at the Red Hills prospect, where secondary, shreddy textured biotite and quartz ± K-feldspar ± pyrite ± chalcopyrite veins (Fig. 4C) are locally abundant. In contrast to the potassic alteration that is located in proximal positions, sodic, sodic (-calcic), and rare calcic assemblages are locally present to the north and widespread to the south of the zone of most intense potassic alteration near the cupola of the hornblende-biotite granodiorite (Figs. 2, 3). Sodic assemblages contain quartz, albite, chlorite, and epidote, whereas actinolite-oligoclase-epidote and local garnet, in areas of leaching of quartz, comprise sodic (-calcic) assemblages (Fig. 4D; Barton et al., 2005; Seedorff et al., 2008). Acid alteration Hydrothermal veins containing muscovite with feldspardestructive envelopes are widespread in each of the several hydrothermal systems associated with the Tea Cup pluton. Sericitic (Fig. 4E) and greisen (Fig. 4F) alteration styles are both characterized by secondary muscovite developed at the expense of feldspars but are distinguished by the relatively coarse grain size in greisen (Seedorff et al., 2005b, 2008). Pyrite is a common accessory phase, whereas minerals such as magnetite (-hematite), secondary K-feldspar, chalcopyrite, and molybdenite were only locally observed. The best developed sericitic alteration is spread over an area east of the Tea Cup pluton (Figs. 2, 3). In this area, sericitic alteration contains 1 to 5 vol percent pyrite and is associated with sparse porphyry dikes. Exposures near the towns of Kelvin and Riverside contain zones of sheeted to pervasive, fracture-controlled sericitic alteration (Fig. 3) and include breccia pipes that are cemented by pyrite and quartz with or without chalcopyrite. East of and overlapping with potassic alteration near Mount Grayback (Fig. 3), sericitic alteration takes the form of coarse-grained quartz-muscovite-chalcopyrite-molybdenitepyrite veins and greisen. East of the Red Hills prospect (Figs. 2, 3), locally intense quartz-sericite-pyrite alteration is present in ~20-m-long, steeply dipping, east-west striking zones in the Ruin Granite (Fig. 4E). Greisen-style muscovite ± pyrite ± quartz alteration (Fig. 4F) is widely developed west of Mount Grayback and, rarely, in the hornblende-biotite granodiorite located 1 to 2 km west of the Tea Cup cupola (Figs. 2, 3). Greisen locally forms narrow muscovite-rich veinlets, whereas other zones have widths of tens of centimeters. The character of the greisen varies along strike from solely muscovite-rich alteration envelopes to quartz-sulfide rich cores with muscovite-rich envelopes; locally, both have outer envelopes of K-feldspar (Seedorff et al., 2008) /98/000/ $

7 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 305 FIG. 4. Photographs of Laramide hydrothermal veins. A. K-feldspar-quartz veins in the cupola of the hornblende-biotite granodiorite. B. Quartz veins with a K-feldspar envelope just east of Mount Grayback in the biotite-granite. C. Oxidized pyrite + chalcopyrite ± quartz vein with K-feldspar envelope cutting the Ruin Granite in the Red Hills prospect. D. Albiteepidote-chlorite ± garnet sodic (-calcic) alteration of the Ruin Granite on the southeastern flank of the Tea Cup pluton. E. Intense porphyry-style sericitic alteration with oxidized pyrite cubes surrounded by muscovite in the Ruin Granite near Box- O Wash. F. Quartz vein surrounded by coarse-grained muscovite in the biotite-muscovite ± garnet granodiorite west of Mount Grayback. G. Specular hematite-quartz vein in the Red Hills prospect. H. Specular hematite-quartz-sericite breccia from the Red Hills prospect /98/000/ $

8 306 NICKERSON ET AL. TABLE 2. Description and Location of Alteration Style, Assemblages, and Veins Deposit type Alteration style Mineral assemblages Veins Location Porphyry Potassic K-spar, Qtz, Bt ± Cpy, K-spar-Qtz, Bt, Qtz, Cupolas of hornblende-biotite granodiorite Mol Mt-Qtz and biotite-muscovite ± garnet granodiorite phases of Tea Cup Pluton; Red Hills Prospect Sericitic Ser, Pyr, Qtz, ±Cpy Qtz,-Pyr ±Ser, Qtz, Ser East of Tea Cup Pluton; Red Hills Greisen Qtz, Musc Qtz-Musc Grayback Mountain Propylitic Chl, Ept, Qtz, Carb Chl, Ept, Qtz, Carb Widespread IOCG Sodic, Sodic-Calcic Alb, Ept, Chl, Ept, Chl Southeast flank of the Tea Cup pluton Sericitic Hem, Chl, Qtz ± Mt Hem-Chl-Qtz, Qtz-Chl, Red Hills-Box-O Wash; Ripsey Wash Hem, Mt, Chl, Notes: Abbreviations: Alb = albite, Bt = biotite, Carb = carbonate minerals, Chl = chlorite, Cpy = chalcopyrite, Ept = epidote, Hb = hornblende, Hem = hematite, Mt = magnetite, Mol = molybdenum, Musc = muscovite, Pyr = pyrite, Qtz = quartz In contrast to the pyritic sericitic and greisen alteration described above, sulfide-poor sericitic alteration is intensely developed in the Ruin Granite near the eastern margin of the Tea Cup pluton and in the area in and east of the Red Hills prospect (Figs. 2, 3). The sulfide-poor nature of this sericitic alteration, like the