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Geology and Mineralization of the Undeveloped Cochise Supergene Porphyry Copper Deposit, Warren (Bisbee) Mining District, Cochise County, Arizona

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Authors Lewis, Kyle

Citation Lewis, Kyle. (2021). Geology and Mineralization of the Undeveloped Cochise Supergene Porphyry Copper Deposit, Warren (Bisbee) Mining District, Cochise County, Arizona (Master's thesis, University of Arizona, Tucson, USA).

Publisher The University of Arizona.

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GEOLOGY AND MINERALIZATION OF THE UNDEVELOPED COCHISE SUPERGENE PORPHYRY COPPER DEPOSIT, WARREN (BISBEE) MINING DISTRICT, COCHISE COUNTY, ARIZONA

Kyle A. Lewis

______

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements

For the Degree of

PROFESSIONAL SCIENCE MASTERS

ECONOMIC GEOLOGY

In the Graduate College

THE UNIVERSITY OF ARIZONA

2021

ACKNOWLEDGMENTS This study was funded by Freeport-McMoRan and inspired by geologists of that company during a field trip to Bisbee and Cochise in the summer of 2019. Many individuals deserve my gratitude for their cooperation and input. Michele Anthony, Mac Canby, and Paul Albers granted me time and permission to complete the field work. Ralph Stegen, Matthew Wetzel, Wolfram

Schuh, Carli Balogh, Nicholas Dize, and David First offered their geologic insight and moral support. Christopher Svenson, Rob Williams, and members of the Copper Queen branch in

Bisbee looked after my health and safety. I want to thank Ernie Wright in particular for sharing his valuable knowledge of the district that he accumulated via decades of dedicated work.

My research would have remained an amorphous, meandering adventure without the guidance of my principal advisor, Dr. Eric Seedorff. It was a true honor to be supervised by someone with his unparalleled “savage geologic competence” (as a former coworker of mine aptly phrased it). Effective scientific communication, the importance of culture and friendship in academia and the minerals industry, and “thinking like a geologist” are principles that will stick with me for my lifetime. Dr. Mark Barton, Dr. Isabel F. Barton, and Dr. Frank Mazdab were also incredibly helpful; I appreciate their time, patience, and dedication to their trade. My colleagues,

Dylan Carlini, Eytan Bos Orent, Alec Martin, and Lydia Bailey were terrific companions, especially during the trying times of 2020. I owe much of my remaining sanity to them.

My final thanks go to my family, in particular my mother. She is loving and helpful, and she always has open ears and an open mind when I need her advice and reassurance.

TABLE OF CONTENTS

LIST OF FIGURES AND TABLES ...... 6

ABSTRACT ...... 7

INTRODUCTION...... 8

LOCATION AND REGIONAL GEOLOGIC SETTING ...... 10 Porphyry copper deposits of southwestern North America ...... 12

HISTORY OF THE WARREN MINING DISTRICT ...... 13 Exploration of the Cochise deposit ...... 15

GEOLOGY OF THE WARREN MINING DISTRICT...... 16 Proterozoic Pinal Schist and Paleozoic strata ...... 17 The Sacramento Hill intrusive complex (SHIC) ...... 18 Breccias of the SHIC ...... 20 Other intrusions in the Mule Mountains ...... 21 Late Jurassic to Early Cretaceous Bisbee Group ...... 21 Structure ...... 22

MINERALIZATION OF THE WARREN DISTRICT ...... 24 Carbonate replacement deposits (CRDs) ...... 24 Mineralization in the Lavender and Sacramento pits ...... 25

METHODS ...... 26

GEOLOGY AND MINERALIZATION OF THE COCHISE DEPOSIT...... 27 Rock types ...... 27 Hypogene alteration ...... 29 Mineralization ...... 32 Leached cap characteristics (supergene alteration) ...... 33

INTERPRETATIONS ...... 35 Sequence of alteration and mineralization ...... 35 Presence or absence of advanced argillic alteration at Cochise ...... 36 Relationship of the Cochise porphyry to Lavender pit intrusions ...... 37 The Cochise deposit as the “roots” of Lavender pit porphyry and CRD mineralization ...... 38 Origin of breccias at Cochise ...... 39 Copper content of sericite alteration ...... 41 Estimation of supergene enrichment factor and volume of leached rock ...... 43

DISCUSSION ...... 44 Comparison to other supergene porphyry Cu deposits ...... 44 The paucity of Jurassic porphyry deposits in southwestern North America ...... 48 Exploration implications ...... 51

CONCLUSIONS ...... 53

FIGURES AND TABLES ...... 55

APPENDIX A - EXPANDED GEOLOGY, MINERALIZATION, AND HISTORY OF THE WARREN MINING DISTRICT ...... 77 Proterozoic Pinal Schist ...... 77 Phanerozoic sedimentary rocks ...... 78 The Sacramento Hill intrusive complex (SHIC) ...... 79 Older quartz porphyry ...... 81 Younger feldspar-quartz porphyry ...... 82 Contact / “intrusion” breccia ...... 83 Hydrothermal / “intrusive” breccias ...... 85 Other intrusions in the Mule Mountains ...... 86 Structure ...... 87 Carbonate replacement deposits (CRDs) ...... 89 Alteration of limestone ...... 91 Disseminated Cu mineralization in the Lavender and Sacramento pits ...... 92 Alteration in the Lavender pit ...... 93

REFERENCES ...... 96

LIST OF FIGURES AND TABLES

Figure 1: Location of the Cochise deposit and Warren (Bisbee) mining district ……..……. 55

Figure 2: Geology and location of the southern Mule Mountains ………………………….56

Figure 3: Geologic map of the Sacramento Hill intrusive complex (SHIC)………….……..57

Figure 4: Cross sections A-A’ and B-B’ through the SHIC ………………………….…..…59

Figure 5: Photos of important rock types at the Cochise deposit ……..…….………..……...61

Figure 6: Photos and photomicrographs of sericitic alteration ………………………………63

Figure 7: Photos and photomicrographs of deeper (orthoclase-stable) alteration ….…..…...65

Figure 8: Photos and photomicrographs of mineralization styles at Cochise ………...….….67

Figure 9: Plan-view limonite and sericite alteration maps of the Cochise deposit ….…….….69

Figure 10: Photos of leached cap features ………………………………………..….…..…..71

Figure 11: Simple reconstruction of Dividend fault movement along B-B’ …………...... ….73

Figure 12: Schematic illustration of enrichment profile and a representative drill hole.. ...…74

Table 1: Compilation of rock descriptions from previous studies …………...………….…..75

ABSTRACT

The Cochise porphyry copper deposit, owned by Freeport-McMoRan, is located outside of

Bisbee, Arizona, immediately northeast of the historic Lavender pit. This supergene deposit is one of the few known unmined chalcocite blankets in North America and thus is a valuable natural geologic laboratory. This study synthesizes information from existing publications and unpublished feasibility studies with new mapping, petrography, and SEM data to provide a more comprehensive account of the geology and exploration history of the deposit.

Base and precious metal mineralization in the Warren district is related to the ~200-Ma

Sacramento Hill Intrusive Complex (SHIC), consisting of quartz monzonitic to granodioritic intrusions and hydrothermal breccias. The post-mineral Dividend fault down-dropped the southwestern portion of the SHIC and extensive networks of surrounding carbonate replacement deposits in Paleozoic rocks. The footwall contains the Cochise deposit, hosted by the northeastern portion of the SHIC and Proterozoic Pinal Schist. The Cochise chalcocite resource of 262 Mt averaging 0.46% Cu overlies hundreds of millions of tons of low-grade (0.1-0.2% Cu) hypogene mineralization centered on the Cochise feldspar-quartz-biotite porphyry stock and crosscutting heterolithic hydrothermal (phreatic) breccias. Shallow sericite-pyrite ± chlorite and intense sericite-silica-pyrite alteration contains weak chalcopyrite and trace bornite. At depth, a phaneritic biotite granodiorite stock hosts weak chalcopyrite-molybdenite-pyrite mineralization associated with quartz-orthoclase ± tourmaline-siderite veins and pervasive chlorite with a variable white to green sericite-pyrite-chalcopyrite ± bornite overprint. Metasomatism extends into the Pinal Schist but its intensity wanes rapidly.

