Dismembered Porphyry Systems near Wickenburg, Arizona: District

Transcription

©2016 Society of Economic Geologists, Inc.
Economic Geology, v. 111, pp. 447–466
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 greisenpotassic 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 resembles other porphyry belts and the distribution of active volcanoes at convergent margins.
Introduction
In southwestern North America, the Cenozoic Basin and
Range extensional province is superimposed upon the
Laramide (80–50 Ma) magmatic arc (Wilkins and Heidrick,
1995; Barton, 1996). The Laramide magmatic arc contains
many well-studied porphyry systems (e.g., Titley and Hicks,
1966; Titley, 1982a; Pierce and Bolm, 1995; Fig. 1). However,
few previous investigations consider the effect that postmineralization normal faulting has had on spatial relationships at
the deposit or district scale (e.g., Lowell, 1968; Wilkins and
Heidrick, 1995; Wodzicki, 1995; Stavast et al., 2008), and
especially at the scale of the magmatic arc (Richard, 1994;
Staude and Barton, 2001).
At the deposit and district scale, the superposition of
normal faults and porphyry systems creates challenges and
benefits for the study of both extensional and hydrothermal
processes. Challenges can arise where hydrothermal alteration obscures subtle distinctions in certain stratigraphic units
that might serve as important structural markers, or where
orebodies are dismembered by normal faults. Benefits of
this superposition arise when products of one of the geologic
processes can be used to help constrain aspects of the other
process. For example, predictable patterns in hydrothermal
alteration zoning can be used as geologic markers that may
better constrain structural reconstructions (e.g., Nickerson et al., 2010), and, in turn, aid in better discriminating
between different styles of extension. Conversely, ore-forming processes can be better constrained where deep levels
† Corresponding
author: e-mail, [email protected]
*Present address: Bronco Creek Exploration, Inc., 1815 E. Winsett Street,
Tucson, Arizona 85719-6547.
of ore-forming systems are exhumed and can be examined
at the surface (Proffett, 1979; Carten, 1986; Dilles and Einaudi, 1992; Seedorff et al., 2008).
Regional-scale reconstructions commonly subdivide regions
into extensional domains and then restore extension in each of
the extended domains (e.g., McQuarrie and Wernicke, 2005).
At the arc scale, such reconstructions can aid in understanding tectonic processes or, as attempted here, the original distribution of porphyry deposits along a magmatic arc.
This study focuses on a poorly understood segment of the
Laramide porphyry copper belt in the White Picacho and
Sheep Mountain districts (Keith et al., 1983), near the town
of Wickenburg in central Arizona (Fig. 2), and provides the
first public description, notwithstanding company reports, of
hydrothermal features observed in the Copper Basin (Crown
King) and Sheep Mountain porphyry systems. Previous
detailed mapping of rocks types and structural geology (Peterson, 1985; Capps et al., 1986, Stimac et al., 1987; R. Powers,
unpub. map) is combined with original, reconnaissance-scale
mapping of hydrothermal alteration and examination of areas
critical to a structural interpretation of the area, which was
made possible by helicopter-assisted access. The data for rock
types, structure, and alteration are used to make a structural
analysis of the area, including a palinspastic reconstruction
of the Oligo-Miocene extension. The reconstruction demonstrates that extension was produced by five superimposed sets
of normal faults, and the district-scale reconstruction reveals
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
and McKenzie (1983) to generate a new reconstruction of
the preextension distribution of porphyry deposits along the
Laramide arc.
0361-0128/16/4384/447-20447
Submitted: December 22, 2013
Accepted: September 25, 2015
448
NICKERSON AND SEEDORFF
114°W
111°W
108°W
Mineral
● Park
Bagdad
●
Copper Basin ●
(Prescott)
Fig. 2
●
●
Arizona
California
Porphyry Systems of the
Laramide Magmatic Arc
Copper Basin
(Crown King)
Sheep Mountain
New Mexico
Arizona
Nevada
36°N
Las Vegas
Globe-Miami
District
Phoenix
●
●●●
33°N
Resolution ●
● Christmas Morenci
Sacaton
● Ray ●
●● ●
●
●
Santa Cruz
Poston
Copper ●
Butte
● ● Creek Safford
Vekol ●
District
San ManuelLakeshore ●
Kalamazoo
●
Ajo ●
Silver Bell
Hillsboro
●
Tyrone
●
●
Santa
Rita
ora
● Cananea
Chihuahua
of
Ca
lifo
rni
La Caridad
0
a
30°N
So n
lf
Gu
Baja California
Tucson
Pima ●●
Peach-Elgin
District ●● ●● Rosemont
● Red Mountain
●
100 km Opodepe
●
●
Cumobabi
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
Porphyry deposits in Arizona are among the largest and best
studied deposits in the world (e.g., Cooke et al., 2005), and
many have been productive mines for over a century (e.g.,
Miami, Inspiration, Ray, and Morenci; Parsons, 1933; Fig. 1).
Nearly all of the known porphyry deposits in Arizona formed
during Laramide time (~80–50 Ma) when NE-directed
subduction of the Farallon plate beneath the North American plate produced a NW-SE-striking magmatic arc (Titley,
1982b; Lang and Titley, 1998; Leveille and Stegen, 2012). The
district-scale portion of this study examines a segment of the
Laramide arc located between the Globe-Miami district and
the Bagdad deposit, within which a major economic deposit
has yet to be identified (Fig. 1).
Extension in western Arizona
The study area near Wickenburg is located between the
highly extended Harcuvar and Harquahala metamorphic core
complexes (Reynolds and Spencer, 1985) to the west and the
unextended Bradshaw Mountains (Rehrig et al., 1980) to the
east (Fig. 2) in the Basin and Range extensional province. Previous workers in the Vulture Mountains, ~10 km west of the
study area (Fig. 2), proposed that SW-dipping, listric normal
faults were responsible for the observed repetition of steeply
dipping (up to ~85°) Tertiary sedimentary and volcanic rocks
exposed in the Vulture Mountains, as well as for the slightly
less tilted (up to ~65°) Tertiary sedimentary and volcanic
rocks exposed in the study area (Rehrig et al., 1980).
Subsequent detailed geologic mapping in the study area
(Capps et al., 1986; Stimac et al., 1987) at 1:24,000 scale,
however, revealed that normal faults are nearly planar
where exposed at the surface across multiple kilometers of
paleodepth and that higher angle faults cut lower angle faults.
Determining the geometry, slip, and relative timing of normal
faults (i.e., the style of extension) near Wickenburg is essential to reconstructing the geology of the district. Furthermore,
reconstruction of extension in the Laramide porphyry belt is
of particular interest to economic geologists, because it can
lead to the identification of offset and covered pieces of porphyry systems (e.g., Lowell, 1968) that might be attractive
exploration targets.
449
DISMEMBERED PORPHYRY SYSTEMS NEAR WICKENBURG, AZ
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
ad
rcu
Ha
Yavapai
Maricopa
US
r qu
Ha
a
la
ha
Wickenburg ●
60
MC
113°30'W
Hwy 60
Target
ture
Vul tains
un
Mo
Newsboy
C
Mo
un
tai
Basin
Copper
(Crown King)
Buckhorn
Creek Target
ns
g
bur
k en ins
c
i
W unta
Mo
.
Fig
Sheep
Mountain
3
Vulture
Mine
Big Horn
Mountains
33°40'N
w
C
La Paz
34°N
va
C
rM
sh
a
113°W
US
20
60
km
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,
magnetite, and up to 5% sphene, zircon, and other accessory
TANA, AND
The geologic map of the study area is shown in Figure 3. Pre- minerals (Stimac et al., 1987).