aforementioned sodic (-calcic) alteration, resembles alteration recognized in IOCG systems (e.g., Barton and Johnson, 2000; Williams et al., 2005). Veins of specular hematite-chlorite-quartz ± magnetite ± pyrite (Fig. 4G, H) are pervasive, and locally areas of quartz + specular hematite completely destroy preexisting texture. Surrounding the most intense areas of sulfide-poor sericite-bearing alteration are zones of chlorite ± quartz alteration. Other styles of alteration Several styles of low-temperature alteration were observed, although they are not shown on the alteration map of Figure 3 due to their widespread nature. Propylitic alteration, characterized by chlorite, epidote, and albite, is sporadically developed across the entire study area. Carbonate veins occur west of Donnelly Wash, where they cut quartz veins in the Ruin Granite. Copper mineralization Although old prospecting pits are scattered across the entire study area, the best developed porphyry copper-style mineralization is located at the Kelvin prospect and eastward toward the towns of Kelvin and Riverside (Fig. 2, 3; Schmidt, 1971; Zelinski, 1973; Corn and Ahern, 1994; Wilkins and Heidrick, 1995). Drilling at the Kelvin prospect in the 1970s intercepted zones of mineralization that locally exceeded 0.6 percent copper. These zones of significant mineralization were less than 100 m thick and are bounded by low-angle faults across which copper grades decrease dramatically (Corn and Ahern, 1994). Hydrothermally altered breccia pipes, with or without copper mineralization, occur locally surrounding the eastern and southern exposures of the Tea Cup center. The Wooley breccia pipe, drilled in 1974 by ALCOA, is located south of the Kelvin prospect on the west side of Ripsey Wash (Fig. 2; Corn and Ahern, 1994). The pipe is ~90 m across and extends ~360 m in an east-west direction. Outcrops exhibit angular fragments that are cemented by quartz and sulfide minerals, with a pyrite to chalcopyrite ratio of ~3 to 1. The rock fragments and the surrounding country rock lack pervasive or disseminated pyritic alteration. The Sultana breccia, in the Riverside area, produced minor copper in about 1900, but this is poorly documented. Other, subhorizontal, east-striking, cigar-shaped breccia bodies that cut Ruin Granite south of Riverside contain only pyrite-quartz-sericite alteration (Barton et al., 2005). Low-grade porphyry molybdenum (-copper) mineralization is exposed over several km 2 on the northern, eastern, and southern sides of Mount Grayback (Fig. 3). Mineralization in this area is spatially and temporally associated with the biotite-muscovite ± garnet granodiorite phase of the Tea Cup pluton and the associated potassic to greisen alteration. Small amounts of molybdenite, in both quartz-poor and quartz-rich veinlets and disseminations, are widespread at the surface. However during mapping, no surface disturbances have been noted that indicate past exploration. The occurrence of mineralization at Mount Grayback is notable because porphyry molybdenum mineralization is rarely associated with strongly peraluminous plutons. Indeed, none of the six subclasses of porphyry molybdenum deposits of Seedorff et al. (2005a) is linked to two-mica granites. The Red Hills prospect (Figs. 2, 3) contains a large, very low grade copper resource that is associated with both sulfide-rich porphyry copper- and sulfide-poor IOCG-style alteration (Williams and Forrester, 1995). The Red Hills have been explored sporadically since the early 1900s, beginning with numerous small pits and shafts that explored for oxidized copper ores. Since the 1960s, some copper has been produced from small leach pads. Considerable drilling surrounding the Red Hills identified a low-grade resource that might contain as much as 450 million tons (Mt) of 0.10 percent copper hosted mainly by porphyry-copper style quartz ± K- feldspar ± pyrite ± chalcopyrite veins (Williams and Forrester, 1995). In addition, copper oxides are exposed within several fault blocks to the east of the Red Hills. Here, potassic alteration decreases abruptly in intensity (Fig. 3), whereas IOCG-style sulfide-poor, specular hematite-bearing sericite (-chlorite) alteration becomes more widespread to the east. Faulting During mapping (Figs. 2, 3, 6) faults were directly observed in outcrop, and their orientations were measured (Figs. 5 7). In cases where fault rocks (i.e., gouge, cataclasite, and breccia) were not exposed, the presence of additional faults could be reasonably inferred on the basis of a wide variety of geologic evidence. For example, contrasts in rock types (Fig. 7A) or structurally controlled abrupt changes in the style and /98/000/ $

