Dismembered Porphyry Systems Near Wickenburg, Arizona: District-Scale Reconstruction with an Arc-Scale Context

Dismembered Porphyry Systems near Wickenburg, Arizona: District-Scale Reconstruction with an Arc-Scale Context

Phillip A. Nickerson†,* and Eric Seedorff Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona, 1040 East Fourth Street, Tucson, Arizona 85721-0077

Abstract This study combines results from reconnaissance-scale mapping of hydrothermal alteration, rock types, and structures to provide a district-scale cross section and associated palinspastic reconstruction of an area with two previously undescribed Laramide (~70 Ma) porphyry systems at Sheep Mountain and Copper Basin (Crown King). Extension at the district scale is placed in an arc-scale context using an original compilation of strike and dip measurements on Tertiary rocks to reconstruct the Laramide porphyry belt prior to extension. The study area contains five sequential, partially superimposed sets of normal faults that are nearly planar where exposed. Dips of all normal faults initiated at 60° to 70° and rotated during slip to angles as gentle as 20°. A palinspastic reconstruction reveals that two, spatially distinct hydrothermal systems overlie different cupolas of a Late Cretaceous pluton. Hydrothermal alteration is zoned from greisen to potassic to transitional greisen- potassic assemblages from deep to shallow structural levels. The reconstruction is used to identify two covered exploration targets. The prospects may be porphyry molybdenum systems of the quartz monzonitic-granitic porphyry Mo-Cu subclass, joining others in an arc that is best known for porphyry copper deposits. The Laramide porphyry belt prior to extension displays a variably well-defined axis, ~100 km wide, with gaps and clusters of deposits along its 700-km strike length. The majority of deposits lie along the axis, but others lie in fore- or rear-arc positions. The interpreted preextensional geometry of the Laramide porphyry belt resem- bles other porphyry belts and the distribution of active volcanoes at convergent margins.

Introduction of ore-forming systems are exhumed and can be examined In southwestern North America, the Cenozoic Basin and at the surface (Proffett, 1979; Carten, 1986; Dilles and Ein- Range extensional province is superimposed upon the audi, 1992; Seedorff et al., 2008). Laramide (80–50 Ma) magmatic arc (Wilkins and Heidrick, Regional-scale reconstructions commonly subdivide regions 1995; Barton, 1996). The Laramide magmatic arc contains into extensional domains and then restore extension in each of many well-studied porphyry systems (e.g., Titley and Hicks, the extended domains (e.g., McQuarrie and Wernicke, 2005). 1966; Titley, 1982a; Pierce and Bolm, 1995; Fig. 1). However, At the arc scale, such reconstructions can aid in understand- few previous investigations consider the effect that postmin- ing tectonic processes or, as attempted here, the original dis- eralization normal faulting has had on spatial relationships at tribution of porphyry deposits along a magmatic arc. the deposit or district scale (e.g., Lowell, 1968; Wilkins and This study focuses on a poorly understood segment of the Heidrick, 1995; Wodzicki, 1995; Stavast et al., 2008), and Laramide porphyry copper belt in the White Picacho and especially at the scale of the magmatic arc (Richard, 1994; Sheep Mountain districts (Keith et al., 1983), near the town Staude and Barton, 2001). of Wickenburg in central Arizona (Fig. 2), and provides the At the deposit and district scale, the superposition of first public description, notwithstanding company reports, of normal faults and porphyry systems creates challenges and hydrothermal features observed in the Copper Basin (Crown benefits for the study of both extensional and hydrothermal King) and Sheep Mountain porphyry systems. Previous processes. Challenges can arise where hydrothermal altera- detailed mapping of rocks types and structural geology (Peter- tion obscures subtle distinctions in certain stratigraphic units son, 1985; Capps et al., 1986, Stimac et al., 1987; R. Powers, that might serve as important structural markers, or where unpub. map) is combined with original, reconnaissance-scale orebodies are dismembered by normal faults. Benefits of mapping of hydrothermal alteration and examination of areas this superposition arise when products of one of the geologic critical to a structural interpretation of the area, which was processes can be used to help constrain aspects of the other made possible by helicopter-assisted access. The data for rock process. For example, predictable patterns in hydrothermal types, structure, and alteration are used to make a structural alteration zoning can be used as geologic markers that may analysis of the area, including a palinspastic reconstruction better constrain structural reconstructions (e.g., Nicker- of the Oligo-Miocene extension. The reconstruction demon- son et al., 2010), and, in turn, aid in better discriminating strates that extension was produced by five superimposed sets between different styles of extension. Conversely, ore-form- of normal faults, and the district-scale reconstruction reveals ing processes can be better constrained where deep levels two new porphyry exploration targets centered on potassic alteration. The results from the Wickenburg area are placed in an arc-scale context by using the equations of Jackson † Corresponding author: e-mail, [email protected] and McKenzie (1983) to generate a new reconstruction of *Present address: Bronco Creek Exploration, Inc., 1815 E. Winsett Street, the preextension distribution of porphyry deposits along the Tucson, Arizona 85719-6547. Laramide arc.

Submitted: December 22, 2013 0361-0128/16/4384/447-20 447 Accepted: September 25, 2015 448 NICKERSON AND SEEDORFF

114°W 111°W 108°W

Las Vegas Porphyry Systems of the a 36°N Laramide Magmatic Arc Arizona Mineral

Nevad ● Park New Mexico

Bagdad ● Copper Basin ● Copper Basin (Prescott) (Crown King) ● Arizona ● California Fig. 2 Sheep Mountain Globe-Miami District ● Phoenix ●●● Resolution ● ● Christmas Morenci Sacaton ● Ray ● ● ● Hillsboro ●● ●● ● 33°N Santa Cruz Poston Copper Butte ● Safford ● Vekol ● ● Creek ● Santa San Manuel- District Lakeshore ● Tyrone Rita Ajo ● ● Kalamazoo Silver Bell Tucson Pima ●● ●● ● Peach-Elgin District ● Rosemont Sono ra ● Red Mountain

G u Chihuahua lf ● Cananea o f

Baja California C a l La Caridad if o ● r n 0100 km i Opodepe a Cumobabi 30°N ● ●

Fig. 1. Index map of porphyry deposits in the Laramide porphyry copper belt. Modified from Titley (1982b). The dashed box indicates the location of Figure 2.

Geologic Setting east (Fig. 2) in the Basin and Range extensional province. Pre- Porphyry deposits in Arizona are among the largest and best vious workers in the Vulture Mountains, ~10 km west of the study area (Fig. 2), proposed that SW-dipping, listric normal studied deposits in the world (e.g., Cooke et al., 2005), and faults were responsible for the observed repetition of steeply many have been productive mines for over a century (e.g., dipping (up to ~85°) Tertiary sedimentary and volcanic rocks Miami, Inspiration, Ray, and Morenci; Parsons, 1933; Fig. 1). exposed in the Vulture Mountains, as well as for the slightly Nearly all of the known porphyry deposits in Arizona formed less tilted (up to ~65°) Tertiary sedimentary and volcanic during Laramide time (~80–50 Ma) when NE-directed rocks exposed in the study area (Rehrig et al., 1980). subduction of the Farallon plate beneath the North Ameri- Subsequent detailed geologic mapping in the study area can plate produced a NW-SE-striking magmatic arc (Titley, (Capps et al., 1986; Stimac et al., 1987) at 1:24,000 scale, 1982b; Lang and Titley, 1998; Leveille and Stegen, 2012). The however, revealed that normal faults are nearly planar district-scale portion of this study examines a segment of the where exposed at the surface across multiple kilometers of Laramide arc located between the Globe-Miami district and paleodepth and that higher angle faults cut lower angle faults. the Bagdad deposit, within which a major economic deposit Determining the geometry, slip, and relative timing of normal has yet to be identified (Fig. 1). faults (i.e., the style of extension) near Wickenburg is essen- tial to reconstructing the geology of the district. Furthermore, Extension in western Arizona reconstruction of extension in the Laramide porphyry belt is The study area near Wickenburg is located between the of particular interest to economic geologists, because it can highly extended Harcuvar and Harquahala metamorphic core lead to the identification of offset and covered pieces of por- complexes (Reynolds and Spencer, 1985) to the west and the phyry systems (e.g., Lowell, 1968) that might be attractive unextended Bradshaw Mountains (Rehrig et al., 1980) to the exploration targets. DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 449

Geologic Map of West-Central Arizona

Highway Tertiary volcanic and sedimentary rocks Early and Mesoproterozoic intrusive rocks County border Cretaceous and Tertiary intrusive rocks Early Proterozoic schist Mine or resource Mesozoic and Paleozoic sedimentary rocks Early Proterozoic metamorphosed sedimentary rocks Exploration target Quaternary undifferentiated Early Proterozoic metamorphosed volcanic rocks Br ads h Copper Basin aw M (Crown King) ounta C Buckhorn ins

r MC z Creek Target a Yavapai rcuv Ha Pa rg 34°N a

L Maricopa enbu Sheep Wickenburg ● Hwy 60 ains Wick Mountain Target Mount 60 US lture Vu ns . 3 untai Fig C Mo Newsboy MC la ha ua rq Vulture Ha Mine U S 60 Big Horn Mountains 20 33°40'N km

113°30'W 113°W 112°30'W Fig. 2. Generalized geologic map of western Arizona, showing location of the district-scale study area of Figure 3 (gray box) relative to county boundaries and nearby mountain ranges, mines, resources, and exploration targets discussed in the text. MCC = metamorphic core complex (geology from Reynolds, 1988).

Service Layer Credits: Sources: USGS, ESRI, Rock types TANA, AND magnetite, and up to 5% sphene, zircon, and other accessory The geologic map of the study area is shown in Figure 3. Pre- minerals (Stimac et al., 1987). vious geologic maps are heavily utilized in its creation (Peter- The crystalline Proterozoic and Cretaceous rocks are over- son, 1985; Capps et al., 1986; Stimac et al., 1987; R. Powers, lain by Tertiary units, which include siliciclastic sedimentary unpub. map), especially for distribution of rock types. New rocks, volcanic rocks, debris flows, and conglomerates. The oldest Tertiary unit is a red to brown conglomerate containing mapping and field checking influenced interpretation of the pebble- to boulder-sized clasts of older crystalline rocks and nature of many contacts, and structural interpretation and dis- lesser volcanic rocks. The conglomerate is similar in appear- tribution of alteration are entirely a result of this study. ance to the synextensional red-bed conglomerates of the Rocks in the study area consist of Proterozoic amphibo- Whitetail and Cloudburst Formations in southeastern Arizona lite, gneiss, schist, granite, and pegmatite, intruded by Late (Dickinson, 1991). The conglomerate varies in thickness from Cretaceous granite, and overlain by late Oligocene and Mio- 1 m to 10s of m and consistently dips steeply at ~55° to 75° to cene volcanic rocks (Stimac et al., 1987; Fig. 3). Proterozoic, the northeast at its base. Near Buckhorn Creek (Fig. 3), the Paleozoic, and Mesozoic sedimentary rocks, which are locally unit contains clasts of crystalline rocks exhibiting porphyry- important ore hosts in certain Laramide porphyry deposits style alteration and Cu oxide mineralization. (e.g., Resolution; Manske and Paul, 2002), were denuded in Tertiary volcanic and sedimentary rocks overlie the basal the study area, most likely by erosion during uplift in the Late Tertiary conglomerate. The volcanic rocks are approximately Cretaceous (Flowers et al., 2008). 2 km thick near the Wickenburg Mountains but thicken to 4 The metamorphic Proterozoic rocks exposed in the study to 5 km near Buckhorn Creek (Fig. 3). The Tertiary rocks dis- area belong to the Yavapai Supergroup (DeWitt et al., 2008). play shallowing dips from the bottom to the top of the Tertiary In the Bradshaw Mountains (Fig. 2), the Yavapai Supergroup section, indicating that they are synextensional deposits. The displays a consistent N-S-striking, moderate to steeply dip- geologic map and cross section in Figure 4 depict a portion ping foliation. Variations in the foliation are used later in this of the Tertiary volcanic and sedimentary sequence mapped study to constrain Tertiary deformation in the metamorphic at 1:24,000 scale near the Wickenburg Mountains (Stimac et rocks. Late Cretaceous granite, dated at 68.4 ± 1.7 Ma (K-Ar al., 1987). biotite) 10 km west of the study area in the Vulture Mountains Tertiary volcanic and sedimentary rocks have not been dated (Rehrig et al., 1980), crops out predominantly in the western in the study area. However, volcanism in the nearby Vulture half of the study area (Fig. 3). The granite is porphyritic to Mountains (Fig. 2) is known to have occurred between ~25 equigranular in texture, containing 30 to 40% orthoclase, 20 to 15 Ma (Rehrig et al., 1980). The oldest volcanic unit is a to 30% plagioclase, 20 to 30% quartz, 3 to 5% biotite, 1 to 2% basaltic lava flow (Tlb) that in some places is interbedded with 450 NICKERSON AND SEEDORFF 2°20'W 34°5'N 11 34°N s East Shee p aw Mountain Tg

