Gregg Canyon Intrusive Center, Sonoma Range, Nevada: Magmastism and Porphyry-Style Alteration-Mineralization

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Authors Wallenberg, Alexandra Leigh

Publisher The University of Arizona.

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Download date 04/10/2021 12:47:02

Link to Item http://hdl.handle.net/10150/642064 GREGG CANYON INTRUSIVE CENTER, SONOMA RANGE, NEVADA:

MAGMASTISM AND PORPHYRY-STYLE ALTERATION-MINERALIZATION

Alexandra Leigh Wallenberg

______

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

2020

Acknowledgments

We extend our gratitude to organizations that funded this project, including an

SEG Graduate Student Fellowship from the Society of Economic Geologists Foundation sponsored by AngloAmerican, a Courtright Scholarship from the Arizona Geological

Society, summer research funding from the J. David Lowell Scholarship in the

Department of Geosciences at the University of Arizona, a Geological Society of

America Research Grant, and a Geological Society of Nevada Elko Chapter

Scholarship. We thank Ralph Stegen of Freeport Exploration, Steve Shaver, Clancy

Wendt, Odin Christensen, and Christin Lucas for providing previous reports and data from the Sonoma property. We acknowledge helpful scientific discussions with Carson

Richardson, Mark Barton, Mihai Ducea, and Steve Shaver. We thank Roger and Nancy

Johnson, nearby ranchers, for their tremendous hospitality, advice, knowledge, and assistance during field work. Jojo and Annemarie Wallenberg provided valued company and assistance during the field season. This work is a part of a M.S. thesis study at the

University of Arizona. We appreciate reviews from Bob Thomas and Fleet Koutz.

Table of Contents ABSTRACT ...... 7 INTRODUCTION ...... 9

PREVIOUS WORK AND EXPLORATION HISTORY ...... 11 REGIONAL TECTONICS AND MAGMATISM ...... 13 GEOLOGIC FRAMEWORK OF THE SONOMA RANGE AND ENVIORNS ...... 16 STRUCTURE ...... 18 MINERAL DEPOSITS ...... 21 METHODS ...... 24

GEOLOGIC MAPPING ...... 24 PETROGRAPHY AND ELECTRON MICROPROBE ANALYSIS ...... 24 WHOLE-ROCK GEOCHEMISTRY AND ASSAYS ...... 24 ROCK TYPES ...... 25

OVERVIEW OF THE GEOLOGY OF GREGG CANYON ...... 25 STRUCTURE ...... 31

PRE-ORE STRUCTURES ...... 31 POST-ORE STRUCTURES ...... 31 HYPOGENE VEINS AND ALTERATION-MINERALIZATION ...... 32

QUARTZ ± K-FELDSPAR AND BIOTITE VEINS AND ASSOCIATED POTASSIC ALTERATION ...... 33 MUSCOVITE ± QUARTZ VEINS WITH GREISEN MUSCOVITE ALTERATION ...... 33 BARREN QUARTZ VEINS ...... 34 QUARTZ-SERICITE-PYRITE VEINS AND ASSOCIATED SERICITIC AND INTERMEDIATE ARGILLIC ALTERATION ...... 34 CHLORITE AND WEATHERED SULFIDE VEINS AND VEINLETS ...... 34 HYDROTHERMAL BRECCIAS AND MASSIVE HYDROTHERMAL QUARTZ ...... 35 SKARN AND HORNFELS ...... 36 HYDROTHERMAL MINERAL COMPOSITIONS ...... 37 SUPERGENE ALTERATION-MINERALIZATION...... 38

OVERVIEW ...... 38 LEACHED CAPPING ...... 38 CHALCOCITE ...... 38 DISTRIBUTION OF METAL GRADES AND TRACE ELEMENT CONTENTS ...... 38 INTERPRETATIONS ...... 40

SPATIAL RELATION AND TIMING OF FAULTING ...... 40 ECONOMIC RESOURCES AND EXPLORATION POTENTIAL OF THE SONOMA PROSPECT ...... 40 4

DISCUSSION ...... 41

COMPARISON TO OTHER QUARTZ MONZONITIC-GRANITIC PORPHYRY MO-CU SYSTEMS IN THE GREAT BASIN . 41 CONCLUSIONS ...... 42 FIGURE CAPTIONS ...... 44 FIGURES...... 46 TABLES ...... 59 APPENDICES ...... 70 REFERENCES ...... 95

Gregg Canyon intrusive center, Sonoma Range, Nevada: Magmatism and porphyry-style alteration-mineralization

Alexandra Leigh Wallenberg* and Eric Seedorff

Lowell Institute for Mineral Resources, Department of Geosciences University of Arizona, 1040 East Fourth Street Tucson, Arizona 85721-0077 USA

*Corresponding author: e-mail, [email protected]

Published as:

Wallenberg, A. L. and Seedorrf, E., 2020, Gregg Canyon intrusive center, Sonoma

Range, Nevada: magmatism and porphyry-style alteration-mineralization, in

Koutz, F.R., and Pennell, W. M., eds.: Vision for discovery: Geology and Ore

Deposits of the Basin and Range, Reno, Nevada, in press.

Abstract The Gregg Canyon intrusive center is a Cretaceous quartz monzonitic to granitic

porphyry Mo-Cu system located southeast of Winnemucca on the eastern side of the

Sonoma Range, southwest of the Getchell trend. The area was explored for

molybdenum during the 1960s and 1970s and subsequently for gold. This study builds

on previous work and reports results of field mapping, petrography, whole-rock

geochemical analyses, and compositions of igneous and hydrothermal minerals.

Five Mesozoic intrusive centers, including Gregg Canyon, and three regional

thrust faults crop out in the Sonoma Range, two of which crop out at Gregg Canyon.

Whether mineralized or barren, none of the intrusions has geometries that suggest

apparent control by the older thrust faults. Eugeoclinal rocks of the Ordovician Valmy

Formation overlie the transitional facies rocks of the Cambrian Osgood Mountains

Quartzite and Preble Formation along the Roberts Mountains thrust. In turn, those rocks

are overlain by upper Paleozoic eugeoclinal rocks of the Havallah Sequence along the

Golconda thrust. The Gregg Canyon intrusive center occurs near the confluence of the

Roberts Mountains and Golconda thrusts and consists of six major intrusive phases.

The earliest and largest intrusion (107 Ma K-Ar biotite, 94.1 Ma U-Pb zircon) is a barren, equigranular biotite-hornblende granodiorite (~67% SiO2) that hosts younger

granodiorite porphyry (~70% SiO2) phases. Other Cretaceous intrusions include quartz

monzonite porphyries (~74% SiO2), feldspar porphyries, granite porphyry stocks at

depth, and biotite latite porphyries. The normative mineralogy based on chemical

analyses of fresh rocks confirms that the rocks are part of a metaluminous suite. The

attitudes of Cenozoic volcanic rocks and other geologic features suggest that Gregg

Canyon system is largely structurally intact and is not significantly tilted.

A skarn prospect in the southern part of the study area occurs in the Preble

Formation near the contact with granodiorite porphyry. Pre-Cenozoic rocks locally are overlain by mid-Miocene volcanic rocks that form a field of rhyolitic flow-dome complexes whose pyroclastic aprons rest on a subhorizontal erosion surface. A swarm of barren quartz latite porphyry dikes probably is related to the mid-Miocene extrusive rocks. A NNE-trending, high-angle normal fault, the Barrel Spring fault, offsets those volcanic rocks by ~550 m.

Previous exploration at Gregg Canyon outlined a small potential resource containing approximately 34 million tonnes averaging 0.073% Mo. Some of the best grades are associated with breccia pipes that contain massive bodies of hydrothermal quartz. This hydrothermal system exhibits various types of hypogene veins, many of which are gently dipping. The veins include secondary K-feldspar and biotite, greisen muscovite, sericitic and intermediate argillic alteration, and sulfide veins without alteration envelopes. In sericitic alteration, K-feldspar, biotite, and plagioclase are altered to sericite. In intermediate argillic alteration, K-feldspar remains unaltered, but plagioclase is altered to sericite, and biotite may be altered to chlorite or sericite. Gregg

Canyon shares geological similarities to other Cretaceous quartz monzonitic to granitic porphyry Mo-Cu systems in the Great Basin such as Hall (Nevada Moly), Buckingham, and Monte Cristo.

Keywords: Porphyry molybdenum, hydrothermal alteration, geochemistry, exploration,

Sonoma Range, Nevada.

Introduction

The porphyry deposit type contributes most to the world supply of molybdenum, and the porphyry molybdenum and porphyry copper classes both contribute significantly. Westra and Keith (1981), White et al. (1981), Theodore and Menzie

(1984), Shaver (1984a, 1991), Seedorff et al. (2005), Ludington and Plumlee (2009),

Ludington et al. (2009), and Taylor et al. (2012) review the geology of porphyry molybdenum systems. Porphyry molybdenum deposits are spatially and genetically associated with felsic porphyritic intrusions of various compositions. In the older literature, they tended to be classified into two major types (Woodcock and Hollister,

1978; White et al., 1981; Shaver, 1984b), whereas Seedorff et al. (2005) recognized six subclasses of porphyry molybdenum deposits.

Porphyry molybdenum deposits are particularly abundant in western North

America, occurring from northern Alaska to northwestern Mexico (Sinclair, 1995; Carten et al., 1993; Nickerson and Seedorff, 2016). Important examples in the Great Basin include Buckingham), Hall (Nevada Moly), Monte Cristo, Mt. Hope, Spruce Mountain, and Pine Grove, and there are additional less important occurrences, most of which are either Cretaceous or mid-Cenozoic (Barton, 1990; Seedorff, 1991). Buckingham, Hall

(Nevada Moly), and Monte Cristo, all located in central to northern Nevada (Fig. 1), belong to the quartz monzonitic-granitic porphyry Mo-Cu subclass of Seedorff et al.

(2005) and are Cretaceous (Sonnevil, 1979; Shaver, 1984a, 1988, 1991; Putney, 1985;

Shaver and McWilliams, 1987; Theodore et al., 1992; Mears et al., 2000; Keeler, 2010; 9

Seedorff et al., 2015). In contrast, the mid-Cenozoic deposits Mount Hope and Spruce

Mountain, located in central and eastern Nevada, respectively, and Pine Grove, in

western Utah, are members of the rhyolitic porphyry Mo subclass (Keith et al., 1986;

Keith and Shanks, 1988; Westra and Riedell, 1996; Rivera, 2008; Pape et al., 2016;

Stegen, 2016). This subclass includes Climax and Henderson, Colorado (Wallace et al.,

1968; Bookstrom et al., 1988; Carten et al., 1988; Seedorff and Einaudi, 2004).

Gregg Canyon, originally known as the Sonoma property, is a quartz monzonitic-

granitic porphyry Mo-Cu deposit similar to the Buckingham, Hall (Nevada Moly), and

Monte Cristo deposits. Gregg Canyon is located about 30 km southeast of Winnemucca

in the southeastern corner of Humboldt County, Nevada (Fig. 1). The Gregg Canyon

intrusive center crops out on the eastern side of the Sonoma Range, from Gregg

Canyon northward to Granite Canyon. The deposit technically is in the southern part of

the Gold Run (Adelaide) district (Willden, 1964; Tingley, 1992), but Gregg Canyon is

genetically unrelated to the mid-Miocene low-sulfidation-type epithermal Ag-Au veins for which the district is best known (Silberman et al., 1973). Gregg Canyon lies southwest of the Getchell trend (McLachlan et al., 2000) and contains similar lithologies, such as the Cambrian Preble Formation and Cretaceous intrusions. It is thus not surprising that the Gregg Canyon area also has been explored for gold.

The geology of the Sonoma Range is complex, with segments of three regional reverse fault systems, at least five intrusions of Jurassic and Cretaceous ages,

Cenozoic volcanic rocks, and post-mid-Miocene normal faults. This study reports results of field mapping and petrography and documents whole-rock geochemical analyses and compositions of igneous and hydrothermal minerals at Gregg Canyon. We claim that

map patterns provide no evidence for structural control of the intrusive center by pre-ore faults and that the system is structurally intact and is not significantly tilted by post-ore faults. This small porphyry molybdenum deposit exhibits various types of hypogene veins, including ones that exhibit secondary K-feldspar and greisen muscovite, and the deposit contains several breccia pipes. Higher grade examples of this subclass of porphyry molybdenum deposit exhibit greater abundances of quartz veins than are observed at the exposed levels of Gregg Canyon. Hypogene copper mineralization, largely as chalcopyrite, have been redistributed during formation of supergene leached capping and an incipient underlying chalcocite blanket.