The crystalline Proterozoic and Cretaceous rocks are overvious geologic maps are heavily utilized in its creation (Peterlain
by Tertiary units, which include siliciclastic sedimentary
son, 1985; Capps et al., 1986; Stimac et al., 1987; R. Powers,
rocks,
volcanic rocks, debris flows, and conglomerates. The
unpub. map), especially for distribution of rock types. New
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 dislesser volcanic rocks. The conglomerate is similar in appeartribution 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 amphiboWhitetail 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 porphyryimportant 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 disarea 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
Rock types
65
65
112°25'W
Tvl
25
Tvu
29
20
42
69
Tvu
44
Tvl
Xi
66
ult
fa
lt
fau
A'
Xi
112°20'W
Xmvs
Xmvs
Tvl
44
Sheep
Mountain
East
Tg
Xmvs
Bradshaw
Mountains
Fig. 3. Generalized geologic map across the study area depicting rock units, faults, selected bedding orientations, and localities discussed in the text. Tertiary volcanic
rocks in the Hells Gate Formation and older units are grouped in the Tertiary lower volcanic unit. Tertiary volcanic rocks younger than the Hells Gate Formation are
grouped in the Tertiary upper volcanic unit. Note direction of north arrow. Geology from Peterson (1985), Capps et al. (1986), Stimac et al. (1987), R. Powers (unpub.
map), and this study. Dashed box outlines the location of Figure 4.
33°55'N
lt
fau
112°30'W
Tvl
Tvl
sh
y Wa
Trilb
km
Kg
55
Xmvs
47
28
Creek
112°35'W
5
4
Tvl
66
rn
ho
ck
u
B
70
65
Xmvs
Buckhorn
Creek
Castle
Tg
Fig.
Xmvs
71
Kg
24
60 Xi
Sheep
Mountain
West
ain
Tvl
52
Tvl
Kg
Xmvs
Strike and dip
24 of bedding
t
oun
pM
65
60
65
Kg
Mount
Tvu
Vernon
fault
Cross Cut
Fault
Metavolcanic & metasedimentary
Granite & gneiss
Normal faults
ee
Sh
Xmvs
A
Kg
Wickenburg
Mountains
Xi
Xmvs
Lower volcanic and sedimentary units
Tvl
Wickenburg
Mountains
fault
Xi
Upper volcanic and sedimentary units
Yavapai Supergroup
Tvu
Granite
Gravels
Kg
Cretaceous Proterozoic
Tg
Tertiary
Geologic Map of Wickenburg Mountains to Bradshaw Mountains
34°N
112°20'W
34°5'N
450
0
Meters
Thl
ThuTlb
Tc
Tlb
Tc
Tst Tsd3
Tim
Mafic dikes and plugs (Tertiary)
Thu
Upper Hells Gate dacite and
rhyodacite flows (Tertiary)
Thl
Lower Hells Gate flows (Tertiary)
Tht
Hells Gate tuffs (Tertiary)
Tsd
San Domingo Volcanics (Tertiary)
Xs
Tlb
Tlb
Tlb
Xs
Andesite flow (Tertiary)
Tc
Clastic sedimentary rocks (Tertiary)
Kg
Granite (Cretaceous)
Xp
Pegmatite (Early Proterozoic)
Xg
Granitic rocks (Early Proterozoic)
Xs
Schistose metamorphic rocks
(Early Proterozoic)
Xs
Xms
Xms
Xs
34
Kg
Xam
Xg
Xps
Kg
Kg
Tim
68
65
Xms
72
Wickenburg Mountains fault
Qs
63
75
54
65
Kg
26
0
Tif
Kg
73
Xms
Strike and dip of bedding
Strike and dip of foliation
Normal fault showing
amount and direction of dip
0
Feet
1000
Tdf
2000
3000
4000
5000
SW
Qs
2400
Qts
Normal fault covered
Tdf
23
Metavolcanic rocks (Early
Proterozoic)
Metasedimentary rocks
(Early Proterozoic)
Psammitic schist (Early Proterozoic)
Normal fault inferred
47 65
Xms
Metacarbonate (Early Proterozoic)
Geologic contact
0
68
Xms
Xms
2800
0
Xps
Xps
56
Xms
Xam
Kg
Quartz-bearing rhyolite flows
(Tertiary)
Lithic tuffs and related sedimentary
rocks (Tertiary)
Lower basalt, basaltic andesite,
and andesite flows (Tertiary)
Ta
Tc
Tlb
Xs
Xam
45
Tsd
Felsic dikes and plugs (Tertiary)
Tst
Tc
Ta
Tst Tc Thu
Tht
Tc Tst Tc
0
300
60
Tlb
Xg
0
320
Thu
Trilby Wash fault
Qs
55
Tsd3
Tst
Tsd3
Xms
Tif
Tsd3
Tc
Tlb
Thu
TcTst
Tlb
Tht
Xs
Tht
00
34
65
29
69
Xmc
57
Xam
00
30
Xps
Xms
50
00
32
Older alluvium (Quaternary and
Tertiary)
Debris flows and avalanche
deposits (Tertiary)
Upper basalts (Tertiary)
Xmc
Xmc
65
3400
Younger alluvium (Quaternary)
Tub
Xs
Thl
Tc
Tc
Thl
Tht
Thu
Xp
30
Tsd
Thu
Tc
40
Tc
0
320
35
70
Tlb
Tdf
Thu
Tlb Thl
Tsd3
44
47
Thu
Tht
Thu
Tlb
47
Tc
Xs
Tlb
Ta
Tst
78
Xps
Qs
Qts
60
Xam
Geologic Map and Cross Section
after Stimac et al. (1987)
Thu
Mount Vernon fault
Thl
38
00
00
36
Tlb
Xam
00
36
340
0
0
340
Trilby Wash ffault
Tril
ult
00
34
Tsd3
Thl
46
400
0
Tsd3
Tht
70
Thl
Tc
54
4200
Tc
NE
40
00
00
42
Tub
Thu
52
Tlb
45
00
34
3600
3800
00
38
65
50
Thu
4
4200
Tub
44
0
420 0
0
00
40
1000
34
00
38
00
1000 m
0
0
36
Xs
N
451
Fig. 4. Geologic map and cross section, digitized from
Stimac et al. (1987). Location of map shown in Figure
3. Note direction of north arrow. Topographic contours are in feet.
452
the basal conglomerate (Tc; Fig. 4). Above the basalt, rhyolite
lava flows and tuffs of the San Domingo Volcanics occur in
the western half of the study area. The Morgan City Rhyolite,
Spring Valley Rhyolite, and Castle Creek Volcanics occupy a
similar stratigraphic position in the eastern half of the study
area (Capps et al., 1986; Stimac et al., 1987). Dacite to rhyodacite lava flows and tuffs make up the structurally higher
Hells Gate Volcanics. Resting unconformably above the
Hells Gate Volcanics are interbedded basalts, tuffs, volcanic
megabreccias, and debris flow deposits with nearly horizontal bedding attitudes. Northwest-striking and steeply dipping
felsic and mafic dikes locally intrude the crystalline basement
and the Tertiary volcanic rocks. The youngest Tertiary unit
is a brown, consolidated to semiconsolidated conglomerate.
A thin layer of Tertiary-Quaternary gravels locally covers the
conglomerate.