4 Xmvs

4 Bradsh Mountain Tv l A'

n vs

i 2°20'W nta 11 u Xi

o Xm p M ee Sh Xmvs p st

ee 6

untain 6 We lt Sh

Xi u

Xi a Mo f

l 0

4 6 Tv Castle Creek 4

2 of bedding

Normal faults 2

Strike and dip

4 4 9

Tv u 2 6

ult 8

5 fa 2

y 2

6 6

Tv u 5 k n or 6

h 2°25'W ck

u 11

Cree B Tv l Xmvs

Buckhorn

2 5

t 6 Tv u

u & metasedimentar

o 7

l N

n 4

u Xmvs

o a t

f l °55' e u M a V f 33 Metavolcanic Granite & gneiss sh y Wa vapai Supergroup ilb

r l

l T Xi vs Xmvs Tv Tv Proterozoic Ya

6 5 Tv l

1 7 2°30'W Granite 11 s Kg Kg Cretaceous Xi Mountain Xmvs Wickenburg

g i

Kg F km

Tg 2

Kg 5 Tv l Tv l Geologic Map of Wickenburg Mountains to Bradshaw Mountain localities discussed in the text. Tertiary volcanic Fig. 3. Generalized geologic map across the study area depicting rock units, faults, selected bedding orientations, and localities discussed in text. Tertiary volcanic rocks younger than the Hells Gate Formation are lower volcanic unit. Tertiary rocks in the Hells Gate Formation and older units are grouped Tertiary Geology from Peterson (1985), Capps et al. (1986), Stimac (1987), R. Powers (unpub. upper volcanic unit. Note direction of north arrow. grouped in the Tertiary Dashed box outlines the location of Figure 4. map), and this study. 5

r 2°35' W

n 0

i 5

5 t 6

l 6

t 6

u Lower volcanic and sedimentary units Gravel s Upper volcanic and sedimentary units

f k

c o rtiary

i A

M Tg Tv l Xmvs Tv u

W Te DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 451

N Geologic Map and Cross Section s r

400 e 3600360 3400 t after Stimac et al. (1987) 0

3800 0 1000 E

00 Me 4000403838 Qs 000 N Younger alluvium (Quaternary)

4 t

Th u

4200 1000 m 4200

42004

b 00

Th u Older alluvium (Quaternary and 44 Qts Th l 50 Tu Tertiary) rnon faul

Ve Debris flows and avalanche Th l 65 Tdf b deposits (Tertiary) Th u Xs Tu Th u 52 Mount

Th l Tub Upper basalts (Tertiary) 3 Tl b

00 34 Tl b Ts 45 3600 70 380038 Tif Felsic dikes and plugs (Tertiary) Th t 3 3800 Th t d Tc Th l Th l

4000 Ts Tim Mafic dikes and plugs (Tertiary) 54 4200 Tc Upper Hells Gate dacite and Th l Thu 44 3400 46

Tc rhyodacite flows (Tertiary) Th l Xs 3 3 d Thl Lower Hells Gate flows (Tertiary)

3600 d Ts Tc Ts 47 t Tl b Ts Tl b Tht Hells Gate tuffs (Tertiary) Tc

Tl b Tsd Th u San Domingo Volcanics (Tertiary) Tl b Th u Th t Th u 30 Xp t 3 Quartz-bearing rhyolite flows Ts Tl b Tsd Xa m Th t Xs 3600 Th u

Tc (Tertiary) Xs d Tl b Tl b Th u Lithic tuffs and related sedimentary 3400 Ts Tst 47 Tc

ul t rocks (Tertiary) Th t Tl b Th t Tc t 40 Th u sh faul t sh f Ta Tlb Lower basalt, basaltic andesite, Ts Tc 3 Th u Wa Wa and andesite flows (Tertiary) Xs Ta

d Qs Tc Ts

ilby il Ta 55

t Andesite flow (Tertiary) Tr Tl b sh faul t Ts Tc Tl b

340034 Wa Tc Tc Clastic sedimentary rocks (Tertiary) 3200 Xg 65 Xs 3200 ilby Tc Tr 78 35 Xs Kg

Th u Granite (Cretaceous)

3 t

0 Tc 340 d Tc 70 Ts t Tl b Ts

Ts Xp Pegmatite (Early Proterozoic) 60 30003 Tl b Xs Tc 60 Xg Granitic rocks (Early Proterozoic) Xms 29 45 Xa m 69

d Schistose metamorphic rocks Xps Xs Ts Xmc Xms (Early Proterozoic) Xms Xa m Xmc

Xs Metacarbonate (Early Proterozoic) 34 Metavolcanic rocks (Early

65 Xam Kg Xmc Proterozoic) 3400

0 Xg 50 Metasedimentary rocks Xp s 2800 Xps Xms

3200 Xms (Early Proterozoic) Xms Qs 63 Xps Psammitic schist (Early Proterozoic) 57

Xps 72 3000 Xa m

56 65

Kg Strike and dip of bedding Kg Xms 54 Strike and dip of foliation 75 Xa m m Xms Geologic contact ickenburg Mountains fault Ti W 68 68

65 Normal fault showing 00 f 26 Ti Kg 73 amount and direction of dip Xms Kg

Xms Normal fault inferred 65 Kg

47 Normal fault covered

Qt s Fig. 4. Geologic map and cross section, digitized from Tdf Tdf Stimac et al. (1987). Location of map shown in Figure SW 23 t Qs

0 3. Note direction of north arrow. Topographic con-

2400 000 000 tours are in feet. Fee 5 4 300 0 100 0 2000 452 NICKERSON AND SEEDORFF the basal conglomerate (Tc; Fig. 4). Above the basalt, rhyolite The Squaw Peak porphyry system, 60 km to the northeast, lava flows and tuffs of the San Domingo Volcanics occur in is hosted in the similar aged Proterozoic Cherry batholith the western half of the study area. The Morgan City Rhyolite, and has been determined to have an age of ~1.7 Ga based on Spring Valley Rhyolite, and Castle Creek Volcanics occupy a Re-Os dating of molybdenite (Sillitoe et al., 2014). At Squaw similar stratigraphic position in the eastern half of the study Peak some of the veins in the porphyry system display ductile area (Capps et al., 1986; Stimac et al., 1987). Dacite to rhyo- fabrics near their margins indicative of postmineral deforma- dacite lava flows and tuffs make up the structurally higher tion. At Copper Basin (Crown King) veins do not display duc- Hells Gate Volcanics. Resting unconformably above the tile fabrics near their margins, and thus the porphyry system Hells Gate Volcanics are interbedded basalts, tuffs, volcanic is considered to be Laramide aged. Drilling conducted in the megabreccias, and debris flow deposits with nearly horizon- late 1960s and early 1970s produced a resource estimate of tal bedding attitudes. Northwest-striking and steeply dipping one billion tons of 0.16% Cu and 0.031% Mo based on eight felsic and mafic dikes locally intrude the crystalline basement drill holes (ASARCO, unpub. report, 1974). and the Tertiary volcanic rocks. The youngest Tertiary unit Near Sheep Mountain (Fig. 3), two porphyry prospects is a brown, consolidated to semiconsolidated conglomerate. have been identified. The larger of the two prospects is A thin layer of Tertiary-Quaternary gravels locally covers the known as Sheep Mountain (Wilkins and Heidrick, 1995), or conglomerate. Sheep Mountain East (Ullmer, 2007). The prospect contains a resource of 40 million metric tons (Mt) of 1.6% Cu and Economic Geology 0.035% Mo (Ullmer, 2007). The mineralization lies under- The study area contains mineralization that is related to sev- neath as much as 700 m of Tertiary volcanic and sedimentary eral genetic types of deposits and formed at distinctly differ- rocks. Molybdenite from drill core at the prospect (hole CC-1 ent times (DeWitt et al., 2008; this study). Gold and copper at 611-m depth) has been dated using the Re-Os technique associated with small, past-producing, volcanogenic massive and yielded a Laramide age of 70.34 ± 0.36 Ma (H. Stein, sulfide systems are hosted in metamorphosed Proterozoic written commun., 2010). rocks. Pegmatite dikes of Proterozoic age have been investi- Approximately 5 km to the west is the Sheep Mountain gated for their beryllium and lithium potential (Jahns, 1952; West prospect, where several outcrops of intensely altered London and Burt, 1978). Epithermal mineralization is locally Proterozoic granite are exposed beneath the Tertiary-Protero- hosted in Tertiary volcanic rocks. Many of the washes in the zoic unconformity in tilted fault blocks (Figs. 3, 5). Several study area produced, and continue to produce, gold from holes have been drilled in the last decade exploring for super- small placer deposits hosted in Tertiary and Quaternary grav- gene copper mineralization, but a significant resource has not els. The Buckhorn Creek area (Fig. 3) has been estimated to been identified. contain 100,000 ounces (oz) of gold in Tertiary and Quater- nary gravels (Montclerg Resources Ltd., unpub. report, 1995). Hydrothermal Alteration The Vulture lode mine, located just west of the study area in Hydrothermal alteration was mapped at reconnaissance scale the Vulture Mountains (Fig. 2), produced 340,000 oz Au and across the study area (Fig. 5). Three important styles of altera- 260,000 oz Ag between 1863 and 1942 (White, 1989). Native tion have been identified: greisen, potassic, and transitional gold and electrum hosted in Proterozoic and Cretaceous crys- greisen-potassic. The term greisen is used here to describe talline rocks is interpreted to be genetically related to a Creta- hydrothermal alteration assemblages where coarse-grained ceous dike (Spencer et al., 2004).The Newsboy prospect (Fig. (>0.5 mm) white mica is an important constituent (e.g., 2), also in the Vulture Mountains (Fig. 2), has a resource of Shaver, 1991; Seedorff et al., 2005a). Greisen is not widely 123,000 oz Au and 2,121,800 oz Ag hosted predominantly at recognized in porphyry copper systems, but in the porphyry the contact between Proterozoic schist and Tertiary rhyolite copper systems in which it has been documented, it occurs lava flows (Hastings et al., 2014). at deep levels, generally beneath the level of the orebodies, Several Laramide porphyry systems near Wickenburg have beneath the region of most abundant quartz veins and most defined mineral resources, but there is no significant past or intense potassic alteration, and well below the level where current production from these systems. The largest resource sericitic alteration develops (Seedorff et al., 2005a, 2008). In is the Copper Basin prospect, here referred to as the Copper contrast, greisen can occur in porphyry molybdenum deposits Basin (Crown King) system (e.g., Ball and Closs, 1983), which of the Mo-Cu subclass in a location that is within and above is located 20 km north of the study area in the Silver Moun- orebodies and the most intense potassic alteration, analogous tain mining district of the southern Bradshaw Mountains (Fig. to the position of sericitic alteration in many porphyry copper 2). This prospect is distinct from the Copper Basin district systems in Arizona (Shaver, 1991; Seedorff et al., 2005a). near Prescott, Arizona, here referred to as the Copper Basin (Prescott) system, which is described by Johnston and Lowell Greisen (1961; Fig. 1). Hosted within the Cretaceous granite (Fig. 6A), NE-striking The Copper Basin (Crown King) system is located 8 km veins of quartz + white mica + pyrite ± chalcopyrite ± K-feld- south of the town of Crown King, Arizona. Soldiers stationed spar with white mica ± pyrite ± chalcopyrite envelopes (Fig. at nearby Fort Misery at the end of the 19th century were the 6B) commonly compose 1 to 5% of outcrops in the central first to identify the prospect (Tognoni, 1969). Chalcopyrite Wickenburg Mountains (Fig. 5). Locally, veins and envelopes and molybdenite, as well as spectacular Cu oxide seeps in the of greisen are so intensely developed that they constitute up drainages, are exposed at the surface hosted in weakly foli- to 20 vol % of outcrops. Vein fillings range from 1 to 200 mm ated Proterozoic Crazy Basin granite (DeWitt et al., 2008). wide and have envelopes with a total width of 5 to 50 mm. DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 453