9 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 307 FIG. 5. Geologic map of Box-O Wash area. In map pattern, faults clearly laterally offset porphyry dikes. Lateral offset of a distinctive muscovite-bearing quartz latite porphyry dike (Kmpd; dips 80 N in Box-O Wash) helped constraining the vertical offset of faults for palinspastic reconstruction. The dip of the fault from set 5 was not measured in outcrop. This fault was assigned to set 5 based on its crosscutting relationship (right-lateral offset) with a fault from set 3 in Box-O Wash. The right lateral offset of a fault with a measured westward dip indicates that the younger fault must dip to the east. Only faults from set 5 could produce such offset. A dip of 70 is inferred for the east-dipping fault based on structural measurements on other faults of the same set. intensity of hydrothermal alteration within a single lithologic unit were observed (Fig. 7B; also compare Fig. 6 with Fig. 3). Laramide At least one Laramide reverse fault characterized by exposed zones of hard, flinty cataclasite crops out near the eastern limit of the Tea Cup pluton (Figs. 2, 6). This fault strikes roughly north-south and has a steep eastward dip. The diabase sheets are repeated across this zone, but the cataclastic zone is cut in several places by Laramide porphyry dikes and by the cupola of the hornblende-biotite phase of the Tea Cup pluton (Figs. 2, 6). This relationship suggests a minimum age of ~73 Ma for the fault. Tertiary In contrast to the flinty cataclasite associated with the Laramide reverse fault, all other exposed faults are marked by clay gouge and breccias. These faults commonly offset porphyry dikes. The faults locally cut sedimentary rock sections, and some are clearly synsedimentary in nature and define the margins of Tertiary basins (Fig. 2). Immediately east of the Tea Cup pluton, one of these younger faults intersects and offsets the Laramide reverse fault (Fig. 6). Where contrasting rock types are juxtaposed, the faults generally place younger rocks on older rocks, as reflected by omission of parts of the stratigraphic or structural section. Where the rock types on either side of the fault are similar, but the hydrothermal alteration is dissimilar, the typical relationship places higher levels of the system over lower levels, which is thus another form of omission of some of the structural section. The field observations thus indicate that the late Oligocene and younger faults are all normal faults. Based on fault orientations and relative age relationships, the post-laramide faults have been grouped into five fault /98/000/ $

10 308 NICKERSON ET AL. FIG. 6. Map of reverse faults (Laramide) and normal faults (Tertiary) in the study area. The normal faults are grouped into fault sets. The cross section A to A' is shown in Figure /98/000/ $

11 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 309 FIG. 7. A. Exposure of the first (oldest) set of normal faults near Ripsey Wash, just south of the study area. Outcrops of Paleozoic (P) limestone rest on Proterozoic diabase sheets (Ydb). The contact (blue line) between the two rock types dips shallowly to the southeast. B. A low-angle normal fault that bounds the eastern side of the Red Hills prospect and belongs to the second set of faults. In this photograph, the fault places intense hematite and/or magnetite-chlorite-quartz alteration adjacent to weak chlorite-quartz alteration. (Photo taken by George Davis.) C. A fault belonging to the third set of normal faults exposed in Donnelly Wash and placing Tertiary volcanic units on Proterozoic Ruin Granite. sets (Figs. 5, 6). In most cases, these assignments can be made unambiguously. If the dip of the fault could not be determined in outcrop, then it was inferred from measurements made on faults with a similar relative age (Fig. 5). A brief description of each fault set, listed in order of relative age from oldest to youngest, is given below. Set 1: Faults belonging to this first fault set strike generally northward and dip ~15 E. A fault of this generation crops out near Ripsey Wash, and another may be present west of Mount Grayback (Fig. 6). Just outside the study area near the southern end of Ripsey Wash, spectacular exposures of a gently southeast-dipping normal fault of this set places Paleozoic limestone on Proterozoic diabase (Fig. 7A). The offset on faults of this generation is poorly constrained but appears to be as great as 3 km. Set 2: Faults of this set strike generally northward with a dip of ~15 W where measured (Fig. 6). Four faults crop out in the study area (Fig. 6): one immediately east of the Red Hills prospect, the Walnut Canyon normal fault (Richard and Spencer, 1997), one east of the Kelvin prospect, and one east of Ripsey Wash. The fault immediately east of the Red Hills prospect places Ruin Granite displaying intense sulfide poor IOCG-style sericitic alteration in the hanging wall on Ruin Granite with moderate propylitic alteration in the footwall. The fault has a dip of 15 W (Fig. 7B). The amount of slip on faults of this set ranges from 2 to 7 km, using the diabase sills as markers. Set 3: Three faults of this set crop out in the study area (Fig. 6): one in Box-O Wash, one