Previous Work and Exploration History

Ferguson et al. (1951), Gilluly (1967) Stahl (1987a, 1987b, 1989) have mapped and described the geology of the Sonoma Range, and igneous rocks have been dated by Silberman and McKee (1971) and McKee et al. (1971). A reconnaissance investigation focusing on Miocene volcanic rocks of the northern Sonoma Range by

Richardson (2019) and Richardson et al. (2019) addressed the magnitude of potential

Cenozoic tilting of the range associated with normal faulting. More detailed geologic investigations of the Gregg Canyon area were accomplished by geologists conducting mineral exploration. Bonham et al. (1985) noted exploration for molybdenum, and

Wendt and Albino (1992) classify Gregg Canyon as a calc-alkaline molybdenum occurrence.

Wood and Rosta (1976) and Wood and Huyck (1977) summarize the exploration history of Gregg Canyon. The Cerro Corporation explored Gregg Canyon for molybdenum from 1969 to 1971. Cerro Corporation drilled 149 air-trac holes averaging

21 m in depth in 1969. Promising results from this drilling and surface rock chip

sampling led Cerro to drill eight holes totaling 2383 m of core from 1970 to 1971. The

property was acquired by Galli Exploration Inc. in 1973, and they collected additional

surface rock chip samples in 1974. Freeport Exploration Co. (1975) and International

Minerals and Chemical Corporation (1976) also made brief examinations. Through a

joint venture with Galli Exploration, AMAX, Inc. conducted Induced Polarization (IP)

surveys, sampling, and geologic mapping in 1976. In 1977, AMAX drilled 529 m of core

in two drill holes. One of the holes was in the best surface geochemical anomaly, and

the average grade of the mineralized interval was 0.12% eMoS2 (0.07% Mo); a second

hole that targeted an IP anomaly was unmineralized (Wood and Huyck, 1977).

AMAX Inc. and Climax Molybdenum Company were major producers of

molybdenite concentrates and frequently expressed molybdenum assays as %MoS2,

instead of %Mo. The conversions are approximately 1% MoS2 = 0.6% Mo; conversely,

1% Mo = 1.67% MoS2. The implicit assumption is that all of the Mo is present as the

sulfide molybdenite instead of in the tungstate solid solution series powellite (CaMoO4)

– scheelite (CaWO4) or end-member powellite. Hence, AMAX and Climax commonly

used “eMoS2” to underscore that this conversion has that underlying assumption (J.D.

Wood, written commun., 2020). The hypogene Cu grade at Gregg Canyon typically is

low (100-300 ppm Cu or 0.01-0.03% Cu).

Mo and Cu values from Cerro’s air-trac drilling were contoured, along with Cerro

(600) and AMAX (45) rock-chip samples. This exercise revealed three geochemical

anomalies that have Mo-rich cores and more extensive Cu anomalies, each of which is

at least partially coextensive with breccia pipes. The best anomaly is about one

kilometer in diameter, with a core with over 400 ppm Mo surrounded by a fringe containing 300 ppm Cu.

Regional Tectonics and Magmatism

Rifting of the supercontinent of Rodinia formed a new continental margin in the late Precambrian, with a miogeocline to the east and a transition to a eugeocline farther west (Poole et al., 1992). By the early Cambrian, the Laurentian paleogeography included passive margin sequences and the formation of a transcontinental arch. This simple paleogeography became more complex on the northeastern margin of Laurentia during the Ordovician, in which Caledonian tectonism resulted in the southward transport of off-shelf assemblages by transform faults, exposing basement rocks outboard of the Cordilleran margin (Gehrels and Pecha, 2014).

Regional shortening in the Late Devonian-Early Mississippian associated with the

Antler orogeny (Fig. 1) folded lower Paleozoic strata of the eugeocline, consisting of shales, cherts, and mafic volcanic sequences, and emplaced them eastward over carbonate and transitional assemblages of the miogeocline along the Roberts

Mountains thrust (Roberts et al., 1958; Poole, 1974; Miller et al., 1992; Gehrels et al.,

2000). The Roberts Mountains thrust cut gently up section to the east, such that the

Ordovician rocks of the transitional facies are exposed in the immediate footwall in westernmost exposures of the fault in central Nevada west of Battle Mountain. Higher stratigraphic levels are exposed in the immediate footwall farther to the east, reaching

Upper Devonian rocks in easternmost exposures of the thrust in the vicinity of Carlin and Eureka, Nevada (Fig. 8 of Reynolds, 1977). The angle of discordancy between the

thrust fault and bedding in its lower plate is small, ranging from 0 to 15° (Fig. 9 of

Reynolds, 1977), presumably reflecting the angle of the west-dipping thrust ramp.

During the late Paleozoic, erosion of the Antler highlands deposited strata along the eastern margin of the allochthon in the foreland basin, and rocks of the Antler overlap sequence were deposited across the Roberts Mountains thrust (Roberts et al.,

1958; Miller et al., 1992; Gehrels and Dickinson, 1995). Deformation continued during the Pennsylvanian within the Ancestral Rocky Mountains and extended westward, affecting parts of the Antler foreland (Kluth and Coney, 1981; Dickinson, 2006). During the Sonoma orogeny of the Late Permian to Early Triassic (Fig. 1), defined originally by

Muller et al. (1951), eugeoclinal strata again were thrust eastward onto the continental shelf, this time over rocks of the Antler overlap sequence (Silberling, 1975; Miller et al,

1982). The Golconda allochthon contains upper Paleozoic eugeoclinal strata of marine, volcanic, and clastic assemblages of the Havallah sequence, including mafic volcanic rocks formerly referred to as the Pumpernickel Formation.

Periods of crustal shortening also occurred during the Mesozoic, albeit with local periods of extension and translation and a recent increase in controversy over the tectonic drivers (Dickinson, 2006; Hildebrand, 2013; Sigloch and Mihalynuk, 2017;

Pavlis et al., 2019). Arc magmatism initiated during the Permian and continued into the

Triassic (Kistler, 1974; Barton, 1996). Jurassic magmas are particularly diverse compositionally (Barton et al., 2011). Major periods of shortening during the Jurassic

(Fig. 1) included the east-vergent Luning-Fencemaker thrust belt in western Nevada

and the west-vergent Willow Creek thrust farther east, in and near the Sonoma Range

(Oldow, 1984; Stahl, 1987a; Ellison and Speed, 1989).

Additional periods of crustal shortening occurred during the Cretaceous but

farther to the east (Fig. 1) in the Central Nevada (Eureka) and Sevier fold-and-thrust belts (DeCelles, 2004; Dickinson, 2006; Long et al., 2014). Magmatism in the Great

Basin became more intense and more felsic during the Cretaceous (Barton, 1990,

1996). As the Farallon plate shallowed beneath the continental lithosphere, the Sierra

Nevada arc shut down toward the end of the Cretaceous, and magmatism jumped inboard during the Late Cretaceous, continuing into the Paleogene, broadly concurrent

with development of basement-cored uplifts during the Laramide orogeny (Dickinson

and Snyder, 1978; Jacobson et al., 2017; Seedorff et al., 2019).

Magmatism resumed in the Great Basin in the late Eocene, at first migrating

rapidly southward in the Eocene, stagnating during the ignimbrite flare up of the

Oligocene, then migrating gradually to the southwest during the early Miocene,

becoming more sparse thereafter except along the proto-Cascade arc and the trace of

the Yellowstone hot spot (Stewart and Carlson, 1976a; Wernicke et al., 1987; Seedorff,

1991; Dickinson, 2006). Extension, especially in the Eocene through Miocene,

contributed to exhumation of mid-crustal rocks in Cordilleran metamorphic core

complexes (Dickinson, 2002). The complexity of Cenozoic magmatic migration in the

Great Basin has been attributed to slab rupture and/or flexure from slab rollback;

moreover, a strike-slip margin was initiated at about 25 Ma with birth of the San

Andreas fault (Dickinson, 2006). As the Mendocino triple junction migrated northward

with time, some of the relative motion between the Pacific and North American plates was taken up farther to the east, such as in the Walker Lane (Dickinson, 2006, 2013).

Geologic Framework of the Sonoma Range and Enviorns

The geology of the Sonoma Range and its geologic units are described by

Ferguson et al. (1951), Silberling (1975), Gilluly (1967), Stahl (1987a, b, 1989) and

(Ketner, 2008). A simplified map of the range is shown as Figure 2. The range contains numerous geologic units and many superimposed geologic events, resulting in complicated geologic relations.

Paleozoic Strata

The Cambrian Osgood Mountain Quartzite and overlying Preble Formation represent the lower Paleozoic transitional facies rocks that occur in the footwall of the mid-Paleozoic Roberts Mountains thrust on the eastern side of the range. The

Ordovician Valmy Formation is widespread, occurring in the Roberts Mountains allochthon and in the hanging wall of the Jurassic Willow Canyon thrust. The Upper

Devonian to Mississippian Harmony Formation occurs in both the hanging wall and footwall of the Willow Canyon thrust in the northern and western parts of the range.

Various lithologies of the Pennsylvanian-Permian Havallah sequence, including basaltic rocks formerly assigned to the Pumpernickel Formation, occur in the hanging wall of the

Golconda thrust.

Mesozoic Strata

The footwall of the Jurassic Willow Canyon thrust contains the Harmony

Formation and Mesozoic strata (Fig. 2). The Mesozoic stratigraphy in the Sonoma

Range includes a myriad of carbonate and siliceous units, including the Triassic-age

China Mountain, Prida, Panther Canyon, Natchez Pass, and Augusta Mountain

Formations. Dark-gray to black shales, slate, quartzites, limestones, and cross-bedded dolomites of the Upper Triassic Grass Valley, Dun Glen, Winnemucca, and Raspberry

Formations occur higher in the section.

Mesozoic Intrusions

There are five intrusive centers with areas of >1 km2 in the Sonoma Range (Fig.

2) The center at Gregg Canyon, which in most places is hydrothermally altered, was

dated at 107 ± 2 Ma (K-Ar biotite) by Silberman and McKee (1971; recalculated from

104 ± 2 Ma using Dalrymple, 1979). Stony Basin is another felsic center with

hydrothermally altered rocks; it is located about 5 km northwest of Gregg Canyon and is

also assumed to be Cretaceous (Stewart and Carlson, 1976b). The Barrel Spring

pluton, located about 5 km southwest of Gregg Canyon, has not been dated (Stewart

and Carlson, 1976b). The Grand Trunk pluton, located 5 km further to the southwest, is

Jurassic (Silberling, 1975; 155 ± 3 Ma, K-Ar biotite; 168 ± 3 Ma, K-Ar hornblende), as confirmed by a recent U-Pb zircon age of about 160 Ma (S. J. Wyld, oral commun.,

2019). The Clear Creek pluton, located about 5 km northwest of the Grand Trunk pluton or about 10 km east of the Barrel Spring pluton, is also presumed to be Jurassic

(Stewart and Carlson, 1976b; Johnson, 1977).

All but the Grand Trunk center crop out as fairly equant plutons, albeit with irregular contacts (Fig. 2). The Grand Trunk center, though covered to the west, seems to have a northeasterly elongation.

Cenozoic Volcanic Rocks

The Sonoma Range contains widespread exposures of Cenozoic rhyolitic

volcanic rocks as lava flows, tuffs, and dikes (Ferguson et al., 1951; Gilluly, 1967).

There are small exposures of Cenozoic andesite and basalt in the northern part of the

range (Ferguson et al., 1951).

The rhyolitic volcanic rocks in the northern part of the range were observed in

several days of reconnaissance in the area in 2016 (C.A. Richardson and E. Seedorff,

unpub. data), which constitute a field of rhyolitic flow-dome complexes and associated felsic lava flows in the northern and western parts of the range. Rhyolitic lavas, which form prominent steep bluffs, were deposited on fall and local surge deposits of unwelded tuff, which in turn rest on a subhorizontal erosion surface on pre-Cenozoic rocks. The rhyolites are dated at 15.7 ± 0.8 Ma (McKee et al., 1971; recalculated from

15.3 ± 0.8 Ma, using Dalrymple, 1979; see also Gilluly, 1967).

Structure

Thrust Faults Produced by Paleozoic and Mesozoic Shortening

The Sonoma Range contains the traces of three regional thrust faults (the

Roberts Mountains, Golconda, and Willow Creek thrusts; produced during three

different orogenies, ranging in age from Late Devonian to Jurassic. Ferguson et al.

