Nickerson Et Al:Layout 1

© 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 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, 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 porphyry 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). 0361-0128/98/000/000-00 $6.00 300 RECONSTRUCTION OF THE TEA CUP PORPHYRY SYSTEM 301 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

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