White mica grains range in size from 0.5 to 5 mm and are found in both the vein filling and alteration halo. Quartz in the vein filling is commonly milky white in color. Sulfides are vs

→ found predominantly in the vein filling but also in the altera- Xm A' Tv l

8 tion halo. The sulfides range in size from 1 to 15 mm and have 5 Greisen a pyrite to chalcopyrite ratio of approximately 10:1. K-feldspar is rarely observed in the greisen veins, where it comprises 2°25'W p Alteration 11

st <1% of the vein filling. 0

We 7 Shee Potassic ssic

Mountain

0 5 Xi

Xi In the Buckhorn Creek area, west of Sheep Mountain (Fig. 0

Po ta 8 Xmvs 5), NE-striking veins of quartz + K-feldspar ± white mica ± Cre ek fault pyrite ± chalcopyrite with biotitic envelopes cut Yavapai Schist Castle l

Tv that contains abundant metamorphic muscovite (Fig. 6C). Hydrothermal

c Veins vary in size from 1 to 15 mm wide with alteration halos

Tv l <10 mm wide. Quartz is the dominant mineral in the vein filling (~65%), accompanied by K-feldspar (~20%), white mica u

Tv (~10%), and sulfides (~5%) consisting of pyrite and lesser

Tv l chalcopyrite. White mica ranges in size from 0.1 to 2 mm. Vein k density increases to the northeast until it reaches 5 vol % adja-

5 3

Tv u 8 5 cent to the Tertiary-Proterozoic unconformity that bounds the ult fa northeastern side of altered Yavapai Schist (Fig. 5). Alteration

Xmvs gradually decreases in intensity to the northwest of the Buck-

ckhorn Cr n horn Creek area until the Yavapai Schist is unaltered.

r rike and dip of hydrothermal vein o ansitional Greisen-Potassi Bu kh St c Tr 30'W Bu Transitional greisen-potassic 2°

11 5

Reconnaissance Map of This term is used here with the meaning of Shaver (1991), 8 Tv u Tv l who first described this type of coarse-grained, K-feldspar and white mica-bearing style of alteration at the Hall (Nevada Moly) deposit, Nevada, where it overlies potassic alteration

t and is regarded as a coarser grained analog of sericitic altera-

l Xmvs Xmvs

u tion (Shaver, 1991). At Sheep Mountain (Fig. 5), veins of

f e quartz + K-feldspar ± white mica ± sulfide (1–5 mm wide)

M V are cut by quartz + K-feldspar + white mica + sulfide veins (1–5 mm wide) with white mica halos (<5 mm wide; Fig 6D).

Kg These veins differ from potassic veins, because they lack Tv l biotite, and white mica is more abundant. They differ from

0 greisen veins because K-feldspar is much more abundant in 8

Tv l 0 the vein fill (up to 50%) and is present in the vein envelope. 8 The vein density is intense in several areas on Sheep Moun- tain, where veins + halos constitute 5 to 10 vol % of outcrops. vs

s White mica varies in size from 0.1 to 1 mm. Envelopes sur-

Xm rounding quartz of the vein filling of both vein types conspicu-

Xi ously change back and forth along strike between white mica Xi 12°35'W km 1 Xmvs and K-feldspar. Mountain Wickenburg Structural Geology Tertiary sedimentary and volcanic units, the Tertiary-Protero- Kg

Kg zoic unconformity, contacts between Proterozoic units, and

5 styles of hydrothermal alteration are structural markers in the 7 Kg 5 study area, and the repetition of these markers was determined

n u by previous studies to be the result of movement on Tertiary

l Kg

t n (~25–15 Ma) normal faults (Rehrig et al., 1980; Peterson,

u Tv l

f k 1985; Capps et al., 1986; Stimac et al., 1987). By scrutinizing A

c o →

M crosscutting relationships between the normal faults exposed Fig. 5. Reconnaissance map of hydrothermal alteration in the study area. Colors faded background are rock units shown Figure 3. See 3 for key to units. Note direction of north arrow. W in the study area, relative ages can be determined, which is

N critical to constraining the style of deformation and is neces- sary for subsequently grouping faults into sets. 33°55' Examination of the normal faults in map view reveals that five distinct sets of Tertiary normal faults, defined by similar 454 NICKERSON AND SEEDORFF

A B K-feldspar Quartz

Oxidized sulfid e White mica

C Quartz D Biotite halo Crosscutting vein White mica K-feldspar

Sulfide site

Fig. 6. Photographs of styles of veins and associated alteration in the study area. A. Unaltered Cretaceous granite. B. Quartz- muscovite-pyrite ± chalcopyrite ± K-feldspar vein hosted in Cretaceous granite; an example of greisen-style alteration. C. Quartz + K-feldspar + white mica + pyrite ± chalcopyrite vein with a biotite envelope cutting the Yavapai Schist at Buck- horn Creek; an example of potassic alteration. D. Quartz + K-feldspar ± white mica ± sulfide veins cut by quartz + K-feldspar + white mica + sulfide veins with white mica halos hosted in Proterozoic gneiss at Sheep Mountain West prospect. The crosscutting vein is an example of the transitional greisen-potassic style of alteration. strikes and dips and crosscutting relationships, are present in Wickenburg Mountains fault (Fig. 8), the fault dip decreases the study area (Fig. 7). Unless they significantly influence the only slightly from 5.1° to 3.3° (curvature of 0.5°/km). The map pattern, faults determined to have less than 500 m of structure contour map also reveals a NE-SW-striking trough offset are not shown in the geologic map (Fig. 3) and were not in the plane of the Mount Vernon fault. Such mullions are assigned to a set of faults (Fig. 7). Many dozens of such small- commonly observed in normal faults (e.g., Proffett, 1977; offset normal faults were identified in previous work across John, 1987a; Wong and Gans, 2008). the study area (e.g., Stimac et al., 1987; Fig. 4). Set 2: The NE-SW-striking faults of this set strike nearly Set 1: Two faults from this set crop out at the surface in the perpendicular to faults in all other sets. One fault from this Wickenburg Mountains (Fig. 7), the Wickenburg Mountains set, named the Cross Cut fault, is exposed in the central por- fault and the Mount Vernon fault. These faults have sinuous tion of the study area, where it has a measured dip in outcrop expressions at the modern surface produced by the intersec- of 45° to the southeast (Fig. 7). The Cross Cut fault places tions of their present-day gentle dips with the modern topog- Tertiary volcanic and sedimentary rocks on Proterozoic rocks. raphy. A structure contour map, generated by contouring the Offset on the Cross Cut fault is not well constrained, but intersection of the fault surface and topography, reveals that apparent offset is estimated to be 1.5 km. the Wickenburg Mountains fault and the Mount Vernon fault Set 3: Faults from this set crop out across the study area. have azimuths of ~125° and dip ~4° to the southwest (Fig. They strike with an azimuth of ~150° and have dips mea- 8). The planes of the two faults are contoured at similar ele- sured in outcrop that range from ~35° to 50° to the south- vations, which might lead one to believe that the two faults west. Ten faults from this set are shown on the map (Fig. are one fault plane. However, the faults are separated by the 7). Typical offsets on faults from this set are approximately Trilby Wash fault that has approximately 1 km of displacement 1 km. The Castle Creek fault, which bounds the western side (Fig. 4), so the similarity in elevations is happenstance. of Sheep Mountain, and the Trilby Wash fault belong to this Previous mapping by Stimac et al. (1987) indicates that por- set (Fig. 7). In the Wickenburg Mountains, certain contacts tions of the contact between Late Cretaceous granite and Pro- between granite and metamorphic rocks were reinterpreted terozoic metamorphic rocks in the Wickenburg Mountains are here as faults belonging to set three, as opposed to intrusive an intrusive contact, not a fault trace. This contact in places is contacts, because the contacts are on the projections from reinterpreted here to be a fault contact because it separates the southeast of faults from set three, and contrasts in styles hydrothermally altered granite from fresh metamorphic rock of hydrothermal alteration are observed across the contacts (Fig. 5), and the contact is nearly planar as revealed by the (Fig. 5). structure contour map (Fig. 8), which would be atypical for Set 4: Members of the second youngest set of faults have an intrusive contact. Across 6 km of downdip exposure on the an azimuth of ~150° and dip steeply to the southwest at ~60° DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 455

to 70°. Dozens of faults from this set crop out across the study area (Stimac et al., 1987); however, few have significant

amounts of offset and only six are shown in Figure 7. The fault

with the largest offset is the Buckhorn fault (Fig. 7), which has

1 : 1.5 km of slip in the central portion of the study area.

Set Set 5: Faults belonging to the youngest set have azimuths : 0 of ~330° and dip steeply to the northeast at ~70°. Nearly all : faults from this set have offsets less than 500 m, and only two faults from this set are depicted in Figure 7. 2

Set Structural Interpretation and Palinspastic

34°5'N 70

: 45 Reconstruction of Normal Faults ! :

: :

The original observations and compilation of crosscutting :

2°25' W : 3 : relationships between normal faults (Fig. 7), structure con-

: lt

: tour maps of normal faults (Fig. 8), and analysis of Tertiary

u Set

! : fa tilting (Fig. 9) are combined below to interpret the style of 70 ! : 40 : extension in the study area. The interpretation provides the Castle Creek

42 means to palinspastically reconstruct Tertiary extension in a

4 20-km-long cross section through the study area (Fig. 3).