Economic Geology
The study area contains mineralization that is related to several genetic types of deposits and formed at distinctly different times (DeWitt et al., 2008; this study). Gold and copper
associated with small, past-producing, volcanogenic massive
sulfide systems are hosted in metamorphosed Proterozoic
rocks. Pegmatite dikes of Proterozoic age have been investigated for their beryllium and lithium potential (Jahns, 1952;
London and Burt, 1978). Epithermal mineralization is locally
hosted in Tertiary volcanic rocks. Many of the washes in the
study area produced, and continue to produce, gold from
small placer deposits hosted in Tertiary and Quaternary gravels. The Buckhorn Creek area (Fig. 3) has been estimated to
contain 100,000 ounces (oz) of gold in Tertiary and Quaternary gravels (Montclerg Resources Ltd., unpub. report, 1995).
The Vulture lode mine, located just west of the study area in
the Vulture Mountains (Fig. 2), produced 340,000 oz Au and
260,000 oz Ag between 1863 and 1942 (White, 1989). Native
gold and electrum hosted in Proterozoic and Cretaceous crystalline rocks is interpreted to be genetically related to a Cretaceous dike (Spencer et al., 2004).The Newsboy prospect (Fig.
2), also in the Vulture Mountains (Fig. 2), has a resource of
123,000 oz Au and 2,121,800 oz Ag hosted predominantly at
the contact between Proterozoic schist and Tertiary rhyolite
lava flows (Hastings et al., 2014).
Several Laramide porphyry systems near Wickenburg have
defined mineral resources, but there is no significant past or
current production from these systems. The largest resource
is the Copper Basin prospect, here referred to as the Copper
Basin (Crown King) system (e.g., Ball and Closs, 1983), which
is located 20 km north of the study area in the Silver Mountain mining district of the southern Bradshaw Mountains (Fig.
2). This prospect is distinct from the Copper Basin district
near Prescott, Arizona, here referred to as the Copper Basin
(Prescott) system, which is described by Johnston and Lowell
(1961; Fig. 1).
The Copper Basin (Crown King) system is located 8 km
south of the town of Crown King, Arizona. Soldiers stationed
at nearby Fort Misery at the end of the 19th century were the
first to identify the prospect (Tognoni, 1969). Chalcopyrite
and molybdenite, as well as spectacular Cu oxide seeps in the
drainages, are exposed at the surface hosted in weakly foliated Proterozoic Crazy Basin granite (DeWitt et al., 2008).
The Squaw Peak porphyry system, 60 km to the northeast,
is hosted in the similar aged Proterozoic Cherry batholith
and has been determined to have an age of ~1.7 Ga based on
Re-Os dating of molybdenite (Sillitoe et al., 2014). At Squaw
Peak some of the veins in the porphyry system display ductile
fabrics near their margins indicative of postmineral deformation. At Copper Basin (Crown King) veins do not display ductile fabrics near their margins, and thus the porphyry system
is considered to be Laramide aged. Drilling conducted in the
late 1960s and early 1970s produced a resource estimate of
one billion tons of 0.16% Cu and 0.031% Mo based on eight
drill holes (ASARCO, unpub. report, 1974).
Near Sheep Mountain (Fig. 3), two porphyry prospects
have been identified. The larger of the two prospects is
known as Sheep Mountain (Wilkins and Heidrick, 1995), or
Sheep Mountain East (Ullmer, 2007). The prospect contains
a resource of 40 million metric tons (Mt) of 1.6% Cu and
0.035% Mo (Ullmer, 2007). The mineralization lies underneath as much as 700 m of Tertiary volcanic and sedimentary
rocks. Molybdenite from drill core at the prospect (hole CC-1
at 611-m depth) has been dated using the Re-Os technique
and yielded a Laramide age of 70.34 ± 0.36 Ma (H. Stein,
written commun., 2010).
Approximately 5 km to the west is the Sheep Mountain
West prospect, where several outcrops of intensely altered
Proterozoic granite are exposed beneath the Tertiary-Proterozoic unconformity in tilted fault blocks (Figs. 3, 5). Several
holes have been drilled in the last decade exploring for supergene copper mineralization, but a significant resource has not
been identified.
Hydrothermal Alteration
Hydrothermal alteration was mapped at reconnaissance scale
across the study area (Fig. 5). Three important styles of alteration have been identified: greisen, potassic, and transitional
greisen-potassic. The term greisen is used here to describe
hydrothermal alteration assemblages where coarse-grained
(>0.5 mm) white mica is an important constituent (e.g.,
Shaver, 1991; Seedorff et al., 2005a). Greisen is not widely
recognized in porphyry copper systems, but in the porphyry
copper systems in which it has been documented, it occurs
at deep levels, generally beneath the level of the orebodies,
beneath the region of most abundant quartz veins and most
intense potassic alteration, and well below the level where
sericitic alteration develops (Seedorff et al., 2005a, 2008). In
contrast, greisen can occur in porphyry molybdenum deposits
of the Mo-Cu subclass in a location that is within and above
orebodies and the most intense potassic alteration, analogous
to the position of sericitic alteration in many porphyry copper
systems in Arizona (Shaver, 1991; Seedorff et al., 2005a).
Greisen
Hosted within the Cretaceous granite (Fig. 6A), NE-striking
veins of quartz + white mica + pyrite ± chalcopyrite ± K-feldspar with white mica ± pyrite ± chalcopyrite envelopes (Fig.
6B) commonly compose 1 to 5% of outcrops in the central
Wickenburg Mountains (Fig. 5). Locally, veins and envelopes
of greisen are so intensely developed that they constitute up
to 20 vol % of outcrops. Vein fillings range from 1 to 200 mm
wide and have envelopes with a total width of 5 to 50 mm.
Kg
5
75
A
Tvl
Kg
33°55'N
Kg
Fig. 5. Reconnaissance map of hydrothermal alteration in the study area. Colors faded in background are rock units shown in Figure 3. See Figure 3 for key to rock units.
Note direction of north arrow.
Greisen
Potassic
km
Xmvs
Xmvs
Xi
Wickenburg
Mountains
Xi
Kg
→
Wickenburg
Mountains
fault
112°35'W
80
Tvl
80
Tvl
Kg
Mount
Vernon
fault
Xmvs
Xmvs
Tvl
85
orn
kh
c
Bu
Transitional Greisen-Potassic
Tvu
Tvl
53
85
lt
fau
Tvu
112°30'W
Xmvs
Buckhorn Creek
Tvu
Tvl
Tvl
fault
Strike and dip of hydrothermal vein
Xi
80
Creek
Castle
Reconnaissance Map of Hydrothermal Alteration
A'→
58
70
50
Xi
Xmvs
Sheep
Mountain
West
112°25'W
Tvl
Xmvs
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
found predominantly in the vein filling but also in the alteration halo. The sulfides range in size from 1 to 15 mm and have
a pyrite to chalcopyrite ratio of approximately 10:1. K-feldspar
is rarely observed in the greisen veins, where it comprises
<1% of the vein filling.
Potassic
In the Buckhorn Creek area, west of Sheep Mountain (Fig.
5), NE-striking veins of quartz + K-feldspar ± white mica ±
pyrite ± chalcopyrite with biotitic envelopes cut Yavapai Schist
that contains abundant metamorphic muscovite (Fig. 6C).
Veins vary in size from 1 to 15 mm wide with alteration halos
<10 mm wide. Quartz is the dominant mineral in the vein filling (~65%), accompanied by K-feldspar (~20%), white mica
(~10%), and sulfides (~5%) consisting of pyrite and lesser
chalcopyrite. White mica ranges in size from 0.1 to 2 mm. Vein
density increases to the northeast until it reaches 5 vol % adjacent to the Tertiary-Proterozoic unconformity that bounds the
northeastern side of altered Yavapai Schist (Fig. 5). Alteration
gradually decreases in intensity to the northwest of the Buckhorn Creek area until the Yavapai Schist is unaltered.
