© 2010 Society of Economic Geologists, Inc. Special Publication 15, pp. 299–316

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 85721-0077

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 sys- tems in the study area that are associated with widespread potassic, sericitic, greisen, sodic (-calcic), and propy- litic alteration. The alteration envelops both porphyry copper and porphyry molybdenum (-copper) mineral- ization. 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 hy- drothermal 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 sys- tems and generally extends 2 to 4 km away from the center of the hydrothermal systems. Greisen-style alter- ation 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, demonstrat- ing that palinspastic reconstruction represents a powerful exploration technique in a district with more than 100 years of exploration history.

Introduction the dismembered pieces of the hydrothermal systems are put PORPHYRY copper deposits in the Laramide (ca. 80−50 Ma) back into context. The exposure of different paleodepths in province of southwestern North America have received con- the multiple fault blocks then becomes an aid to understand- siderable attention at the scale of individual orebodies (Titley ing timing and processes of copper porphyry formation. For and Hicks, 1966; Titley, 1982a). Recent recognition of alter- example, root zones beneath mineralization, which would ation assemblages formed on the distal flanks and roots of the most likely never be drilled in an upright system, can be ex- systems (Seedorff et al., 2008) suggests, however, that a sys- humed and, therefore, examined in footwall blocks of normal tem-scale understanding of their evolution still remains to be faults (Seedorff et al., 2008). Exploration in the Basin and developed. One fact that has hindered a system-scale under- Range province can utilize the fact that extension often pre- standing so far is the complex normal faulting that dismem- serves and hides pieces of mineral deposits, leaving vectors bered most Laramide porphyry systems after their emplace- toward mineralization to the structurally aware geologist. ment. Faulting commonly separates originally adjacent parts The structural complexities in highly faulted areas of the of porphyry systems by kilometers and juxtaposes unrelated Basin and Range province have been studied extensively (e.g., parts of the systems (Proffett, 1977; Stavast et al., 2008). Crittenden et al., 1980; Dickinson, 1991; Davis et al., 2004) System-scale understanding of dismembered copper por- and have produced long-standing controversies that center on phyry systems requires careful attention to postmineral the geometry, timing, and magnitude of normal faulting structural geology. The effects of extension can be removed (Wernicke, 1981; Lister and Davis, 1989; Miller et al., 1999; through palinspastic reconstruction of the systems whereby Maher, 2008). Economic geologists working in extended por- phyry systems provided some of the first insights into this de- † Corresponding author: e-mail, [email protected] bate (Lowell, 1968; Proffett, 1977) and continue to refine the

299 300 NICKERSON ET AL. understanding of this issue utilizing knowledge gained from Geologic Setting drill hole data and geophysical methods, which are not com- The Tea Cup porphyry system is located in the northern monly available to structural geologists (Dilles and Gans, Tortilla Mountains of east-central Arizona, the heart of the 1995; Wilkins and Heidrick, 1995; Stavast et al., 2008). porphyry copper belt of southwestern North America (Fig. This study describes the rock types, structure, and hydro - 1), and within an area that has been the focus of long-stand- thermal alteration and provides a palinspastic reconstruction of ing investigation (e.g., Ransome, 1903; Cornwall, 1982; the Laramide Tea Cup porphyry system in the northern Tortilla Maher, 2008). Major nearby porphyry copper districts in- Mountains, Pinal County, Arizona. The results are based on clude Ray (Phillips et al., 1974), Globe-Miami (Peterson, new field work, combined with earlier mapping by Schmidt 1962), Superior (Hammer and Peterson, 1968; Manske and (1971), Cornwall and Krieger (1975a, b), Bradfish (1979), Paul, 2002), Poston Butte (Nason et al., 1982), Christmas Richard and Spencer (1997), and Barton et al. (2005). The new (Koski and Cook, 1982), Mission-Pima (Barter and Kelly, mapping for this study and the work of Barton et al. (2005) 1982), and Sierrita-Esperanza (West and Aiken, 1982). Ex- paid particular attention to the characterization of hydrother- ploration for porphyry copper deposits in the region has mal alteration, the recognition of internal variations in the ig- ebbed and flowed in tandem with copper prices (Lowell, neous rocks, and the structural geology of the area. These fea- 1978; Paul and Manske, 2005), with times of intense explo- tures, in combination with the general geologic patterns ration mainly in the late 19th and middle 20th centuries. The reported in earlier work, are critical to interpreting the struc- discovery of the Resolution deposit near Superior in the mid- tural evolution of the study area and thereby reconstructing 1990s has renewed the interest of both junior and major min- the magmatic and related hydrothermal systems at Tea Cup. ing 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).