The supergene blanket covers an area of ~1,000 x 800 m and is 15-180 m thick. Chalcocite and covellite coat and replace pyrite and chalcopyrite. Abundant pyrite (5-15%) and hypogene sericitization of feldspars and mafic minerals lowered the acid-buffering capacity of the rock and 7

allowed efficient leaching and transport of Cu from the low-grade protore and reprecipitation in

the supergene blanket during weathering. The overlying leached cap contains supergene alunite

and kaolinite, is virtually barren (100-400 ppm Cu) with almost no Cu oxides or carbonates, and

changes from hematite- to jarosite-dominated limonite near the enrichment interface. An

estimated 2 million m3 of protore material was leached and accumulated into the current resource

with an enrichment factor of 3X that of hypogene tenor. Restoring normal movement on the

Dividend fault places the hanging wall block 1-1.5 km above and ~1 km laterally offset from the

Cochise deposit, suggesting that the Cochise deposit corresponds, prior to faulting, to an original deep flank of the magmatic-hydrothermal system.

A comparison to other supergene deposits within the North and South American Cordillera

emphasizes the importance of suitable pyrite:chalcopyrite ratios and hydrolytic alteration of a

porphyry deposit to enable efficient leaching and appreciable enrichment. A paucity of Jurassic

porphyry deposits in southwestern North America despite preservation of oxidized and hydrous

intermediate to alkaline plutons suggests a possible tectonic impediment.

INTRODUCTION

Porphyry Cu deposits contain 75% of the total worldwide resources and reserves of Cu

(Mudd and Jowitt, 2018). Production by heap leaching supergene sulfide and oxide ore has

steadily increased since 1970 and is especially important in the arid and semi-arid regions of

Arizona, Mexico, Chile, Peru, and elsewhere (Titley and Marozas, 1995; Sillitoe, 2005; Mudd

and Jowitt, 2018). Supergene enrichment processes can concentrate Cu grades up to 2-10 times

that of the hypogene sulfide protore. In addition, supergene ore is commonly amenable to solvent extraction-electrowinning (SX-EW) recovery which can create environmental, metallurgical, and

economic advantages. Because of these attractive features, most known metallogenic belts

hosting supergene-enriched porphyry copper deposits have been explored relatively thoroughly,

and few known deposits remain undisturbed by mining (Lopez and Titley, 1995).

The Cochise deposit, located outside of Bisbee, Arizona (Fig. 1), is one such unmined

deposit and is thus a preserved natural laboratory. However, previous studies of the Warren

mining district (e.g., Ransome, 1904; Bryant and Metz, 1966) largely ignored Cochise because of

its perceived subeconomic potential and, consequently, many unresolved issues remain regarding the hydrothermal history at Cochise and its structural-magmatic relationship to mineralization in the Lavender pit and underground workings. Renewed interest in the deposit and recognition of its resource potential by Phelps Dodge and later Freeport-McMoRan staff warranted a more comprehensive review of the deposit and an appreciation for its local and regional significance.

The products of supergene environments depend on the mineralogy and structure of the oxidizing sulfide system. Previous workers (e.g., Blanchard, 1968; Anderson, 1982; Titley and

Marozas, 1995; Sillitoe, 2005) devised methods to predict underlying hypogene features, such as sulfide ratios and Cu grade, from the characteristics of the leached cap. Testing the efficacy of such methods against actual deposits is possible by constructing limonite maps and documenting the mineralogy, textures, and abundance of iron oxides, Cu minerals, and secondary sulfates in the residual capping and comparing these findings to underlying unoxidized hypogene features, often from drill core. Only by understanding these relationships can the economic and metallurgical impacts of supergene alteration of individual porphyry deposits be appreciated. The

Warren (Bisbee) mining district hosts multiple styles of alteration-mineralization formed by a wide range hydrothermal fluid temperature and chemistry, host rock lithology, and structural control; large mineral systems like this can only be understood as a holistic system if the

9 structure, stratigraphy, intrusion history, and distribution of alteration and mineralization of individual study areas are properly documented and ultimately tied together.

This study unifies fragmented information on the Cochise deposit from existing publications on the greater Warren mining district along with data from unpublished feasibility studies by

Phelps Dodge and Freeport-McMoRan. The results of new mapping, petrography, and scanning electron microscope (SEM) investigations help to illustrate the characteristics and timing of intrusive and hydrothermal events at Cochise. Interpretations of these new findings, while referring to analogs in the published domain, seek to recognize the significance of the features of

Cochise and their relevance to ore deposit geology and exploration. Looking to other provinces with an uneven temporal distribution of porphyry deposits and considering the Jurassic framework into which Cochise was introduced allows speculation into the paucity of Jurassic porphyry deposits in southwestern North America.

LOCATION AND REGIONAL GEOLOGIC SETTING

The Cochise deposit is located in the Warren mining district east of Bisbee, Arizona, 128 km southeast of Tucson and 10 km north of the Mexico-United States international border (Fig. 1).

Despite its rich mining history, modern Bisbee is supported mostly by tourism and retirees.

Access to the town is via State Highway 80 which separates the Lavender pit to the southwest and the unmined Cochise deposit to the northeast (Fig 2). The surrounding Mule Mountains are situated in the southern Basin and Range Province of southwestern North America and northern

Mexico. Topography is influenced by late Tertiary and Quaternary extensional faulting

(Dickinson, 2002) that was accompanied by syn-extensional sedimentation and volcanism

(Scarborough, 1989).

The salient geologic features of the Mule Mountains (Fig. 2) have been documented by

Ransome (1904), Bonillas et al. (1916), Hayes and Landis (1964), and Bryant and Metz (1966).

Paleoproterozoic Pinal Schist forms the basement and is composed of metasedimentary and

lesser metavolcanic rocks that were accreted onto the North American craton as the Mazatzal

accretionary province at 1.65-1.7 Ga (Silver, 1978; Karlstrom and Bowring, 1988; Meijer, 2014).

Proterozoic rocks that are common in many parts of Arizona, such as the granite emplaced at

~1.4 Ga (e.g., Ruin Granite; Anderson, 1989) and the sedimentary rocks of the Apache Group

deposited at ~1.3-1.1 Ga (Stewart et al., 2001; Timmons et al., 2005), are absent near Bisbee, and

the ~1.1 Ga diabase intrusions are present (Bonillas et al., 1916) but rare. The Pinal Schist was

overlain by Paleozoic siliciclastic and carbonate sedimentary rocks (Ransome, 1904) deposited

along a passive margin in the early and middle Paleozoic and within basins formed in the late

Paleozoic during the amalgamation of Pangea (Dickinson, 1981; Kluth and Coney, 1981).

Multiple episodes of eastward- to northeastward-vergent subduction along the western

margin of North America from the Triassic to early Cenozoic caused crustal shortening and

magmatism associated with magmatic-hydrothermal mineralization throughout the region

(Damon et al., 1981; Dickinson, 1981; Tosdal et al., 1989; Staude and Barton, 2001; Greig and

Barton, 2019). Jurassic calc-alkaline to alkaline felsic to intermediate magmatism was the most

voluminous magmatic period of the Mesozoic as part of a ~200 to 150 Ma arc roughly

continuous from northern Mexico to western Canada (Tosdal et al., 1989).