Set : Normal Faults Sets Tertiary tilting 65 : An examination of the strikes and dips of the oldest Tertiary fault rocks above the Tertiary-Proterozoic unconformity (Fig. 3) 5

n shows that, in nearly all instances, these rocks dip ~65° (±5°) or

kh Set to the northeast. This tilting is the result of slip and concur-

: B

70 rent tilting on the SW-dipping normal faults (Stimac et al.,

2°30'W 1987). Orientations of foliation in the Proterozoic Yavapai

t 11 u : t : Schist in the study area can also be used to constrain the

u s magnitude of Tertiary tilting recorded in crystalline rocks

t !

o n

r t beneath the Tertiary-Proterozoic unconformity. The orien-

o 45

r tation of foliation in the Yavapai Schist is commonly con-

: :

V ! sistent over distances of 10s of kilometers (e.g., DeWitt et

40 : al., 2008). Thus, any changes observed in the orientation of ult

: : fa foliation are likely the result of deformation subsequent to h : 0

: ! : as the Proterozoic foliation-forming event. The most probable

W candidate for reorienting foliation in the study area is tilting

! Trilby : caused by Tertiary extension.

37 : : Foliation measurements of the Yavapai Schist in the Wick-

8 : enburg Mountains, which have been highly extended (Sti-

2°35'W . : ig : mac et al., 1987), are compared to foliation measurements 11 F ! (DeWitt et al., 2008) of the Yavapai Schist 20 km to the north

40 :

34°N ! in the Bradshaw Mountains (Fig. 2), an area for which there

: 40

: is no evidence for significant Tertiary extension and tilting : !

: (Rehrig et al., 1980). Measurements of foliation in the two ! ! 65 areas are compared in contoured equal-area stereographic km 45 : 30 projection in Figure 9A and B, and the average foliation var-

: ies significantly between the two areas. To test the hypothesis

: : that the rigid body rotation was caused by tilting associated

g s : r with Tertiary extension, the foliation data from the Wicken-

u 5

a l burg Mountains are rotated 65° clockwise about a horizon-

a tal axis trending 150° (Fig. 9C). This rotation would restore

f k

c o i the amount of Tertiary tilting recorded by the oldest Tertiary

M 2°40'W

W A

11 volcanic and sedimentary rocks across the study area. The mean plane of the rotated Wickenburg Mountains foliation N data (Fig. 9C) plots within a few degrees of the mean plane Fig. 7. Descriptive classification of normal faults. Normal faults in study area grouped into five sets, numbered from oldest to youngest. Faults within each set have similar have set each within Faults youngest. to oldest from numbered sets, five into grouped area study in faults Normal faults. normal of classification Descriptive 7. Fig. Dashed box is location of Figure 8. strikes and dips, as well common crosscutting relationships. Note direction of north arrow.

2°40'W of the foliation data from the unextended Bradshaw Moun- 33°55' 11 tains, suggesting that the Yavapai Schist and other Proterozoic rocks record the same magnitude of Tertiary tilting observed in the oldest Tertiary volcanic and sedimentary rocks across the study area. 456 NICKERSON AND SEEDORFF

3800 3600 4000

Wickenburg Mountains3200 faul 3600 3800 Trilby 3600 3400 Wash 3600 fault 3400 Dip = 5.1º Mount Vernon fault

t 3200

3000 3200 3400

3200 3000 00 30

2600 Dip = 3.3º 2800 Structure Countour Map

2800 3400 Fault surface contour (feet)

Set 1 faults Younger faults N

1000 ft 1000 m

Fig. 8. Structure contour map of two faults from Set 1 exposed in the Wickenburg Mountains. Map generated by contouring the elevation of the intersection of the low-angle fault planes with topography. A. Structure contour map of Mount Vernon and Wickenburg Mountains faults with topography as a base layer. Dips are calculated across two intervals of the Wickenburg Mountains low-angle fault in the southwestern part of the map, illustrating that the dips of the two faults change only slightly over 6 km of nearly continuous downdip exposure. The calculated curvature is 0.5°/km. See Figure 7 for location in the map in the study area.

Style of extension in the transport direction, regardless of whether the faults As mentioned above, debate surrounds the style of exten- moved concurrently, sequentially in the transport direction, sion in the study area, i.e., listric faults (Rehrig et al., 1980) or another order as long as they do not crosscut one another. versus more planar, “domino-style” normal faults (Stimac et The progressive change in dips across successive faults can be counteracted, to a certain degree, by the development of drag al., 1987). The threshold of curvature which defines a “listric” folds in the hanging wall of the listric faults or by the develop- normal fault is not universally defined. Buck (1988) showed ment of antithetic listric faults (e.g., Ramsay and Huber, 1987, initial listric fault curvature of 4.5°/km in his “rolling hinge” p. 520). If present, either of the latter mechanisms should be model for extension, and other authors have proposed cur- evident in field. vature greater than 10°/km (e.g., Rehrig et al., 1980). Predic- Observations in map view (Fig. 3) and stereographic analy- tions of the competing models are outlined here and tested sis (Fig. 9A-C) are not consistent with the predictions of lis- against observations in the study area. tric fault geometry for rocks in the Wickenburg area. The For one set of nearly planar, domino-style normal faults, Yavapai Schist and the oldest Tertiary volcanic and sedimen- the amount of tilting is the same from one fault panel to the tary rocks in the study area were tilted ~65° NE during Ter- next, even if there are many faults in the set (e.g., Ramsay tiary extension. Furthermore, where exposure of fault planes and Huber, 1987, p. 518). In contrast, listric normal faults is extensive enough at the surface in the downdip direction produce greater amounts of tilting in the hanging wall than to determine the curvature (Fig. 8), the calculated curvature in the underlying footwall as rocks in the hanging wall slip is indeed low (~0.5°/km). Large-scale drag folds or antithetic down a concave-upward fault plane equal to the amount listric faults are not observed in the study area, so it is unlikely of curvature on the fault multiplied by the displacement of that increased tilting in the transport direction caused by lis- the fault. Where a set of multiple, subparallel listric normal tric faults has been counteracted by such mechanisms. Taken faults are present, the dip of beds in the hanging walls of suc- together, evidence in the study area controverts the involve- cessive fault blocks should show progressively steeper dips ment of strongly listric normal faults in extension and suggests DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 457

A B

N = N = C

Fig. 9. Contoured poles to planes of foliation measurements in the Yavapai Schist depicted on stereonets, using equal-area lower hemi- sphere projections. A. Foliation measurements in the Yavapai Schist from the unextended southern Bradshaw Mountains, 20 km north of Sheep Mountain. Data from DeWitt et al. (2008). B. Foliation measurements in the Yavapai Schist from the Wickenburg Mountains. Data from Stimac et al. (1987). C. Rotation of the data in panel B 65° clockwise about a horizontal axis trending 150°. Rotation restores Tertiary tilting in the study area. Rotated data from Wickenburg Mountains closely match data from the Bradshaw Mountains, indicating that congruent amounts of tilting are recorded in the Yavapai Schist, the Proterozoic- N = Tertiary unconformity, and Tertiary sedimentary and volcanic rocks. that the observed brittle extension was accommodated by of foliation observed in the Yavapai Schist between extended superimposed sets of nearly planar, domino-style faults. There and unextended terranes (Fig. 9). is no direct evidence in the study area for how extension was accommodated at depth, but in principle, there is no geomet- Approach to restoring movement on normal faults ric requirement that these upper crustal normal faults neces- Figure 10 shows the palinspastic reconstruction of the 20-km- sarily merge into detachment faults at depth (e.g., Seedorff long cross section in the study area (Fig. 3) and an interpre- and Richardson, 2014). tation of that reconstruction. In the reconstruction, faults are modeled as perfectly planar, whereas field evidence indicates Interpretation of the normal faults that the faults are slightly curved in the downdip direction. It The grouping of the Tertiary normal faults into fault sets, is likely that subtle drag folds or small faults in the hanging each set with a distinct relative age, suggests that each set wall of the normal faults counteracted the slight differential can be viewed as a sequential generation of faults. Hence, tilting produced by the slip on the gently curved fault planes. each generation is defined as a set of similarly oriented faults Displacement along the normal faults was removed in sequen- that moved more or less contemporaneously during specific tial order, from the youngest to the oldest generations of nor- time intervals, as evidenced by their consistent crosscutting mal faults (panels A-F, Fig. 10), as determined by relative ages relationships, i.e., each generation of faults operated as a and dip measurements. The magnitude of slip on individual system of normal faults. As the normal faults within a given faults was constrained using structural markers, including the set cut and extended the crystalline and supracrustal rocks in Proterozoic-Tertiary unconformity, contacts between various the study area, the dip of the active fault planes rotated to lithologies of Proterozoic metamorphic rocks, the Tertiary lower angles. Once the fault planes of a fault set rotated to stratigraphy, and hydrothermal alteration assemblages. Ter- angles that were kinematically unfavorable for slippage (less tiary sedimentary and volcanic rocks were rotated to horizon- than ~30°; Byerlee, 1978; Sibson, 1994), a new fault set with tal in the time slice of the reconstruction in which they were new fault planes formed. Faults of the new set cut and con- deposited. The relative age of the Tertiary sedimentary and tinued passively rotating rocks of older fault blocks, faults of volcanic rocks was determined by examining crosscutting and older fault sets, and any other contained geologic elements, onlapping relationships between the faults and the Tertiary including porphyry systems. This repeated sequence of events sedimentary and volcanic rocks (i.e., faults either being cut by produced a cumulative northeastward tilting of ~65°, as evi- or being mantled by Tertiary sedimentary and volcanic rocks). denced by the present-day dips of the oldest Tertiary volca- Due to the abundance of crystalline rocks in the study area, nic and sedimentary rocks and the difference in orientation and thus the paucity of structural markers in certain areas, a 458 NICKERSON AND SEEDORFF

Wickenburg Mtns. Buckhorn Creek Buckhorn Sheep Mtn. A A Creek Target A’

Buckhorn BCCreek fault

Trilby Wash Castle Creek fault fault E Cross Cut D fault

Wickenburg F Mountains Mount G fault Vernon fault SW Buckhorn NE Creek Target Sheep Buckhorn Hwy 60 Mountain Target Creek Tvl