(1951) mapped a contact in the northeastern part of the range between the Sonoma

Range Formation, which was reinterpreted by Gilluly (1967) and subsequent workers as

the Valmy Formation, on the west with the Preble Formation on the east as a west-

dipping thrust fault. Gilluly (1967) interpreted the Valmy-Preble contact in the

northwestern part of the range (Fig. 2) as a steeply west-dipping normal fault, the

Adelaide fault. Stahl (1987b) fit a plane to fault traces of the Adelaide fault that suggests that it dips 20°W, at least locally, and he called it the Roberts Mountains thrust. It is likely that the fault is the gently west-dipping Roberts Mountains thrust fault, perhaps locally offset by one or more steeply west-dipping normal faults that correspond to the

Adelaide fault of Gilluly (1967). From Gregg Canyon northward (Fig. 2), the Roberts

Mountains thrust fault placed Ordovician Valmy Formation over transitional facies rocks of the Cambrian Osgood Mountains Quartzite and Preble Formation during the Antler orogeny. This might be one of the deepest exposures of the Roberts Mountains thrust in northern Nevada (cf., Fig. 8 of Reynolds, 1977).

Rocks of both the footwall and hanging wall of the Roberts Mountains thrust were cut off at high angles by the Golconda thrust during the Sonoma orogeny and overridden by upper Paleozoic eugeoclinal rocks of the Havallah sequence (Silberling,

1975; Stahl, 1987b). This portion of the Golconda thrust dips to the southeast and can be traced from Gregg Canyon southwesterly to Grand Trunk Canyon (Fig. 2). The intersection of the two faults projects into the Gregg Canyon intrusive center at the current level of exposure and rakes southwesterly at depth.

Crustal shortening in the Sonoma Range occurred again along the Willow

Canyon thrust, which crops out in the northwestern part of the Sonoma Range. The

Willow Canyon is a west-vergent Jurassic thrust system, broadly coeval with east- vergent Luning-Fencemaker thrust system. The footwall consists of autochthonous

Harmony Formation that lacks west-vergent folds and crops out on the northeastern and northwestern parts of the range (Stahl, 1989), as well as Permian and Triassic strata

that are exposed farther south on the western edge of the range and in a window a few

kilometers to the east (Fig. 2). The hanging wall consists of allochthonous Harmony

Formation with regional-scale, west-vergent folds and Ordovician Valmy Formation

(Stahl, 1989).

The various thrust faults are shown in Figure 2, and Figure 3 is a simplified

structural column that shows the temporal relationships between sedimentation, thrust

faults, and volcanism. Gregg Canyon and four other Mesozoic intrusive centers crop out

in the central part of the range, and their geometries show no apparent control by any of

the older thrust faults.

Normal Faults Produced by Cenozoic Extension

The Sonoma Range lacks prominent young fault scarps on either sides of the

range (Ferguson et al., 1951). Fault scarps related to the 1915 Pleasant Valley

earthquake, however, crop out a few tens of kilometers south of Gregg Canyon,

marking the northern end of the central Nevada seismic belt (Wallace, 1984).

Wernicke et al. (1987) hypothesized that the Sonoma Range may have been

subjected to large-magnitude extension since the mid-Miocene because Gilluly (1967)

recorded steep dips, though of variable attitude, on rhyolitic tuffs in the northern part of

the range. The authors, however, interpret the attitudes of the tuffs as primary features

in rocks comprising the pyroclastic aprons underlying associated lavas of the flow-dome complexes: bedding in the fall deposits mantles the irregular underlying topography, and cross beds occur in surge deposits (Richardson, 2019; Richardson et al., 2019). It is the authors’ opinion that the steep attitudes are not the result of tilting. West-dipping normal

faults do locally place rhyolitic lavas directly against pre-Cenozoic rocks, such as the

Barrel Springs fault west of Gregg Canyon. These faults have tilted the base of the lavas since they were erupted at about 16 Ma, but only about 5° eastward (Ferguson et al., 1951). Likewise, the attitudes of older rocks and structures (Ferguson et al., 1951;

Gilluly, 1967) suggest that it is unlikely that there was significant extension in the

Sonoma Range prior to 16 Ma.

The implication of these relations is that Mesozoic intrusions and associated hydrothermal systems should have remained nearly upright, in contrast to many others elsewhere in the Basin and Range province that are significantly tilted by post-mineral extension (e.g., Wilkins and Heidrick, 1995; Seedorff et al., 2005, p. 276-277).

Mineral Deposits

Mineral deposits on the eastern side of the Sonoma Range vary in deposit type(s), age, and amount of prior production. Commodities include gold, silver, copper, molybdenum, manganese, and lead from various mines, the most prominent being from the Gold Run (Adelaide) district.

The Gold Run (Adelaide) district is located at the foot of the eastern slope of the range around Gold Run Creek, extending about 13 km north-south from Rock Creek on the north to Gregg Canyon on the south (Vanderburg, 1938; Ferguson et al., 1951;

Willden, 1964; Tingley, 1992). The district consists principally of the Adelaide and

Crown (Adelaide Crown) mines that produced mainly between 1897 and 1910. A small amount of placer production was also recorded (Bonham et al., 1985). The recorded output from 1907-1936 was $567,402 (Vanderburg, 1938); its current value would be

about $10 million (CPI Inflation Calculator, 2019). Limestones, quartzites, and shales of

the Preble Formation are the primary host rocks in both mines.

The Adelaide mine is located near the fault separating the Valmy Formation on

the west from Preble Formation on the east and has the most historical production, as

Au, Ag, Cu, and Pb. The principal opaque minerals are pyrrhotite, chalcopyrite,

sphalerite, and galena. Descriptions suggest that the ore was hosted by steep east- dipping beds of limestone and calcareous phyllite, probably principally as Cu-Zn skarns containing garnet, vesuvianite, and pyroxene (Ransome, 1909). The nearby

Cumberland mine is a Pb-Zn-Ag skarn. The age of these deposits is not known. Though there are no intrusions shown on published maps, the geology suggests that the source of fluids to form calc-silicate and ore minerals is an intrusion, presumably at depth, which regional maps suggest could be Jurassic, Cretaceous, or even Eocene.

The Crown (Adelaide Crown) is located about 3 km to the east. The ore is hosted

by silicified, brecciated, pyritic fault zones (mostly weathered to limonite) that are 3 to 25

m wide (Vanderburg, 1938). These constitute low-sulfidation-type epithermal quartz- adularia veins, and the Crown vein dips approximately 70° west. Combined production from open pit and underground workings from about 1920 into the early 1940s was

19,000 oz Au and 345,000 oz Ag (Ag/Au = 18) (Abrams, 2011). A small open pit south of the Crown mine produced another 4917 oz Au and 53,474 oz Ag (Ag/Au = 11) from

1988 to 1991 (Abrams, 2011). The area around the Crown mine has been explored since 1980 by at least four companies, who have drilled >200 holes. A structural zone with strong continuity of high-grade intercepts persists for at least 275 m along strike

(Abrams, 2011). The Crown (Adelaide-Crown) mine has a K-Ar adularia age of 14.7 ±

0.4 Ma (Silberman et al., 1973; recalculated from 14.3 ± 0.4 Ma using Dalrymple, 1979).

Although the local geology does not require it, these veins may have projected upward into overlying, now-eroded, volcanic rocks. The age of adularia from the vein is 1.0 m.y. younger than the one date on a rhyolite lava in the district (McKee et al., 1971).

The Gregg Canyon intrusive center, the subject of this study, crops out in Gregg

Canyon and extends northward into Granite Canyon. Technically it is located in the

southern part of the Gold Run (Adelaide) district (Tingley, 1992) but is located 8 – 10 km

south of the Adelaide and Crown mines (e.g., Wood and Huyck, 1977). Gregg Canyon

is a Cretaceous quartz monzonitic to granitic porphyry Mo-Cu system, which also has a

small amount of mineralized skarn on its margin.

The Black Diablo mine is located another 5 km south of Gregg Canyon on the

eastern side of the Sonoma Range, straddling the Humboldt-Pershing County line,

which is usually assigned to the Black Diablo district (Tingley, 1992). It produced

approximately 57,281 tons of manganese (braunite) ore associated with lens-shaped

bodies of jasper. Ore is associated with argillites, cherts, and greenstones that were

once assigned to the Pumpernickel Formation (Muller et al., 1951) but are now

assigned to the Havallah sequence. Descriptions suggest that the manganese may

have a distal exhalative origin, formed on the upper Paleozoic sea floor during

submarine mafic volcanic eruptions.

Methods

Geologic Mapping

Gregg Canyon and adjacent areas of the Sonoma property, 15 km2, were

mapped in May and June 2019, at a 1:5000 scale utilizing the Anaconda method

(Brimhall et al., 2006), with layers for rock type, structure, and alteration–mineralization.

The Anaconda mapping system was adapted for the geologic mapping in Gregg

Canyon by plotting vein orientations, size, mineralogy, and alteration envelopes. In

addition, recording the density of veins within an outcrop involved measuring the width

and average spacing of a vein type, for example, 0.7/35 cm reports a 0.7-cm-wide vein

spaced 35 cm apart. The resultant density then for that vein is 2%, reported as volume

percent.

Petrography and Electron Microprobe Analysis

Polished thin sections of samples of unaltered and altered rocks and distinct vein

types were examined using a Nikon petrographic microscope assisted in plane

polarized, cross-polarized, and reflected light. Igneous and hydrothermal minerals were

analyzed using a Cameca SX100 electron microprobe at the Michael J. Drake Electron

Microprobe Laboratory at the University of Arizona.

Whole-rock Geochemistry and Assays

Samples selected for whole-rock geochemical analysis included unaltered and

altered rocks from each unit with identifiable mineralization on the surface, in addition to

specific vein types. After cutting, polishing, and cleaning 20-50 g of each sample, ALS

geochemical laboratory analyzed the abundance of major oxides and trace elements in

each sample. Additional geochemical data generated during prior exploration programs were also used in this study, along with historical geology and metallurgical reports.

Data used here include information from all prior operators mentioned above in the

Previous Work and Exploration History section. Historic data were made available courtesy of Odin Christensen, Clancy Wendt, and Ralph Stegen and Freeport

Exploration, 2019).

Rock Types

Overview of the Geology of Gregg Canyon

Figure 4 is a geologic map of the Gregg Canyon area, showing rock type and structure. The Paleozoic lithologies consist primarily of transitional facies autochthonous units (Preble Formation) and overriding allochthonous eugeoclinal units (Valmy

Formation) that were transported eastward along the Roberts Mountains thrust during the Antler orogeny (Roberts et al., 1958; Reynolds, 1977; Gehrels et al., 2000). These rocks were in turn overridden by upper Paleozoic eugeoclinal rocks during the Sonoma orogeny along the Golconda thrust (Roberts et al., 1958; Gehrels and Dickinson, 1995).

Mesozoic intrusions at Gregg Canyon postdate major crustal shortening within the study area and vary in composition, mineralogy, and alteration-mineralization. Two areas located in the northeastern and southwestern portions of the map possess skarn alteration and contain the highest levels of Cu and Zn mineralization. These two areas are associated with two small, mineralized. Normal faults within the region, such as the

Barrel Spring Fault on the western edge of the property, show substantial displacement

(over 500 m).

Paleozoic strata

The Paleozoic units observed in the study area include the Preble Formation

(footwall of the RMT), Valmy Formation (hanging wall of RMT), the Pennsylvanian-

Permian Havallah Sequence (hanging wall of GT), including rocks previously assigned

to the Pumpernickel Formation, and the Koipato Formation. These units are

predominantly comprised of shales, slates, quartzite, chert, and interbedded limestones,

but the Koipato Formation includes flow-banded rhyolites, andesites, and breccias that

are volcanic in origin (Ferguson et al., 1951). The limits of the Preble and Havallah are

difficult to differentiate due to poor exposure in the Golconda thrust fault zone; however,

quartzite of the Valmy Formation forms cliffs that are resistant to weathering. The

limestone beds of the Preble Formation and rocks previously assigned to the

Pumpernickel Formation host the copper-oxide-stained skarn outcrops on the fringes of the Gregg Canyon intrusive center.

Intrusive Rocks

There are six igneous intrusive events observed at Gregg Canyon; however, only four contain molybdenum mineralization. Save for the earliest intrusion, each event contributed to the emplacement of veins and alteration of the surrounding host rocks.