Crustal shortening associated with the Laramide orogeny (~80 to 45 Ma) in southeastern

Arizona has been incompletely characterized, in part because of subsequent extensional

deformation and sedimentary cover. Certain geologists inferred thin-skinned deformation

characterized by low-angle thrust faults, flats, ramps, and fault-bend folds (e.g., Drewes, 1978;

Richard and Spencer, 1998). In contrast, Davis (1979) argued against thin-skinned deformation and interpreted thick-skinned deformation associated with basement-cored uplifts bound by

moderate-angle reverse faults and related fault-propagation folds with overturned beds in the

hanging wall and footwall, including in the Mule Mountains. Recent work supports the latter

interpretation based on cut-off angles of faults and identification of fault-propagation folds and

overturned beds which indicates that virtually all reverse faults in southeastern Arizona are

associated with thick-skinned deformation and basement-cored uplifts (Favorito and Seedorff,

2017, 2021; Seedorff et al., 2019). Because of dismemberment, tilting, and sedimentation during

Tertiary extension, structures caused by Laramide shortening and their relationship to Laramide porphyry Cu deposits are poorly understood outside of select areas of study (Favorito and

Seedorff, 2020).

Porphyry copper deposits of southwestern North America

Southwestern North America contains some of the largest, best-studied, and longest-mined porphyry Cu±Au-Mo deposits in the world (Fig. 1). At least 40 deposits in this region are major ore bodies with significant production in the past (e.g., Ajo, Miami, Bisbee), present (e.g., Ray,

Morenci, and Cananea), and possibly the near future (e.g., Resolution, Rosemont). From 1870 through 2011, Cu production in southwestern North America totaled ~75 Mt Cu, which is 13.5% of the total world production during that period (Leveille and Stegen, 2012). At its peak in 1919, this region produced 28% of the Cu worldwide. Of the 85 dated porphyry copper deposits in southwestern North America, 81 are associated with the Laramide arc (80-45 Ma) and only 3 deposits are Jurassic: Bisbee and Courtland-Gleeson, Arizona, and El Arco, Baja California,

Mexico (Lang et al., 2001; Leveille and Stegen, 2012). In Arizona, despite numerous occurrences of Jurassic volcanic and intrusive rocks (e.g., Tosdal et al., 1989), Bisbee is the only

magmatic-hydrothermal system with a significant production history. The rarity of economic

porphyry and epithermal mineralization in the Jurassic arc of southwestern North America is

discussed below.

The Courtland-Gleeson district is 35 km northeast of Bisbee (Fig. 1) and from the 1870’s to

1920’s produced minor amounts of Cu, Pb, Ag, and Au from oxidized manto-style sulfide

replacement bodies in folded Paleozoic limestones intruded by a quartz monzonite porphyry

(Wilson, 1927) dated by Lang et al. (2001) at 201.3 ± 0.7 Ma; it is thus, like Bisbee, Early

Jurassic. The Star Hill deposit in this district was drilled in 1988-89 by Santa Fe Pacific Mining who delineated a small resource of 2.8 Mt averaging ~0.9% Cu (Riesmeyer, 1989). Drilling revealed that the structural block of mineralized Cambrian Abrigo Limestone and Jurassic porphyry intrusions are allochthonous and were reverse faulted on top of the Cretaceous Bisbee

Group.

The El Arco porphyry Cu deposit of Baja California, Mexico (Fig. 1), is the only other significant Jurassic porphyry deposit in addition to Bisbee in southwestern North America (if we exclude Nevada from that geographic area). A U-Pb zircon age of 164.7 ± 6.5 Ma and Re-Os molybdenite ages averaging 164.1 ± 0.4 Ma (Valencia et al., 2006) indicates mineralization at El

Arco occurred during the Middle Jurassic, approximately 35 m.y. after Bisbee. El Arco is currently being developed, and total proven and probable sulfide ore reserves stand at 2,411 Mt averaging 0.42% Cu and 0.007% Mo (Southern Copper Corporation, 2021). Coolbaugh et al.,

(1995) provide a short geologic description of El Arco.

HISTORY OF THE WARREN MINING DISTRICT

Mineral claims were first staked in 1877 and by 1880 mining of the Copper Queen ore body commenced from small surface pits and shallow underground workings (Graeme, 1981). Ore

averaged 23% Cu and was shipped directly to a smelter. Early mining success and the original geologic investigators of the district (Ransome, 1904; Bonillas et al., 1916) attracted developers and investors. Advances in smelting techniques allowed the sulfide ores of the district to become commercially important in the early 1890’s (Graeme, 1981). Declining grades (now only 8 to

12% Cu), low metal prices, and high freight charges necessitated a more direct connection of ore to the growing network of railroads in the United States, so Phelps, Dodge, and Co., the parent company of Copper Queen, constructed the El Paso and Southwestern Railroad. A smelter was built 42 km away in the town of Douglas, Arizona, reducing pollution in the confined town of

Bisbee. By 1902, over 380 million lbs of copper had been produced in the district. Phelps Dodge acquired its competitors’ properties and by 1947 had consolidated most of the district. Copper, lead, zinc, and manganese production was particularly important during both World Wars.

A colorful description of the living conditions of Bisbee in the early 1900’s by Ransome

(1904; p. 15) juxtaposes the amenities afforded by profitable mining operations with the dangers of floods, typhoid, and the cavalier nature of a western mining town during those times. Graeme

(1981) describes robberies, murders, hangings, and attacks by Native Americans.

Production through the early 1900’s focused on high-grade carbonate replacement deposits

(CRDs) at a time when the lower-grade porphyry-style mineralization was subeconomic.

Underground mining was accompanied and eventually superseded by open pit mining of disseminated mineralization in the Sacramento pit from 1919 to 1929 and the Lavender pit from

1951 to 1974. Operations ceased by 1975, but leaching of existing stockpiles continued into the

1990’s (Graeme, 1993). Total metal production from 1880 to 1975 was 7.7 Glbs (3.5 M tonnes) of copper, 2.7 Moz (84 tonnes) gold, 100 Moz (3.1 k tonnes) of silver, 355 Mlbs (161 k tonnes) of zinc, 324 Mlbs (147 k tonnes) of lead, and 11 Mlbs (5 k tonnes) of manganese (Graeme,

1981), making the Warren district one of the most important producers of base and precious

metals in Arizona. Underground operations from 1880-1975 produced 53 Mt averaging 6% Cu

and open pit mining of the Lavender and Sacramento pits produced ~223 Mt averaging 0.63%

Cu (Stegen et al., 2005). Freeport-McMoRan acquired Phelps Dodge in 2007 and continues to

oversee reclamation, exploration, and security of the district.

Exploration of the Cochise deposit

The Cochise deposit crops out as a conspicuous red leached cap in a well-studied mining district, so its “discovery” can be attributed to reevaluation and drilling of previously subeconomic mineralization and advances in Cu leaching technology (SX-EW) rather than leached cap appraisal, geochemical sampling, geophysics, or other more complicated exploration

methods. In the 1890’s the Cochise and Copper King shafts were sunk into the footwall of the

Dividend fault, where miners encountered sub-economic chalcocite in pyritic schist (Graeme,

1993). The Dividend fault was thereafter considered the boundary of ore in the district (Fig. 2) and thus a boundary to detailed geologic inquiry and exploration for almost a century.

Consequently, the Cochise resource remained largely unexplored until the 1970’s.

Exploration holes drilled in 1913 north of the Dividend fault encountered weak Cu

mineralization that did not warrant further interest at the that time (Phelps Dodge Corporation,

1989). From 1975 - 1976, Occidental Minerals drilled 22 core holes on the northwestern side of

the Cochise deposit and intercepted significant chalcocite mineralization overlying a porphyry

Cu system. Four of these drill holes reached ~920 m depth, the deepest to date. Occidental

terminated their lease and Phelps Dodge acquired this land adjacent to their existing operations.