Xmvs

Modern Wickenburg Surface Mountains

Fig. 10. Panels depicting the palinspastic reconstruction of cross section line A to A' in Figure 3. Locations of endpoints and key to rock units are located in Figure 3. Bold faults extending above and below the section are the faults being restored, and dashed faults are faults that have already been restored in each panel. A. Modern cross section. B. Restoration of the 5th set of normal faults. C. Restoration of the 4th set of normal faults. D. Restoration of the 3rd set of normal faults. E. Restoration of the 2nd set of normal faults. The fault in this set strikes nearly perpendicular to the line of section. F. Restoration of the 1st set of normal faults. G. An interpretive cross section constrained by the palinspastic reconstruction, including reconstruction of hydrothermal alteration. The interpretation is faded in the background, and pieces of the reconstruction from panel F are shown in the foreground. Geographic locations discussed in the text are indicated with thin lines. The Buckhorn Creek and Highway 60 exploration targets are identified. The targets are located directly above the two cupolas of the Cretaceous pluton. The location of the cupolas is interpreted based on mapped patterns in the zonation of hydrothermal alteration. DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 459 number of uncertainties remain in the restoration. The three- of postmineralization Tertiary extension in the Basin and dimensional shape of the igneous bodies is unknown; thus the Range province began to change exploration strategies. A few form chosen here is based on relationships that are plausible orebodies in the Basin and Range province (e.g., Yerington considering constraints imposed by restoration of hydrother- and Hall, Nevada) were previously known to be tilted and dis- mal alteration patterns (Fig. 10G). As previously mentioned, membered (J. Proffett, pers. commun., 2014); however, wide- dips measurements are not available on some faults, either spread recognition of tilting and dismemberment in orebodies from this study or previous work, due to lack of exposure. In in the Basin and Range province postdated discovery of Kala- these cases, crosscutting relationships were used to assess the mazoo (e.g., Proffett, 1977; Shaver and McWilliams, 1987; generation to which such faults belonged, and then dips mea- Seedorff, 1991; Seedorff et al., 1996). Eventually, Wilkins and sured from other faults of the same fault set were used. Heidrick (1995) suggested that all porphyry deposits in the In addition, normal faults in the reconstruction are shown Basin and Range province should be assumed to be faulted locally extending to depths of approximately 20 km below the and tilted until proven otherwise. During the past decade, the Tertiary-Proterozoic unconformity (Fig. 10F), whereas expo- importance of tilting of orebodies across the Basin and Range sures of the faults at the surface likely only reach paleodepths province has continued to be emphasized (e.g., Seedorff et al., less than 9 km (Fig. 10G). It is likely that fault displacement 2005a, p. 276–277; Maher, 2008; Stavast et al., 2008; Nicker- switched from a brittle to ductile regime somewhere between son et al., 2010) and is again demonstrated here. the deepest exposures of the normal faults at the surface and Despite at least two drilling campaigns in the 1960s and the bottom of the cross section in Figure 10F. Modeling duc- 2000s, economic mineralization has not yet been located at the tile movement is not possible using the rigid reconstruction Sheep Mountain West prospect. The palinspastic reconstruc- approach employed in this study, and instead brittle move- tion demonstrates that sulfide-bearing transitional greisen- ment was projected below deepest surface exposures. Thus, potassic alteration at Sheep Mountain and potassic alteration any portions of the cross section beneath the deepest surface at Buckhorn Creek are both pieces of a larger, dismembered exposure, which are not central to the interpretation of the porphyry system. Potassically altered pieces of the same por- reconstruction presented below, should be viewed as loosely phyry system are “structurally covered” (as used by Corn and constrained and speculative. Ahern, 1994) by Tertiary volcanic and sedimentary rocks west of Sheep Mountain in the modern cross section (Fig. 10G). To Examination of the district-scale reconstruction our knowledge, this target, which we name Buckhorn Creek, The reconstruction indicates that two distinct hydrothermal has not been tested with a drill hole, but significant mineral- systems formed, each centered on a separate cupola of the ization could be associated with potassic alteration. Late Cretaceous granite pluton (Fig. 10G). The cupolas do Additionally, it is likely that intense greisen alteration not crop out, and their locations were assigned based on the exposed in the Wickenburg Mountains is the expression of intensity of alteration in fault blocks observed at the surface. a porphyry system, and we name this target the Highway 60 The pluton intrudes metasedimentary and metavolcanic rocks target. Whether the intensity of greisen alteration has any in the west and metaplutonic rocks in the east. Potassic and correlation to the development of structurally higher level transitional potassic-greisen hydrothermal alteration exposed alteration, including sulfide mineralization, remains uncer- at Buckhorn Creek and Sheep Mountain are shown to be tain. Outcrops in the Wickenburg Mountains, however, dem- sourced from the easternmost cupola of the pluton, whereas onstrate that significant quantities of magmatic hydrothermal the greisen alteration hosted in Late Cretaceous granite in fluids were released at least locally from that portion of the the Wickenburg Mountains is part of a separate hydrother- Late Cretaceous pluton. Structurally higher levels that may mal system centered on the western cupola of the pluton (Fig. contain porphyry mineralization are not located in the line of 10G). Hydrothermal alteration is zoned upward from greisen section but may lie underneath Quaternary and Tertiary cover to potassic to transitional greisen-potassic assemblages (Fig. southwest of the study area near U.S. Highway 60 (Fig. 2). 10G). The uppermost levels of the system are eroded, as is The targets generated by the district-scale palinspastic evidenced by the presence of porphyry-style alteration and reconstruction provide an example of a geologically based Cu oxides in clasts of the Tertiary conglomerate at the base of method for exploring beneath postmineralization cover rocks. the Tertiary section near Buckhorn Creek. In the Laramide porphyry province, geologically driven The Tertiary volcanic section reaches a maximum thick- exploration underneath postmineralization cover has yielded ness of approximately 5 km. This thickness is greater than the several discoveries (e.g., Kalamazoo, Lowell, 1968; Resolu- thickest exposed section of lower volcanic and sedimentary tion, Paul and Manske, 2005). Continued exploration in the rocks exposed in the map, which is approximately 4 km. The province should incorporate structural interpretations when greater thickness in the reconstruction is suggested by res- designing exploration programs, which may also involve geo- toration of intrusive contacts between granite and metamor- physical and geochemical techniques. phic rocks in the Wickenburg Mountains. Restoration of those contacts requires slip on the Wickenburg Mountains fault that Arc-Scale Reconstruction of Tertiary Extension in elevates part of the lower volcanic and sedimentary rock unit the Laramide Porphyry Copper Belt above the thickest exposed section. The detailed examination and reconstruction of the porphyry systems near Wickenburg fills a gap between the Globe- Exploration targets Miami district and Bagdad in a previous compilation of por- Leading up to, and subsequent to, the discovery of the Kala- phyry deposits in the Laramide magmatic arc (Titley, 1982b). mazoo orebody (Lowell, 1968), knowledge and understanding These porphyry systems are now placed in their preextension 460 NICKERSON AND SEEDORFF context at the scale of the entire Laramide porphyry copper by the bedding attitudes provides a basis for grouping regions belt in order to better understand the original spatial relation- where the magnitude of extension was similar. These data ships between the porphyry systems. To this end, a regional- are then adapted to estimate a β factor utilizing the following scale reconstruction of Cenozoic extension is presented for equation from Jackson and McKenzie (1983): the relevant portions of Arizona, New Mexico, and Sonora sin θ (Figs. 11–12), revisiting a topic addressed earlier by Richard β = ——– (1) sin θ' (1994) and Staude and Barton (2001) but using a different approach. where θ is the dip of a normal fault at its inception and θ' is the dip of the normal fault after fault motion ceases. Sev- Methodology eral assumptions are made in the calculations: (1) the dips An original compilation of strikes and dips of the oldest pre- of syn- and postextension Tertiary rocks record only the and synextension Oligocene and Miocene sedimentary and effects of Cenozoic extension; (2) normal faults were tilted volcanic strata across the porphyry belt serves as the data for to lower angles by the same amount that Tertiary beds were the reconstruction (Fig. 11). The tilting information recorded tilted to steeper dips; (3) single sets of faults accommodated

Domains of Cenozoic Extension

Fig. 11. Map showing a compilation of strike and dip data of Tertiary sedimentary and volcanic rocks in the vicinity of the Laramide magmatic arc, contoured domains of Cenozoic extension with a similar β factor, and the present and restored locations of porphyry deposits. Strike and dip data are compiled from Anderson (1977, 1978), Arizona Bureau of Mines (1959), Banks et al. (1977), Blacet and Miller (1978), Blacet et al. (1978), Brooks (1985), Capps et al. (1986), Carr (1991), Cooper (1959, 1960), Cox et al. (2006), Dickinson (1987), Dockter and Keith (1978), Gray et al. (1985), Grubensky (1989), Gruben- sky and Demsey (1991); Grubensky et al. (1995), John (1987b), Keith and Theodore (1975), Maher (2008), Reynolds and Skotnicki (1993), Richter et al. (1982), Rytuba et al. (1978), Sherrod and Tosdal (1991), Spencer (1989), Stavast et al. (2008), Stewart and Roldán-Quintana (1994), Stimac et al. (1994), Tosdal et al. (1986), Wilson (1960), Wilson and Moore (1959), Wilson et al. (1959, 1960), and Wolfe (1983). DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 461 s C k

rc e d -a p ear erso n

R To olcanic Center r Americ a Simcoe Mountain 120º W Glacier Pea Hackamor Goat Rock s Newberry Broken Medicine Lake Mount Rainier Mount Hoo Mount Je ff Middle Sister Dittmar V (Crater Lake) Mount Mazam a Mount Cayley s Mount Meage earc or Adams is F ax c Mount Baker Ar Mount South Siste r olcanic Center Snow Mountain olcanic Center Rainbow Mountain Northwestern North Cappy Mountain 125º W Mount Garibaldi Mount Shast a Quaternary Cascade (2-0 Ma),

Mount Saint Helen N Maidu V 49º N 42º N 100 km Lassen V e r B 68º W ? e o a Arcs o Paramillos Nort ca s Paramillos Su Va Mo-(Cu) Mo-(Cu) -arc

ll Rear Santa Clar st Wa Cerro Blanc lcanic Fields Azule s ) nque Cerro Mercedari Pimento n We xis Rio de Las

Alta r a Yu Los c Los Bargres Sur r olcanic Centers A El e s s - niente Major V Cu-(Mo) Rear-Arc Vo Cu-(Au-Mo Miocene-Early Pliocene (16-4 Ma), Central Chil zcachitas Te Vi El

El Pachon N Amos-Andre Piuquenes 100 km Rio Blanco Los Bronce Forearc Comparison of Magmatic Los Pelambres Rosario de Rengo 34º S 30º S Cerro Bayo de Cobr

72º W

c c c c c r r r r r a a a a a - - - - - r r r r r r r a a a a a a e e e e e R R R R A R o k Chin ford Distric t o yrone Morenci T Saf District d Hillsbor Mineral Park Globe-Miami America

Copper Cree Copper Basin (Prescott)

Copper Basin (Crown King)