Most of these phases are felsic in composition and plot on a simple fractionation trend

(Wood and Huyck, 1977); more differentiated rocks occur in the brecciated zones. Other

intermediate or mafic phases, such as andesite dikes, postdate mineralization (Wood

and Huyck, 1977). Figure 5 shows rock type and alteration cross sections from A to A’

(see Fig. 4 for location), showing the bedrock geology, location of drill holes, and alteration at depth.

The mineralogy of the intrusions consists primarily of plagioclase, K-feldspar, and quartz; the more mafic phases contain hornblende, but some of the others may contain only biotite. Although Gregg Canyon contains vein-related hydrothermal muscovite, there is no primary (igneous) muscovite, no phenocrystic almandine-spessartine garnet, and no other aluminous phases. Hence, the mineralogic evidence suggest that the intrusive rocks are part of a metaluminous suite, rather than a peraluminous suite.

Table 1 shows whole-rock analyses of the least altered samples from Gregg

Canyon for major and trace elements. The main observations of the analyses include silica contents ranging from 67% to 74% SiO2, and rubidium contents extend from ~90 ppm to ~143 ppm Rb, whereas strontium contents range from 253 ppm to 428 ppm Sr.

These analyses indicate that, in spite of the relatively high silica contents, the igneous rocks are not highly evolved, for instance compared to high-silica rhyolites of rhyolitic porphyry Mo deposits such as Climax and Henderson (e.g., White et al., 1981; Keith et al., 1993; Seedorff et al., 2005).

Table 2 shows the normative composition of the three least altered samples. The low normative corundum values (≤0.18) confirm the mineralogic evidence that the rocks are metaluminous.

The earliest intrusion is a Cretaceous biotite-hornblende granodiorite with accompanying granodiorite porphyry phases (~70% SiO2, Kgd and Kgdp in Fig. 4) with feldspar phenocrysts up to 15 mm in length. The granodiorite is the pluton that as dated at 107 ± 2 Ma (K-Ar biotite) by Silberman and McKee (1971; recalculated from 104 ± 2

Ma using Dalrymple, 1979), but preliminary zircon geochronology yielded an age of 94.1

+ 0.4 Ma. Though the main granodiorite is not a mineralizing intrusion, it is a wall rock

for younger, mineralized stocks and dikes. As a result, in most places the granodiorite is

at least weakly hydrothermally altered.

The granodiorite is light gray, equigranular, and contains 5-15% quartz, 5-10% potassium feldspar, 30-40% plagioclase, 0-15% hornblende, and 0-15% biotite. Its SiO2,

content is ~67%, and the K2O, and Na2O contents are approximately 2.95%, and

3.98%, respectively. The main intrusive phase shows no sign of economic molybdenum

mineralization; however, the later porphyritic phases may be intensely altered and

mineralized, with Cu-bearing sulfides, such as chalcopyrite. These later porphyritic

phases are similar in mineralogy and composition to the main pluton but are

distinguished by K-feldspar phenocrysts that are up to 15 mm in length (Wood and

Huyck, 1977).

Biotite quartz monzonite porphyry stocks (Kqmp in Fig. 4) intrude the

granodiorite. The biotite quartz monzonite stocks (~74% SiO2) are medium grained and

have rounded quartz phenocrysts that range from 2 to 5 mm in diameter and comprise

20% of the rock. Additional minerals include 15-30% potassium feldspar, 30-40%

plagioclase, 2-5% hornblende, and 5-15% biotite.

Feldspar-biotite monzonite porphyry dikes occur within two km of the main

intrusive body. This unit is highly sericitized and pyritic in certain outcrops, producing

positive I.P. anomalies. The unit may be purple, yellow, or green and is very fine

grained. The feldspar phenocrysts range from 2-4 mm in length, whereas the less

prominent biotite phenocrysts average 2 mm. The mineral composition includes up to

75% feldspar and 25% mafic minerals, with little to no quartz. This unit composes most

of the matrix material found in the main breccia pipe; however, and the absence of

monzonite porphyry clasts within the breccia unit suggests that its emplacement may

coincide with the development of the breccia (Wood and Huyck, 1977).

Drill logs report a light pink, granite porphyry that contains up to 15% biotite, 15% plagioclase, up to 45% potassium feldspar, and as much as 25% rounded quartz phenocrysts that are similar in morphology to the phenocrysts in the quartz monzonite porphyry. Although the unit does not crop out at the surface, drill holes indicate that the granite porphyry intrudes the main breccia pipe and the quartz monzonite porphyry.

This unit has associated rhyolite and aplite dikes.

A late biotite latite porphyry (Tlp in Figure 4) is a medium gray, partially chloritized rock containing 5% quartz, 10-30% potassium feldspar, 30-40% plagioclase,

and 10-25% variably chloritized biotite. It is the youngest mineralized intrusive phase, is

magnetite-bearing, and commonly contains secondary biotite. It may be highly magnetic

but contributes only minor molybdenite (Wood and Huyck, 1977).

The final intrusions are several quartz latite porphyry dikes (too small to show on

Fig. 4) that contain alkali feldspar phenocrysts up to 40 mm in length. Radiometric

dating indicates that the unit has an age of 15.3 + 0.8 Ma (Parkison, 1976). No

mineralization occurs with these dike swarms (Wood and Huyck, 1977), which may be

cogenetic with the mid-Miocene felsic lava flows that crop out in this part of the Sonoma

Range.

Igneous Mineral Compositions

Electron microprobe analysis identified the compositions of fresh igneous

minerals from the three main intrusive sequences: the granodiorite (Kgd), the quartz

monzonite porphyry (Kqmp), and the granodiorite porphyry (Kgdp). The microprobe

analyses were normalized using methods from Brady and Perkins (2007, 2017). Table 3

outlines the compositions of biotite, feldspar, and amphibole from selected samples of

least altered rocks, with calculated mole fractions of end-member compositions. All

analyses will be reported in appendices of a future M. S. thesis by A. L. Wallenberg.

Compositions of minerals were similar within each of the intrusions, with minor differences between the granodiorite and the quartz monzonite porphyry.

K-feldspars range in composition from Or79-Or96. Plagioclase grains, especially

within the granodiorite porphyry, are compositionally zoned. Compositions range mostly

from oligoclase to sodic andesine (An9-An37; Ab75-Ab52).

Biotite in the main intrusions are intermediate between phlogopite and

oxyphlogopite, where the composition is approximately KFeMg2Si3AlO10(OH)2. The

amphiboles fall between magnesiohornblende and ferritschermakite, depending on the

sample. Magnesiohornblende appears in all three main intrusive sequences; however,

the quartz monzonite porphyry is the only unit identified to contain ferritschermakite.

TiO2 contents of amphibole are 0.16 to 0.98 wt%, and the overall approximate mineral

formulae are Ca2Mg3Fe2Si7AlO22OH2 and Ca2Mg2Fe2Si6Al2O22OH2.

Structure

Pre-ore Structures

The Sonoma Range contains the traces of three regional thrust faults produced during three different orogenies, though exposures of the thrusts are poor in the study area. During the Antler orogeny, eugeoclinal rocks of the Ordovician Valmy Formation overrode the transitional facies rocks of the Cambrian Osgood Mountains Quartzite and

Preble Formation along the Roberts Mountains thrust. Structure contours indicate that in the Sonoma Range the thrust strikes northerly and dips 20° west (Stahl,1987b), and the trace of the thrust extends on the northeastern side of the range from Gregg Canyon northward, though locally offset in the Miocene by the Adelaide normal fault. These rocks were overridden by upper Paleozoic eugeoclinal rocks of the Havallah sequence during the Sonoma orogeny along the Golconda thrust. In the Sonoma Range, the

Golconda thrust strikes northeasterly and dips southeast and can be traced from Gregg

Canyon southwesterly to Grand Trunk Canyon. Further shortening occurred during the

Jurassic along the west-vergent Willow Canyon thrust, which crops out in the

northwestern part of the Sonoma Range.

Post-ore Structures

Pre-Cenozoic rocks locally are overlain by Cenozoic volcanic rocks that form a

mid-Miocene field of rhyolitic flow-dome complexes with subhorizontal pyroclastic

aprons. Several NNE-trending, west-dipping high-angle normal faults may show

substantial displacement. For instance, the Barrel Spring fault on the western portion of

the study area (Fig. 4) has approximately 550 m of post-mid-Miocene displacement.

The Front Range fault occurs near the eastern side of the property parallel to the

Pumpernickel Valley (Fig. 4). Other fault systems within the study area strike N30°W

and N65°E.

The attitudes of Cenozoic volcanic rocks and older features suggest that the

Gregg Canyon system is largely structurally intact and is not significantly tilted.

Evidence, including the fact that the young, steeply dipping faults are predominantly

west-dipping, suggests that rocks of the northern Sonoma Range, including the Gregg

Canyon porphyry molybdenum system probably is very slightly east-tilted, but only by about 5 degrees (Ferguson et al., 1951).

Hypogene Veins and Alteration-Mineralization

Whole-rock analyses of hydrothermally altered samples are presented in Table 4,

for comparison with the unaltered analyses in Table 1.

In porphyry systems, wall-rock alteration generally is strongly associated with

veins and their alteration envelopes. Vein intensity and hydrothermal alteration also

closely relate to the distribution and zonation of metals in porphyry systems (Meyer and

Hemley, 1967; Carten et al., 1988; Seedorff and Einaudi, 2004; Seedorff et al., 2005;

Gruen et al., 2010). In Gregg Canyon, several intrusions produced a multiplicity of types

and generations of veins and associated wall-rock alteration. Field photographs of

representative veins are shown in Figure 6, and the mapped distribution of alteration is

shown in Figure 7. Figure 8 shows stereonets of vein orientations, which shows that

many of the veins dip gently.

Field observation and synthesis of previous work indicate the following styles of

alteration at the surface in intrusive rocks: potassic, greisen muscovite, sericitic and

intermediate argillic, and propylitic in intrusive rocks and skarn in carbonate wall rocks.

Quartz ± K-feldspar and Biotite Veins and Associated Potassic Alteration

Veins of quartz ± K-feldspar with variably developed K-feldspar envelopes crop out at the surface (Fig. 6C), though nowhere in great abundance except around breccia pipes. Silicic alteration envelopes were not observed. These veins generally lack sulfides at the surface (copper oxides are present rarely), but unweathered rock exposed in drill core contain molybdenite ± pyrite ± (chalcopyrite), with one occurrence of fluorite observed (Wood and Huyck, 1977). K-feldspar-rich veins also have been observed. The abundance of quartz ± K-feldspar veins is greater at depth, where molybdenite grades also are higher. Secondary biotite and biotitic veins are most abundant beneath the shells of higher Mo grades. Hairline fracture coatings of molybdenite-specularite occur distally hosted in quartzite (Fig. 6A).

Muscovite ± Quartz Veins with Greisen Muscovite Alteration

Greisen muscovite ± quartz veins are widespread at the surface at Gregg

Canyon (Fig. 6B). The term greisen can refer to an alteration type and to a type of mineral deposit, and there are a variety of definitions (e.g., Shaver, 1991, p. 320-321;

Seedorff, 1988, p. 389-391; Seedorff et al., 2005, p. 263; 2008, p. 941; Runyon et al.,

2019, p. 3). Where referring to an alteration type, we prefer a definition that emphasizes grain size: veins and associated alteration envelopes consisting of coarse-grained (>1 mm) white mica, generally muscovite, and quartz. This definition contrasts greisen with sericitic alteration envelopes, which contain fine-grained (<1 mm) white mica.

Greisen veins commonly contain variably oriented, platy aggregates of

muscovite, accompanied, prior to weathering, by pyrite and perhaps other sulfides and

quartz. The distinction between open-space vein filling and wall-rock alteration envelope

can be difficult to discern, but the veins seem to have an important wall-rock

replacement component (Fig. 6B).

Barren Quartz Veins

Barren quartz veins are common in all geologic units in the study area, and their

widths measure 0.1 to 7 cm (Fig. 9). K-feldspar + quartz veins are cut and offset by

barren quartz veins but in turn are cut and offset by quartz + sericite + pyrite veins.