From 1987-1988, Phelps Dodge further delineated the resource with 76 drill holes up to ~325 m deep and averaging 190 m. Initial tests indicated the deposit was amenable to heap leaching and

15 most rock types crushed easily (Phelps Dodge Corporation, 1989). Additional infill drilling campaigns were completed from 1995-1997 and 2007-2008, and to date over 42,000 m have been drilled at Cochise.

Scoping and feasibility studies from 1989-2007 incorporated the results of drilling and core logging, surficial mapping, trenching, geologic and resource modelling, metallurgical testing, mine planning, financial analyses, and environmental / social considerations. The current resource stands at 262 Mt averaging 0.46% total Cu (Freeport-McMoRan Inc., 2021). Mine plans envision an open pit with a production life of 10-12 years and a low stripping ratio of 0.4:1 waste to ore. Leaching of three 10,000-ton test piles of representative chalcocite ore indicated that ore crushed in 2-3 stages to minus-1 inch and heap leached would recover ~65% of total Cu (Phelps

Dodge Corporation, 2002), amounting to 1.7 billion lbs of recoverable Cu.

GEOLOGY OF THE WARREN MINING DISTRICT

To provide geologic context for the Cochise deposit, a geologic summary of the district is provided here, compiled in Table 1, and Appendix A is an expanded version of it. Unlike the

Cochise deposit specifically, the greater Warren district has received considerable geologic attention over the last 120 years. Major contributions by Ransome (1904), Bonilllas et al. (1916),

Bryant (1964; 1968), Bryant and Metz (1966), Nye (1968), Lang et al. (2001), and Stegen et al.

(2005) provide a robust geologic framework for the open pits and underground workings, including the structure, the sequence and petrography of multiple porphyritic intrusions and associated breccias, distribution of alteration types, and detailed characteristics of multiple styles of mineralization.

Proterozoic Pinal Schist and Paleozoic strata

The Pinal Schist, a quartz-muscovite schist originally named and described by Ransome

(1904), forms the basement rocks of the district. These metamorphosed siltstones, wackes,

sandstones, and lesser volcanic rocks of predominantly greenschist-facies mineral assemblages

(Meijer, 2014) comprise the ~1.65-1.75-Ga Mazatzal accretionary province that form the basement of parts of Arizona, New Mexico, and Colorado (Karlstrom and Bowring, 1988). In the

Mule Mountains, the schist underlies Paleozoic and younger sedimentary rocks and was intruded by granites and felsic porphyritic stocks, dikes, and sills in the Early and Middle Jurassic.

Schistosity is well developed but can be cryptic in more massive facies. Microscopic investigations show mineral content to be predominantly quartz and fine muscovite with accessory tourmaline, garnet, zircon, chlorite, and magnetite or ilmenite (Ransome, 1904;

Meijer, 2014). The schist is mineralized at Cochise but constitutes a very minor portion of the overall resource.

Resting unconformably on the Pinal Schist are Paleozoic calcareous and siliciclastic rocks with an aggregate thickness of ~1,600 m in the Mule Mountains (Ransome, 1904). These include the Cambrian Bolsa Quartzite, Cambrian Abrigo Limestone, Devonian Martin Limestone,

Mississippian Escabrosa Limestone, and Pennsylvanian-Permian Naco Group limestones.

Detailed descriptions of the thickness, structure, lithology, and fossil content of these strata are provided by Ransome (1904), Bonillas et al. (1916) and Nye (1968). All carbonate-bearing strata host CRD mineralization (Bryant, 1964), although certain formations or individual horizons are preferentially mineralized.

The Sacramento Hill intrusive complex (SHIC)

Base and precious metal mineralization in the Warren district formed within and around the

~1.5 km wide Sacramento Hill intrusive complex (SHIC) (Fig. 2, 3). The southwestern portion of the SHIC in the hanging wall of the Dividend fault was studied intensely compared to the

Cochise deposit, but because mineralization in the two structural blocks is linked, the geology in and adjacent to the Lavender pit serves as crucial context for understanding the geology of the

Cochise deposit itself. Early workers noted the spatial relationship of the highly altered SHIC to mineralization (Ransome, 1904; Bonillas et al., 1916), and subsequent studies by Bryant (1964;

1968), Bryant and Metz (1966), Lang et al. (2001), and Stegen et al. (2005) mostly converge on a sequence of igneous and hydrothermal events to explain the geologic features of the district.

Salient components are shown in Table 1 and more information on lithologies, alteration, mineralization, and controversies around their understanding can be found in Appendix A.

The older quartz porphyry (or “Sacramento porphyry”), with a U-Pb zircon age of 198.9±1.6

Ma (Lang et al., 2001), is the oldest intrusion of the SHIC based on crosscutting relations

(Bryant, 1964; Bryant and Metz, 1966). It contains ~5% embayed quartz phenocrysts and accessory apatite and zircon as the only preserved igneous minerals after intense, texturally- destructive hydrolytic alteration, silicification, and pyritization (15-25% pyrite). Advanced argillic alteration minerals pyrophyllite, dickite, and alunite were reported by some workers

(Bryant, 1964; Bryant and Metz, 1966; Lang et al., 2001) whereas Stegen et al. (2005) argued that advanced argillic alteration is restricted to the breccia units at the bottom of the Lavender pit, and that quartz-sericite-pyrite alone defines the alteration of the older quartz porphyry. What is agreed upon is the presence of intense silicification, hydrolytic alteration, and pyritization of this unit which may have occurred contemporaneous with Cu-poor silica-pyrite replacement bodies in the limestones (Bryant and Metz, 1966; Einaudi, 1982). The older quartz porphyry 18 forms the prominent Sacramento Hill in the Lavender pit, and some geologic maps show it on the footwall (Cochise) side of the Dividend fault (Bryant and Metz, 1966; Lang et al., 2001; unpublished map by E. Wright in Fluor Mining and Metals, 2007) whereas others restrict it to the

Lavender pit (Bryant, 1964; Phelps Dodge Corporation, 1989; this study).

The younger feldspar-quartz porphyry (U-Pb zircon age of 199.9±0.8 Ma; Lang et al., 2001) forms masses, dikes, and sills in the Lavender pit and in underground workings and cuts the older quartz porphyry and contact / “intrusion” breccia (Bonillas et al., 1916; Bryant, 1964). It contains phenocrysts of embayed quartz, sericitized feldspar, and chloritized biotite in a groundmass of quartz and sericite (Bryant, 1964; Stegen et al., 2005). It is ubiquitously sericitized with minor pyrite (~1%). Most publications (except Lang et al., 2001) emphasize a lack of advanced argillic alteration and silicification (Bryant, 1964, 1968; Bryant and Metz,

1966; Stegen et al., 2005). This led Bryant and Metz (1966) to state that alteration affecting the older quartz porphyry occurred prior to the emplacement of the younger feldspar-quartz porphyry. Most authors consider this unit to be pre-main stage Cu mineralization because of its crosscutting relations, common association with CRDs (e.g., Nye, 1968), and itself being variably mineralized in underground exposures (Bonillas et al., 1916; Bryant, 1964).

Descriptions of the Cochise porphyry from this study and previous work most closely resemble those of the younger feldspar-quartz porphyry in the Lavender pit. These two intrusions contain notable differences in pyrite content (~5-15% at Cochise vs. ~1% in the younger feldspar-quartz porphyry in the Lavender pit), but both units have similar phenocryst mineralogy, size, and abundance, so the two intrusions may be genetically related. The Cochise porphyry was studied during drilling campaigns by Phelps Dodge personnel and has a U-Pb zircon age of 200.0

± 0.8 Ma (Lang et al., 2001).

A third possible intrusion encountered during deep drilling named the Warren porphyry has

an age of 201.4 ± 0.8 Ma and was considered to be the deeper equivalent of the younger

feldspar-quartz porphyry (Lang et al., 2001). Stegen et al. (2005) believed the Warren porphyry

to be a distinct stock-like mass beneath the Lavender pit that may connect with a biotite-quartz monzonite dike in the Cochise area.