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

0361-0128/98/000/000-00 $6.00 301 302 NICKERSON ET AL. the primary host of Laramide hydrothermal alteration and grained leucocratic granitoids with 40 percent 3- to 10-mm- mineralization. The oldest of these is the ca. 1.4 Ga Ruin diam anhedral quartz, 25 percent pink subhedral 3-to 4-mm- Granite (Dickinson, 1991) usually recognized by 1- to 5-cm- diam K-feldspar intergrown with 30 to 40 percent subhedral long K-feldspar crystals but is locally represented by an white plagioclase in 2- to 4-mm-diam grains, and 2 to 3 per- equigranular, fine-grained phase (Richard and Spencer, cent 1-mm-diam biotite (Richard and Spencer, 1997). The bi- 1997). The Ruin Granite is overlain by sedimentary rocks of otite-muscovite ± garnet granodiorite is an equigranular, the Apache Group comprised of siltstones and sandstones of medium- to medium-fine−grained, locally aplitic unit. It con- the Pioneer Formation, the Dripping Spring Quartzite, sists of 30 percent quartz in 2- to 4-mm-diam anhedral grains, Mescal Limestone, and a cap of basaltic flows. The younger 60 to 70 percent feldspar, mostly plagioclase, in 2-to 6-mm- Troy Quartzite crops out in the eastern one-third of the study diam, anhedral grains, and 5 to 7 percent mica, with a variable area (grouped with the Apache Group in Fig. 2), as described muscovite to biotite ratio (Richard and Spencer, 1997). by Cornwall and Krieger (1975b). Porphyry dikes: Several varieties of Laramide dikes occur The ca. 1.1 Ga Sierra Ancha Diabase crops out as sills, flat in the study area (Table 1). Most dikes are strongly por- sheets, and dikes. The unit commonly displays a strong sub- phyritic with varied contents of quartz, feldspar, hornblende, ophitic, diabasic texture. Mineralogically, the diabase consists and mica (biotite ± rare muscovite) phenocrysts. Porphyritic of plagioclase laths in a black groundmass of pyroxene and dikes are concentrated in two swarms, one east of the Tea magnetite. In the study area, the sheets and sills dip vertically Cup pluton and another centered on Box-O Wash (Fig. 2). and in the region served as reactive hosts for hydrothermal al- Several dike types are found in both dike swarms. The dikes teration and mineralization (Force, 1998). have a preferred orientation striking 070°, with steep dips, The Paleozoic section in the study area reaches a thickness and are structural markers. of approximately 1 to 1.5 km. These mostly carbonate rocks are restricted in the eastern one-third of the study area (Fig. Tertiary sedimentary and volcanic rocks 2) and consist of the Cambrian Bolsa Quartzite and Abrigo Variably dipping late Oligocene to Miocene sedimentary Formation, the Devonian Martin Limestone, the Mississip- rocks (Fig. 2) of the Whitetail, Cloudburst, and San Manuel pian Escabrosa Limestone, and the carbonate-dominated Formations are found across the study area. Locally, thin tuff Pennsylvanian to Permian Naco Group (Cornwall and Krieger, beds occur within the sedimentary sequences. They constitute 1975b). evidence for synextensional sedimentation and provide a key element in reconstructing the history and timing of faulting. Laramide igneous rocks Ripsey and Hackberry Wash: The largest exposures of Ter- Laramide volcanic and plutonic rocks are widely distrib- tiary rocks in the study area occur near Ripsey Wash and uted in the northern Tortilla Mountains and adjacent areas. Hackberry Wash. In Ripsey Wash, the Tertiary rocks are part The intrusive complexes, including the Tea Cup pluton, lo- of the San Manuel Formation, which is comprised of gray to calized and generated hydrothermal systems and associated buff strata of alluvial fan and braided plain facies, with local mineralization. megabreccia bodies and interstratified silicic tuffs (Dickin- Williamson Canyon Volcanics: This unit crops out west of son, 1991). Exposures of this unit dip as steeply as 45° east in the Gila River in the southeastern portion of the study area Ripsey Wash (Fig. 2). Potassium-argon dating of the silicic (Fig. 2). It consists of porphyritic, basaltic andesitic volcanic tuffs yielded ages of 20.3 and 17.5 Ma in this section (Dickin- breccias, with abundant xenoliths of Troy Quartzite and son, 1991). unidentified rock fragments (Willden, 1964; Koski and Cook, Hackberry Wash contains exposures of the San Manuel 1982). The basaltic andesite was extruded as mafic eruptions Formation and the older Cloudburst Formation. The Cloud- during the early phases of construction of the Laramide arc. burst Formation is lithologically similar to the San Manuel Tea Cup pluton: In its principal exposures, the pluton (Fig. Formation but is reddish in color. Exposures of the Cloud- 2) is crudely zoned from its outermost marginal phase of burst Formation have been tilted as much as 100° to the east minor biotite-hornblende quartz monzodiorite to main inte- in Hackberry Wash (Maher et al., 2004). A K-Ar date of a tuff rior phases of hornblende-biotite granodiorite, biotite gran- near the basal contact of the Cloudburst Formation yielded ite, and westernmost biotite-muscovite ± garnet granodiorite an age of 25.4 Ma just west of Hackberry Wash (Dickinson, (Barton et al., 2005). The biotite-hornblende quartz monzo- 1991). diorite is relatively fine grained (1−4 mm), with 15 to 30 per- Red Hills area and Donnelly Wash: A small exposure of cent mafic minerals consisting of hornblende greater than bi- Whitetail Conglomerate was mapped near the Red Hills otite; it makes up a small mass on the southeastern edge of prospect. It is a dark reddish brown, poorly sorted to massive the pluton and is the oldest of the intrusive phases (Fig. 2). conglomerate that probably correlates with the Hackberry The hornblende-biotite granodiorite has varied grain size Member of the Cloudburst Formation (Dickinson, 1991; (2−8 mm), contains 10 to 15 percent mafic minerals with bi- Maher et al., 2004). Cobbles and rare boulders are comprised otite subequal to hornblende, and is mainly equigranular to of granitoids, schist, and less abundant felsic volcanic or hy- seriate, with sparse larger (5−15 mm) feldspar and quartz pabyssal rocks. crystals in its more porphyritic phases. This unit comprises An avalanche breccia is located in and near Donnelly Wash the eastern part of the Tea Cup pluton and forms its promi- (Fig. 2). The unit is comprised of a monolithologic breccia nent cupola (Fig. 2). The biotite granite occupies the central consisting of unsorted angular clasts of mostly Ruin Granite part of the pluton and also crops out southwest of the main (Richard and Spencer, 1997). Similar deposits have been pluton. The unit is comprised of equigranular, medium- mapped in the eastern part of the study area by Krieger