San Manuel-Kalamazo

Bagda Ray

s s s s s s i i i i i Resolution i x x x x x a a a a a

c c c c c c r r r A A A A A A

Red Mountain

c c c c c r r r r r a a a a a e e e e e e e r r r r r r o o o o o F F F F F F Sacaton Casa Grande N ko l Ve La Caridad 100 km Ajo Silver Bell Fig. 12. Comparison of porphyry systems the reconstructed Laramide copper belt to features in other magmatic arcs. A. Reconstructed location 2008) and is and Ranney, at 50 Ma (Blakey North America of southwestern is representative background in the arc. Topography magmatic of the Laramide systems The topography and the location of selected porphyry deposits were restored using different approaches. B. Porphyry copper systems intended for general reference only. of the Miocene-early Pliocene magmatic arc central Chile (after Sillitoe and Perelló, 2005). C. Quaternary Cascade northwestern North America Hildreth, 2007), showing major volcanic centers. Note the change in scale from panels A and B to panel C. Lakeshore Cananea Pima District Rosemont Sheep Mountai Peach Elgin Southwestern North Opodepe Restored Laramide (75-55 Ma), Cumobabi 462 NICKERSON AND SEEDORFF a maximum of 30° of tilting and initiated with 60° dips; (4) (Titley, 1982b; Leveille and Stegen, 2012). Hence, when dis- all extension was NE-SW directed; (5) tilting was unidirec- cussing the geometry of the porphyry copper belt, it is appro- tional; and (6) any post-Laramide strike-slip faulting did not priate to make comparisons to continental arcs. Nonetheless, significantly alter locations of porphyry deposits. It is unlikely the ages of Laramide porphyry deposits span ~75 to 55 Ma that these conditions are met across the entire region consid- (Titley, 1982b; Seedorff et al., 2005b) so their distribution is ered here; however, the generalization of Cenozoic extension a time-integrated pattern rather than a snapshot of an active in this manner allows for a palinspastic reconstruction of the arc. porphyry systems of the Laramide porphyry copper belt that The reconstructed distribution of porphyry deposits of the is more representative of its original form than the current, Laramide arc (Fig. 12A) yields a variably well-defined arc axis, postextension distribution. with gaps and clusters of deposits along the 700 km of strike Jackson and McKenzie (1983) demonstrated geometrically length parallel to the Laramide plate margin. The apparent that a normal fault that is tilted from 60° to 30° produces a β time-integrated axis of the porphyry copper belt (dashed, Fig. factor of 1.73, equivalent to 73% extension. In the strike and 12A) extends from Mineral Park to Red Mountain. Nearly two dip compilation used here, dips are assigned to three groups dozen deposits lie along the axis of the arc. Fore-arc depos- of dips, 0° to 30°, 30° to 60°, and 60° to 90°. An average its include those in the Ajo, Cananea, La Caridad, Opodepe, amount of extension was assigned for each domain (Fig. 11), and Cumobabi districts, and rear-arc deposits include those in and a regional contour map of the magnitude of extension was the Safford, Morenci, Tyrone, Chino, and Hillsboro districts. created. Locations of porphyry deposits were restored to the Along-axis spacing between the known deposits is variable. northeast by removing the cumulative amount of extension Deposits in the Globe-Miami, Safford, and Pima districts are between the modern location of a deposit and the unextended separated by only a few kilometers, whereas Sheep Moun- terrane outside the Basin and Range (either the Colorado Pla- tain and Ajo are separated by an apparent gap of more than teau or the southern Rocky Mountains). 100 km along the axis of the magmatic arc. In addition to the uncertainties introduced from the Across-axis spacing of known deposits is also highly vari- assumptions made in its construction, further uncertainties able. Resolution lies 20 km from the deposits of the Globe- about the distribution of porphyry deposits arise from their Miami district, but Opodepe is separated from Hillsboro by degree of preservation and the limited modern exposure of 325 km. Casa Grande and Vekol, as well as deposits within the the porphyry copper belt. Denudation prior to or during Globe-Miami and Pima districts, are separated by less than Tertiary extension in Arizona could have completely eroded 10 km across axis. In the Globe-Miami and Pima districts, the porphyry systems in the arc (Barton, 1996). Furthermore, clustering of deposits reflects, in part, the dismemberment Quaternary and Tertiary sedimentary and volcanic rocks cover of porphyry systems by Tertiary normal faults (Stavast, 2006; greater than 70% of the surface in the Basin and Range prov- Maher, 2008; Stavast et al., 2008). That is, the named deposits ince of Arizona (Reynolds, 1988). These younger rocks likely identified today were once parts of larger porphyry systems, conceal additional Laramide porphyry systems. as is the case in the classic example of San Manuel-Kalamazoo As previously mentioned, this study follows earlier arc-scale (Lowell, 1968). reconstructions of porphyry systems in the Laramide magmatic arc by Richard (1994), and Staude and Barton (2001). Discussion Staude and Barton (2001) considered a larger portion of southwestern North America in their reconstruction and restored Classification of porphyry systems near Wickenburg more generalized domains of extension based on the loca- Because an orebody has not been located in the study area, tions and dominant extension directions of metamorphic core it is speculative to classify the two identified porphyry sys- complexes. Similar to this study, Richard (1994) delineated tems based on an economically dominant metal. Nonetheless, extensional domains using a compilation of tilting information deposit classes are at least partially constrained by composi- recorded in Tertiary rocks and restored extension using the tions of igneous source rocks and may display some distinc- equations of Jackson and McKenzie (1983). The present study tive styles of hydrothermal alteration (Seedorff et al., 2005a). uses an independent, albeit partially overlapping set of dip ori- For example, porphyry Au deposits are normally associated entations used by Richard (1994), including data generated with dioritic host rocks, whereas porphyry W and Sn depos- since publication of the earlier study. Moreover, the earlier its are associated with rhyolitic and rhyodacitic source rocks, work by Richard (1994) averaged the dips of Tertiary rocks respectively. to determine tilting within domains, as opposed to estimat- The granitic composition of the inferred Late Cretaceous ing tilting using the dips of only the oldest pre- or synexten- source rocks for the porphyry systems in the study area (Fig. sion Tertiary rocks. By averaging the dips of all Tertiary rocks, 11) may be indicative of porphyry molybdenum deposits of calculations of extension are influenced by rocks which only the quartz monzonitic-granitic porphyry Mo-Cu or granitic record a portion of the extension with a domain. Thus, the porphyry Mo subclasses (Seedorff et al., 2005a). Nonethe- earlier calculations by Richard (1994) significantly underesti- less, porphyry Cu-(Mo) deposits in the Globe-Miami dis- mated the amount of extension across the porphyry belt. trict (Fig. 1) are sourced from the Schultze Granite, which is compositionally similar (Stavast, 2006; Maher, 2008) to the Examination of the Arc-Scale Reconstruction of the Late Cretaceous granite in the study area. The conspicuous Laramide Porphyry Belt absence of large numbers of porphyry dikes in the study area The Laramide porphyry copper belt is a manifestation of is also commonly observed in porphyry Mo-Cu systems (e.g., the Laramide magmatic arc of southwestern North America Hall (Nevada Moly), Shaver, 1991; Buckingham, Loucks and DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 463

Johnson, 1992). The transitional greisen-potassic style of alter- clustered magmatic activity, as well as apparent gaps in mag- ation documented on Sheep Mountain is perhaps suggestive matic activity greater than 100 km. One clear difference is the of a porphyry Mo-Cu system as well, because the only other breadth of the Laramide porphyry copper belt south of Red well-documented instance of this distinctive style of high- Mountain, where it reaches a maximum width of 325 km. The level alteration is the Hall (Nevada Moly) porphyry Mo-Cu maximum across-axis width between magmatic features in the system (Shaver, 1991). The high Mo contents observed in the other arcs is less than 150 km. nearby porphyry resources at Sheep Mountain East (40 Mt @ Clearly, there is no blueprint for magmatic features in con- 1.6% Cu and 0.035% Mo; Ullmer, 2007) and Copper Basin vergent oceanic-continental plate margin arc. Differences in (Crown King) (1 Gt @ 0.16% Cu and 0.031% Mo; ASARCO, the interaction between downgoing slabs, mantle wedges, and unpub. report, 1974) are also suggestive of possible porphyry continental crust will make each arc distinctive. However, Mo-Cu affinities. when compared at the entire arc scale, differences in geom- Together, this evidence suggests that the porphyry sys- etry between the reconstructed Laramide magmatic arc (as tems identified here are perhaps part of a cluster of porphyry revealed by the porphyry copper belt) and other arcs appear Mo-Cu systems located in the middle of what was previously to be minimal. thought to be a large gap in the Laramide porphyry belt (Titley, 1982b). These systems join other quartz monzonitic- Conclusions granitic porphyry Mo-Cu deposits, e.g., El Crestón/Opodepe The effect of Tertiary extension on the geometry of Laramide (León and Miller, 1981) and Cumobabi (Scherkenbach et al., porphyry systems has been demonstrated at the district and 1985) and Mo-rich porphyry copper deposits of the quartz arc scale. Original reconnaissance mapping is combined with monzodioritic-granitic Cu-(Mo) deposits, e.g., Sierrita (Aiken previous detailed mapping as the basis for a 20-km-long pal- and Baugh, 2007), in the arc, as well as deposits that have inspastic reconstruction. The reconstruction reveals that characteristics transitional between those two subclasses, e.g., two porphyry-style hydrothermal systems emanate from a Mineral Park (Wilkinson et al., 1982). Laramide pluton exposed in the study area. Hydrothermal alteration is zoned from greisen at deeper levels, to potassic Comparison of the scale and geometry of the and transitional greisen-potassic at higher levels. Five super- Laramide magmatic arc to other arcs imposed sets of normal faults, which initially developed at Whereas exploration in the Laramide porphyry copper belt high angles and then rotated to lower angles during exten- has evolved to consider deposit-scale (e.g., Lowell, 1968; sion, dismembered the porphyry systems. Potentially well- Wilkins and Heidrick, 1995), district-scale (e.g., Wodzicki, mineralized fault blocks are buried underneath Tertiary and 1995; Stavast et al., 2008), and sometimes regional-scale Quaternary volcanic and sedimentary rocks. Two exploration extension (e.g., Maher, 2008), little attention has been given targets, Buckhorn Creek and Highway 60, are worthy of fur- to the effects of extension at the scale of the Laramide por- ther investigation and perhaps drill testing. phyry belt (Richard, 1994; Staude and Barton, 2001), and a Extension at the scale of the Laramide porphyry belt is comparison of the preextension geometry of the Laramide quantified using a compilation of strikes and dips documented porphyry copper belt to magmatic features in other well-stud- in pre- and synextension rocks across the porphyry belt. The ied arcs is lacking. tilting recorded by the attitudes provides a basis to group The length, breath, and spacing between porphyry cen- regions where the magnitude of extension is similar and then ters in the reconstructed Laramide porphyry copper belt is is adapted to estimate a β factor across the southern Basin compared to other magmatic features in the prolific central and Range province. Tertiary extension is restored quantita- Chilean porphyry copper sub-belt in the Miocene-early Plio- tively to reveal the preextension geometry of the porphyry cene magmatic arc (Sillitoe and Perelló, 2005) and the Qua- belt, where the majority of porphyry deposits clearly define ternary Cascade volcanic arc of northwestern North America a 100-km-wide axis and others lie in fore- or rear-arc settings. (Hildreth, 2007) in Figure 12A-C. The magmatic features in The arc geometry, once extension is restored, closely resem- each of the arcs are distinctive. For example, as previously bles other magmatic arcs formed at convergent oceanic-conti- mentioned, the Laramide porphyry belt spans about 20 m.y. nental plate boundaries. of activity (~75–55 Ma, Lang and Titley, 1998; Seedorff et al., 2005b) and was not necessarily stationary during that inter- Acknowledgments val. The central Chilean sub-belt—somewhat arbitrarily con- We would like to thank Bronco Creek Exploration for logis- strained in length—was active for about 12 m.y. (16–4 Ma, tical and financial support of this project, including helicop- Sillitoe and Perelló, 2005). The Quaternary portion of the ter support at Sheep Mountain. Additional financial support Cascade arc includes only its last 2 m.y. of activity (Hildreth, came from Science Foundation Arizona and an award from 2007), and this portion is not yet sufficiently eroded to reveal the Society of Economic Geologists student research fund. much about its potential for porphyry mineralization (e.g., Dave Maher and David Johnson introduced us to the study John et al., 2005). area, and discussions in the field with Dave Maher, Doug The length of the Laramide porphyry copper belt, at Kreiner, Mike McCarrel, and Isaac Nelson helped mold our 700 km, is greater than the 400-km-long Miocene-early Plio- thinking. Early reviews of this manuscript by Mark Barton, cene porphyry copper sub-belt of central Chile, but shorter George Davis, Charles Ferguson, and Peter Reiners, and than the 1,250-km-long chain of volcanic fields in the Qua- later reviews by John Proffett, Jeremy Richards, John-Mark ternary Cascade arc. Spacing between deposits (and volcanic Staude, Carson Richardson, and Joe Colgan greatly improved fields) along strike is similar, with all three arcs having areas of its quality. 464 NICKERSON AND SEEDORFF