Breccias of the SHIC

In addition to coherent intrusions, the SHIC also contains widespread and mineralized heterolithic breccias with granule to boulder-sized, angular to subrounded clasts in a clastic rock flour matrix. The “contact” or “intrusion” breccia was considered distinct due to emplacement at the contact between the older quartz porphyry and Paleozoic sedimentary rocks, its locally sheared or mylonitic textures, and its large, isolated clasts of massive pyrite-bornite (Bonillas et al., 1916; Bryant, 1964; Stegen et al., 2005). The genesis and timing of this older breccia relative to CRD mineralization are unresolved (see Appendix A). Stegen et al. (2005) consider the massive sulfide lenses to be clasts of previously formed mantos, whereas Bryant (1964) believed them to be limestone clasts replaced by sulfide in situ after brecciation.

An additional variety of breccia termed the “intrusive” breccia was emplaced as thin films, dikes, sills, and masses, which was variably cemented by quartz, calcite, and sulfide minerals, and was commonly associated with disseminated Cu in the open pits and CRDs in underground workings (Bryant, 1964, 1968; Nye, 1968). Some clasts were apparently transported on the order of 1,000 m, and their rounded shapes and variable orientations recorded a chaotic and turbulent transport environment. These breccias were considered hydrothermal in origin by Bryant and

Nye and classified as phreatic by Sillitoe (1985). Breccias at Cochise differ from these

“intrusive” breccias only in minor detail and were probably formed by similar processes.

Other intrusions in the Mule Mountains

In addition to mineralized intrusions of the SHIC, thin dikes (<5 m) of approximate andesitic

composition cut the SHIC locally. They are fine grained, equigranular to porphyritic, variably altered to clay-chlorite-iron oxides, and were emplaced after mineralization at 190.8 ± 0.5 Ma

(Lang et al., 2001). The 175-Ma Juniper Flat granite forms a large, elongate stock and associated dikes north of Tombstone Canyon and along Escabrosa Ridge northwest and west of Bisbee (Fig.

2). It is the largest intrusion in the Mule Mountains (Hayes and Landis, 1964) and displays minor hydrothermal alteration (Lang et al., 2001), including of the sodic-calcic variety (M. D. Barton, pers. comm., 2021).

Late Jurassic to Early Cretaceous Bisbee Group

The Bisbee Group rests unconformably on a Late Jurassic erosional surface of Proterozoic,

Paleozoic, and Jurassic SHIC rocks. In southeastern Arizona, it records sedimentation mainly in fluvial systems in a region where the Cordilleran foreland basin coincided with a west- northwesterly extension of the Chihuahua trough, an arm of the Gulf of Mexico basin (Bilodeau and Lindberg, 1983; Dickinson, 1989; Blakey, 2014). Ransome (1904) subdivided the group into four different formations: the basal Glance Conglomerate and overlying Morita Formation,

Mural Limestone, and Cintura Formation. The total thickness of the Bisbee Group is estimated to approach or exceed 1,500 m, depending on the extremely variable local thickness of the Glance

Conglomerate (Ransome, 1904; Hayes and Landis, 1964).

Features of the basal Glance Conglomerate provide key insight into the dynamics of post- mineral faulting and supergene enrichment history of the district. Northeast of the Dividend fault

(i.e., in the footwall block where the Cochise deposit is located) the Glance is fairly homogeneous and 0 to 20 m thick, indicating deposition on a low-relief erosional surface of

Pinal Schist and the northeast portion of the mineralized SHIC. Southwest of the Dividend fault, the Glance contains clasts of SHIC and Paleozoic sedimentary rocks, and its thickness ranges from 0 to over 1,000 m thick, indicating substantial relief at that time (Ransome, 1904; Bonillas et al., 1916; Bryant and Metz, 1966). Rounded clasts of gossanous material with chalcocite (Nye,

1968; Stegen et al., 2005) indicate uplift, exposure, oxidation, and some degree of supergene enrichment of the SHIC during the Middle to Late Jurassic.

Structure

The Mule Mountains are divided into two geologic blocks on either side of the northwest- southeast-trending Tombstone Canyon, along which Arizona Highway 80 runs (Figs. 2 and 3).

The northeastern block consists of the Bisbee Group resting on Proterozoic schist and the

Cochise portion of the SHIC; the southwestern block contains densely-faulted Paleozoic rocks and windows of schist all cut by Jurassic dikes and sills. Thus, Jurassic intrusions are exposed on both sides of the canyon, but the preserved sedimentary sections are markedly different.

Paleozoic beds dip gently or moderately away from the Juniper Flat granite, suggested by Bryant and Metz (1966) to indicate anticlinal doming from its emplacement. However, the 10-30° E to

NE dips on post-intrusion Bisbee Group strata (Hayes and Landis, 1964) suggest alternative mechanisms for the modest tilting in the district. Near the ore deposits, most Paleozoic rocks dip

25° to 35° east (Stegen et al., 2005).

The Dividend normal fault parallels Tombstone Canyon and dissects the SHIC. Timing of its movement has implications for the overall geology of the Mule Mountains as well as for supergene mineralization history of the district and depositional environment of the Glance

Conglomerate. Dip measurements on the fault range from nearly vertical (Ransome, 1904), to

60-70° S-SW (Bryant, 1964; Bryant and Metz, 1966), to a range from ~55° S to 90° (Bonillas et

al., 1916; Stegen et al., 2005). Displacement is constrained by Paleozoic sedimentary rocks on

either side of the fault and varies from 600 m to 1,500 m along strike (Bryant, 1964; Stegen et al., 2005). The difference in throw estimations has been attributed to additional displacement on parallel synthetic faults (Bryant and Metz, 1966; Nye, 1968), not to tipping out or tapering displacement on the Dividend fault itself. Between the Cochise deposit and Lavender pit, a displacement of ~1,000 to 1,200 m is probably appropriate.

Substantial movement on the Dividend fault is constrained to the Early or Middle Jurassic, considering that the fault dissects the ~200-Ma SHIC and there are drastic variations in thickness and clast composition in the Glance on either side of the fault (Bryant and Metz, 1966). The

footwall was eroded flat and stripped of its Paleozoic cover before Glance deposition, whereas

Paleozoic rocks preserved in the hanging wall (Lavender pit vicinity) were eroded into hilly

topography and filled by the Glance. These observations, in conjunction with the presence of iron-oxide- and chalcocite-bearing clasts of porphyry in the Glance, indicate substantial movement after the assemblage of the SHIC and mineralization.

Other normal faults of lesser displacement within the hanging wall of the Dividend fault include the northwest-trending Abrigo, Gold Hill, and Bisbee West faults (Ransome, 1904).

Northeast-striking normal faults are conspicuous in the Cretaceous beds northeast of Bisbee

(Hayes and Landis, 1964), and numerous northeast-striking faults within Paleozoic beds to the south of Bisbee, including the Quarry and Czar faults, are commonly associated with ore bodies in the limestones (Ransome, 1904; Bryant and Metz, 1966).

Approximately 10 km southeast of Bisbee (Fig. 2), a reverse fault that crops out near Gold hill has a moderate southwesterly dip and places limestone of the Naco Group on top of Bisbee

Group strata. The reverse fault in turn is cut and offset by a more steeply southwest-dipping

normal fault (Ransome, 1904; Hayes and Landis, 1964). In the vicinity of the reverse fault, the

Bisbee Group beds are commonly folded with steep to overturned dips, and the fault displays

moderate- to steep cutoff angles with bedding and displays a footwall syncline that extends for at

least 10 km (Hayes and Landis, 1964, cross section D-D’).