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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) con- sodic, sodic (-calcic), and propylitic alteration formed in and sidered these rocks to be younger than the Whitetail Con- surrounding the Tea Cup pluton (Figs. 3, 4; Table 2; alter- glomerate. However, Maher (2008) found evidence that ation terminology is the same as in Seedorff et al., 2005b). All similar deposits to the east represented the base of the cor- but one of these systems have porphyry copper-style alter- relative Cloudburst Formation. ation and mineralization and are localized on cupolas of the Located structurally above the rock avalanche breccia at Tea Cup pluton. The other distinctive system is distal from Donnelly Wash are volcanic units that crop out extensively in the most intense potassic and sericitic alteration and contains the western one-third of the study area (Fig. 2). These in- locally intense alteration and mineralization akin to iron clude several varieties of tuff and basalt. K-Ar and Ar-Ar age oxide-copper-gold (IOCG) systems (e.g., Barton and Johnson, dating suggest that these volcanic units range between 19 and 2000; Williams et al., 2005). 15 Ma in age (Richard and Spencer, 1997). Overlying the volcanic units is an unnamed conglomerate Potassic and sodic (-calcic) alteration (Fig. 2). It consists of massive, crudely to moderately well Two types of feldspar-stable alteration are prevalent in the bedded, cobble to boulder conglomerate and poorly sorted study area. The core of the hydrothermal system is defined by gravels. It also contains local synsedimentary volcanic de- porphyry copper-style potassic alteration, whereas sodic (-cal- posits (Richard and Spencer, 1997). cic) alteration similar to IOCG systems (Barton and Johnson, 2000; Williams et al., 2005) is developed in a distal position. Hydrothermal Alteration and Mineralization Potassic alteration, which is typified by secondary biotite At least three, and possibly four, hydrothermal systems con- and/or K-feldspar plus quartz veins, formed in several distinct taining distinctive combinations of potassic, sericitic, greisen, centers. The most intense potassic alteration in the study area

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is near the cupola of the hornblende-biotite granodiorite, im- mediately west of the Kelvin prospect (Figs. 2, 3), where sul- fide-poor veins of quartz-K-feldspar, biotite, quartz, and mag- netite-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 associ- ated with aplitic to pegmatitic phases of the two-mica granite. Finally, to the west, weak to moderate potassic alteration oc- curs at the Red Hills prospect, where secondary, shreddy tex- tured 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 as- semblages 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, chlo- rite, 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 feldspar- destructive 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 as- sociated 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 . 3. Map illustrating the distribution of hydrothermal alteration in the Tea Cup area. . 3. Map illustrating the distribution of hydrothermal alteration in Tea

IG with or without chalcopyrite. F East of and overlapping with potassic alteration near Mount Grayback (Fig. 3), sericitic alteration takes the form of coarse-grained quartz-muscovite-chalcopyrite-molybdenite- pyrite veins and greisen. East of the Red Hills prospect (Figs. 2, 3), locally intense quartz-sericite-pyrite alteration is pre- sent 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 nar- row 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).

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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. Albite- epidote-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.