Quartz-sericite-pyrite Veins and Associated Sericitic and Intermediate Argillic

Alteration

Veins of pyrite ± quartz ± (chalcopyrite) with quartz + sericite + pyrite +

(chalcopyrite) envelopes (Fig. 6D) are common and range from 0.1 to 4 cm in thickness

and are irregular to planar. The abundance of this type of vein is dependent on the host

rock. The volume may approach 2% in the original granodiorite host but increases to

10% in the best mineralized areas of granodiorite porphyry, quartz monzonite porphyry, and monzonite porphyry.

Chlorite and Weathered Sulfide Veins and Veinlets

Chlorite veinlets are sparse, less than one volume percent on outcrops of the

granodiorite, and do not exceed 2 mm in thickness. Chlorite veinlets are also spatially

associated with weathered sulfide veinlets (now iron oxides), 2 mm wide, that lack

alteration envelopes. The latter type of vein constitutes up to five volume percent of an outcrop in all geologic units.

Hydrothermal Breccias and Massive Hydrothermal Quartz

Gregg Canyon contains several breccia pipes, which vary in size, morphology, age, and origin. The breccia pipes are closely associated with the margins of the quartz monzonite porphyry and have a circular morphology around intrusions of the granite porphyry.

The main breccia pipe, located in the north-central portion of the map, is approximately 460 m wide and 1200 m in length and extends vertically over 300 m. The breccia includes subangular to angular fragments of white to light brown quartzite and argillites. Drill logs indicate that fragments of monzonite porphyry occur at depth and that granite porphyries intrude the breccia pipe at depth. This area coincides with where molybdenite veinlets are observed on the surface. In the upper portions, massive hydrothermal quartz cements the breccia; the amount of igneous matrix increases with depth (Wood and Huyck, 1977).

In addition to the main breccia pipe, two small, ovoid breccias, located in the northeastern and southwestern portions of the property, do not exceed 300 m in diameter. These breccias are similar in age, but the genetic relation between them and the main breccia pipe is inconclusive. The breccias in these regions contain limestone, argillite, and quartzite fragments with iron-stained clay and monzonitic matrix. The breccia pipes possess anomalously high Mo/Zn and Mo/Cu values, although molybdenite is not observed at the surface.

Figure 10 shows massive milky white, sometimes iron-stained, quartz veins that

occur along the margins of these breccia pipes. These quartz veins range from 3 cm to

10 m in thickness with no preferred orientation. The milky, bull quartz veins may

constitute 5 to 15% of an outcrop. There is no evidence for economic mineralization

near these outcrops; however, the alteration commonly observed on the surrounding

outcrops is sericitic.

Skarn and Hornfels

Small silver-tungsten-manganese-copper prospects occur distal to the main intrusive center in the northeastern and southeastern portions of the property in skarn and hornfels (Fig. 7). Although both skarns occur near breccias in calcareous units, the two skarns have contrasting mineralogy and metal concentrations.

The southeastern skarn is in the Preble Formation at the contact with the granodiorite porphyry; the skarn covers an area of 300 m by 400 m. It contains garnet, quartz, actinolite, chlorite, muscovite, and minor epidote. Copper oxides appear at the surface.

The northeastern skarn is approximately 400 by 300 m in area and occurs in certain beds in the Pumpernickel Formation of the Havallah Sequence and in the

Koipato Formation, at the contact with the quartz monzonite porphyry near the smallest breccia pipe. This area has higher metal grades than the southeastern skarn, with abundant manganese oxide mineralization (500 ppm Mn) occurring with extensive quartz veins along bedding planes. The mineralogy includes quartz, chlorite, epidote, calcite, and andalusite (?).

Hydrothermal Mineral Compositions

Whole-rock analyses of altered samples are presented in Table 4

The electron microprobe was also used to analyze the compositions of the hydrothermal minerals; sericite, chlorite, epidote, feldspars, and actinolite, and data is provided in Table 5.

The sericite is prevalent in hydrolytically altered intrusions, especially the monzonite porphyry. The sericite may grow as radial growths within the matrix or pseudomorph biotite.

Chloritization of biotite is common in altered assemblages, often interfering with analyses. Normalization involved using seven cations per formula, eleven oxygens, and assuming octahedral sites as full. Epidote occurs within altered granodiorite and in rocks of the Havallah sequence that were formerly assigned to the Pumpernickel

Formation, with a formula of Ca2FeAl2(Si2O7)O(OH). Additionally, appreciable amounts

of Mn are detected from these analyses, ranging from 0.25 to 0.61 wt% MnO.

Skarn in the southeastern portion of the Sonoma property contains actinolite with

a range of compositions, Ca2Mg2.21-2.63Fe1.75-2.43Si8O22OH2. None of the amphiboles,

fresh or altered, possessed a significant amount of fluorine. The garnets in the skarn are

andraditic.

Supergene Alteration-Mineralization

Overview

In Gregg Canyon, supergene alteration varies in intensity and mineralogy depending on the host rock. Igneous hosts, such as quartz monzonite porphyry, produce goethite, jarosite, and rare chalcocite. Oxidized copper minerals are rare overall, but carbonate rocks, such as in the Preble Formation, contain minor copper oxides and limonites.

Leached Capping

The surficial environment is largely devoid of sulfide minerals. Weathering has produced a limonitic leached cap, mostly containing goethite, with lesser jarosite and hematite.

Chalcocite

A few outcrops at the surface contain chalcocite that coats or pseudomorphs pyrite within the intrusion-hosted veins in Gregg Canyon, especially in and near breccia pipes. Drill indicates that chalcocite occurs as coatings on pyrite at the water table level, i.e., an incipient blanket at the base of oxidation, which occurs at depths of less than

100 m (Wood and Huyck, 1977).

Distribution of Metal Grades and Trace Element Contents

Mo and Cu values from Cerro’s air-trac drilling were contoured, along with Cerro

(600) and AMAX (45) rock-chip samples. This exercise revealed three geochemical anomalies that have Mo-rich cores and more extensive Cu anomalies, each of which is at least partially coextensive with breccia pipes. The best anomaly is about one kilometer in diameter, with a core with over 400 ppm Mo surrounded by a fringe

containing 300 ppm Cu (Figs. 11 and 12). Molybdenum and copper distribution at depth

are illustrated along a representative cross section in Figure 13.

The foci for Mo and Cu mineralization are in the central portion of the property

and within or around brecciated areas. Copper anomalies at the surface locally exceed

4,000 ppm, suggesting that significant copper at the surface may be contained in

limonite. In contrast to copper, molybdenum values do not exceed 500 ppm.

This relative grade relation inverts in cross section, as indicated in Figure 13,

where Mo grades increase with depth whereas copper grades decrease with depth. Drill

log data indicates that the surficial depletion in molybdenum ends less than 20 m

beneath the surface, where eMoS2 grades rise to >0.05%. Molybdenite mineralization

decreases distally from the main breccia pipe.

AMAX’s hole S-1, drilled in 1977, passed through three rock types. It was

assayed on 50-ft (15-m) composites for Cu, Mo, and various trace elements and had an

intercept of ~15 m of 0.1% Mo (0.17% eMoS2) near the bottom of the hole (Wood and

Huyck, 1977). The trace elements generally do not show consistent trends down the hole. The composites in and around (mostly above) the intercept thus provide representative values of other elements in this system. Expressed in terms of the mean

(standard deviation) in parts per million for two significant figures, the values are: Mo

100 (230); Cu 130 (120), W 14 (6); F 300 (100); Li 22 (15); Zn 42 (51), and Sn 15 (9).

Interpretations

Spatial Relation and Timing of Faulting

The Gregg Canyon center and four other Mesozoic intrusive centers crop out in the central part of the range, and their geometries show no apparent control by any of the older thrust faults. The Gregg Canyon intrusive center is the only one that crops out across a thrust fault at the present surface, and it occurs where the gently dipping

Golconda thrust cuts off the earlier Roberts Mountains thrust at the current erosion surface. Map patterns provide no evidence for structural control by either of those pre- ore faults.

The only post-ore faults are high-angle faults, of which the Barrell Spring fault

(Figs. 4, 5) has 550 m of displacement. Cenozoic volcanic rocks dip only gently, and there is no other evidence for significant tilting. Thus, the porphyry system is not significantly tilted and is largely structurally intact.

Economic Resources and Exploration Potential of the Sonoma Prospect

Molybdenum and copper concentrations provide the most economic potential of the Sonoma property. Copper grades are low, including in an incipient chalcocite blanket at the depth of the water table. The most widespread, economically significant sulfide in this area is molybdenite, which is abundant at depth and present in multiple intrusive phases.

Four drill holes crudely define a volume containing approximately 34 million metric tonnes (37 million tons) of material with a mean grade of 0.073% Mo (0.12% eMoS2). The geometry of the mineralized zone resembles a portion of an inverted shell

or saucer. Assuming the shape is symmetrical, the entire shell or saucer has potential

for 73 million metric tonnes (80 million tons) of +0.1% eMoS2 within 300 m of the

surface (Wood and Huyck, 1977). The deposit is uneconomic because of its small size

and the depth of overburden.

Discussion

Comparison to Other Quartz Monzonitic-Granitic Porphyry Mo-Cu Systems in the

Great Basin

The pre-mineral intrusion at Gregg Canyon is a granodiorite (67% SiO2), and the mineralizing intrusions are more silicic: granodiorite porphyry, quartz monzonite porphyry, and granite porphyries (>70% SiO2). Silicic intrusions are present at the other

Cretaceous quartz monzonitic-granitic porphyry Mo-Cu systems in the Great Basin,

such as Buckingham (Theodore et al., 1992), Hall (Shaver, 1991), and Monte Cristo

(Sonnevil, 1979; Putney, 1985).

The hydrothermal system at Gregg Canyon contains veins with secondary K- feldspar and biotite envelopes, greisen muscovite, sericitic and intermediate argillic alteration, and sulfide veins without alteration envelopes. The other quartz monzonitic- granitic porphyry Mo-Cu systems in the Great Basin contain most of these vein and alteration types. Hall (Nevada Moly) also contains well developed greisen muscovite

(Shaver, 1991); Buckingham (Loucks and Johnson, 1992) and Monte Cristo (Sonnevil,

1979; Putney, 1985) contain sericitic alteration, without reported greisen; and Gregg

Canyon contains both greisen and sericitic alteration. Both Monte Cristo and Gregg

Canyon contain widespread intermediate argillic alteration, in some cases clearly as selvages on sulfide veins (e.g., Putney, 1985), and in other cases with less clear relation

to veins. 41

Gregg Canyon is largely intact and untilted by post-ore normal faults, but the

other Cretaceous quartz monzonitic-granitic porphyry Mo-Cu systems in the Great

Basin were impacted more by post-ore normal faults. Hall (Nevada Moly) is highly dismembered and significantly tilted (Shaver and McWilliams, 1987); Buckingham

(Keeler and Seedorff, 2007; Keeler, 2010) and Monte Cristo (E. Seedorff, unpub. data) are moderately tilted and dismembered.

Weathering of the upper, copper-bearing parts of the systems produced supergene chalcocite blankets that were mined at Buckingham and Hall (Nevada Moly).

At Buckingham, the largest of the supergene chalcocite deposits was the Contention deposit in the Copper Basin area in the northern Battle Mountain district (Blake, 1992;

Keeler, 2010); but the large molybdenum deposit (Loucks and Johnson, 1992) has yet to be mined. At Hall, the supergene chalcocite blanket was mined in a separate open pit by the Tonopah Copper operation (Mears et al., 2000) from the hypogene molybdenum deposit mined earlier by Anaconda and Cyprus. Gregg Canyon has only an incipiently developed chalcocite blanket located at the present water table.

Conclusions

Gregg Canyon is a center of a metaluminous suite of felsic igneous rocks that produced a weakly mineralized Cretaceous quartz monzonitic to granitic porphyry Mo-

Cu system with similarities to Buckingham, Hall (Nevada Moly), and Monte Cristo. The intrusive center occurs where the gently dipping Golconda thrust cuts off the earlier

Roberts Mountains thrust at the current erosion surface, but map patterns provide no evidence for structural control by either of those pre-ore faults. Although there are

steeply dipping post-ore normal faults, the system is not significantly tilted and is structurally intact.

Grades rarely exceed 0.07% Mo. Four drill holes roughly define a volume containing 34 million metric tonnes of material with an average grade of 0.073% Mo.