MINERALIZATION OF THE WARREN DISTRICT

Carbonate replacement deposits (CRDs)

CRDs were the first ores mined in the district and provided ~70% of the district’s total Cu

production and nearly all of its Pb, Zn, Ag, and Au. The replacement bodies formed crudely

stratabound mantos along favorable Paleozoic beds and chimneys at the intersections of faults

commonly associated with porphyry intrusions and hydrothermal breccias (Bryant, 1964; Nye,

1968). Altered intrusions near ore bodies rarely carried economic grades in the early days of the

district, and the advance of mining cross-cuts was stopped when large bodies of porphyry were encountered (Ransome, 1904). Breccias and fault zones also enhanced weathering processes and thus were effective guides to both hypogene and supergene ore. Individual deposits ranged from a few hundred tons to over 1 million tons (Bryant, 1964) and formed a semi-circle radiating outward from the SHIC (Fig. 2; note shaft locations in Fig. 3).

The hypogene mineralogy is zoned from cores of siliceous pyrite, outward to copper sulfides, to lead-zinc sulfides, to outermost specular hematite and manganese masses (Bonillas et al., 1916; Bryant, 1964; Schumer, 2017). Limestone was recrystallized adjacent to CRDs, and skarn alteration is rare (Ransome, 1904; Bonillas et al., 1916; Bryant, 1964). Oxidized ore contained chalcocite, native copper, cuprite, malachite, azurite, chrysocolla, brochantite, and tenorite.

Mineralization in the Lavender and Sacramento pits

The remainder of the district’s Cu production came from open-pit mining of supergene

chalcocite developed in the porphyry intrusions and breccias. The chalcocite blanket ranged from

15-120 m thick and was continuous with supergene enrichment at Cochise, undisrupted by the

Dividend fault (Fig. 4; Cook, 1994; Stegen et al., 2005). The silica-pyrite-altered older quartz porphyry and sericitized younger feldspar-quartz porphyry both contain weak disseminated Cu sulfide and low-grade secondary chalcocite except within permeable fractured zones (Bryant and

Metz, 1966). The contact / “intrusion” breccia contained isolated lenses of massive bornite-pyrite interpreted as limestone clasts replaced in situ (Bryant, 1964) or clasts of previously formed

CRDs (Stegen et al., 2005). Mineralization in the porphyries and breccias are associated with discrete zones of sericitic, advanced argillic, and deep potassic alteration assemblages, although their distribution and relationship to mineralization are contested (see Appendix A; Bryant, 1964;

Bryant and Metz, 1966; Lang et al., 2001; Stegen et al., 2005).

Geologic and geochronologic evidence indicates at least two periods of enrichment. Clasts of mineralized SHIC rocks in the Glance Conglomerate (Bryant and Metz, 1966; Stegen et al.,

2005) require erosion and oxidation of the SHIC during the Middle to Late Jurassic. Evidence for late Cenozoic enrichment includes the roughly conformable geometry of the chalcocite blanket to current topography (Fig. 4), the lack of blanket offset by the Dividend fault, and K-Ar ages of

9.08 ± 0.22 Ma and 3.5 ± 0.33 Ma on supergene alunite and jarosite, respectively (Cook, 1994;

Cook and Porter, 2005). Reddish to maroon hematitic horizons in the leached caps of the

Lavender pit and Cochise deposit suggest multiple enrichment cycles according to researchers who have shown that hematitic horizons indicate the oxidation of former chalcocite blankets

(Blanchard, 1968; Anderson, 1982).

METHODS

During the summer of 2020, the author spent three weeks at the Cochise deposit mapping at

a scale of 1:5,000 with a focus on the distribution of major rock types, the abundance and mineralogy of limonite in the leached cap, and the intensity of hydrolytic alteration. Along outcrops and roadcuts, I identified hematite, goethite, and jarosite by their habit and streak color and generalized the overall limonite character to fit the map scale using the overall hue of

outcrops. In addition, my supervisor and I examined core from six drill holes from the Cochise

deposit; many core records were incomplete due to multiple past assaying campaigns. Polished

thin sections of 20 samples collected from outcrops and core were studied using a petrographic

microscope and a JEOL 6010LA scanning electron microscope (SEM) with energy dispersive X-

ray spectroscopy (EDS). These new data were integrated with prior results from Phelps Dodge

and Freeport-McMoRan studies completed in the late 1980’s through the early 2000’s to capture

the salient geologic features of the Cochise deposit.

Geologic maps of the Cochise deposit from previous studies show important discrepancies.

An effort was made to rectify these disagreements by constructing a new map (Fig. 3) which

resembles an unpublished Phelps Dodge map of the Cochise area completed in 1988. A more

recent map by Ernie Wright (Fluor Mining and Metals, 2007) is also similar but shows “older

porphyry” north of the Dividend fault where the former two maps show only breccia and altered

feldspar-quartz porphyry. Other maps of the SHIC by Bryant and Metz (1966) and in Schumer

(2017) show only older quartz porphyry and no breccias at Cochise. Bryant (1964) shows only

younger feldspar-quartz porphyry and no breccias. Figure 3.7 in Lang et al. (2001) is a recreation

of the 1988 Phelps Dodge map but the units appear to have been symbolized incorrectly.

GEOLOGY AND MINERALIZATION OF THE COCHISE DEPOSIT

Rock types

In the area encompassing the Cochise deposit, the footwall of the Dividend fault is composed of Pinal Schist intruded by one or more feldspar-quartz-biotite porphyry stocks and irregular masses and dikes of breccia (Figs. 3 and 4). All of these units are mineralized and

altered.

The Pinal Schist is distinctly foliated with 1 to >10 cm bands of varying proportions of quartz and fine-grained muscovite and local layers with minor 0.1-0.2 mm red garnet. In outcrop,

schistosity is often accentuated in sulfide-stable zones exposed to mechanical weathering but is

less conspicuous above in the limonitic leached cap. Foliations generally strike north-south ± 20°

with variable dips and in places are cut by wavy, folded quartz veins (Fig. 5A). The schist is

white to slightly greenish and glossy on fresh faces away from oxidized sulfides. In general,

alteration is poorly developed in the schist compared to the other lithologies and mineralization

is typically weak.

The Cochise porphyry and breccia complex was the focus of hydrothermal activity at the

Cochise deposit. This biotite-quartz-feldspar porphyry forms an irregularly-shaped, crudely

northwest-elongate stock with occasional dikes into the Pinal Schist and contains large xenoliths

or roof pendants of schist locally. Other than uncommon thin (<2m) dikes of post-mineralization

andesite, the Cochise porphyry is the only voluminous intrusion at the surface. It is porphyritic-

aphanitic (Fig. 5B), with phenocrysts of ~30% 2-5 mm euhedral to subhedral sericitized feldspar,

5-10% 2-5 mm embayed round quartz, and 2-5% 0.5-3 mm euhedral chloritized and/or

sericitized biotite in a very fine-grained groundmass of sericitized feldspar, quartz, and accessory

rutile and zircon. If another distinct intrusive phase crops out at Cochise, it was not identified.

Deeper samples from drill core are obscured by pervasive K-silicate and sericitic alteration and

are medium-grained and seriate in texture with ~40% plagioclase, 20% orthoclase feldspar, 25-

30% quartz, and 10-15% biotite.

Drill hole CCSC-5 at 340 m depth intersected a dike(?) of altered and mineralized fine-

grained quartz diorite(?) with 0.2-1 mm quartz-pyrite-chalcopyrite veinlets and pervasive chlorite, sericite, and preserved biotite. Vein-hosted and disseminated chalcopyrite content is

elevated (~1%) compared to the adjacent, more felsic Cochise porphyry. Most drill logs do not

document this more mafic dike and it was not studied in detail here.