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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 de- produced minor copper in about 1900, but this is poorly doc- scribed above, sulfide-poor sericitic alteration is intensely de- umented. Other, subhorizontal, east-striking, cigar-shaped veloped in the Ruin Granite near the eastern margin of the breccia bodies that cut Ruin Granite south of Riverside con- Tea Cup pluton and in the area in and east of the Red Hills tain only pyrite-quartz-sericite alteration (Barton et al., 2005). prospect (Figs. 2, 3). The sulfide-poor nature of this sericitic Low-grade porphyry molybdenum (-copper) mineralization alteration, like the aforementioned sodic (-calcic) alteration, is exposed over several km2 on the northern, eastern, and resembles alteration recognized in IOCG systems (e.g., Bar- southern sides of Mount Grayback (Fig. 3). Mineralization in ton and Johnson, 2000; Williams et al., 2005). Veins of specu- this area is spatially and temporally associated with the bi- lar hematite-chlorite-quartz ± magnetite ± pyrite (Fig. 4G, otite-muscovite ± garnet granodiorite phase of the Tea Cup H) are pervasive, and locally areas of quartz + specular pluton and the associated potassic to greisen alteration. Small hematite completely destroy preexisting texture. Surrounding amounts of molybdenite, in both quartz-poor and quartz-rich the most intense areas of sulfide-poor sericite-bearing alter- veinlets and disseminations, are widespread at the surface. ation are zones of chlorite ± quartz alteration. However during mapping, no surface disturbances have been noted that indicate past exploration. The occurrence of min- Other styles of alteration eralization at Mount Grayback is notable because porphyry Several styles of low-temperature alteration were observed, molybdenum mineralization is rarely associated with strongly although they are not shown on the alteration map of Figure peraluminous plutons. Indeed, none of the six subclasses of 3 due to their widespread nature. Propylitic alteration, char- porphyry molybdenum deposits of Seedorff et al. (2005a) is acterized by chlorite, epidote, and albite, is sporadically de- linked to two-mica granites. veloped across the entire study area. Carbonate veins occur The Red Hills prospect (Figs. 2, 3) contains a large, very west of Donnelly Wash, where they cut quartz veins in the low grade copper resource that is associated with both sul- Ruin Granite. fide-rich porphyry copper- and sulfide-poor IOCG-style al- teration (Williams and Forrester, 1995). The Red Hills have Copper mineralization been explored sporadically since the early 1900s, beginning Although old prospecting pits are scattered across the entire with numerous small pits and shafts that explored for oxidized study area, the best developed porphyry copper-style mineral- copper ores. Since the 1960s, some copper has been pro- ization is located at the Kelvin prospect and eastward toward duced from small leach pads. Considerable drilling surround- the towns of Kelvin and Riverside (Fig. 2, 3; Schmidt, 1971; ing the Red Hills identified a low-grade resource that might Zelinski, 1973; Corn and Ahern, 1994; Wilkins and Heidrick, contain as much as 450 million tons (Mt) of 0.10 percent cop- 1995). Drilling at the Kelvin prospect in the 1970s intercepted per hosted mainly by porphyry-copper−style quartz ± K- zones of mineralization that locally exceeded 0.6 percent cop- feldspar ± pyrite ± chalcopyrite veins (Williams and For- per. These zones of significant mineralization were less than rester, 1995). In addition, copper oxides are exposed within 100 m thick and are bounded by low-angle faults across which several fault blocks to the east of the Red Hills. Here, potas- copper grades decrease dramatically (Corn and Ahern, 1994). sic alteration decreases abruptly in intensity (Fig. 3), whereas Hydrothermally altered breccia pipes, with or without cop- IOCG-style sulfide-poor, specular hematite-bearing sericite per mineralization, occur locally surrounding the eastern and (-chlorite) alteration becomes more widespread to the east. southern exposures of the Tea Cup center. The Wooley breccia pipe, drilled in 1974 by ALCOA, is located south of the Kelvin Faulting prospect on the west side of Ripsey Wash (Fig. 2; Corn and During mapping (Figs. 2, 3, 6) faults were directly observed Ahern, 1994). The pipe is ~90 m across and extends ~360 m in outcrop, and their orientations were measured (Figs. 5−7). in an east-west direction. Outcrops exhibit angular fragments In cases where fault rocks (i.e., gouge, cataclasite, and brec- that are cemented by quartz and sulfide minerals, with a pyrite cia) were not exposed, the presence of additional faults could to chalcopyrite ratio of ~3 to 1. The rock fragments and the be reasonably inferred on the basis of a wide variety of geo- surrounding country rock lack pervasive or disseminated logic evidence. For example, contrasts in rock types (Fig. 7A) pyritic alteration. The Sultana breccia, in the Riverside area, or structurally controlled abrupt changes in the style and

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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 ver- tical 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 clay gouge and breccias. These faults commonly offset por- unit were observed (Fig. 7B; also compare Fig. 6 with Fig. 3). phyry dikes. The faults locally cut sedimentary rock sections, and some are clearly synsedimentary in nature and define the Laramide margins of Tertiary basins (Fig. 2). Immediately east of the At least one Laramide reverse fault characterized by ex- Tea Cup pluton, one of these younger faults intersects and posed zones of hard, flinty cataclasite crops out near the east- offsets the Laramide reverse fault (Fig. 6). Where contrasting ern limit of the Tea Cup pluton (Figs. 2, 6). This fault strikes rock types are juxtaposed, the faults generally place younger roughly north-south and has a steep eastward dip. The dia- rocks on older rocks, as reflected by omission of parts of the base sheets are repeated across this zone, but the cataclastic stratigraphic or structural section. Where the rock types on zone is cut in several places by Laramide porphyry dikes and either side of the fault are similar, but the hydrothermal al- by the cupola of the hornblende-biotite phase of the Tea Cup teration is dissimilar, the typical relationship places higher pluton (Figs. 2, 6). This relationship suggests a minimum age levels of the system over lower levels, which is thus another of ~73 Ma for the fault. form of omission of some of the structural section. The field observations thus indicate that the late Oligocene and Tertiary younger faults are all normal faults. In contrast to the flinty cataclasite associated with the Based on fault orientations and relative age relationships, Laramide reverse fault, all other exposed faults are marked by the post-Laramide faults have been grouped into five fault

0361-0128/98/000/000-00 $6.00 307 308 NICKERSON ET AL. . 6. The cross section A to A' is in the study area. The normal faults are grouped into fault sets. Map of reverse faults (Laramide) and normal (Tertiary) IG F shown in Figure 8.