Transitional greisen-potassic
This term is used here with the meaning of Shaver (1991),
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
and is regarded as a coarser grained analog of sericitic alteration (Shaver, 1991). At Sheep Mountain (Fig. 5), veins of
quartz + K-feldspar ± white mica ± sulfide (1–5 mm wide)
are cut by quartz + K-feldspar + white mica + sulfide veins
(1–5 mm wide) with white mica halos (<5 mm wide; Fig 6D).
These veins differ from potassic veins, because they lack
biotite, and white mica is more abundant. They differ from
greisen veins because K-feldspar is much more abundant in
the vein fill (up to 50%) and is present in the vein envelope.
The vein density is intense in several areas on Sheep Mountain, where veins + halos constitute 5 to 10 vol % of outcrops.
White mica varies in size from 0.1 to 1 mm. Envelopes surrounding quartz of the vein filling of both vein types conspicuously change back and forth along strike between white mica
and K-feldspar.
Structural Geology
Tertiary sedimentary and volcanic units, the Tertiary-Proterozoic unconformity, contacts between Proterozoic units, and
styles of hydrothermal alteration are structural markers in the
study area, and the repetition of these markers was determined
by previous studies to be the result of movement on Tertiary
(~25–15 Ma) normal faults (Rehrig et al., 1980; Peterson,
1985; Capps et al., 1986; Stimac et al., 1987). By scrutinizing
crosscutting relationships between the normal faults exposed
in the study area, relative ages can be determined, which is
critical to constraining the style of deformation and is necessary for subsequently grouping faults into sets.
Examination of the normal faults in map view reveals that
five distinct sets of Tertiary normal faults, defined by similar
454
A
B
K-feld
spar
Quart
z
Oxidiz
ed
sulfid
e
C
Biotite
halo
par
te
e si
mica
D
z
Quart
ica
te m
Whi
White
Crosscutting
vein
K-felds
id
Sulf
Fig. 6. Photographs of styles of veins and associated alteration in the study area. A. Unaltered Cretaceous granite. B. Quartzmuscovite-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 Buckhorn 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
the study area (Fig. 7). Unless they significantly influence the
map pattern, faults determined to have less than 500 m of
offset are not shown in the geologic map (Fig. 3) and were not
assigned to a set of faults (Fig. 7). Many dozens of such smalloffset normal faults were identified in previous work across
the study area (e.g., Stimac et al., 1987; Fig. 4).
Set 1: Two faults from this set crop out at the surface in the
Wickenburg Mountains (Fig. 7), the Wickenburg Mountains
fault and the Mount Vernon fault. These faults have sinuous
expressions at the modern surface produced by the intersections of their present-day gentle dips with the modern topography. A structure contour map, generated by contouring the
intersection of the fault surface and topography, reveals that
the Wickenburg Mountains fault and the Mount Vernon fault
have azimuths of ~125° and dip ~4° to the southwest (Fig.
8). The planes of the two faults are contoured at similar elevations, which might lead one to believe that the two faults
are one fault plane. However, the faults are separated by the
Trilby Wash fault that has approximately 1 km of displacement
(Fig. 4), so the similarity in elevations is happenstance.
Previous mapping by Stimac et al. (1987) indicates that portions of the contact between Late Cretaceous granite and Proterozoic metamorphic rocks in the Wickenburg Mountains are
an intrusive contact, not a fault trace. This contact in places is
reinterpreted here to be a fault contact because it separates
hydrothermally altered granite from fresh metamorphic rock
(Fig. 5), and the contact is nearly planar as revealed by the
structure contour map (Fig. 8), which would be atypical for
an intrusive contact. Across 6 km of downdip exposure on the
Wickenburg Mountains fault (Fig. 8), the fault dip decreases
only slightly from 5.1° to 3.3° (curvature of 0.5°/km). The
structure contour map also reveals a NE-SW-striking trough
in the plane of the Mount Vernon fault. Such mullions are
commonly observed in normal faults (e.g., Proffett, 1977;
John, 1987a; Wong and Gans, 2008).
Set 2: The NE-SW-striking faults of this set strike nearly
perpendicular to faults in all other sets. One fault from this
set, named the Cross Cut fault, is exposed in the central portion of the study area, where it has a measured dip in outcrop
of 45° to the southeast (Fig. 7). The Cross Cut fault places
Tertiary volcanic and sedimentary rocks on Proterozoic rocks.
Offset on the Cross Cut fault is not well constrained, but
apparent offset is estimated to be 1.5 km.
Set 3: Faults from this set crop out across the study area.
They strike with an azimuth of ~150° and have dips measured in outcrop that range from ~35° to 50° to the southwest. Ten faults from this set are shown on the map (Fig.
7). Typical offsets on faults from this set are approximately
1 km. The Castle Creek fault, which bounds the western side
of Sheep Mountain, and the Trilby Wash fault belong to this
set (Fig. 7). In the Wickenburg Mountains, certain contacts
between granite and metamorphic rocks were reinterpreted
here as faults belonging to set three, as opposed to intrusive
contacts, because the contacts are on the projections from
the southeast of faults from set three, and contrasts in styles
of hydrothermal alteration are observed across the contacts
(Fig. 5).
Set 4: Members of the second youngest set of faults have
an azimuth of ~150° and dip steeply to the southwest at ~60°
:
:
Set 2
45
:
Set 3
40
:
Set 4
:
:
45
40
:
!0
:
: : : : !: !
!
Trilby
37
!
Fig
.8
Set 5
112°25'W
:
Mount
Vernon
fault
Cross Cut
fault
:
:
:
:
:
: :
!
!
fault
65
km
30
:
:
:
!
:
!
45
40
40
h
as
W
:
::
:
34°N 112°35'W
Normal Faults Sets
70
70
42
rn
ho
k
c
fault
lt
65
! 70
34°5'N
A'
:
112°30'W
:
: :
!
Bu
Creek
fau
:
:
:
::
!
:
Castle
:
0
:
:
5
A
33°55'N
112°40'W
Wickenburg
Mountains
fault
:
112°40'W
:
:
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
strikes and dips, as well as common crosscutting relationships. Note direction of north arrow. Dashed box is location of Figure 8.
Set 1
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
1.5 km of slip in the central portion of the study area.
Set 5: Faults belonging to the youngest set have azimuths
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.
Structural Interpretation and Palinspastic
Reconstruction of Normal Faults
The original observations and compilation of crosscutting
relationships between normal faults (Fig. 7), structure contour maps of normal faults (Fig. 8), and analysis of Tertiary
tilting (Fig. 9) are combined below to interpret the style of
extension in the study area. The interpretation provides the
means to palinspastically reconstruct Tertiary extension in a
20-km-long cross section through the study area (Fig. 3).
Tertiary tilting
An examination of the strikes and dips of the oldest Tertiary
rocks above the Tertiary-Proterozoic unconformity (Fig. 3)
shows that, in nearly all instances, these rocks dip ~65° (±5°)
to the northeast. This tilting is the result of slip and concurrent tilting on the SW-dipping normal faults (Stimac et al.,
1987). Orientations of foliation in the Proterozoic Yavapai
Schist in the study area can also be used to constrain the
magnitude of Tertiary tilting recorded in crystalline rocks
beneath the Tertiary-Proterozoic unconformity. The orientation of foliation in the Yavapai Schist is commonly consistent over distances of 10s of kilometers (e.g., DeWitt et
al., 2008). Thus, any changes observed in the orientation of
foliation are likely the result of deformation subsequent to
the Proterozoic foliation-forming event. The most probable
candidate for reorienting foliation in the study area is tilting
caused by Tertiary extension.