Dikes and large irregular masses of mineralized breccia occur at the surface and at depth at

the Cochise deposit, similar to the Lavender pit. They form at the contacts between the Cochise

porphyry and the schist and as dikes and irregular masses within those units. Contacts are locally

sharp but elsewhere appear cryptic and diffuse. Significant volumes of breccia occupy the

southwestern margin of the Cochise porphyry near the Dividend fault. Breccia is juxtaposed with

the older quartz porphyry of the Lavender pit in the hanging wall (Figs. 3 and 4). Subangular to

rounded clasts of Cochise porphyry, schist, and rare broken quartz veins range from sand-sized

up to 30 cm in length, set in a fine-grained matrix of pulverized quartz, sericite, iron oxides and

clay (Figs. 5C – 5E). No clasts of Paleozoic sedimentary rocks, gossanous material, or massive

sulfide were observed. In one sample of core studied, angular fragments of quartz-molybdenite

veins suggests at least one pre-brecciation mineralizing event, but the majority of mineralization

appears to have occurred after the formation of the breccias. Core sample BC-60-652’ has K-

feldspar flooding of the breccia matrix indicating potassic alteration occurred, at least in part,

syn- to post-breccia formation. Pyrite and lesser chalcopyrite and bornite are disseminated

irregularly throughout the matrix as well as in veins cutting across both the matrix and clasts.

Locally, the breccia is silicified and resistant to erosion but elsewhere is composed mostly of

pulverized rock flour, sericite, and iron oxides and is somewhat soft.

Post-mineral dikes striking E-W ±20° cut the Cochise porphyry and schist in numerous

places. They are up to 3 m wide, light gray and aphanitic or have subtle outlines of former 1-2

mm feldspar phenocrysts. The rock is thoroughly altered to very fine-grained sericite, chlorite,

clay and ~10-20% iron oxides. Exotic limonite coats fractures on outcrops, but there is no

indication of sulfide mineralization. These are probably the andesitic dikes for which Lang et al.

(2001) reported a U-Pb zircon age of 191 Ma. The lack of quartz phenocrysts and prevalence of

fine Fe and Ti oxides is consistent with an intermediate composition.

Hypogene alteration

Sericitic (fine-grained muscovite) alteration is ubiquitous at the surface and at shallow levels

of the system. It becomes increasingly intense and texturally-destructive towards the center of the deposit, where large bodies of hydrothermal breccia contact the Cochise porphyry and schist.

Petrographic work from this study and by Phelps Dodge staff in the 1980’s show that the upper

~300 m of the deposit contains 25% to 80% sericite. Sericite forms halos (Fig. 6A) along quartz-

pyrite ± chalcopyrite-bornite veins in all lithologies and replaces feldspars phenocrysts and

groundmass minerals in the Cochise porphyry. Biotite phenocrysts are replaced by chlorite

and/or sericite and host rutile and sulfide or hematite along altered cleavage sheets. In the quartz-

muscovite Pinal Schist, sericitic alteration is mostly cryptic or undiscernible with the exception

of slightly coarser muscovite halos around quartz-pyrite veins. The matrix of the hydrothermal breccias contains very fine sericite as well as coarser crystals and radiating clusters around quartz-pyrite veins cutting the matrix (Figs. 5E and 6B), indicating post-brecciation growth of sericite and pyrite. Local zones of stockwork quartz veins, silicification, 10-20% pyrite, and

intense hydrolytic alteration (Figs. 6C and 6D) suspected of having undergone advanced argillic

alteration were instead masses of almost entirely sericite, quartz, and pyrite. At depth, sericitic alteration wanes in intensity but persists to at least 900 m (the deepest drill holes) as a partial overprint of former orthoclase-chlorite±tourmaline alteration (Fig. 6E). Sericite-pyrite alteration extends laterally at least 1 km in all directions from the center of the system into the schist where it becomes increasingly subtle.

Core samples of altered porphyry contain variable amounts of green sericite replacing

feldspar and groundmass minerals (Fig. 6F). Semi-quantitative EDS analyses indicate these

green-tinted micas contain 1-4% Mg ± Fe, indicating a substantial phengitic component. No

SWIR or XRD analyses were performed to discern crystallographic properties of these micas,

but the K, Mg, Fe, and Al ratios are appropriate for the phengite solid solution series between

end-members of muscovite (K(Al)2(AlSi3)O10(OH)2) and Mg celadonite

(K(Mg,Al)2(Si4)O10(OH)2) with minor Fe. Sericite immediately adjacent to veins is typically colorless, which may indicate multiple sericitic alteration events or dependence of mica color and chemistry on distance from the veins (i.e., a fluid-rock chemical interaction gradient).

Potassic alteration occurs in all lithologies below depths of ~300 m, where sericitic overprinting was incomplete. In the biotite granodiorite, quartz veins have orthoclase halos with variable K-feldspar flooding of the groundmass and chlorite after biotite (Figs. 7A and 7B).

Quartz veins in this zone are associated with weak to moderate (~0.5 to 3% total sulfide) pyrite- chalcopyrite-molybdenite mineralization. Select core samples of breccia with clasts of schist and fragmented quartz-molybdenite veins contained granular orthoclase-quartz replacement of the matrix and samples of schist on the west side of the deposit adjacent to the Cochise stock at depths around 300 m had secondary orthoclase as thin sheets or fingers along foliation planes. It

is possible that potassic alteration extended to levels of the system now exposed at the surface.

Compelling textural evidence for this was not observed, but given the degree of textural

destruction by sericitic alteration towards the center of the deposit, the possibility remains.

The CCSC series drill holes by Occidental in 1975 are restricted to the northwestern quarter

of the deposit and provide the only samples deeper than 350m. Core samples CCSC-5-840m and

CCSC-3-916m both contain tourmaline within and adjacent to quartz-orthoclase veins in the granodiorite (Figs. 7B - 7D). Tourmaline is black in hand sample and bluish green in thin section and forms anhedral granular clusters as well as radiating fans. EDS measurements indicate

2+ compositions between schorl and dravite (Na(Fe ,Mg)3Al6(Si6O18)(BO3)3(OH)3(OH)). These

deep samples also contain siderite as small clusters of rhombohedral grains between radiating

tourmaline needles and in pockets in granular quartz veins (Figs. 7C and 7D). Hematite is common along siderite grains boundaries and cleavage planes.

Advanced argillic alteration assemblages (quartz-alunite±pyrophyllite-dickite) were documented in the Lavender pit by Bryant (1964) and Stegen et al. (2005) but do not appear to be significant within the Cochise system contrary to comments by Lang et al. (2001). Field mapping and petrography on 20 samples did not identify hypogene occurrences of pyrophyllite, alunite, or other assemblages formed at low aK+/aH+. Interference figures of colorless micaceous grains thought to be pyrophyllite had 2V angles that were too small to be pyrophyllite, and EDS measurements on grains resembling pyrophyllite macroscopically all contained potassium. Late irregular veins of monomineralic white to pinkish alunite with no halos occur in the leached cap and enriched zones and minor kaolinite occurs in fault zones and in the weathered leached cap, but these are supergene in origin. X-ray diffraction analysis of core from metallurgical drill holes

at Cochise by Phelps Dodge in 1997 and petrography by Phelps Dodge in the 1980’s indicated

abundant quartz, sericite, and pyrite, but minor amounts of kaolinite and no pyrophyllite.

Mineralization

The Cochise porphyry, breccias, and schist all host hypogene and supergene sulfide

mineralization. The Cochise resource of 262 Mt averaging 0.46% Cu (Freeport-McMoRan Inc.,

2021) is entirely based on the secondary chalcocite-covellite zone. The Pinal Schist represents a very minor portion of this because hypogene and supergene Cu mineralization wanes rapidly outward from the porphyry-breccia complex.