in Donnelly Wash (Fig. 7C), and one east of Ripsey Wash. They strike generally northward and have dips of ~45 W. The slip on these faults can be as great as 2 km. Near Box-O Wash a fault of this generation cuts and offsets a distinctive muscovite-bearing quartz latite porphyry dike that has a dip of 80 N (Fig. 5). Judging from the offset, the slip at this location was ~700 m. Set 4: At least eight faults of this set have been observed in the study area with strikes of ~010 and dips of ~70 W. The slip on these faults is usually less than 1 km. A well-exposed fault of this generation occurs immediately south of the Red Hills prospect. Here the fault dips 72 W and offsets the same distinctive muscovite-bearing quartz latite porphyry dike mentioned above by ~250 m. Set 5: Numerous faults of this last fault set crop out in the study area with strikes of ~350 and dips of ~70 E. The slip on these faults is usually less than 1 km. For clarity, Figure 6 only shows faults of this fault set having offsets exceeding /98/000/ $

12 310 NICKERSON ET AL. m. Examples of faults of this fault set are well exposed in the Tertiary volcanic units west of Donnelly Wash (Richard and Spencer, 1997). Structural Interpretation and Palinspastic Reconstruction of Normal Faults Interpretation Laramide: Diabase sheets are repeated across the northsouth striking, steeply dipping Laramide fault zone marked by cataclasites (Figs. 2, 6). The lack of a normal fault in an appropriate location to create this repetition led Barton et al. (2005) to conclude that the repetition was caused by the Laramide fault. Maher (2008) correlated the Laramide fault zone with the Walnut Canyon thrust fault, which is not to be confused with the Walnut Canyon normal fault, exposed several kilometers north of the study area. Breccia pipes exposed in the Ruin Granite east of Ripsey Wash locally contain sedimentary rocks and may indicate upward transport of clasts from overthrust supracrustal rocks at the time of mineralization (Barton et al., 2005). Indeed, the reverse fault presently exposed about 15 km to the east at Romero Wash (Krieger, 1974; Dickinson, 1991) may be the updip continuation of the faults in the Ruin Granite near the Tea Cup pluton that are marked by cataclasite. This fault zone belongs to a belt of reverse faults that is discontinuously exposed from Superior south-southeastward to the Johnny Lyon Hills (Dickinson, 1991). Tertiary: Because the Tertiary normal faults are grouped in fault sets of different relative ages, each fault set can be interpreted as a sequential generation of faults. Hence, each generation is defined as a set of similarly oriented faults that moved more or less contemporaneously. As the normal faults within a set cut and extended the Tea Cup porphyry system and surrounding area, the dip of each fault plane rotated to shallower angles. Once the fault planes of a fault set rotated to angles that were kinematically unfavorable for slippage (less than ~30 ; Anderson, 1951), a new fault set with new faults planes formed, and it cut and progressively rotated the older fault sets and the porphyry system. This repeated sequence of events produced a cumulative eastward tilting of ~90 as evidenced by the present-day dips of the oldest synextensional Tertiary rocks and the diabase sills. The lack of a significant penetrative fabric near the mapped normal faults leads us to conclude that slip and tilting along the five sets of normal faults were accommodated in a rigid fashion at the scale of our reconstruction. Approach to restoring movement on normal faults Figures 8 and 9 show palinspastic reconstructions of the 32- km-long cross section (Fig. 6) across the study area, for both rock types and hydrothermal alteration. Displacement along the normal faults was removed in sequential order, from the youngest to the oldest generation of normal faults (panels A through F, Fig. 8), as determined by dip measurements and relative ages. Slip on faults was constrained using observed lateral offsets of dikes and faults and drill hole data. Key markers that were restored include diabase sheets and sills, various phases of the Tea Cup pluton, textural units of the Ruin Granite, and alteration assemblages. In addition, unconformities of the same age were placed at similar structural levels. Tertiary sedimentary rocks were restored to horizontal at the time they were deposited. Small offset faults (slip <200 m) were not considered in the reconstruction. Due to the predominance of igneous rocks in the study area, a number of uncertainties remain in the restoration. The three-dimensional shape of the igneous bodies is unknown, thus the form chosen here is based on plausible relationships with the restoration of hydrothermal alteration (Fig. 9) and symmetry with the map pattern. Also, the tilting of intrusive rocks across the study area is assumed to be nearly 90 based on the observation that the diabase sheets now dip vertically. However, these