Foliation measurements of the Yavapai Schist in the Wickenburg Mountains, which have been highly extended (Stimac et al., 1987), are compared to foliation measurements
(DeWitt et al., 2008) of the Yavapai Schist 20 km to the north
in the Bradshaw Mountains (Fig. 2), an area for which there
is no evidence for significant Tertiary extension and tilting
(Rehrig et al., 1980). Measurements of foliation in the two
areas are compared in contoured equal-area stereographic
projection in Figure 9A and B, and the average foliation varies significantly between the two areas. To test the hypothesis
that the rigid body rotation was caused by tilting associated
with Tertiary extension, the foliation data from the Wickenburg Mountains are rotated 65° clockwise about a horizontal axis trending 150° (Fig. 9C). This rotation would restore
the amount of Tertiary tilting recorded by the oldest Tertiary
volcanic and sedimentary rocks across the study area. The
mean plane of the rotated Wickenburg Mountains foliation
data (Fig. 9C) plots within a few degrees of the mean plane
of the foliation data from the unextended Bradshaw Mountains, 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
3800
rg
3600
Mo
3400
un
tai
ns
fau
lt
360
Tri
lby 0
Wa
sh
lt
34
00
Mount Vernon fault
0
0
340
3200
3.
3º
3200
00
30
Di
p
=
00
30
00
3600
fau
320
0
300
26
3800
5.
1º
bu
=
en
Di
p
ck
3200
Wi
4000
3600
Structure Countour Map
2800
2800
0
340
Fault surface contour
(feet)
Set 1 faults
1000 ft
Younger faults N
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
As mentioned above, debate surrounds the style of extension in the study area, i.e., listric faults (Rehrig et al., 1980)
versus more planar, “domino-style” normal faults (Stimac et
al., 1987). The threshold of curvature which defines a “listric”
normal fault is not universally defined. Buck (1988) showed
initial listric fault curvature of 4.5°/km in his “rolling hinge”
model for extension, and other authors have proposed curvature greater than 10°/km (e.g., Rehrig et al., 1980). Predictions of the competing models are outlined here and tested
against observations in the study area.
For one set of nearly planar, domino-style normal faults,
the amount of tilting is the same from one fault panel to the
next, even if there are many faults in the set (e.g., Ramsay
and Huber, 1987, p. 518). In contrast, listric normal faults
produce greater amounts of tilting in the hanging wall than
in the underlying footwall as rocks in the hanging wall slip
down a concave-upward fault plane equal to the amount
of curvature on the fault multiplied by the displacement of
the fault. Where a set of multiple, subparallel listric normal
faults are present, the dip of beds in the hanging walls of successive fault blocks should show progressively steeper dips
in the transport direction, regardless of whether the faults
moved concurrently, sequentially in the transport direction,
or another order as long as they do not crosscut one another.
The progressive change in dips across successive faults can be
counteracted, to a certain degree, by the development of drag
folds in the hanging wall of the listric faults or by the development of antithetic listric faults (e.g., Ramsay and Huber, 1987,
p. 520). If present, either of the latter mechanisms should be
evident in field.
Observations in map view (Fig. 3) and stereographic analysis (Fig. 9A-C) are not consistent with the predictions of listric fault geometry for rocks in the Wickenburg area. The
Yavapai Schist and the oldest Tertiary volcanic and sedimentary rocks in the study area were tilted ~65° NE during Tertiary extension. Furthermore, where exposure of fault planes
is extensive enough at the surface in the downdip direction
to determine the curvature (Fig. 8), the calculated curvature
is indeed low (~0.5°/km). Large-scale drag folds or antithetic
listric faults are not observed in the study area, so it is unlikely
that increased tilting in the transport direction caused by listric faults has been counteracted by such mechanisms. Taken
together, evidence in the study area controverts the involvement of strongly listric normal faults in extension and suggests
457
B
A
N=
N=
C
N=
that the observed brittle extension was accommodated by
superimposed sets of nearly planar, domino-style faults. There
is no direct evidence in the study area for how extension was
accommodated at depth, but in principle, there is no geometric requirement that these upper crustal normal faults necessarily merge into detachment faults at depth (e.g., Seedorff
and Richardson, 2014).
Interpretation of the normal faults
The grouping of the Tertiary normal faults into fault sets,
each set with a distinct relative age, suggests that each set
can be viewed as a sequential generation of faults. Hence,
each generation is defined as a set of similarly oriented faults
that moved more or less contemporaneously during specific
time intervals, as evidenced by their consistent crosscutting
relationships, i.e., each generation of faults operated as a
system of normal faults. As the normal faults within a given
set cut and extended the crystalline and supracrustal rocks in
the study area, the dip of the active fault planes rotated to
lower angles. Once the fault planes of a fault set rotated to
angles that were kinematically unfavorable for slippage (less
than ~30°; Byerlee, 1978; Sibson, 1994), a new fault set with
new fault planes formed. Faults of the new set cut and continued passively rotating rocks of older fault blocks, faults of
older fault sets, and any other contained geologic elements,
including porphyry systems. This repeated sequence of events
produced a cumulative northeastward tilting of ~65°, as evidenced by the present-day dips of the oldest Tertiary volcanic and sedimentary rocks and the difference in orientation
Fig. 9. Contoured poles to planes of foliation measurements in the
Yavapai Schist depicted on stereonets, using equal-area lower hemisphere 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 ProterozoicTertiary unconformity, and Tertiary sedimentary and volcanic rocks.
of foliation observed in the Yavapai Schist between extended
and unextended terranes (Fig. 9).
Approach to restoring movement on normal faults
Figure 10 shows the palinspastic reconstruction of the 20-kmlong cross section in the study area (Fig. 3) and an interpretation of that reconstruction. In the reconstruction, faults are
modeled as perfectly planar, whereas field evidence indicates
that the faults are slightly curved in the downdip direction. It
is likely that subtle drag folds or small faults in the hanging
wall of the normal faults counteracted the slight differential
tilting produced by the slip on the gently curved fault planes.
Displacement along the normal faults was removed in sequential order, from the youngest to the oldest generations of normal faults (panels A-F, Fig. 10), as determined by relative ages
and dip measurements. The magnitude of slip on individual
faults was constrained using structural markers, including the
Proterozoic-Tertiary unconformity, contacts between various
lithologies of Proterozoic metamorphic rocks, the Tertiary
stratigraphy, and hydrothermal alteration assemblages. Tertiary sedimentary and volcanic rocks were rotated to horizontal in the time slice of the reconstruction in which they were
deposited. The relative age of the Tertiary sedimentary and
volcanic rocks was determined by examining crosscutting and
onlapping relationships between the faults and the Tertiary
sedimentary and volcanic rocks (i.e., faults either being cut by
or being mantled by Tertiary sedimentary and volcanic rocks).
Due to the abundance of crystalline rocks in the study area,
and thus the paucity of structural markers in certain areas, a
458
A
A
Wickenburg Mtns.
B
D
F
Buckhorn Creek Buckhorn Sheep Mtn.