The small porphyry molybdenum deposit exhibits various types of hypogene veins, including ones that exhibit secondary K-feldspar and biotite, greisen muscovite alteration, sericitic alteration, and sulfide veins without alteration envelopes, i.e., it is an example of a quartz monzonitic to granitic porphyry Mo-Cu system with both coarse- grained and fine-grained hydrothermal white micas (greisen muscovite and sericite, respectively). The deposit contains several breccia pipes. Quartz vein abundances only locally exceed 10 vol%, and silicic envelopes were not observed. Examples of this subclass of porphyry molybdenum deposit with higher hypogene molybdenum and copper grades exhibit greater abundances of quartz veins than observed at the exposed levels of Gregg Canyon.

Figure Captions Fig. 1. Map of the western United States. showing approximate traces of Luning-

Fencemaker, Golconda, Roberts Mountains and Sevier thrusts, from Dickinson (2006) and distribution and percentage abundance of Mesozoic from Barton et al. (1988).

Fig. 2. Geologic map of the Sonoma Range, with inset showing approximate location in

Nevada. Adapted from Ferguson et al. (1951). Abbreviations: RMT = Roberts Mountains thrust; GT = Golconda thrust; WCT = Willow Canyon thrust. See comments in text.

Some of the print in the explanation boxes is very hard to read.

Fig. 3. Generalized structural column of the Sonoma Range.

Fig. 4. Geologic map of the Sonoma property, including Gregg Canyon, showing structure, rock type, and areas of brecciation.

Fig. 5. Bent cross section from A to A’ with drill hole control, adapted from Wood and

Huyck (1977). Location of section shown in Figure 4. A. Rock types. B. Alteration.

Fig. 6. Field photographs of different vein types in Gregg Canyon, geologic hammer for scale. A. Molybdenite + specularite veinlets in quartzite. B. Large greisen veins in

Kqmp. C. Quartz veins with K-feldspar envelope in Kmp. D. Quartz + oxidized pyrite cubes in Kmp.

Fig. 7. Alteration map of the Sonoma property, mainly in Gregg Canyon. Note that the alteration is patchy and discontinuous. Propylitic alteration is the most extensive.

Fig. 8. Stereonets of specific vein types in Gregg Canyon. A. Quartz + oxidized pyrite + sericite with greisen envelopes. B. Potassium feldspar and quartz with potassium

feldspar envelopes. C. Quartz veins in granodiorite. D. Quartz veins in granite porphyry.

E. Quartz veins in quartz monzonite porphyry. F. Quartz veins in monzonite porphyry.

Note that two populations emerge, showing shallow, east-west orientations and a

northwest orientation. The legend shows Kamb contours in standard deviations.

Fig. 9. Photographs of two shallowly dipping veins in outcrop. A. Quartz vein. B.

Oxidized pyrite + quartz vein.

Fig. 10. Field photographs of bull quartz veins. A. Outcrop scale; at about 10% vein density. B. Close up view, no orientation observed.

Fig. 11. Molybdenum distribution map in Gregg Canyon. Contoured from 764 assayed samples, including rock chip and air-trac holes, compiled from Cerro Corporation and samples from this study. Note that the concentration of molybdenum at the surface is less than that of copper.

Fig. 12. Copper distribution map in Gregg Canyon, with Cu assays in ppm. Contoured from 764 assayed samples, including rock chip and air-trac holes, compiled from Cerro

Corporation and samples from this study.

Fig. 13. Bent cross section of metal grades through Gregg Canyon, approximately east-

west, looking north. Location of section shown in Figures 11 and 12. Adapted from

cross sections from Wood and Rosta (1977), with metal grades contoured from assays

of drill core.

Figures Wallenberg and Seedorff, Fig. 1.

Wallenberg and Seedorff, Fig. 2.

Wallenberg and Seedorff, Fig. 3.

Wallenberg and Seedorff, Fig. 4.

Wallenberg and Seedorff, Fig. 5.

Wallenberg and Seedorff, Fig. 6.

Wallenberg and Seedorff, Fig. 7.

Wallenberg and Seedorff, Fig. 8.

Wallenberg and Seedorff, Fig. 9.

Wallenberg and Seedorff, Fig. 10.

Wallenberg and Seedorff, Fig. 11.

Wallenberg and Seedorff, Fig. 12.

Wallenberg and Seedorff, Fig. 13.

Tables

Table 1. Whole-rock analyses of least altered intrusive rocks.

Table 2. Normative analyses of least altered intrusive rocks.

Table 3. Electron microprobe compositions of igneous minerals.

Table 4. Whole-rock analyses of altered samples.

Table 5. Electron microprobe compositions of hydrothermal minerals.

Table 1. Whole-rock analyses of least altered intrusive rocks Sample no. GC-1 GC-103 GC-105 Rock unit 1Kgd 2Kqmp 3Kgdp Major element (wt% oxides) SiO2 67.1 74.3 70.1 Al2O3 15.00 13.15 14.60 FeO 2.86 1.81 2.39 CaO 3.24 1.43 2.06 MgO 1.73 0.37 0.86 Na2O 3.98 3.48 4.13 K2O 2.95 4.33 3.68 Cr2O3 0.006 <0.002 <0.002 TiO2 0.42 0.20 0.28 MnO 0.06 0.02 0.04 P2O5 0.16 0.09 0.10 SrO 0.05 0.03 0.04 BaO 0.11 0.11 0.14 LOI 0.98 0.68 0.79 Total 98.65 100.00 99.22 Minor element (ppm) Ba 1070 989 1325 Ce 48.2 52.8 55.2 Cr 40 10 10 Cs 1.33 1.45 3.73 Dy 1.75 1.40 1.62 Er 0.97 0.93 0.93 Eu 0.92 0.52 0.63 Ga 21.4 16.6 19.6 Gd 2.68 1.75 2.03 Ge <5 <5 <5 Hf 3.90 3.40 3.30 Ho 0.34 0.25 0.32 La 26.2 31.0 31.4 Lu 0.13 0.14 0.17 Nb 10.5 14.5 14.3 Nd 19.8 18.0 19.6 Pr 5.35 5.26 5.53 Rb 90.1 113.0 143.0 Sm 3.69 2.67 3.23 Sn 1 1 1 Sr 428 253 349

Table 1. (Cont.) Sample no. GC-1 GC-103 GC-105 Ta 0.9 1.5 1.3 Tb 0.32 0.25 0.28 Th 8.35 19.25 15.35 Tm 0.13 0.11 0.17 U 2.37 3.29 1.27 V 58 20 35 W 1 3 2 Y 9.30 7.60 8.40 Yb 0.93 0.87 1.08 Zr 145 108 122 As 0.4 1.2 0.9 Bi 0.02 0.15 0.03 Hg <0.005 <0.005 <0.005 In 0.01 <0.005 0.01 Re <0.001 <0.001 <0.001 Sb 0.05 0.25 0.18 Se <0.2 0.3 <0.2 Te <0.01 0.05 <0.01 Tl 0.11 0.12 0.25 Ag <0.5 <0.5 <0.5 Cd <0.5 <0.5 <0.5 Co 7 2 4 Cu 28 55 4 Li 20 10 20 Mo <1 14 3 Ni 11 3 8 Pb <2 7 5 Sc 6 2 3 Zn 38 15 80 Au <0.001 <0.001 <0.001 1Granodiorite 2Quartz monzonite porphyry 3Granodiorite porphyry

Table 2. Normative analyses of least altered intrusive rocks Sample no. GC-1 GC-103 GC-105 Rock unit 1Kgd 2Kqmp 3Kgdp Name (wt %)4 Quartz 22.18 32.04 23.79 Plagioclase 47.59 36.03 44.62 Orthoclase 18.30 26.39 22.84 Corundum 0.00 0.18 0.08 Diopside 1.01 0.00 0.00 Hypersthene 6.09 4.27 6.58 Ilmenite 0.80 0.38 0.53 Magnetite 1.38 1.75 2.32 Hematite 0.00 0.00 0.00 Apatite 0.37 0.21 0.23 Zircon 0.03 0.01 0.03 Chromite 0.01 0.00 0.00 Pyrite 0.02 0.00 0.00 Total 100.76 101.26 101.02 1Granodiorite 2Quartz monzonite porphyry 3Granodiorite porphyry 4Weight norms calculated from CIPW spreadsheet from M. N. Ducea, pers. commun., 2020; calculation steps after Johannsen, 1931

Table 3. Electron microprobe compositions of igneous minerals Sample no. GC-1b GC-103 GC-105 GC-1 GC-29 GC-103 GC-105 GC-1a GC-29a *Mineral 1Phl Phl Phl 2Or Ab Plag Or 3Mghbl Fehbl Sample ID GC-1b-bt-2 GC-103-bt-2 GC-105-bt-1 GC-1a-a GC-29a-a GC-103-b GC-105-c GC-1a-1 GC-29a-1

*Rock unit Kgd Kqmp Kgdp Kgd Kqmp Kqmp Kgdp Kgd Kqmp Oxide (wt %) SiO2 36.32 35.94 37.09 64.78 65.57 61.21 63.80 49.45 42.98 TiO2 4.30 3.79 3.82 0.03 0.00 0.00 0.02 0.98 0.16 Al2O3 14.17 15.69 14.02 18.30 21.24 24.01 18.80 4.72 9.17 Cr2O3 ------0.01 0.01 0.01 0.01 ------Fe2O3 ------0.00 0.08 0.07 0.05 11.68 19.86 FeO 16.93 17.30 15.90 0.23 0.00 0.00 0.00 1.20 1.75 MnO 0.40 0.38 0.62 0.00 0.00 0.00 0.00 ------MgO 12.60 12.39 13.78 0.00 0.01 0.02 0.01 15.84 9.32 CaO 0.00 0.02 0.02 0.00 2.00 4.84 0.00 11.56 11.88 BaO ------0.14 0.00 0.08 0.21 ------Na2O 0.15 0.19 0.17 0.49 10.37 8.68 1.13 0.96 1.09 K2O 9.39 9.51 9.27 15.88 0.10 0.44 15.08 0.37 1.07 F ------0.12 0.36 H2O ------1.81 1.83 O-F ------0.05 -0.15 Total 94.28 95.22 94.68 99.85 99.38 99.36 99.09 98.65 99.31 Cation (apfu) Fe3+/∑Fe ------0.90 0.91 IVSi 2.43 2.38 2.45 3.00 0.06 0.06 0.06 7.09 6.33 IVTi ------0.00 0.00 0.00 0.00 ------IVAl 1.12 1.23 1.09 1.00 2.62 2.76 2.52 0.80 1.59 IVAs,IVP,IVS ------0.11 0.08 IVCr ------0.00 0.00 0.00 0.00 ------IVFe+3 ------0.00 0.01 0.01 0.00 ------IVFe+2 ------0.01 0.00 0.00 0.00 ------IVMn ------0.00 0.00 0.00 0.00 ------IVMg ------0.00 0.00 0.00 0.00 ------∑(IV) 3.55 3.61 3.54 4.01 2.69 2.83 2.59 8.00 8.00 VITi 0.22 0.19 0.19 ------0.11 0.02

Table 3. (Cont.) Sample no. GC-1b GC-103 GC-105 GC-1 GC-29 GC-103 GC-105 GC-1a GC-29a *Mineral 1Phl Phl Phl 2Or Ab Plag Or 3Mghbl Fehbl Sample ID GC-1b-bt-2 GC-103-bt-2 GC-105-bt-1 GC-1a-a GC-29a-a GC-103-b GC-105-c GC-1a-1 GC-29a-1 *Rock unit Kgd Kqmp Kgdp Kgd Kqmp Kqmp Kgdp Kgd Kqmp Oxide (wt %) VIMg 1.26 1.22 1.36 ------3.39 2.04 VIMetal ------0.11 0.52 ∑(VI) 2.45 2.39 2.46 ------5.00 5.00 VIIIFe2+ ------0.00 0.00 VIIIMn ------0.00 0.00 VIIIMg ------0.00 0.00 VIIICa ------1.78 1.87 VIIINa ------0.22 0.13 ∑(VIII) ------2.00 2.00 Aca 0.00 0.00 0.00 0.00 0.22 0.51 0.00 ------Aba ------0.00 0.00 0.00 0.01 ------Ana 0.02 0.02 0.02 0.04 2.11 1.64 0.25 0.04 0.18 AK 0.80 0.80 0.78 0.94 0.01 0.05 2.19 0.07 0.20 AVacancy ------0.89 0.61 ∑(A) 0.82 0.83 0.80 0.99 2.34 2.21 2.44 1.00 1.00 XOH 2.00 2.00 2.00 ------1.73 1.80 XF ------0.06 0.17 XO ------0.21 0.04 ∑(X) 2.00 2.00 2.00 ------2.00 2.00 2Endmember (mole %) An ------0.00 9.57 23.00 0.00 ------Ab ------4.45 89.87 74.53 10.19 ------Or ------95.55 0.56 2.47 89.81 ------Data normalized by Excel spreadsheets developed by Brady and Perkins (2007, 2017) 1Used six cations per formula, eleven oxygens, and assumed no trioctahedral substitutions 2Recalculated Fe3+ from all Fe and FeO and estimated endmember compositions. 3Fe3+/∑Fe calculated by normalizing Fe3+ and Fe2+; elements used for normalization: F, Na, Mg, Al, Si, K, Ca, Ti, Fe, P, S, As, Zn, Sc, V, Cr, Mn, and Ni *Abbreviations used: Phl = Phlogopite, Or = orthoclase, Ab = albite, An = anorthite, Plag = plagioclase; Mghbl = magnesiohornblende, Fehbl = ferritschermakite, Kgd = granodiorite, Kqmp = quartz monzonite porphyry, Kgdp = granodiorite porphyry