sheets have irregular edges and their attitudes are difficult to measure in outcrop. Observing the exposure across topography in map pattern is a more useful means of determining their dip. As previously mentioned, it was impossible to measure the dips of some faults in outcrop. In these cases, crosscutting relationships were used to determine the relative age of the fault, and dips measured from other faults of the same fault set were used (Fig. 5). Results and uncertainties of the palinspastic reconstruction Panels A through F of Figure 8 show the stepwise reconstruction of cross section A-A'. Figure 9A shows the interpreted reconstructed geology with some lateral extrapolation for illustrative purposes, and Figure 9B is the interpreted hydrothermal overlay. The reconstruction of rock types reveals a composite pluton intruding the eastern margin of a basement-cored uplift (Figs. 8, 9). The geometry of the intrusion is not influenced by Laramide reverse faults, which it postdates. In the restored section, the hornblende-biotite granodiorite is the shallowest. It is intruded at deeper levels by the biotite granite and at the deepest levels by the biotite-muscovite ± garnet granite. Outcrops of the biotite-hornblende quartz monzodiorite lie out of the line of section. The cupola of the hornblende-biotite granodiorite is shown to lie just beneath the modern surface, although it could lie substantially farther east or west. The biotite granite is interpreted to have been shallowest beneath the Red Hills prospect. This configuration follows from the abundance of biotite-bearing, hornblende-absent porphyry dikes in the Red Hills, suggesting they emanated from the biotite granite. The cupola of the biotite-muscovite ± garnet granodiorite apparently formed deeper than the cupolas of the other phases of the pluton based on the present distribution of hydrothermal features with this phase, and the inference that hydrothermal fluid release formed near the cupola. In the palinspastic restoration of hydrothermal alteration, three centers of porphyry-related alteration are shown (Fig. 9B). Potassic alteration, marked by K-feldspar quartz and biotitic alteration, restores to areas near the cupolas of the hornblende-biotite, biotite, and two-mica phases of the Tea Cup pluton. Sericitic alteration is structurally higher and overlaps the zones of potassic alteration in all three systems. Greisen alteration is restricted to lower levels of the Tea Cup pluton at depths of 7 to 10 km. As described above, intense sulfide poor IOCG-style sericitic alteration flanks the porphyry-style hydrothermal systems at /98/000/ $

13 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 311 FIG. 8. Sequential palinspastic reconstruction looking north from A-A'. There is no vertical exaggeration. A. Modern cross section. B. Removal of 5 th set of normal faults (orange faults). C. Removal of 4 th set of normal faults (brown faults). D. Removal of 3 rd set of normal faults (light blue faults). E. Removal of the 2 nd set of normal faults (green faults). F. Removal of the 1 st set of normal faults (dark blue faults). roughly the same depth as the cupola of the hornblende-biotite granodiorite. In Figure 9, the distribution of IOCG-like features reflects the map observations, with intense IOCGstyle alteration lying mostly in the 2- to 4-km depth range north and south of the cupola of the hornblende-biotite phase of the Tea Cup pluton. At the Red Hills prospect, IOCG-style alteration crosscuts and is crosscut by moderately intense porphyry style potassic alteration. The Kelvin prospect (Figs. 2, 3) can be restored to a position approximately 1 km above and slightly east of the cupola of the hornblende-biotite granodiorite (Figs. 8, 9). The Red Hills prospect would be above the inferred cupola of the biotite granite for reasons discussed above. However, the relatively weak potassic alteration and abundant porphyry dikes found in the Red Hills may indicate that the prospect flanks, rather than lies directly above, the center of a biotite granite-related cupola of a porphyry system. Another possibility is that it could represent the distal flank of a porphyry system created by the hornblende-biotite granodiorite. This appears unlikely, however, because potassic alteration, and the abundance of porphyry dikes, diminishes to the east toward the cupola of the hornblende-biotite granodiorite. In either case, the proximity of the Red Hills prospect to the Tea Cup pluton in the reconstruction strongly suggests that the porphyry dikes and alteration originate from a phase of the Tea Cup pluton. Discussion Comparisons to other Laramide porphyry systems Previous studies of Laramide porphyry systems in the southwestern United States have focused on the spatial and temporal relationships between igneous and related hydrothermal activity (e.g., Titley and Hicks, 1966; Titley, 1982a; Pierce and Bolm, 1995). Here, we focus on a discussion of the age of the intrusions, the distribution of mineralization with depth, the occurrence of different styles of hydrothermal alteration, the depth and level of exposure of the porphyry systems, and the style of extension. Age of intrusions: Laramide intrusions in the northern Tortilla and nearby Dripping Spring Mountains range from 75 to /98/000/ $