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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 Granite with moderate propylitic alteration in the footwall. made unambiguously. If the dip of the fault could not be de- The fault has a dip of 15° W (Fig. 7B). The amount of slip on termined in outcrop, then it was inferred from measurements faults of this set ranges from 2 to 7 km, using the diabase sills made on faults with a similar relative age (Fig. 5). A brief de- as markers. scription of each fault set, listed in order of relative age from Set 3: Three faults of this set crop out in the study area (Fig. oldest to youngest, is given below. 6): one in Box-O Wash, one in Donnelly Wash (Fig. 7C), and Set 1: Faults belonging to this first fault set strike generally one east of Ripsey Wash. They strike generally northward and northward and dip ~15° E. A fault of this generation crops have dips of ~45° W. The slip on these faults can be as great out near Ripsey Wash, and another may be present west of as 2 km. Near Box-O Wash a fault of this generation cuts and Mount Grayback (Fig. 6). Just outside the study area near the offsets a distinctive muscovite-bearing quartz latite porphyry southern end of Ripsey Wash, spectacular exposures of a gen- dike that has a dip of 80° N (Fig. 5). Judging from the offset, tly southeast-dipping normal fault of this set places Paleozoic the slip at this location was ~700 m. limestone on Proterozoic diabase (Fig. 7A). The offset on Set 4: At least eight faults of this set have been observed in faults of this generation is poorly constrained but appears to the study area with strikes of ~010° and dips of ~70° W. The be as great as 3 km. slip on these faults is usually less than 1 km. A well-exposed Set 2: Faults of this set strike generally northward with a dip fault of this generation occurs immediately south of the Red of ~15° W where measured (Fig. 6). Four faults crop out in Hills prospect. Here the fault dips 72° W and offsets the same the study area (Fig. 6): one immediately east of the Red Hills distinctive muscovite-bearing quartz latite porphyry dike prospect, the Walnut Canyon normal fault (Richard and mentioned above by ~250 m. Spencer, 1997), one east of the Kelvin prospect, and one east Set 5: Numerous faults of this last fault set crop out in the of Ripsey Wash. The fault immediately east of the Red Hills study area with strikes of ~350° and dips of ~70° E. The slip prospect places Ruin Granite displaying intense sulfide poor on these faults is usually less than 1 km. For clarity, Figure 6 IOCG-style sericitic alteration in the hanging wall on Ruin only shows faults of this fault set having offsets exceeding 200

0361-0128/98/000/000-00 $6.00 309 310 NICKERSON ET AL. m. Examples of faults of this fault set are well exposed in the the same age were placed at similar structural levels. Tertiary Tertiary volcanic units west of Donnelly Wash (Richard and sedimentary rocks were restored to horizontal at the time Spencer, 1997). they were deposited. Small offset faults (slip <200 m) were not considered in the reconstruction. Structural Interpretation and Due to the predominance of igneous rocks in the study Palinspastic Reconstruction of Normal Faults area, a number of uncertainties remain in the restoration. The three-dimensional shape of the igneous bodies is unknown, Interpretation thus the form chosen here is based on plausible relationships Laramide: Diabase sheets are repeated across the north- with the restoration of hydrothermal alteration (Fig. 9) and south−striking, steeply dipping Laramide fault zone marked symmetry with the map pattern. Also, the tilting of intrusive by cataclasites (Figs. 2, 6). The lack of a normal fault in an ap- rocks across the study area is assumed to be nearly 90° based propriate location to create this repetition led Barton et al. on the observation that the diabase sheets now dip vertically. (2005) to conclude that the repetition was caused by the However, these sheets have irregular edges and their atti- Laramide fault. Maher (2008) correlated the Laramide fault tudes are difficult to measure in outcrop. Observing the ex- zone with the Walnut Canyon thrust fault, which is not to be posure across topography in map pattern is a more useful confused with the Walnut Canyon normal fault, exposed sev- means of determining their dip. As previously mentioned, it eral kilometers north of the study area. Breccia pipes exposed was impossible to measure the dips of some faults in outcrop. in the Ruin Granite east of Ripsey Wash locally contain sedi- In these cases, crosscutting relationships were used to deter- mentary rocks and may indicate upward transport of clasts mine the relative age of the fault, and dips measured from from overthrust supracrustal rocks at the time of mineraliza- other faults of the same fault set were used (Fig. 5). tion (Barton et al., 2005). Indeed, the reverse fault presently exposed about 15 km to the east at Romero Wash (Krieger, Results and uncertainties of the palinspastic reconstruction 1974; Dickinson, 1991) may be the updip continuation of the Panels A through F of Figure 8 show the stepwise recon- faults in the Ruin Granite near the Tea Cup pluton that are struction of cross section A-A'. Figure 9A shows the inter- marked by cataclasite. This fault zone belongs to a belt of re- preted reconstructed geology with some lateral extrapolation verse faults that is discontinuously exposed from Superior for illustrative purposes, and Figure 9B is the interpreted hy- south-southeastward to the Johnny Lyon Hills (Dickinson, drothermal overlay. 1991). The reconstruction of rock types reveals a composite pluton Tertiary: Because the Tertiary normal faults are grouped in intruding the eastern margin of a basement-cored uplift fault sets of different relative ages, each fault set can be in- (Figs. 8, 9). The geometry of the intrusion is not influenced terpreted as a sequential generation of faults. Hence, each by Laramide reverse faults, which it postdates. In the re- generation is defined as a set of similarly oriented faults that stored section, the hornblende-biotite granodiorite is the moved more or less contemporaneously. As the normal faults shallowest. It is intruded at deeper levels by the biotite gran- within a set cut and extended the Tea Cup porphyry system ite and at the deepest levels by the biotite-muscovite ± garnet and surrounding area, the dip of each fault plane rotated to granite. Outcrops of the biotite-hornblende quartz monzodi- shallower angles. Once the fault planes of a fault set rotated orite lie out of the line of section. The cupola of the horn- to angles that were kinematically unfavorable for slippage blende-biotite granodiorite is shown to lie just beneath the (less than ~30°; Anderson, 1951), a new fault set with new modern surface, although it could lie substantially farther east faults planes formed, and it cut and progressively rotated the or west. older fault sets and the porphyry system. This repeated se- The biotite granite is interpreted to have been shallowest quence of events produced a cumulative eastward tilting of beneath the Red Hills prospect. This configuration follows ~90° as evidenced by the present-day dips of the oldest synex- from the abundance of biotite-bearing, hornblende-absent tensional Tertiary rocks and the diabase sills. The lack of a sig- porphyry dikes in the Red Hills, suggesting they emanated nificant penetrative fabric near the mapped normal faults from the biotite granite. The cupola of the biotite-muscovite leads us to conclude that slip and tilting along the five sets of ± garnet granodiorite apparently formed deeper than the normal faults were accommodated in a rigid fashion at the cupolas of the other phases of the pluton based on the present scale of our reconstruction. distribution of hydrothermal features with this phase, and the inference that hydrothermal fluid release formed near the Approach to restoring movement on normal faults cupola. Figures 8 and 9 show palinspastic reconstructions of the 32- In the palinspastic restoration of hydrothermal alteration, km-long cross section (Fig. 6) across the study area, for both three centers of porphyry-related alteration are shown (Fig. rock types and hydrothermal alteration. Displacement along 9B). Potassic alteration, marked by K-feldspar–quartz and bi- the normal faults was removed in sequential order, from the otitic alteration, restores to areas near the cupolas of the youngest to the oldest generation of normal faults (panels A hornblende-biotite, biotite, and two-mica phases of the Tea through F, Fig. 8), as determined by dip measurements and Cup pluton. Sericitic alteration is structurally higher and relative ages. Slip on faults was constrained using observed overlaps the zones of potassic alteration in all three systems. lateral offsets of dikes and faults and drill hole data. Key mark- Greisen alteration is restricted to lower levels of the Tea Cup ers that were restored include diabase sheets and sills, various pluton at depths of 7 to 10 km. phases of the Tea Cup pluton, textural units of the Ruin Gran- As described above, intense sulfide poor IOCG-style sericitic ite, and alteration assemblages. In addition, unconformities of alteration flanks the porphyry-style hydrothermal systems at