Table 4. Whole-rock analyses of altered samples Sample no. GC-8a GC-29b GC-45 GC-57a GC-68d GC-73e GC-75a GC-92a GC-96d GC-97b GC-99 Rock unit 1Kgdp 2Kqmp 3Kgd Kgd Kqmp Kgdp Kgd 4Ꞓp Kqmp Kqmp Kqmp Major Element (wt% oxides) SiO2 71.30 65.50 28.50 73.80 68.30 62.80 50.20 58.50 72.60 75.40 72.50 Al2O3 14.20 11.70 6.53 14.50 16.15 10.50 10.00 9.77 13.85 11.85 10.95 Fe2O3 5.51 12.35 50.50 3.15 3.51 16.55 23.10 20.00 2.09 4.26 8.62 CaO 0.11 0.14 0.11 0.16 0.05 0.14 0.18 0.11 0.16 0.10 0.12 MgO 1.04 0.47 0.24 0.80 0.74 0.81 0.31 0.50 0.07 0.38 0.42 Na2O 0.06 0.18 0.28 1.33 0.67 0.07 0.57 0.14 1.75 0.49 1.73 K2O 5.22 3.73 5.01 4.98 5.30 3.79 3.53 3.09 9.23 4.95 3.06 Cr2O3 0.01 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO2 0.49 0.21 0.23 0.39 0.49 0.32 0.16 0.21 0.16 0.25 0.29 MnO 0.03 0.01 0.01 0.01 0.01 0.05 0.01 0.01 <0.01 <0.01 0.01 P2O5 0.19 0.29 0.09 0.14 0.03 0.41 0.71 0.35 0.09 0.07 0.20 SrO <0.01 <0.01 0.01 <0.01 <0.01 <0.01 0.06 <0.01 0.02 <0.01 <0.01 BaO 0.06 0.09 0.10 0.04 0.11 0.05 0.08 0.06 0.17 0.08 0.07 LOI 3.56 4.38 7.88 2.38 3.91 4.79 10.30 6.17 0.85 2.30 3.00 Total 101.78 99.05 99.50 101.69 99.27 100.28 99.21 98.91 101.04 100.13 100.97 Minor Element (ppm) Ba 481.00 874.00 845.00 412.00 1105.00 439.00 769.00 618.00 1585.00 763.00 699.00 Ce 33.90 46.50 8.00 42.60 32.10 32.50 79.10 30.00 46.30 33.40 36.60 Cr 40.00 20.00 40.00 40.00 30.00 30.00 20.00 10.00 10.00 20.00 20.00 Cs 3.30 2.39 1.28 3.19 5.47 4.89 2.64 1.98 2.28 4.44 5.94 Dy 0.82 2.57 1.90 1.63 0.91 3.04 1.26 1.27 1.20 1.19 1.60 Er 0.39 1.51 1.27 0.88 0.45 2.45 0.76 0.79 0.83 0.78 0.82 Eu 0.36 0.50 0.53 0.55 0.39 0.81 0.34 0.39 0.49 0.27 0.55 Ga 16.90 23.10 6.10 23.30 20.40 16.00 19.80 20.20 13.20 20.20 12.80 Gd 1.13 2.31 1.68 1.93 1.66 3.00 1.17 1.62 1.29 1.60 1.84

Table 4. (Cont.) Sample no. GC-8a GC-29b GC-45 GC-57a GC-68d GC-73e GC-75a GC-92a GC-96d GC-97b GC-99 Hf 2.80 3.30 1.70 3.40 4.80 3.10 0.60 2.20 3.70 3.30 3.40 Ho 0.13 0.48 0.43 0.29 0.17 0.65 0.25 0.26 0.24 0.23 0.29 La 21.80 25.60 5.30 23.10 18.80 17.90 42.60 16.00 38.70 17.60 20.40 Lu 0.07 0.26 0.20 0.13 0.09 0.33 0.13 0.13 0.14 0.13 0.14 Nb 8.50 13.10 8.90 10.10 11.40 8.50 4.70 10.80 8.50 15.20 10.70 Nd 11.60 18.00 5.10 16.80 12.90 15.40 19.30 11.80 13.30 12.70 13.80 Pr 3.25 5.01 1.19 4.67 3.65 3.78 7.28 3.19 4.13 3.70 3.75 Rb 237.00 150.00 123.00 261.00 227.00 240.00 136.00 106.50 233.00 189.50 145.50 Sm 2.01 3.09 1.29 3.12 2.26 3.39 2.07 2.15 2.00 2.11 2.49 Sn 3.00 18.00 4.00 3.00 4.00 2.00 17.00 14.00 1.00 4.00 4.00 Sr 56.80 23.70 97.00 38.60 40.70 18.00 579.00 28.50 178.00 68.20 66.90 Ta 0.80 1.40 0.50 1.00 1.10 0.90 0.40 1.00 0.80 1.50 1.20 Tb 0.16 0.38 0.27 0.26 0.17 0.48 0.22 0.18 0.20 0.24 0.23 Th 5.91 14.50 3.89 10.60 4.05 8.94 7.21 10.55 16.65 5.71 6.88 Tm 0.04 0.26 0.18 0.11 0.07 0.34 0.13 0.11 0.11 0.13 0.15 U 4.31 31.50 26.50 7.21 1.56 9.40 14.95 8.36 7.11 1.87 3.34 V 65.00 55.00 58.00 87.00 119.00 67.00 190.00 42.00 30.00 38.00 58.00 W 46.00 89.00 37.00 42.00 63.00 65.00 27.00 134.00 15.00 30.00 17.00 Y 4.10 14.00 15.20 8.50 5.20 21.20 6.80 8.00 8.50 7.10 8.60 Yb 0.35 1.90 1.14 0.71 0.59 2.35 0.80 0.76 0.84 0.80 0.80 Zr 112.00 118.00 66.00 122.00 178.00 108.00 18.00 85.00 119.00 118.00 115.00 As 10.30 66.80 39.80 111.50 6.40 85.60 55.50 >250 7.30 44.50 13.30 Bi 0.48 2.22 1.45 2.18 0.94 0.36 0.20 1.26 0.81 1.97 0.97 Hg 0.01 0.05 0.03 0.01 0.02 0.06 0.02 0.05 <0.005 0.05 0.01 In 0.01 0.03 0.01 0.02 0.03 0.04 0.02 0.15 0.06 0.01 <0.005 Re <0.001 <0.001 0.00 <0.001 0.00 0.00 <0.001 <0.001 <0.001 0.00 0.00 Sb 1.41 2.67 0.61 3.02 0.20 37.00 1.77 54.00 1.13 2.48 0.85 Se 0.70 2.70 59.20 0.20 1.60 1.20 4.90 1.10 0.40 1.80 3.00 Te 0.01 0.27 1.72 0.07 0.04 0.12 0.05 0.20 0.14 0.07 0.07 Tl 0.23 0.19 0.12 0.28 0.51 0.40 0.20 0.16 0.14 0.24 0.33 Ag <0.5 0.60 3.80 <0.5 <0.5 <0.5 <0.5 0.50 <0.5 3.70 <0.5

Table 4. (Cont.) Sample GC-8a GC-29b GC-45 GC-57a GC-68d GC-73e GC-75a GC-92a GC-96d GC-97d GC-99 no. Cd 0.80 10.80 <0.5 <0.5 <0.5 1.40 0.70 20.50 <0.5 <0.5 <0.5 Co 2.00 2.00 39.00 1.00 1.00 9.00 1.00 <1 3.00 <1 5.00 Cu 319.00 1160.00 12.00 57.00 44.00 590.00 1030.00 4070.00 60.00 89.00 174.00 Li 10.00 10.00 <10 10.00 20.00 10.00 20.00 10.00 <10 20.00 10.00 Mo 58.00 51.00 579.00 82.00 26.00 945.00 102.00 28.00 72.00 863.00 165.00 Ni 6.00 5.00 57.00 15.00 3.00 29.00 9.00 3.00 4.00 3.00 9.00 Pb 63.00 6.00 16.00 18.00 2.00 57.00 21.00 48.00 43.00 15.00 4.00 Sc 6.00 3.00 2.00 5.00 7.00 7.00 10.00 3.00 1.00 2.00 3.00 Zn 105.00 895.00 64.00 56.00 25.00 193.00 111.00 1370.00 9.00 25.00 117.00 Au <0.001 0.00 0.37 0.01 <0.001 <0.001 <0.001 <0.001 <0.001 0.00 <0.001 1Granodiorite porphyry 2Quartz monzonite porphyry 3Granodiorite 4Preble Formation

Table 5. Electron microprobe compositions of hydrothermal minerals Sample no. GC-45c GC-73e GC-79 GC-29a GC-26b GC-26b GC-26b GC-45c GC-73e GC-92 GC-92 GC-92 GC-92 *Mineral 1Sericite Sericite Sericite 2Chl Chl 3Ep Ep 4Or Or 5Act Act 6An An Sample ID GC-45c-s-3 GC-73e-s-5 GC-79d-s-2 GC-26b-chl-1 GC-29a-chl-1 GC-26b-ep-1 GC-26b-ep-2 GC-45c-alt-a GC-73e-alt-a GC-92-act-b GC-92-act-e GC-92-gt-a GC-92-gt-b *Rock unit Kgd Kgdp Pn Kqmp Kgd Kgd Kgd Kgd Kgdp Cp Cp Cp Cp Oxide (wt %) SiO2 52.06 45.30 48.55 28.48 26.05 36.69 35.99 63.31 63.81 51.47 52.63 34.54 34.80 TiO2 0.29 0.38 0.36 0.04 0.06 0.18 0.13 0.00 0.01 0.02 0.00 0.00 0.01 Al2O3 33.43 28.73 31.84 18.13 19.12 24.63 18.14 19.09 18.91 2.03 0.84 0.18 0.02 Cr2O3 ------0.00 0.01 ------0.00 0.01 Fe2O3 0.00 0.00 0.00 0.00 0.00 12.25 19.99 0.03 0.00 3.23 1.13 31.21 31.33 FeO 3.16 5.60 2.03 20.96 24.03 0.00 0.00 0.00 0.00 14.09 19.41 -0.38 -0.16 MnO 0.01 0.04 0.01 1.22 6.35 0.61 0.25 0.00 0.00 ------0.76 0.69 MgO 1.93 3.18 2.57 19.35 11.12 0.05 0.02 0.01 0.00 11.86 9.91 0.00 0.04 CaO 0.00 0.00 0.00 0.07 0.07 22.27 21.61 0.01 0.00 11.61 11.60 31.93 32.02 BaO ------0.32 0.37 ------Na2O 0.02 0.17 0.14 0.01 0.03 0.01 0.05 1.44 0.47 0.25 0.21 ------K2O 5.24 10.86 10.23 0.04 0.04 0.00 0.00 14.66 15.95 0.11 0.09 ------F ------0.03 0.03 ------H2O ------2.00 1.99 ------O-F ------0.01 -0.01 ------Total 96.13 94.25 95.73 88.30 86.88 95.47 94.16 98.88 99.53 96.68 97.82 98.26 98.75 Cation (apfu) Fe3+/∑Fe ------0.17 0.05 ------IVSi 3.00 3.00 3.00 2.05 2.01 2.95 2.99 0.06 0.06 7.66 7.87 2.98 2.99 IVTi ------1.54 1.74 0.01 0.01 0.00 0.00 0.34 ----- 0.00 0.00 IVAl ------2.53 2.54 0.00 0.13 ------IVAs,IVP,IVS ------0.00 ------IVCr ------0.00 0.00 ------IVFe+3 ------0.00 0.00 ------IVFe+2 ------0.00 0.00 ------IVMn ------0.00 0.00 ------IVMg ------0.00 0.00 ------∑(IV) 3.00 3.00 3.00 3.59 3.75 2.96 3.00 2.60 2.60 8.00 8.00 2.98 2.99 VITi 0.01 0.02 0.02 0.00 0.00 ------0.00 0.00 ------VIAl 2.27 2.24 2.32 ------0.02 0.02 0.02 0.00 VICr ------0.00 0.00 VIFe+3 0.15 0.31 0.10 1.26 1.55 ------0.36 0.13 2.02 2.02 VIFe+2 0.00 0.00 0.00 0.00 0.00 ------1.75 2.43 ------VIMn 0.00 0.00 0.00 0.07 0.41 ------VIMg 0.17 0.31 0.24 2.08 1.28 ------2.63 2.21 ------VIMetal ------0.24 0.22 ------∑(VI) 2.60 2.89 2.68 3.41 3.25 ------5.00 5.00 2.04 2.03 VIIIAl ------2.33 1.78 ------VIIIFe3+ ------0.74 1.25 ------VIIIFe2+ ------0.00 0.00 ------0.00 0.00 -0.03 -0.01 VIIIMn ------0.04 0.02 ------0.08 0.08 0.06 0.05 VIIIMg ------0.01 0.00 ------0.00 0.00 0.00 0.00 VIIICa ------1.85 1.86 2.95 2.94 VIIINa ------0.07 0.06 ------