14 312 NICKERSON ET AL. FIG. 9. A. Hypothetical Laramide cross section showing the composite Tea Cup pluton intruding a basement-cored uplift. Thick black lines represent the modern surface. Maroon lines mark the paleosurface at 73 and 25 Ma. Abbreviations: Khbg = hornblende-biotite granodiorite, Kbg = biotite granite, Khbg = biotite-hornblende quartz monzodiorite, Kmbg = biotite-muscovite ± garnet granodiorite, P = Paleozoic sedimentary rocks, Q = undifferentiated Quaternary deposits, Tc = Tertiary conglomerate, Ts= Oligo-Miocene sedimentary rocks, Ya = Proterozoic Apache Group, Yr = Proterozoic Ruin Granite. B. Reconstruction of hydrothermal alteration. No vertical exaggeration. 61 Ma, as demonstrated by recent U-Pb zircon dating (Seedorff et al., 2005a). The age range of magmatism and mineralization in a cluster of deposits north of the study area near the Schultze Granite, including the Globe-Miami, Pinto Valley, and Resolution deposits, is from ~69 to 61 Ma. To the south in the Tortilla and Dripping Spring Mountains, the porphyry systems range from ~73 Ma in the Tea Cup pluton to ~69 to 66 Ma at the Granite Mountain pluton (Ray deposit), and ~65 Ma at Christmas. The most important mineralized porphyry systems in eastcentral Arizona are associated with the Schultze Granite and Granite Mountain plutons (Maher, 2008). These plutons are felsic, compositionally uniform, and formed by several, thermally discrete intrusive events (Seedorff et al., 2005a; Maher, 2008). In contrast, the Tea Cup system is composite and prominently zoned over a wide compositional range. The U- Pb zircon ages indicate that this heterogeneity was achieved several million years prior to the formation of the Schultze Granite and Granite Mountain plutons in a single thermal event of no more than about 1 m.y. duration. Mineralization and depth of emplacement: Similar to other porphyry systems in east-central Arizona, the highest copper concentrations (Kelvin and Red Hills prospects) are located structurally above several cupolas of the Tea Cup intrusive center. The cupolas of the hornblende-biotite granodiorite and biotite granite can be restored to a position of about 6 km below the 73 Ma paleosurface, which is within the 1- to 6-km paleodepth range of formation for most cupolas of porphyry systems (Seedorff et al., 2005b). The top of the biotite-muscovite ± garnet granodiorite is not well constrained but most likely formed at 5 to 10 km. The molybdenum (-copper) mineralization formed near the apex of the biotite-muscovite ± garnet granodiorite. Although this geometry is typical, the molybdenum occurrence is notable by its difference from the ordinary subclasses of the porphyry molybdenum class of deposits (cf. Seedorff et al., 2005b). However, it has some similarities to the Cretaceous Harvey Creek occurrence in northeastern Washington (Miller and Theodore, 1982). Styles of hydrothermal alteration: Both the feldspar-stable potassic alteration and the feldspar-destructive acid alteration observed in the Tea Cup porphyry system are common to porphyry systems (Seedorff et al., 2005b). Whereas greisen alteration encountered west of Mount Grayback and locally to the west of the cupola of the hornblende-biotite granodiorite is only rarely reported from porphyry systems, it is typical of the root zones beneath the Miami-Inspiration, Sierrita-Esperanza, and Ray porphyry systems (Seedorff et al., 2008). The IOCG-style sodic (-calcic) and sulfide-poor sericitic alteration observed in and around the Tea Cup pluton is not commonly reported in Laramide porphyry systems. However, by combining results of this study with observations from Sierrita-Esperanza (Stavast et al., 2008) and other areas in Arizona, it appears that IOCG-style alteration may be a widespread, albeit typically minor feature of many districts in the Laramide province and not an oddity of a few Jurassic porphyry systems such as Yerington and other systems (Battles and Barton, 1995; Dilles et al., 2000). Furthermore, the pres /98/000/ $