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

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FIG. 9. A. Hypothetical Laramide cross section showing the composite Tea Cup pluton intruding a basement-cored up- lift. 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 = bi- otite-muscovite ± garnet granodiorite, P = Paleozoic sedimentary rocks, Q = undifferentiated Quaternary deposits, Tc = Ter- tiary 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 (See- The top of the biotite-muscovite ± garnet granodiorite is dorff et al., 2005a). The age range of magmatism and miner- not well constrained but most likely formed at 5 to 10 km. alization in a cluster of deposits north of the study area near The molybdenum (-copper) mineralization formed near the the Schultze Granite, including the Globe-Miami, Pinto Val- apex of the biotite-muscovite ± garnet granodiorite. Although ley, and Resolution deposits, is from ~69 to 61 Ma. To the this geometry is typical, the molybdenum occurrence is no- south in the Tortilla and Dripping Spring Mountains, the por- table by its difference from the ordinary subclasses of the por- phyry systems range from ~73 Ma in the Tea Cup pluton to phyry molybdenum class of deposits (cf. Seedorff et al., ~69 to 66 Ma at the Granite Mountain pluton (Ray deposit), 2005b). However, it has some similarities to the Cretaceous and ~65 Ma at Christmas. Harvey Creek occurrence in northeastern Washington The most important mineralized porphyry systems in east- (Miller and Theodore, 1982). central Arizona are associated with the Schultze Granite and Styles of hydrothermal alteration: Both the feldspar-stable Granite Mountain plutons (Maher, 2008). These plutons are potassic alteration and the feldspar-destructive acid alteration felsic, compositionally uniform, and formed by several, ther- observed in the Tea Cup porphyry system are common to mally discrete intrusive events (Seedorff et al., 2005a; Maher, porphyry systems (Seedorff et al., 2005b). Whereas greisen 2008). In contrast, the Tea Cup system is composite and alteration encountered west of Mount Grayback and locally to prominently zoned over a wide compositional range. The U- the west of the cupola of the hornblende-biotite granodiorite Pb zircon ages indicate that this heterogeneity was achieved is only rarely reported from porphyry systems, it is typical of several million years prior to the formation of the Schultze the root zones beneath the Miami-Inspiration, Sierrita-Es- Granite and Granite Mountain plutons in a single thermal peranza, and Ray porphyry systems (Seedorff et al., 2008). event of no more than about 1 m.y. duration. The IOCG-style sodic (-calcic) and sulfide-poor sericitic al- Mineralization and depth of emplacement: Similar to other teration observed in and around the Tea Cup pluton is not porphyry systems in east-central Arizona, the highest copper commonly reported in Laramide porphyry systems. However, concentrations (Kelvin and Red Hills prospects) are located by combining results of this study with observations from structurally above several cupolas of the Tea Cup intrusive Sierrita-Esperanza (Stavast et al., 2008) and other areas in center. The cupolas of the hornblende-biotite granodiorite Arizona, it appears that IOCG-style alteration may be a wide- and biotite granite can be restored to a position of about 6 km spread, albeit typically minor feature of many districts in the below the 73 Ma paleosurface, which is within the 1- to 6-km Laramide province and not an oddity of a few Jurassic por- paleodepth range of formation for most cupolas of porphyry phyry systems such as Yerington and other systems (Battles systems (Seedorff et al., 2005b). and Barton, 1995; Dilles et al., 2000). Furthermore, the pres-