REFERENCES Dickinson, W.R., 1987, General geologic map of Catalina core complex and Aiken, D.M., and Baugh, G.A., 2007, The Sierrita copper-molybdenum San Pedro trough: Arizona Bureau of Geology and Mineral Technology deposit: An updated report, Pima mining district, Pima County, AZ: Ari- Miscellaneous Map Series 87-A, scale 1:62,500. zona Geological Society, Symposium, Tucson, September 2007, Field Trip ——1991, Tectonic setting of faulted Tertiary strata associated with the Cata- 8 Guidebook, 24 p. lina core complex in southern Arizona: Geological Society of America Spe- Anderson, R.E., 1977, Geologic map of the Boulder City 15–minute quad- cial Paper 264, 106 p., map scale 1:125,000. rangle, Clark County, Nevada: U.S. Geological Survey Geological Quad- Dilles, J.H., and Einaudi, M.T., 1992, Wall-rock alteration and hydrothermal rangle Map GQ-1395, scale 1:62,500. flow paths about the Ann-Mason porphyry copper deposit, Nevada: a 6-km ——1978, Geologic map of the Black Canyon 15–minute quadrangle, vertical reconstruction: Economic Geology, v. 87, p. 1963–2001. Mohave County, Arizona, and Clark County, Nevada: U.S. Geological Sur- Dockter, R.D., and Keith, W.J., 1978, Reconnaissance geologic map of the vey Geological Quadrangle Map GQ-1394, scale 1:62,500. Vekol Mountains quadrangle, Arizona: U.S. Geological Survey Miscella- Arizona Bureau of Mines, 1959, Geologic map of Cochise County, Arizona: neous Field Studies Map MF-931, scale 1:62,500. Arizona Bureau of Mines, 1 sheet, scale 1:375,000. Flowers, R.M., Wernicke, B.P., and Farley, K.A., 2008, Unroofing, inci- sion, and uplift history of the southwestern Colorado Plateau from apatite Ball, T.T., and Closs, L.G., 1983, Geology and fluid inclusion investigation (U-Th)/He thermochronometry: Geological Society of America Bulletin, of the Crown King breccia pipe complex, Yavapai County, Arizona [abs.]: v. 120, p. 571–587. Geological Society of America Abstracts with Programs, v. 15, p. 276–277. Gray, F., Miller, R.J., Peterson, D.W., May, D.J., Tosdal, R.M., and Kahle, K., Banks, N.G., Dockter, R.D., Briskey, J.A., Davis, G.H., Keith, S.B., Budden, 1985, Geologic map of the Growler Mountains, Pima and Maricopa Coun- R.T., Kiven, C.W., and Anderson, P., 1977, Reconnaissance geologic map of ties, Arizona: U.S. Geological Survey Miscellaneous Field Studies Map the Tortolita Mountains quadrangle, Arizona: U.S. Geological Survey Mis- MF-1681, scale 1:62,500. cellaneous Field Studies Map MF-864, scale 1:62,500. Grubensky, M.J., 1989, Geologic map of the Vulture Mountains, west-central Barton, M.D., 1996, Granitic magmatism and metallogeny of southwestern Arizona: Arizona Geological Survey Map M-27, 3 sheets, scale 1:24,000. North America: Transactions of the Royal Society of Edinburgh: Earth Grubensky, M.J., and Demsey, K.A., 1991, Geologic map of the Little Horn Sciences, v. 87, and Geological Society of America Special Paper 315, Mountains 15' quadrangle, southwestern Arizona: Arizona Geological Sur- p. 261–280. vey Map M-29, 1 sheet, scale 1:50,000, 10 p. Blacet, P.M., and Miller, S.T., 1978, Reconnaissance geologic map of the Grubensky, M.J., Haxel, G.B., and Demsey, K.A., 1995, Geologic map of the Jackson Mountain quadrangle, Graham County, Arizona: U.S. Geological southeastern Kofa Mountains and western Tank Mountains, southwestern Survey Miscellaneous Field Studies Map MF-939, scale 1:62,500. Arizona: U.S. Geological Survey Miscellaneous Investigations Series Map Blacet, P.M., Bergquist, J.R., and Miller, S.T., 1978, Reconnaissance geologic I-2454, scale 1:62,500. map of the Silver Reef Mountains quadrangle, Pinal and Pima Counties, Hastings, M., Olin, E., and Mach, L., 2014, Technical report on resources, Arizona: U.S. Geological Survey Miscellaneous Field Studies Map MF-934, Newsboy Gold project, Maricopa County, Arizona—NI 43-101 Technical scale 1:62,500. Report: Bullfrog Gold Corporation Report, 185 p. Blakey, R.C., and Ranney, W., 2008, Ancient landscapes of the Colorado Pla- Hildreth, W., 2007, Quaternary magmatism in the Cascades—geologic per- teau: Grand Canyon, Arizona, Grand Canyon Association, 176 p. spectives: U.S. Geological Survey Professional Paper 1744, 125 p. Brooks, W.E., 1985, Reconnaissance geologic map of part of McLendon vol- Jackson, J.A., and McKenzie, D.P., 1983, The geometrical evolution of nor- cano, Yavapai County, Arizona: U.S. Geological Survey Miscellaneous Field mal fault systems: Journal of Structural Geology, v. 5, p. 471–482. Studies Map MF-1783, scale 1:24,000. Jahns, R.H., 1952, Pegmatite deposits of the White Picacho district, Mari- Buck, W.R., 1988, Flexural rotation of normal faults: Tectonics, v. 7, copa and Yavapai Counties, Arizona: Arizona Bureau of Mines Bulletin 162, p. 959–973. 105 p. Byerlee, J., 1978, Friction of rocks: Pure and Applied Geophysics, v. 116, John, B.E., 1987a, Geometry and evolution of a mid-crustal extensional fault p. 615–626. system: Chemehuevi Mountains, southeastern California: Geological Soci- Capps, R.C., Reynolds, S.J., Kortemeier, C.P., and Scott, E.A., 1986, Geologic ety, London, Special Publication 28, p. 313–335. map of the northeastern Hieroglyphic Mountains, central Arizona: Arizona ——1987b, Geologic map of the Chemehuevi Mountains area San Ber- Bureau of Geology and Mineral Technology Open-File Report 86-10, 16 p., nardino County, California and Mohave County, Arizona: U.S. Geological scale 1:24,000, text 16 p. Survey Open-File Report 87-666, scale 1:24,000. Carr, W.J., 1991, A contribution to the structural history of the Vidal- Parker John, D.A., Rytuba, J.J., Breit, G.N., Clynne, M.A., and Muffler, L.J.P., 2005, region, California and Arizona: U.S. Geological Survey Professional Paper Hydrothermal alteration in Maidu Volcano: A shallow fossil acid-sulfate 1430, Plate 4, scale 1:125,000. magmatic-hydrothermal system in the Lassen Peak area, California: Geo- Carten, R.B., 1986, Sodium-calcium metasomatism: Chemical, temporal, and logical Society of Nevada Symposium 2005, Window to the World, Reno, spatial relationships at the Yerington, Nevada, porphyry copper deposit: Nevada, May 2005, Proceedings, p. 295–313. Economic Geology, v. 81, p. 1495–1519. Johnston, W.P., and Lowell, J.D., 1961, Geology and origin of mineral- Cooke, D.R., Hollings, P., and Walshe, J.L., 2005, Giant porphyry deposits: ized breccia pipes in Copper Basin, Arizona: Economic Geology, v. 56, Characteristics, distribution, and tectonic controls: Economic Geology, p. 916–940. v. 100, p. 801–818. Keith, S.B., Gest, D.E., DeWitt, E., Toll, N.W., and Everson, B.A., 1983, Cooper, J.R., 1959, Reconnaissance geologic map of southeastern Cochise Metallic mineral districts and production in Arizona: Arizona Bureau of County, Arizona: U.S. Geological Survey Miscellaneous Field Studies Map Geology and Mineral Technology, Geological Survey Branch, Bulletin 194, MF-213, scale 1:62,500. 58 p. ——1960, Reconnaissance map of the Willcox, Fisher Hills, Cochise, and Dos Keith, W.J., and Theodore, T.G., 1975, Reconnaissance geologic map of the Cabezas quadrangles, Cochise and Graham Counties, Arizona: U.S. Geo- Arivaca quadrangle, Arizona: U.S. Geological Survey Miscellaneous Field logical Survey Miscellaneous Field Studies Map MF-231, scale 1:62,500. Studies Map MF-678, scale 1:63,360. Corn, R.M., and Ahern, R., 1994, Structural rotation and structural cover at Lang, J.R., and Titley, S.R., 1998, Isotopic and geochemical characteristics of the Kelvin porphyry copper prospect, Pinal County, Arizona: Arizona Geo- Laramide magmatic systems in Arizona and implications for the genesis of logical Society Spring 1994 Field Trip Guidebook, 16 April 1994, p. 31–42. porphyry copper deposits: Economic Geology, v. 93, p. 138–170. Cox, D.P., Force, E.R., Wilkinson, W.H., Jr., More, S.W., Rivera, J.S., and León, F.L., and Miller, C.P., 1981, Geology of the Crestón molybdenum- Wooden, J.L., 2006, The Ajo mining district, Pima County, Arizona—evi- copper deposit, in Ortlieb, L., and Roldán Q., J., eds., Geology of north- dence for middle Cenozoic detachment faulting, plutonism, volcanism, and western Mexico and southern Arizona: Mexico City, Instituto de Geología, hydrothermal alteration: U.S. Geological Survey Professional Paper 1733, U.N.A.M., p. 223–238. plate 1, scale 1:24,000, 46 p. Leveille, R.A., and Stegen, R.J., 2012, The southwestern North America por- DeWitt, E., Langenheim, V.E., Force, E.R., Vance, R.K., Lindberg, P.A., phyry copper province: Society of Economic Geologists Special Publication and Driscoll, R.L., 2008, Geologic map of Prescott National Forest and the 16, p. 361–401. headwaters of the Verde River, Yavapai and Coconino Counties, Arizona: London, D., and Burt, D.M., 1978, Lithium pegmatites of the White Picacho U.S. Geological Survey Scientific Investigations Map 2996, scale 1:100,000, district, Maricopa and Yavapai Counties, Arizona: Arizona Bureau of Geol- 100 p. ogy and Mineral Technology Special Paper 2, p. 61–73. DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ 465