Table 5. (Cont.) Sample no. GC-45c GC-73e GC-79 GC-29a GC-26b GC-26b GC-26b GC-45c GC-73e GC-92 GC-92 GC-92 GC-92 ∑(VIII) ------3.12 3.05 ------2.00 2.00 2.98 2.99 ACa 0.00 0.00 0.00 0.01 0.01 1.92 1.93 0.00 0.00 ------ABa ------0.01 0.02 ------ANa 0.00 0.02 0.02 0.00 0.00 0.00 0.01 0.31 0.10 0.00 0.00 ------AK 0.39 0.92 0.81 0.00 0.00 0.00 0.00 2.11 2.31 0.02 0.02 ------AVacancy ------0.98 0.98 ------∑(A) 0.39 0.94 0.82 0.01 0.01 1.92 1.93 2.43 2.43 1.00 1.00 ------XOH 2.00 2.00 2.00 2.00 2.00 ------1.98 1.99 ------XF ------0.01 0.01 ------XO ------0.00 0.00 ------∑(X) 2.00 2.00 2.00 2.00 2.00 ------2.00 2.00 ------H ------1.00 1.00 ------4Endmember (mole ------%) An ------0.06 0.00 ------Ab ------12.95 4.29 ------Or ------86.99 95.71 ------Data normalized by Excel spreadsheets developed by Brady and Perkins (2007, 2017) 1Used three cations per formula, eleven oxygens, and assumed no phengite component. 2Used seven cations per formula, eleven oxygens, and assumed octahedral sites were full 3Used 12.5 oxygens per formula and converted all iron to Fe3+ 4Recalculated Fe3+ from all Fe and FeO and estimated endmember compositions 5Fe3+/∑Fe calculated by normalizing Fe3+ and Fe2+; elements used for normalization: F, Na, Mg, Al, Si, K, Ca, Ti, Fe, P, S, As, Zn, Sc, V, Cr, Mn, and Ni 6Assume all iron as Fe3+ *Abbreviations used: Chl = chlorite, Or = orthoclase, Ep = epidote, Act = actinolite, An = andradite, Kgd = granodiorite, Kqmp = quartz monzonite porphyry, Kgdp = granodiorite porphyry, Cp = Preble Formation, Pn = Pumpernickel Formation

Appendices

Appendix 1: Electron microprobe analysis of fresh biotite.

Appendix 2: Electron microprobe analysis of fresh feldspars.

Appendix 3: Electron microprobe analysis of fresh amphiboles.

Appendix 4: Electron microprobe analysis of sericite.

Appendix 5: Electron microprobe analysis of altered feldspars.

Appendix 6: Electron microprobe analysis of altered amphiboles.

Appendix 7: Electron microprobe analysis of garnet.

Appendix 8: Whole-rock analysis of altered and unaltered samples.

Appendix 9: Mass gains and losses of elemental oxides normalized to constant Al2O3. A.

Granodiorite (Kgd). B. Quartz monzonite porphyry (Kqmp). C. Granodiorite porphyry (Kgdp).

Appendix 10: Hornblende barometry estimates with temperature, pressure, and depth constraints.

Appendix 11: U-Pb geochronology of intrusive bodies in Northwestern Nevada.

Appendix 9.

Histograms of gains and losses for various types of wall-rock alteration in A. granodiorite, B. quartz monzonite porphyry, and C. granodiorite porphyry.

Appendix 10. Hornblende thermobarometry estimates with temperature, pressure, and depth constraints 1Sample no GC-1a GC-1b GC-1b GC-1b GC-26b GC-29a GC-29a Sample Name GC-1a_7 GC-1b GC-1b_4 GC-1b_6 GC-26b GC-29a_4 GC-29a_A4 Average 2T°C 543 717 759 696 543 657 660 653 P (kb) 1.5 2.3 1.5 1.7 1.5 1.9 1.6 1.7 Depth (km) 4.4 6.8 4.4 5.2 4.5 5.6 4.7 5.1 1Samples all come from the same rock unit, Kgd, granodiorite 2Results based on iteration using Anderson and Smith pressure at various thermometers and using Holland and Blundy Hbld-Plag thermometry calibration reaction edenite + albite = richterite + anorthite; see Anderson (1996)

Mass Gains and Losses

Samples and Analytical Procedural

Table 4 provides the geochemical analysis of eleven samples, consisting of unaltered

intrusive rock and of veins with different alteration mineral assemblages. XRF, LA-ICPMS mass

spectrometry, and colorimetric titration techniques carried out by ALS Geochemistry, Tucson.

Three least altered samples of the granodiorite, quartz monzonite porphyry, and the

granodiorite porphyry provided a baseline for estimating the geochemical gains and losses in

each altered sample. Altered rocks are small, trimmed samples in an attempt to characterize

processes occurring in the alteration envelope on a vein or a in single type of background

alteration (e.g., Ulrich and Heinrich, 2001), rather than geochemical changes that might occur in

much larger volumes of rock that likely contain multiple types of veins and associated types of

wall-rock alteration and intervening areas of relatively unaltered rock.

The gains and losses for major and trace elements are shown through histograms in

Appendix 9. The data were normalized to constant Al2O3, as it was the most immobile element,

and each alteration pattern was shown in accordance with the host rock of the sample.

Results

Results are summarized by host rock phases. In the main intrusion (Appendix 9A), the

granodiorite, Fe, K, Cr, P, and Ba increase for intermediate argillic assemblages hosted, whereas losses occur in Ca, Mg, Na, Mn, and Sr, with little to no change in the Si content. Sericitic alteration associated with the granodiorite is associated with gains in Si, Fe, and K with losses in

Ca, Mg, Na, Cr, Mn, P, Sr, and Ba.

The most altered intrusive phase, the quartz monzonite porphyry (Appendix 9B), contains a multiplicity of alteration styles and the highest levels of Mo compared to other host rocks. The trends associated with intermediate argillic alteration include an increase in K and

Ba; little to no change in Si, Fe, or P; and decrease in Ca, Mg, Na, Ti, Mn, and Sr contents.

Sericitic assemblages show gains in Si, Fe, Mg, Cr, Ti, and P, while losing Ca, Na, K, Mn, Sr, and

Ba. Greisen alteration corresponds with gains in Fe, Mg, Cr, and Ti and losses in Si, Ca, Na, Mn,

P, Sr, and Ba. Propylitic assemblages show gains in Si, Fe, K, Cr, and Ti, with losses in Ca, Na, Mn,

P, Sr, and Ba.

Two samples of granodiorite porphyry with greisen were analyzed ((Appendix 9C).

Greisen alteration associated with displays a similar geochemical behavior with greisen alteration in the quartz monzonite porphyry except for an increase in P.

Interpretations

Gains and losses for are interpreted by type of alteration, based on observed mineralogic changes. The samples analyzed here pertain to greisen muscovite, sericitic, and intermediate argillic (all of which are forms of hydrolytic alteration), as well as propylitic

(typically characterized mainly by volatile addition). Inconsistencies between samples may reflect difficulties in sampling only one type of hypogene alteration or local effects of weathering.

The variability in iron contents largely reflects the amount of sulfide in a sample.

Sericitic alteration, for which K-feldspar, plagioclase, and mafic minerals are altered to sericite, show depletions in Ca, Sr, and Na (from plagioclase) and Mn (presumably from mafic minerals).

Samples of intermediate argillic alteration, where only plagioclase and mafic minerals are

altered to sericite, have similar gains and losses but increases in potassium could reflect replacement of K-poor minerals by the K-bearing sericite. The three greisen samples are similar to the other types of hydrolytic alteration, but further work should look for the cause of addition of Ti and Cr. The one propylitic sample exhibits minimal metasomatism, as expected, except for minor addition of Fe and losses in Na and Ca.

Hornblende Thermobarometry

Data Acquisition

Appendix 10 outlines the hornblende thermobarometry estimates from the Gregg

Canyon intrusive center, showing variations in temperature, pressure, and depth from seven hornblende and plagioclase pairs. These mineral pairs are from the least altered samples of the granodiorite. The electron microprobe was used to analyze the compositions of these mineral pairs, and the results are based on iteration using Anderson and Smith pressure at various thermometers and the Holland and Blundy hornblende-plagioclase thermometry calibration reaction edenite + albite = richterite + anorthite (see Anderson, 1996).

Results

The results of these analyses show crystallization temperatures ranging from ~540—

760°C, pressures from 1.5—2.3 kb, and depths from 4.4—6.8 km, averaging ~650°C, 1.7 kb, and

5.1 km, respectively.

Depth of Emplacement for Porphyry Mo-Cu systems

Unlike porphyry Cu systems, few studies focus on the depth of emplacement of porphyry Mo-Cu systems. The literature commonly reports fluid inclusion homogenization temperatures to estimate entrapment pressures and/or thermobarometry to assess depth in which porphyry deposits form. Cu deposits commonly form above magma chambers emplaced

Most porphyry copper deposits form at depths of 1 to 6 km, though a few form at greater depths (Seedorff et al., 2005). There is some suggestion that deposits emplaced at shallow depths (3-4 km) have higher grades than deeper (>5 km) deposits (Proffett, 2009). Several studies support this hypothesis, including the Catface porphyry (<4 km), Grasberg (0.8 km),

Batu Hijau (2.4 km), and El Teniente (2.9 km) (McInnes et al., 2005; Smith et al., 2012).

The emplacement depth of the Gregg Canyon intrusive center ranges from 4-7 km, corresponding with crystallization temperatures of ~540 to 760 °C. Fluid inclusion analyses conducted in porphyry Mo-Cu systems are typically large, moderately saline (4-16 wt% NaCl), homogenize at temperatures between 250 to 400 °C, and may contain large amounts of CO2

(Theodore et al., 1992). Studies conclude that emplacement depths for Mo-Cu systems range from 2 to 11 km; however, fluorine-deficit porphyry Mo deposits, such as Little Cottonwood, appear to have deeper emplacement depths than Climax-type deposits (Bloom, 1981; John,

1989; Theodore at al., 1992). The data collected here suggest that Gregg Canyon formed at a paleodepth of ~5 km, i.e., toward the deeper end of the typical range of porphyry copper deposits.

Appendix 11. U-Pb geochronology of intrusive bodies in Northwestern Nevada Coordinates Locality Sample Rock Type Age (Ma) 2σ (Ma) MSWD (Lat/Long)

Gregg Canyon (Sonoma 40°45' 23''N 117° GC-2 Granodiorite 94.1 0.4 1.3 Range) 31' 53''W

41°08' 10''N 117° Osgood Mountains Osgood North Granodiorite 109.4 0.29 0.47 16' 17''W 41°11' 07''N 117° Osgood Mountains Osgood South Granodiorite 95.5 0.39 1.9 15' 15''W

Spanish Basin (Sonoma 40°42' 37''N 117° SPB001 Granodiorite 161.9 0.098 1.4 Range) 33' 31'' W

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