15 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 313 ence of widespread IOCG-style alteration and mineralization indicates that IOCG deposits could exist in the Laramide systems of southwestern North America that are distal to, but broadly contemporaneous with, porphyry copper centers, as is the case at the Pumpkin Hollow (Lyon) deposit in the Yerington district, Nevada (Matlock and Ohlin, 1996; Dilles et al., 2000). Exploration targets in northern Tortilla Mountains Knowledge and understanding of Tertiary extension in the Laramide porphyry province began to change exploration strategies in the late 1960s, thus leading to the discovery of the Kalamazoo orebody (Lowell, 1968). As time progressed, it became clear that tilted and dismembered orebodies were common and that all orebodies in the Basin and Range should be assumed to be faulted and tilted until proven otherwise (Seedorff, 1991a; Wilkins and Heidrick, 1995). During the past decade, the importance of orebody tilting across the Basin and Range province has been emphasized by numerous workers (e.g., Seedorff et al., 1996; Maher, 2008; Stavast et al., 2008). Early exploration at Tea Cup initially centered on outcropping mineralization near Riverside. Beginning in the 1960s, exploration geologists recognized that the Tea Cup porphyry system was dismembered and possibly tilted and that lowangle faults bounded some altered structural blocks, thus opening the possibility for a larger fault-bounded deposit under cover. Related exploration tested various hypotheses and discovered the mineralized fault blocks at the Kelvin prospect. The present study reveals at least two potential exploration targets (Fig. 9A). First, rocks lying under the Tertiary and Quaternary Donnelly Wash basin (Fig. 2) can be restored to structural levels and distances from the cupola of the hornblende-biotite phase of the Tea Cup pluton that are similar to the Kelvin prospect (Fig. 8F). Exposures of the Ruin Granite west of Donnelly Wash (Fig. 2), which are restored beneath the West Grayback basin target (Fig. 8F), are dominantly propylitically altered, but one ~10-m-deep shaft revealed significant copper oxide mineralization. The second potential exploration target is the cupola of the biotite granite phase of the Tea Cup pluton. As discussed above, the Red Hills may represent the upper levels of a porphyry system centered on the biotite granite (Fig. 9A). Lower levels of this system may lie west or east of the Red Hills. The area under cover to the west appears more prospective because potassic alteration exposed in outcrop decreases markedly east of the Red Hills (Figs. 3, 6). Style of extension and broader exploration implications The manner in which the crust is extended continues to be a topic of considerable debate. Most controversies center on the geometry, timing, and magnitude of normal faults in extended terranes (John and Foster, 1993). For example, many investigators have inferred strongly listric faults that flatten at depth and can be kinematically linked to a low-angle mylonitic zone as exposed in metamorphic core complexes (Wernicke, 1981). In contrast, others recognize that normal faults occur in multiple generations and that low-angle faults are cut and offset by younger, high-angle faults (Proffett, 1977). The amount of curvature on fault planes has dramatic consequences on the path of fault blocks during extension and thus controls the locations of pieces of dismembered orebodies after extension. A comparison of the geometry, timing, and magnitude of normal faults between the study area and other nonmylonitic domains of large-magnitude extension in the Basin and Range province is made in Table 3. In the study area, Tertiary normal faults have created ~210 percent extension, based on the palinspastic reconstruction, and ~90 eastward tilting along five sets of normal faults that initiated at high angles of ~55 to 75. The superposition of five generations of normal faults produced greater amounts of tilt and extension than is observed in other localities. The 7-km offset along the Walnut TABLE 3. Comparison of Extended Porphyry Systems and other Areas in Arizona and Nevada Maximum Curvature of Location Tilting of bedding Sets of Dip of oldest set displacement on normal fault Total (reference) due to extension normal faults of normal faults normal faults (km) planes (/km) extension (%) Yerington district, NV (Proffett, 1977) Royston district, NV (Seedorff, 1991b) Caetano Caldera, NV (Colgan et al., 2008) Robinson district, NV 35 west 7 5 (Seedorff et al., 1996) to 65 east GMSRC 1 region, AZ ~ (Maher, 2008) Teacup-Red Hills, overturned AZ (Barton et al., 2005; this study) 1 GMSRC = Globe, Miami, Superior, Ray, Christmas /98/000/ $

16 314 NICKERSON ET AL. Canyon normal fault (Fig. 6) is greater than any offset associated with other faults. However, Quaternary fill in the Donnelly Wash basin covers much of the area west of the Walnut Canyon normal fault (Figs. 2, 6, 8A). Normal faults that would reduce the estimated amount of slip on the Walnut Canyon normal fault may lie under the Quaternary cover but are not incorporated into the present reconstruction. As mentioned earlier, geologic evidence indicates that extension of the Tea Cup hydrothermal systems was accommodated in a rigid fashion at the scale of our reconstruction. Earlier work in the study area hypothesized that extension was accommodated on strongly listric faults (Howard and Foster, 1996). Whereas listric faults produce rotation of fault blocks, the results of the present study (Fig. 