Creek Target
A’
Buckhorn
Creek
fault
C
Trilby Wash
fault
Wickenburg
Mountains
fault
Castle Creek
fault
Mount
Vernon
fault
E
Cross Cut
fault
G
SW
Buckhorn
Creek
Target
Hwy 60 Buckhorn
Creek
Target
Tvl
NE
Sheep
Mountain
Xmvs
Xi
Kg
Modern
Surface
Wickenburg
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.
number of uncertainties remain in the restoration. The threedimensional shape of the igneous bodies is unknown; thus the
form chosen here is based on relationships that are plausible
considering constraints imposed by restoration of hydrothermal alteration patterns (Fig. 10G). As previously mentioned,
dips measurements are not available on some faults, either
from this study or previous work, due to lack of exposure. In
these cases, crosscutting relationships were used to assess the
generation to which such faults belonged, and then dips measured from other faults of the same fault set were used.
In addition, normal faults in the reconstruction are shown
locally extending to depths of approximately 20 km below the
Tertiary-Proterozoic unconformity (Fig. 10F), whereas exposures of the faults at the surface likely only reach paleodepths
less than 9 km (Fig. 10G). It is likely that fault displacement
switched from a brittle to ductile regime somewhere between
the deepest exposures of the normal faults at the surface and
the bottom of the cross section in Figure 10F. Modeling ductile movement is not possible using the rigid reconstruction
approach employed in this study, and instead brittle movement was projected below deepest surface exposures. Thus,
any portions of the cross section beneath the deepest surface
exposure, which are not central to the interpretation of the
reconstruction presented below, should be viewed as loosely
constrained and speculative.
Examination of the district-scale reconstruction
The reconstruction indicates that two distinct hydrothermal
systems formed, each centered on a separate cupola of the
Late Cretaceous granite pluton (Fig. 10G). The cupolas do
not crop out, and their locations were assigned based on the
intensity of alteration in fault blocks observed at the surface.
The pluton intrudes metasedimentary and metavolcanic rocks
in the west and metaplutonic rocks in the east. Potassic and
transitional potassic-greisen hydrothermal alteration exposed
at Buckhorn Creek and Sheep Mountain are shown to be
sourced from the easternmost cupola of the pluton, whereas
the greisen alteration hosted in Late Cretaceous granite in
the Wickenburg Mountains is part of a separate hydrothermal system centered on the western cupola of the pluton (Fig.
10G). Hydrothermal alteration is zoned upward from greisen
to potassic to transitional greisen-potassic assemblages (Fig.
10G). The uppermost levels of the system are eroded, as is
evidenced by the presence of porphyry-style alteration and
Cu oxides in clasts of the Tertiary conglomerate at the base of
the Tertiary section near Buckhorn Creek.
The Tertiary volcanic section reaches a maximum thickness of approximately 5 km. This thickness is greater than the
thickest exposed section of lower volcanic and sedimentary
rocks exposed in the map, which is approximately 4 km. The
greater thickness in the reconstruction is suggested by restoration of intrusive contacts between granite and metamorphic rocks in the Wickenburg Mountains. Restoration of those
contacts requires slip on the Wickenburg Mountains fault that
elevates part of the lower volcanic and sedimentary rock unit
above the thickest exposed section.
Exploration targets
Leading up to, and subsequent to, the discovery of the Kalamazoo orebody (Lowell, 1968), knowledge and understanding
459
of postmineralization Tertiary extension in the Basin and
Range province began to change exploration strategies. A few
orebodies in the Basin and Range province (e.g., Yerington
and Hall, Nevada) were previously known to be tilted and dismembered (J. Proffett, pers. commun., 2014); however, widespread recognition of tilting and dismemberment in orebodies
in the Basin and Range province postdated discovery of Kalamazoo (e.g., Proffett, 1977; Shaver and McWilliams, 1987;
Seedorff, 1991; Seedorff et al., 1996). Eventually, Wilkins and
Heidrick (1995) suggested that all porphyry deposits in the
Basin and Range province should be assumed to be faulted
and tilted until proven otherwise. During the past decade, the
importance of tilting of orebodies across the Basin and Range
province has continued to be emphasized (e.g., Seedorff et al.,
2005a, p. 276–277; Maher, 2008; Stavast et al., 2008; Nickerson et al., 2010) and is again demonstrated here.
Despite at least two drilling campaigns in the 1960s and
2000s, economic mineralization has not yet been located at the
Sheep Mountain West prospect. The palinspastic reconstruction demonstrates that sulfide-bearing transitional greisenpotassic alteration at Sheep Mountain and potassic alteration
at Buckhorn Creek are both pieces of a larger, dismembered
porphyry system. Potassically altered pieces of the same porphyry system are “structurally covered” (as used by Corn and
Ahern, 1994) by Tertiary volcanic and sedimentary rocks west
of Sheep Mountain in the modern cross section (Fig. 10G). To
our knowledge, this target, which we name Buckhorn Creek,
has not been tested with a drill hole, but significant mineralization could be associated with potassic alteration.
Additionally, it is likely that intense greisen alteration
exposed in the Wickenburg Mountains is the expression of
a porphyry system, and we name this target the Highway 60
target. Whether the intensity of greisen alteration has any
correlation to the development of structurally higher level
alteration, including sulfide mineralization, remains uncertain. Outcrops in the Wickenburg Mountains, however, demonstrate that significant quantities of magmatic hydrothermal
fluids were released at least locally from that portion of the
Late Cretaceous pluton. Structurally higher levels that may
contain porphyry mineralization are not located in the line of
section but may lie underneath Quaternary and Tertiary cover
southwest of the study area near U.S. Highway 60 (Fig. 2).
The targets generated by the district-scale palinspastic
reconstruction provide an example of a geologically based
method for exploring beneath postmineralization cover rocks.
In the Laramide porphyry province, geologically driven
exploration underneath postmineralization cover has yielded
several discoveries (e.g., Kalamazoo, Lowell, 1968; Resolution, Paul and Manske, 2005). Continued exploration in the
province should incorporate structural interpretations when
designing exploration programs, which may also involve geophysical and geochemical techniques.
Arc-Scale Reconstruction of Tertiary Extension in
the Laramide Porphyry Copper Belt
The detailed examination and reconstruction of the porphyry
systems near Wickenburg fills a gap between the GlobeMiami district and Bagdad in a previous compilation of porphyry deposits in the Laramide magmatic arc (Titley, 1982b).
These porphyry systems are now placed in their preextension
460
context at the scale of the entire Laramide porphyry copper
belt in order to better understand the original spatial relationships between the porphyry systems. To this end, a regionalscale reconstruction of Cenozoic extension is presented for
the relevant portions of Arizona, New Mexico, and Sonora
(Figs. 11–12), revisiting a topic addressed earlier by Richard
(1994) and Staude and Barton (2001) but using a different
approach.
Methodology
An original compilation of strikes and dips of the oldest preand synextension Oligocene and Miocene sedimentary and
volcanic strata across the porphyry belt serves as the data for
the reconstruction (Fig. 11). The tilting information recorded
by the bedding attitudes provides a basis for grouping regions
where the magnitude of extension was similar. These data
are then adapted to estimate a β factor utilizing the following
equation from Jackson and McKenzie (1983):
θ
β = sin
——–
sin θ'
(1)
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. Several assumptions are made in the calculations: (1) the dips
of syn- and postextension Tertiary rocks record only the
effects of Cenozoic extension; (2) normal faults were tilted
to lower angles by the same amount that Tertiary beds were
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), Grubensky 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).