0361-0128/98/000/000-00 $6.00 312 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 313 ence of widespread IOCG-style alteration and mineralization the West Grayback basin target (Fig. 8F), are dominantly indicates that IOCG deposits could exist in the Laramide sys- propylitically altered, but one ~10-m-deep shaft revealed sig- tems of southwestern North America that are distal to, but nificant copper oxide mineralization. broadly contemporaneous with, porphyry copper centers, as The second potential exploration target is the cupola of the is the case at the Pumpkin Hollow (Lyon) deposit in the Yer- biotite granite phase of the Tea Cup pluton. As discussed ington district, Nevada (Matlock and Ohlin, 1996; Dilles et above, the Red Hills may represent the upper levels of a por- al., 2000). phyry system centered on the biotite granite (Fig. 9A). Lower levels of this system may lie west or east of the Red Hills. The Exploration targets in northern Tortilla Mountains area under cover to the west appears more prospective be- Knowledge and understanding of Tertiary extension in the cause potassic alteration exposed in outcrop decreases Laramide porphyry province began to change exploration markedly east of the Red Hills (Figs. 3, 6). strategies in the late 1960s, thus leading to the discovery of the Kalamazoo orebody (Lowell, 1968). As time progressed, it Style of extension and broader exploration implications became clear that tilted and dismembered orebodies were The manner in which the crust is extended continues to be common and that all orebodies in the Basin and Range should a topic of considerable debate. Most controversies center on be assumed to be faulted and tilted until proven otherwise the geometry, timing, and magnitude of normal faults in ex- (Seedorff, 1991a; Wilkins and Heidrick, 1995). During the tended terranes (John and Foster, 1993). For example, many past decade, the importance of orebody tilting across the investigators have inferred strongly listric faults that flatten at Basin and Range province has been emphasized by numerous depth and can be kinematically linked to a low-angle my- workers (e.g., Seedorff et al., 1996; Maher, 2008; Stavast et lonitic zone as exposed in metamorphic core complexes (Wer- al., 2008). nicke, 1981). In contrast, others recognize that normal faults Early exploration at Tea Cup initially centered on outcrop- occur in multiple generations and that low-angle faults are cut ping mineralization near Riverside. Beginning in the 1960s, and offset by younger, high-angle faults (Proffett, 1977). The exploration geologists recognized that the Tea Cup porphyry amount of curvature on fault planes has dramatic conse- system was dismembered and possibly tilted and that low- quences on the path of fault blocks during extension and thus angle faults bounded some altered structural blocks, thus controls the locations of pieces of dismembered orebodies opening the possibility for a larger fault-bounded deposit after extension. under cover. Related exploration tested various hypotheses A comparison of the geometry, timing, and magnitude of and discovered the mineralized fault blocks at the Kelvin normal faults between the study area and other nonmylonitic prospect. domains of large-magnitude extension in the Basin and Range The present study reveals at least two potential exploration province is made in Table 3. In the study area, Tertiary nor- targets (Fig. 9A). First, rocks lying under the Tertiary and mal faults have created ~210 percent extension, based on the Quaternary Donnelly Wash basin (Fig. 2) can be restored to palinspastic reconstruction, and ~90° eastward tilting along structural levels and distances from the cupola of the horn- five sets of normal faults that initiated at high angles of ~55° blende-biotite phase of the Tea Cup pluton that are similar to to 75°. The superposition of five generations of normal faults the Kelvin prospect (Fig. 8F). Exposures of the Ruin Granite produced greater amounts of tilt and extension than is ob- west of Donnelly Wash (Fig. 2), which are restored beneath served 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, 60° 3 10° 4 3°−7° 137 NV (Proffett, 1977)

Royston district, NV 50° 2 10° 3.5 130 (Seedorff, 1991b)

Caetano Caldera, NV 40-50° 3 5°−20° 6 110 (Colgan et al., 2008)

Robinson district, NV 35 west 7 5° (Seedorff et al., 1996) to 65° east

GMSRC1 region, AZ ~4 1°−3° 20−400 (Maher, 2008)

Teacup-Red Hills, 90° 5 15°overturned 7 1° 210 AZ (Barton et al., 2005; this study)