10), along with the work by Ramsay and Huber (1987), show that cross sections constructed utilizing strongly listric faults do not conserve mass when palinspastically restored in a rigid manner. Thus, utilization of a cross section similar to Howard and Foster s (1996) as an exploration tool in the study area is ill-advised. The amount of extension in the study area (~210%) is as great as or greater than what is estimated for some metamorphic core complexes (e.g., Howard and John, 1987). However, results show that in the study area all higher angle normal faults cut normal faults of lower angles, and that fault geometries were nearly planar, occurred in generations, and had maximum displacements less than tens of kilometers. This demonstrates the ability of superimposed sets of initially high-angle and nearly planar normal faults to dramatically extend the upper crust. It contradicts the notion that extension on a similar scale to what is observed in metamorphic core complexes must be accommodated on strongly listric normal faults with tens of kilometers of slip. FIG. 10. A. Cross section through the study area after Howard and Foster (1996). B. Palinspastic reconstruction of the cross section by Howard and Foster (1996) reveals that the cross section does not conserve mass (i.e., kilometer-scale overlaps and gaps between fault blocks) when rigidly restored. Abbreviations: Ktc = Cretaceous Tea Cup pluton, Ts = Oligo-Miocene sedimentary rocks, Ya = Proterozoic Apache Group, Yr = Ruin Granite. Ultimately, the importance of the style of extension to an exploration geologist rests in how an ore deposit is dismembered. In the northern Tortilla Mountains, mineralization associated with a single intrusive center is spread across more than 20 km along the extension direction, and more pieces may lie under cover farther to the west. Although a significant ore deposit has yet to be discovered after a century of intermittent exploration in the Tea Cup intrusive center, exploration has only locally sought covered targets of the types postulated here. Similar dismemberment has been documented throughout the Basin and Range province (Table 3), which runs from Mexico to Canada. Economic geologists in the province should not only consider all ore deposits to be tilted and extended until proven otherwise (Wilkins and Heidrick, 1995) but must also understand how the deposits were extended in order to shrewdly explore for new deposits. Summary and Conclusions Detailed mapping revealed that the composite Tea Cup pluton was intruded in four phases that created at least three, possibly four, distinct hydrothermal systems. The broadly coeval intrusive phases are zoned from distal and early biotitehornblende quartz monzodiorite to late and deep two-mica granodiorite. Potassic alteration dominates the core of the systems, with several styles of feldspar-destructive acid alteration, most notably sericitic alteration at higher structural levels, sodic (-calcic) alteration 2 to 4 km distal on the flanks of the systems, and propylitic alteration surrounding the other alteration types. The central systems vary from porphyry copper style at high levels to greisenlike molybdenum (-copper) style within the two-mica granodiorite; distal mineralization is IOCG like in its characteristics. Superimposed sets of Tertiary normal faults initially developed at high angles and subsequently dismembered the porphyry system cumulatively creating more than 200 percent extension and 90 eastward tilting. Tilting caused by normal faulting provides an exceptional cross-sectional view at the present surface of the porphyry system, from its distal flanks to paleodepths of >10 km. This exposure displays multiple vectors toward possible mineralization, and utilization of palinspastic reconstruction has created two new exploration targets in an area with more than 100 years of exploration history. The Basin and Range province stretches from Mexico to Canada and contains numerous Laramide porphyry and other types of ore deposits. The manner in which the crust extends has dramatic effects on where fault blocks, which may contain ore deposits, are moved. This requires the exploration geologist to have an understanding of different models of extension and where they are applicable. Here, the power of palinspastic reconstruction utilizing superimposed sets of nearly planar normal faults has been demonstrated in a highly extended terrane, which was previously thought to have been extended along strongly listric normal faults. Reexamination of any extended orebody thought to have been dismembered along listric normal faults should be considered. Although rigorous palinspastic reconstructions of the type shown here are work intensive, they are made easier with the significant drill hole coverage that often is found in brownfields districts, and they present the opportunity to discover new world-class orebodies in districts with long exploration histories /98/000/ $