100 km
N
34º S
Forearc
72º W
Rosario de Rengo
El Teniente
Rio BlancoLos Bronces
Vizcachitas
B
Cerro Blanco
68º W
Paramillos Norte
Paramillos Sur
Rio de Las Vacas
Pimenton
West Wall
Santa Clara
Yunque
Cerro Mercedario
El Altar
is
Arc ax
La Caridad
Chino
Tyrone
Hillsboro
Red Mountain
Morenci
Safford District
San Manuel-Kalamazoo
Copper Creek
Ray
Cerro Bayo de Cobre
Amos-Andres
Los Pelambres
El Pachon
c
Rear-ar
Mount Shasta
Rainbow Mountain
Cappy Mountain
South Sister
Mount Saint Helens
Mount Adams
125º W
Maidu Volcanic Center
C
Mount Jefferson
Middle Sister
Broken Top
Newberry
Mount Hood
Mount Rainier
Goat Rocks
Simcoe Mountains
Glacier Peak
120º W
Dittmar Volcanic Center
Medicine Lake
Hackamore
Mount Mazama
(Crater Lake)
Mount Baker
Mount Garibaldi
Mount Meager
Mount Cayley
Snow Mountain
Lassen Volcanic Center
42º N
49º N
100 km
Fig. 12. Comparison of porphyry systems of the reconstructed Laramide porphyry copper belt to features in other magmatic arcs. A. Reconstructed location of porphyry
systems of the Laramide magmatic arc. Topography in the background is representative of southwestern North America at 50 Ma (Blakey and Ranney, 2008) and is
intended for general reference only. The topography and the location of selected porphyry deposits were restored using different approaches. B. Porphyry copper systems
of the Miocene-early Pliocene magmatic arc of central Chile (after Sillitoe and Perelló, 2005). C. Quaternary Cascade magmatic arc of northwestern North America (after
Hildreth, 2007), showing major volcanic centers. Note the change in scale from panels A and B to panel C.
Opodepe
Cumobabi
Cananea
Pima District
Peach Elgin
Rosemont
Silver Bell
Globe-Miami
District
Resolution
Los Azules
Los Bargres Sur
Miocene-Early Pliocene
(16-4 Ma), Central Chile
Piuquenes
100 km
N
Rear-Arc Volcanic Fields
N
Quaternary Cascade (2-0 Ma),
Northwestern North America
Forearc
Vekol
Lakeshore
Ajo
Sacaton
Casa Grande
Sheep Mountain
Copper Basin
(Crown King)
30º S
Mo-(Cu)?
Mo-(Cu)
Major Volcanic Centers
Cu-(Au-Mo)
Cu-(Mo)
Comparison of Magmatic Arcs
is
ax
Copper Basin
(Prescott)
Bagdad
A
Rear-arc
c
Ar
Forearc
Mineral Park
Restored Laramide (75-55 Ma),
Southwestern North America
arc
Rear-
Arc axis
461
462
a maximum of 30° of tilting and initiated with 60° dips; (4)
all extension was NE-SW directed; (5) tilting was unidirectional; and (6) any post-Laramide strike-slip faulting did not
significantly alter locations of porphyry deposits. It is unlikely
that these conditions are met across the entire region considered here; however, the generalization of Cenozoic extension
in this manner allows for a palinspastic reconstruction of the
porphyry systems of the Laramide porphyry copper belt that
is more representative of its original form than the current,
postextension distribution.
Jackson and McKenzie (1983) demonstrated geometrically
that a normal fault that is tilted from 60° to 30° produces a β
factor of 1.73, equivalent to 73% extension. In the strike and
dip compilation used here, dips are assigned to three groups
of dips, 0° to 30°, 30° to 60°, and 60° to 90°. An average
amount of extension was assigned for each domain (Fig. 11),
and a regional contour map of the magnitude of extension was
created. Locations of porphyry deposits were restored to the
northeast by removing the cumulative amount of extension
between the modern location of a deposit and the unextended
terrane outside the Basin and Range (either the Colorado Plateau or the southern Rocky Mountains).
In addition to the uncertainties introduced from the
assumptions made in its construction, further uncertainties
about the distribution of porphyry deposits arise from their
degree of preservation and the limited modern exposure of
the porphyry copper belt. Denudation prior to or during
Tertiary extension in Arizona could have completely eroded
porphyry systems in the arc (Barton, 1996). Furthermore,
Quaternary and Tertiary sedimentary and volcanic rocks cover
greater than 70% of the surface in the Basin and Range province of Arizona (Reynolds, 1988). These younger rocks likely
conceal additional Laramide porphyry systems.
As previously mentioned, this study follows earlier arc-scale
reconstructions of porphyry systems in the Laramide magmatic arc by Richard (1994), and Staude and Barton (2001).
Staude and Barton (2001) considered a larger portion of southwestern North America in their reconstruction and restored
more generalized domains of extension based on the locations and dominant extension directions of metamorphic core
complexes. Similar to this study, Richard (1994) delineated
extensional domains using a compilation of tilting information
recorded in Tertiary rocks and restored extension using the
equations of Jackson and McKenzie (1983). The present study
uses an independent, albeit partially overlapping set of dip orientations used by Richard (1994), including data generated
since publication of the earlier study. Moreover, the earlier
work by Richard (1994) averaged the dips of Tertiary rocks
to determine tilting within domains, as opposed to estimating tilting using the dips of only the oldest pre- or synextension Tertiary rocks. By averaging the dips of all Tertiary rocks,
calculations of extension are influenced by rocks which only
record a portion of the extension with a domain. Thus, the
earlier calculations by Richard (1994) significantly underestimated the amount of extension across the porphyry belt.
Examination of the Arc-Scale Reconstruction of the
Laramide Porphyry Belt
The Laramide porphyry copper belt is a manifestation of
the Laramide magmatic arc of southwestern North America
(Titley, 1982b; Leveille and Stegen, 2012). Hence, when discussing the geometry of the porphyry copper belt, it is appropriate to make comparisons to continental arcs. Nonetheless,
the ages of Laramide porphyry deposits span ~75 to 55 Ma
(Titley, 1982b; Seedorff et al., 2005b) so their distribution is
a time-integrated pattern rather than a snapshot of an active
arc.
The reconstructed distribution of porphyry deposits of the
Laramide arc (Fig. 12A) yields a variably well-defined arc axis,
with gaps and clusters of deposits along the 700 km of strike
length parallel to the Laramide plate margin. The apparent
time-integrated axis of the porphyry copper belt (dashed, Fig.
12A) extends from Mineral Park to Red Mountain. Nearly two
dozen deposits lie along the axis of the arc. Fore-arc deposits include those in the Ajo, Cananea, La Caridad, Opodepe,
and Cumobabi districts, and rear-arc deposits include those in
the Safford, Morenci, Tyrone, Chino, and Hillsboro districts.
Along-axis spacing between the known deposits is variable.
Deposits in the Globe-Miami, Safford, and Pima districts are
separated by only a few kilometers, whereas Sheep Mountain and Ajo are separated by an apparent gap of more than
100 km along the axis of the magmatic arc.
Across-axis spacing of known deposits is also highly variable. Resolution lies 20 km from the deposits of the GlobeMiami district, but Opodepe is separated from Hillsboro by
325 km. Casa Grande and Vekol, as well as deposits within the
Globe-Miami and Pima districts, are separated by less than
10 km across axis. In the Globe-Miami and Pima districts, the
clustering of deposits reflects, in part, the dismemberment
of porphyry systems by Tertiary normal faults (Stavast, 2006;
Maher, 2008; Stavast et al., 2008). That is, the named deposits
identified today were once parts of larger porphyry systems,
as is the case in the classic example of San Manuel-Kalamazoo
(Lowell, 1968).