Loucks, T.A., and Johnson, C.A., 1992, Economic geology: U. S. Geological Seedorff, E., Dilles, J.H., Proffett, J.M., Jr., Einaudi, M.T., Zurcher, L., Survey Professional Paper 798-D, p. 101–138. Stavast, W.J.A., Johnson, D.A., and Barton, M.D., 2005a, Porphyry depos- Lowell, J.D., 1968, Geology of the Kalamazoo orebody, San Manuel district, its: Characteristics and origin of hypogene features: Economic Geology Arizona: Economic Geology, v. 63, p. 645–654. 100th Anniversary Volume, p. 251–298. Maher, D.J., 2008, Reconstruction of Middle Tertiary extension and Laramide Seedorff, E., Barton, M.D., Gehrels, G.E., Johnson, D.A., Maher, D.J., porphyry copper systems, east-central Arizona: Ph. D. thesis, Tucson, Uni- Stavast, W.J.A., and Flesch, E., 2005b, Implications of new U-Pb dates versity of Arizona, 328 p. from porphyry copper-related plutons in the Superior-Globe-Ray-Christ- Manske, S.L., and Paul, A.H., 2002, Geology of a major new porphyry cop- mas area, Arizona [abs.]: Geological Society of America Abstracts with Pro- per center in the Superior (Pioneer) district, Arizona: Economic Geology, grams, v. 37, no. 7, p. 164. v. 97, p. 197–220. Seedorff, E., Barton, M.D., Stavast, W.J.A., and Maher, D.J., 2008, Root McQuarrie, N., and Wernicke, B.P., 2005, An animated tectonic recon- zones of porphyry systems: Extending the porphyry model to depth: Eco- struction of southwestern North America since 36 Ma: Geosphere, v. 1, nomic Geology, v. 103, p. 939–956. p. 147–172. Shaver, S.A., 1991, Geology, alteration, mineralization and trace element Nickerson, P.A., Barton, M.D., and Seedorff, E., 2010, Characterization and geochemistry of the Hall (Nevada Moly) deposit, Nye County, Nevada, in reconstruction of multiple copper-bearing hydrothermal systems in the Tea Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geol- Cup porphyry system, Pinal County, Arizona: Society of Economic Geolo- ogy and ore deposits of the Great Basin. Symposium Proceedings: Reno, gists Special Publication 15, p. 299–316. Nevada, Geological Society of Nevada, p. 303–332. Parsons, A.B., 1933, The porphyry coppers: New York, American Institute of Shaver, S.A., and McWilliams, M., 1987, Cenozoic extension and tilting Mining and Metallurgical Engineers, 588 p. recorded in Upper Cretaceous and Tertiary rocks at the Hall molybdenum Paul, A.H., and Manske, S.L., 2005, History of exploration at the Magma mine, deposit, northern San Antonio Mountains, Nevada: Geological Society of Superior, Arizona: Geological Society of Nevada Symposium 2005, Window America Bulletin, v. 99, p. 341–353. to the World, Reno, Nevada, May 2005, Proceedings, v. 1, p. 629–638. Sherrod, D.R., and Tosdal, R.M., 1991, Geologic setting and Tertiary struc- Peterson, R.R., 1985, The geology of the Buckhorn Creek area, Yavapai tural evolution of southwestern Arizona and southeastern California: Jour- County, Arizona: M.S. thesis, Flagstaff, Northern Arizona University, 109 p. nal of Geophysical Research, v. 96, p. 12,407–12,423. Pierce, F.W., and Bolm, J.G., eds., 1995, Porphyry copper deposits of the Sibson, R.H., 1994, An assessment of field evidence for “Byerlee” friction: American Cordillera: Arizona Geological Society Digest 20, 656 p. Pure and Applied Geophysics, v. 142, p. 645–662. Proffett, J.M., Jr., 1977, Cenozoic geology of the Yerington district, Nevada, Sillitoe, R.H., and Perelló, J.A., 2005, Andean copper province: Tectonomag- and implications for the nature and origin of Basin and Range faulting: Geo- matic settings, deposit types, metallogeny, exploration, and discovery: Eco- logical Society of America Bulletin, v. 88, p. 247–266. nomic Geology 100th Anniversary Volume, p. 845–890. ——1979, Ore deposits in the western U.S.—a summary: Nevada Bureau of Sillitoe, R.H., Creaser, R.A., Kern, R.R., and Lenters, M.H., 2014, Squam Mines and Geology Report 33, p. 13–32. Peak, Arizona: Paleoproterozoic precursor to the Laramide porphyry cop- Ramsay, J.G., and Huber, M.I., 1987, The techniques of modern structural per province: Economic Geology, v. 109, p. 1171–1177. geology, v. 2: Folds and Fractures: London, Academic Press, 381 p. Spencer, J.E., 1989, Compilation geologic map of the Buckskin and Rawhide Rehrig, W.A., Shafiqullah, M., and Damon, P.E., 1980, Geochronology, geol- Mountains, west-central Arizona: Arizona Geological Survey Bulletin 198, ogy, and listric normal faulting of the Vulture Mountains, Maricopa County, Plate 3, scale 1:100,000. Arizona: Arizona Geological Society Digest 12, p. 89–110. Spencer, J.E., Reynolds, S.J., Grubensky, M.J., Duncan, J.T., and White, Reynolds, S.J., 1988, Geologic map of Arizona: Arizona Geological Survey D.C., 2004, Economic geology of the Vulture mine: Arizona Geological Map 26, scale 1:1,000,000. Survey Digital Geologic Map 41 (DGM-41), scale 1:24,000. Reynolds, S.J., and Skotnicki, S.J., 1993, Geologic map of the Phoenix South Staude, J.-M.G., and Barton, M.D., 2001 Jurassic to Holocene tectonics, 30' × 60' quadrangle, central Arizona: Arizona Geological Survey Open-File Report 93-18, 1 sheet, scale 1:100,000. magmatism, and metallogeny of northwestern Mexico: Geological Society Reynolds, S.J., and Spencer, J.E., 1985, Evidence for large-scale transport of America Bulletin, v. 113, p. 1357–1374. on the Bullard detachment fault, west-central Arizona: Geology, v. 13, Stavast, W.J.A., 2006, Three-dimensional evolution of magmatic hydrother- p. 353–356. mal systems, Schultze Granite and Ruby Star Granodiorite, Arizona: Ph. D. Richard, S.M., 1994, Preliminary reconstruction of Miocene extension in the thesis, Tucson, University of Arizona, 414 p. Basin and Range of Arizona and adjacent areas: Arizona Geological Survey Stavast, W.J.A., Butler, R.F., Seedorff, E., Barton, M.D., and Ferguson, C.A., Open-File Report 94-5, 9 p. 2008, Tertiary tilting and dismemberment of the Laramide arc and related Richter, D.H., Klein, D.P., and Lawrence, V.A., 1982, Mineral resource hydrothermal systems, Sierrita Mountains, Arizona: Economic Geology, potential map of the Gila-San Francisco Wilderness study area, Graham v. 103, p. 629–636. and Greenlee Counties, Arizona: U.S. Geological Survey Miscellaneous Stewart, J.H., and Roldán-Quintana, J., 1994, Map showing late Cenozoic Field Studies Map MF-1315-B, scale 1:62,500. extensional tilt patterns and associated structures in Sonora and adjacent Rytuba, J.J., Till, A.B., Blair, W., and Haxel, G.B., 1978, Reconnaissance geo- areas, Mexico: U.S. Geological Survey Miscellaneous Field Studies Map logic map of the Quijotoa Mountains quadrangle, Pima County, Arizona: MF-2238, scale 1:1,000,000. U. S. Geological Survey Miscellaneous Field Studies Map MF-937, scale Stimac, J.A., Fryxell, J.E., Reynolds, S.J., Richard, S.M., Grubensky, M.J., 1:62,500. and Scott, E.A., 1987, Geologic map of the Wickenburg, southern Buck- Scherkenbach, D.A., Sawkins, F.J., and Seyfried, W.E., Jr., 1985, Geologic, horn, and northwestern Hieroglyphic Mountains, central Arizona: Arizona fluid inclusion, and geochemical studies of the mineralized breccias at Bureau of Geology and Mineral Technology Open-File Report 87-9, 13 p., Cumobabi, Sonora, Mexico: Economic Geology, v. 80, p. 1566–1592. scale 1:24,000. Seedorff, E., 1991, Magmatism, extension, and ore deposits of Eocene to Stimac, J.A., Richard, S.M., Reynolds, S.J., Capps, R.C., Kortemeier, C.P., Holocene age in the Great Basin—mutual effects and preliminary pro- Grubensky, M.J., Allen, G.B., Gray, F., and Miller, R.J., 1994, Geologic posed genetic relationships: Geology and Ore Deposits of the Great Basin: map and cross sections of the Big Horn and Belmont Mountains, west- Geological Society of Nevada, Symposium, Reno/Sparks, April 1990, Pro- central Arizona: Arizona Geological Survey Open-File Report 94-15, 16 p., ceedings, v. 1, p. 133–178. 3 sheets, scale 1:50,000. Seedorff, E., and Richardson, C.A., 2014, Diverse geometries and temporal Titley, S.R., ed., 1982a, Advances in geology of the porphyry copper deposits, relationships of normal faults and fault systems in continental extension: A southwestern North America: Tucson, University of Arizona Press, 560 p. perspective from the Basin and Range province and mineral deposits [ext. ——1982b, Geologic setting of porphyry copper deposits, in Titley, S.R., ed., abs]: Geological Society of London Conference, London, 23–25 June 2014, Advances in geology of the porphyry copper deposits, southwestern North Programme and Abstract Volume, p. 20–22. America: Tucson, University of Arizona Press, p. 37–58. Seedorff, E., Hasler, R.W., Breitrick, R.A., Fahey, P.L., Jeanne, R.A., Shaver, Titley, S.R., and Hicks, C.L., eds., 1966, Geology of the porphyry copper S.A., Stubbe, P., Troutman, T.W., and Manske, S.L., 1996, Overview of the deposits, southwestern North America: Tucson, University of Arizona geology and ore deposits of the Robinson district, with emphasis on its Press, 287 p. post-mineral structure: Geological Society of Nevada Field Trip Guidebook Tognoni, H. C., 1969, Copper Bain, Fort Misery, Yavapai County, Arizona: Compendium, 1995, Reno/Sparks, p. 87–91. Unpublished Mineral Economics Corporation report, 7 p. 466 NICKERSON AND SEEDORFF

Tosdal, R.M., Peterson, D.W., May, D.J., LeVegue, R.A., and Miller, R.J., Wilson, E.D., and Moore, R.T., 1959, Geologic map of Mohave County, Ari- 1986, Reconnaissance geologic map of the Mount Ajo and part of the zona: Arizona Bureau of Mines, 1 sheet, scale 1:375,000. Pisinimo quadrangles, Pima County, Arizona: U.S. Geological Survey Mis- Wilson, E.D., Moore, R.T., and Peirce, H.W., 1959, Geologic map of Gila cellaneous Field Studies Map MF-1820, scale 1:62,500. County, Arizona: Arizona Bureau of Mines, 1 sheet, scale 1:375,000. Ullmer, E., 2007, Sheep Mountain property, Yavapai County, Arizona—NI Wilson, E.D., Moore, R.T., and O’Haire, R.T., 1960, Geologic map of Pima 43-101 Technical Report: Unpublished Lebon Gold Mines Limited report, and Santa Cruz Counties, Arizona: Arizona Bureau of Mines, 1 sheet, scale 19 p. 1:375,000. White, D.C., 1989, Geology of the Vulture mine: Mining Engineering, v. 41, Wodzicki, W.A., 1995, The evolution of Laramide igneous rocks and porphyry p. 1119–1222. copper mineralization in the Cananea district, Sonora, Mexico: Ph. D. the- Wilkins, J., Jr., and Heidrick, T.L., 1995, Post-Laramide extension and rota- sis, Tucson, University of Arizona, 181 p. tion of porphyry copper deposits, southwestern United States: Arizona Wolfe, E.W., 1983, Geologic map of the Arnold Mesa roadless area, Yavapai Geological Society Digest 20, p. 109–127. County, Arizona: U.S. Geological Survey Miscellaneous Field Studies Map Wilkinson, W.H., Jr., Vega, L.A., and Titley, S.R., 1982, Geology and ore MF-1577, scale 1:24,000. deposits at Mineral Park, in Titley, S.R., ed., Advances in geology of the Wong, M.S., and Gans, P.B., 2008, Geologic, structural, and thermochro- porphyry copper deposits, southwestern North America: Tucson, Univer- nologic constraints on the tectonic evolution of the Sierra Mazatán core sity of Arizona Press, p. 523–541. complex, Sonora, Mexico: New insights into metamorphic core complex Wilson, E.D., 1960, Geologic map of Yuma County, Arizona: Arizona Bureau formation: Tectonics, v. 27, no. 4, TC4013, doi:10.1029/2007TC002173. of Mines, 1 sheet, scale 1:375,000.