1 GMSRC = Globe, Miami, Superior, Ray, Christmas

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Canyon normal fault (Fig. 6) is greater than any offset associ- Ultimately, the importance of the style of extension to an ated with other faults. However, Quaternary fill in the Don- exploration geologist rests in how an ore deposit is dismem- nelly Wash basin covers much of the area west of the Walnut bered. In the northern Tortilla Mountains, mineralization as- Canyon normal fault (Figs. 2, 6, 8A). Normal faults that would sociated with a single intrusive center is spread across more reduce the estimated amount of slip on the Walnut Canyon than 20 km along the extension direction, and more pieces normal fault may lie under the Quaternary cover but are not may lie under cover farther to the west. Although a significant incorporated into the present reconstruction. ore deposit has yet to be discovered after a century of inter- As mentioned earlier, geologic evidence indicates that ex- mittent exploration in the Tea Cup intrusive center, explo- tension of the Tea Cup hydrothermal systems was accommo- ration has only locally sought covered targets of the types pos- dated in a rigid fashion at the scale of our reconstruction. Ear- tulated here. Similar dismemberment has been documented lier work in the study area hypothesized that extension was throughout the Basin and Range province (Table 3), which accommodated on strongly listric faults (Howard and Foster, runs from Mexico to Canada. Economic geologists in the 1996). Whereas listric faults produce rotation of fault blocks, province should not only consider all ore deposits to be tilted the results of the present study (Fig. 10), along with the work and extended until proven otherwise (Wilkins and Heidrick, by Ramsay and Huber (1987), show that cross sections con- 1995) but must also understand how the deposits were ex- structed utilizing strongly listric faults do not conserve mass tended in order to shrewdly explore for new deposits. when palinspastically restored in a rigid manner. Thus, uti- lization of a cross section similar to Howard and Foster’s Summary and Conclusions (1996) as an exploration tool in the study area is ill-advised. Detailed mapping revealed that the composite Tea Cup The amount of extension in the study area (~210%) is as pluton was intruded in four phases that created at least three, great as or greater than what is estimated for some metamor- possibly four, distinct hydrothermal systems. The broadly co- phic core complexes (e.g., Howard and John, 1987). How- eval intrusive phases are zoned from distal and early biotite- ever, results show that in the study area all higher angle nor- hornblende quartz monzodiorite to late and deep two-mica mal faults cut normal faults of lower angles, and that fault granodiorite. Potassic alteration dominates the core of the geometries were nearly planar, occurred in generations, and systems, with several styles of feldspar-destructive acid alter- had maximum displacements less than tens of kilometers. ation, most notably sericitic alteration at higher structural lev- This demonstrates the ability of superimposed sets of initially els, sodic (-calcic) alteration 2 to 4 km distal on the flanks of high-angle and nearly planar normal faults to dramatically ex- the systems, and propylitic alteration surrounding the other tend the upper crust. It contradicts the notion that extension alteration types. The central systems vary from porphyry cop- on a similar scale to what is observed in metamorphic core per style at high levels to greisenlike molybdenum (-copper) complexes must be accommodated on strongly listric normal style within the two-mica granodiorite; distal mineralization is faults with tens of kilometers of slip. IOCG like in its characteristics. Superimposed sets of Tertiary normal faults initially devel- oped at high angles and subsequently dismembered the por- phyry system cumulatively creating more than 200 percent ex- tension and 90° eastward tilting. Tilting caused by normal faulting provides an exceptional cross-sectional view at the pre- sent surface of the porphyry system, from its distal flanks to pa- leodepths 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 geolo- gist to have an understanding of different models of extension and where they are applicable. Here, the power of palinspas- tic reconstruction utilizing superimposed sets of nearly planar normal faults has been demonstrated in a highly extended ter- rane, which was previously thought to have been extended along strongly listric normal faults. Reexamination of any ex- tended orebody thought to have been dismembered along listric normal faults should be considered. Although rigorous FIG. 10. A. Cross section through the study area after Howard and Foster palinspastic reconstructions of the type shown here are work (1996). B. Palinspastic reconstruction of the cross section by Howard and intensive, they are made easier with the significant drill hole Foster (1996) reveals that the cross section does not conserve mass (i.e., kilo- meter-scale overlaps and gaps between fault blocks) when rigidly restored. coverage that often is found in brownfields districts, and they Abbreviations: Ktc = Cretaceous Tea Cup pluton, Ts = Oligo-Miocene sedi- present the opportunity to discover new world-class orebod- mentary rocks, Ya = Proterozoic Apache Group, Yr = Ruin Granite. ies in districts with long exploration histories.

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Acknowledgments ——1995, Tertiary stratigraphic and structural relationships in the Copper Butte area, Teapot Mountain quadrangle, Pinal County, Arizona: Arizona Reviews by Steven Kesler, Thomas Monecke, and Jon Geological Survey Contributed Report CR-95-H, 15 p. Spencer improved this manuscript. We thank our coauthors Dilles, J.H., and Gans, P.B., 1995, The chronology of Cenozoic volcanism and on previous work at Tea Cup— J.G. Brown, G.B. Haxel, T.S. deformation in the Yerington area, western Basin and Range and Walker Hayes, E.P. Jensen, D.A. Johnson, R.J. Kamilli, and K.R. Lane: Geological Society of America Bulletin, v. 107, p. 474–486. Dilles, J.H., Einaudi, M.T., Proffett, J., and Barton, M.D., 2000, Overview of Long, as well as D.J. Maher and G.H. Davis for helpful dis- the Yerington porphyry copper district: Magmatic to nonmagmatic sources cussions and critical comments throughout the course of this of hydrothermal fluids: Their flow paths and alteration effects on rocks and study. D.J. Maher helped in the field and provided regional Cu-Mo-Fe-Au ores: Society of Economic Geologists Guidebook Series, v. data. Bronco Creek Exploration provided drill hole data for 32, p. 55–66. Drewes, H., 1976, Tectonic setting of the porphyry copper deposits of south- the area near the Red Hills prospect. PN gratefully acknowl- eastern Arizona and some adjacent areas [abs.]: Arizona Geological Society edges financial support by the Society of Economic Geolo- Digest 11, p. 91-92. gists and the Science Foundation Arizona. The project also Force, E.R., 1998, Laramide alteration of Proterozoic diabase: A likely con- benefited from support provided by the U.S. Geological Sur- tributor of copper to porphyry systems in the Dripping Spring Mountains vey through the Porphyry Copper Life Cycle project and area, southeastern Arizona: ECONOMIC GEOLOGY, v. 93, p 171–193. Hammer, D.F., and Peterson, D.W., 1968, Geology of the Magma mine area, grants 02HQAG0088 and 08HQGR0060, and National Sci- Arizona, in Ridge, J.D., ed., Ore deposits of the United States, 1933–1967 ence Foundation grant EAR-0230091 to MB and ES. (Graton-Sales volume): New York, American Institute of Mining, Metallur- gical, and Petroleum Engineers, v. 2, p. 1282–1310. 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