Discussion
Classification of porphyry systems near Wickenburg
Because an orebody has not been located in the study area,
it is speculative to classify the two identified porphyry systems based on an economically dominant metal. Nonetheless,
deposit classes are at least partially constrained by compositions of igneous source rocks and may display some distinctive styles of hydrothermal alteration (Seedorff et al., 2005a).
For example, porphyry Au deposits are normally associated
with dioritic host rocks, whereas porphyry W and Sn deposits are associated with rhyolitic and rhyodacitic source rocks,
respectively.
The granitic composition of the inferred Late Cretaceous
source rocks for the porphyry systems in the study area (Fig.
11) may be indicative of porphyry molybdenum deposits of
the quartz monzonitic-granitic porphyry Mo-Cu or granitic
porphyry Mo subclasses (Seedorff et al., 2005a). Nonetheless, porphyry Cu-(Mo) deposits in the Globe-Miami district (Fig. 1) are sourced from the Schultze Granite, which
is compositionally similar (Stavast, 2006; Maher, 2008) to the
Late Cretaceous granite in the study area. The conspicuous
absence of large numbers of porphyry dikes in the study area
is also commonly observed in porphyry Mo-Cu systems (e.g.,
Hall (Nevada Moly), Shaver, 1991; Buckingham, Loucks and
Johnson, 1992). The transitional greisen-potassic style of alteration documented on Sheep Mountain is perhaps suggestive
of a porphyry Mo-Cu system as well, because the only other
well-documented instance of this distinctive style of highlevel alteration is the Hall (Nevada Moly) porphyry Mo-Cu
system (Shaver, 1991). The high Mo contents observed in the
nearby porphyry resources at Sheep Mountain East (40 Mt @
1.6% Cu and 0.035% Mo; Ullmer, 2007) and Copper Basin
(Crown King) (1 Gt @ 0.16% Cu and 0.031% Mo; ASARCO,
unpub. report, 1974) are also suggestive of possible porphyry
Mo-Cu affinities.
Together, this evidence suggests that the porphyry systems identified here are perhaps part of a cluster of porphyry
Mo-Cu systems located in the middle of what was previously
thought to be a large gap in the Laramide porphyry belt
(Titley, 1982b). These systems join other quartz monzoniticgranitic porphyry Mo-Cu deposits, e.g., El Crestón/Opodepe
(León and Miller, 1981) and Cumobabi (Scherkenbach et al.,
1985) and Mo-rich porphyry copper deposits of the quartz
monzodioritic-granitic Cu-(Mo) deposits, e.g., Sierrita (Aiken
and Baugh, 2007), in the arc, as well as deposits that have
characteristics transitional between those two subclasses, e.g.,
Mineral Park (Wilkinson et al., 1982).
Comparison of the scale and geometry of the
Laramide magmatic arc to other arcs
Whereas exploration in the Laramide porphyry copper belt
has evolved to consider deposit-scale (e.g., Lowell, 1968;
Wilkins and Heidrick, 1995), district-scale (e.g., Wodzicki,
1995; Stavast et al., 2008), and sometimes regional-scale
extension (e.g., Maher, 2008), little attention has been given
to the effects of extension at the scale of the Laramide porphyry belt (Richard, 1994; Staude and Barton, 2001), and a
comparison of the preextension geometry of the Laramide
porphyry copper belt to magmatic features in other well-studied arcs is lacking.
The length, breath, and spacing between porphyry centers in the reconstructed Laramide porphyry copper belt is
compared to other magmatic features in the prolific central
Chilean porphyry copper sub-belt in the Miocene-early Pliocene magmatic arc (Sillitoe and Perelló, 2005) and the Quaternary Cascade volcanic arc of northwestern North America
(Hildreth, 2007) in Figure 12A-C. The magmatic features in
each of the arcs are distinctive. For example, as previously
mentioned, the Laramide porphyry belt spans about 20 m.y.
of activity (~75–55 Ma, Lang and Titley, 1998; Seedorff et al.,
2005b) and was not necessarily stationary during that interval. The central Chilean sub-belt—somewhat arbitrarily constrained in length—was active for about 12 m.y. (16–4 Ma,
Sillitoe and Perelló, 2005). The Quaternary portion of the
Cascade arc includes only its last 2 m.y. of activity (Hildreth,
2007), and this portion is not yet sufficiently eroded to reveal
much about its potential for porphyry mineralization (e.g.,
John et al., 2005).
The length of the Laramide porphyry copper belt, at
700 km, is greater than the 400-km-long Miocene-early Pliocene porphyry copper sub-belt of central Chile, but shorter
than the 1,250-km-long chain of volcanic fields in the Quaternary Cascade arc. Spacing between deposits (and volcanic
fields) along strike is similar, with all three arcs having areas of
463
clustered magmatic activity, as well as apparent gaps in magmatic activity greater than 100 km. One clear difference is the
breadth of the Laramide porphyry copper belt south of Red
Mountain, where it reaches a maximum width of 325 km. The
maximum across-axis width between magmatic features in the
other arcs is less than 150 km.
Clearly, there is no blueprint for magmatic features in convergent oceanic-continental plate margin arc. Differences in
the interaction between downgoing slabs, mantle wedges, and
continental crust will make each arc distinctive. However,
when compared at the entire arc scale, differences in geometry between the reconstructed Laramide magmatic arc (as
revealed by the porphyry copper belt) and other arcs appear
to be minimal.
Conclusions
The effect of Tertiary extension on the geometry of Laramide
porphyry systems has been demonstrated at the district and
arc scale. Original reconnaissance mapping is combined with
previous detailed mapping as the basis for a 20-km-long palinspastic reconstruction. The reconstruction reveals that
two porphyry-style hydrothermal systems emanate from a
Laramide pluton exposed in the study area. Hydrothermal
alteration is zoned from greisen at deeper levels, to potassic
and transitional greisen-potassic at higher levels. Five superimposed sets of normal faults, which initially developed at
high angles and then rotated to lower angles during extension, dismembered the porphyry systems. Potentially wellmineralized fault blocks are buried underneath Tertiary and
Quaternary volcanic and sedimentary rocks. Two exploration
targets, Buckhorn Creek and Highway 60, are worthy of further investigation and perhaps drill testing.
Extension at the scale of the Laramide porphyry belt is
quantified using a compilation of strikes and dips documented
in pre- and synextension rocks across the porphyry belt. The
tilting recorded by the attitudes provides a basis to group
regions where the magnitude of extension is similar and then
is adapted to estimate a β factor across the southern Basin
and Range province. Tertiary extension is restored quantitatively to reveal the preextension geometry of the porphyry
belt, where the majority of porphyry deposits clearly define
a 100-km-wide axis and others lie in fore- or rear-arc settings.
The arc geometry, once extension is restored, closely resembles other magmatic arcs formed at convergent oceanic-continental plate boundaries.
Acknowledgments
We would like to thank Bronco Creek Exploration for logistical and financial support of this project, including helicopter support at Sheep Mountain. Additional financial support
came from Science Foundation Arizona and an award from
the Society of Economic Geologists student research fund.
Dave Maher and David Johnson introduced us to the study
area, and discussions in the field with Dave Maher, Doug
Kreiner, Mike McCarrel, and Isaac Nelson helped mold our
thinking. Early reviews of this manuscript by Mark Barton,
George Davis, Charles Ferguson, and Peter Reiners, and
later reviews by John Proffett, Jeremy Richards, John-Mark
Staude, Carson Richardson, and Joe Colgan greatly improved
its quality.
464
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Dismembered Porphyry Systems near Wickenburg, Arizona: District

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