Underlying the leached cap and enriched zone are hundreds of millions of tons of hypogene material grading 0.13 to 0.21% Cu with small zones up to 0.5% Cu that are not economic to leach (Fluor Mining and Metals, 2007). The total extent of hypogene mineralization has not been delineated by drilling. Both the potassic-chloritic and the sericitic alteration zones contain copper sulfide mineralization. Quartz-orthoclase-chlorite ± tourmaline alteration was accompanied by weak pyrite-chalcopyrite-molybdenite mineralization with total sulfide contents of 0.5 to 3% and pyrite:chalcopyrite ratios of 1:1 to 3:1. Chalcopyrite and rare bornite are hosted within and adjacent to the quartz-(orthoclase) veins and as disseminations in chloritized igneous biotite.

At shallower levels, sericite-pyrite ± quartz overprints the quartz-orthoclase-chlorite ± tourmaline alteration and carries ~5-12% pyrite and 0.1 to 0.5% chalcopyrite with trace bornite.

Pyrite and minor chalcopyrite occur in sericite veins with or without quartz, in sericite vein halos

(Fig. 8A) and as apparent disseminations in pervasive sericitic alteration of the porphyry. The breccias contain pyrite ± chalcopyrite as veins cutting through both the clasts and matrix as well as round to irregular blebs dispersed throughout the matrix. Large grain clusters of framboidal pyrite are internally fractured but are otherwise unbroken in the breccia (outside of fault zones)

(Fig. 6B). Some of the pyrite grains in the breccias contain 1-3% 2-10 μm round blebs of chalcopyrite, bornite, and rare galena (Fig. 8B). In comparison to the potassic zone, the sericitic alteration carries approximately the same amount of total copper, but the amount of pyrite is substantially higher.

Supergene enrichment is strongest beneath Jones Hill and Copper King Hill and in the canyon between them at the center of the Cochise porphyry and breccia complex. Deeper and especially intense enrichment occurs within faults, and elevated acid-soluble Cu in drill assays commonly represent such features. The chalcocite blanket ranges from 15 to 175 m thick and is thicker beneath topographic highs (Fig. 4) (Stegen et al., 2005). The blanket dips modestly southwest, subparallel to the pre-mining topography, into the Lavender pit and is not offset by the Dividend fault (Cook, 1994). Beneath the jarosite-dominated lower terminus of the leached cap, chalcocite and lesser covellite replace Cu sulfides and coat the outer margins of pyrite (Figs.

8C and 8D). Overall, chalcocite comprises ~80% of the secondary sulfide content and covellite

~20%, although their distribution is variable. The abundance of chrysocolla, malachite, and other

Cu oxide minerals is negligible.

Leached cap characteristics (supergene alteration)

The conspicuous leached cap overlying the Cochise deposit ranges from 5-80 m thick, and its lower boundary with enriched sulfides mimics modern topography, with exceptions (Fig. 4).

A central area ~1,000 by 800 m overlying the zone of substantial chalcocite enrichment (Fig. 9) hosts moderate to intense (~3-10%) hematite as oxidized stockwork veins (Fig. 10A) and crusty maroon aggregates or black masses along fault planes (Fig. 10B). Along ridges and outcrops exposed directly to weathering, hematite forms thin films on fractures and is commonly black but has a red streak. Goethite typically occurs with hematite in subordinate amounts and coats

fractures adjacent to relict sulfide veins occupied by indigenous hematite. The leached cap

column progresses downward from red hematite-rich limonite to yellowish-brown jarosite-rich limonite above the supergene sulfide interface (Figs. 10C and 10E). Fine grained mustard-yellow to greenish-brown jarosite fills fractures immediately above pyritic zones and also forms euhedral crystal-lined cavities in outcrops of schist and breccia. EDS analyses of surface samples

3+ of such jarosite indicate the presence of both K-jarosite (KFe 3(SO4)2(OH)6) and lesser

3+ natrojarosite (NaFe 3(SO4)2(OH)6). One sample of breccia contained minor barite needles

intergrown with jarosite as well as small cores of woodhouseite (CaAl3(PO4)(SO4)(OH)6)

containing trace As, Pb, and Sr content, surrounded by jarosite. Alunite forms granular or

plumose, pearly to chalky white veins (Fig. 10D) in the leached cap and into the underlying

supergene Cu zone to depths up to ~200 m. The alunite veins lack altered halos and are

monomineralic except where wall rock minerals are entrained. Kaolinite occurs in fault zones

and on some weathered surfaces but does not appear to be abundant.

Sulfide is not preserved at the natural surface, but pyrite and trace molybdenite were observed in the mixed jarosite-chalcocite-pyrite zones along road cuts and drill pads. Low concentrations, 100 – 400 ppm Cu, in the leached cap (Phelps Dodge Corporation, 1992) indicate efficient leaching of Cu during the weathering of sericite-altered rocks containing ~5-12% pyrite.

Chrysocolla and malachite were observed as minor occurrences in only two places, in harmony with the insignificance of the oxide Cu ore type in the Cochise resource models (Phelps Dodge

Corporation, 1992).

INTERPRETATIONS

Sequence of alteration and mineralization

Crosscutting relationships between the Cochise porphyry, breccias, and different vein styles indicate the following sequence of events:

1. The Pinal Schist was intruded by the Cochise porphyry, and possibly by an additional phase

at depth, as stocks and dikes at ~200 Ma (Lang et al., 2001) as part of the SHIC.

2. At least one minor set of quartz ± molybdenite veins cut the porphyry and later became

infrequent clasts in the hydrothermal breccias.

3. Breccia dikes and masses with variably rounded fragments of schist and porphyry in a sandy

to very fine-grained matrix of pulverized rock flour formed at moderate to shallow levels.

4. K-silicate alteration manifested as quartz-orthoclase-chlorite ± tourmaline-siderite veins in

the intrusions and local orthoclase flooding in the matrix of breccias and in the schist

adjacent to the intrusions; accompanying sulfide was weak (0.5-3%) as vein-hosted and

disseminated pyrite-chalcopyrite ± molybdenite ± bornite.

5. Sericitic overprinting was both widespread and intense towards the center and current surface

of the deposit and precipitated substantial pyrite (5-12%), chalcopyrite (~0.1-0.5%), and trace

bornite. Sericite overprinted former K-silicate alteration in all rock types, including forming

euhedral flakes and unbroken plumose clusters in the breccia matrix. Pyrite ± chalcopyrite ±

bornite veins cut the consolidated breccia matrix and formed disseminated blebs and

framboidal aggregates. The Cu grade of the K-silicate and sericitic alteration assemblages are

similar, so it is possible that Cu sulfides were reprecipitated during sericitic alteration but not

enhanced or transported appreciably (discussed below).

6. Post-mineral andesitic dikes were emplaced around 191 Ma (Lang et al., 2001), constraining

the age of mineralization to Early Jurassic, shortly after the assembly of the SHIC.

7. Drastic differences in the Glance Conglomerate on either side of the Dividend fault suggest

significant normal movement prior to the deposition of the Glance but after assembly of the

SHIC. Clasts of porphyry with oxidized sulfides in the Glance Conglomerate northeast of the

Cochise deposit indicate uplift, erosion, and possibly an initial enrichment event by the

Middle to Late Jurassic. The area was submerged and buried by rock of the upper formations

of the Bisbee Group.

8. Uplift and erosion during the Late Cretaceous to Cenozoic (Laramide orogeny) again

exposed the bisected SHIC to weathering. Reverse faults in the Mule Mountains were not

mapped near the SHIC so their effect on the geometry of mineralization is unknown but

probably minor. Many of the small-displacement normal faults mapped throughout the

district facilitated strong supergene enrichment and likely formed as a result of Cenozoic

extension. K-Ar dates on supergene alunite and jarosite (Cook, 1994; Cook and Porter, 2005)

and the approximate conformity of the enrichment blanket to modern topography suggest that

much of the enrichment in the district occurred during the Miocene and younger.

All of these temporal relationships are compatible with proposed sequences observed in the

Lavender pit and in underground workings by Bryant (1964; 1968) Bryant and Metz (1966), and

Nye (1968).

Presence or absence of advanced argillic alteration at Cochise

Hypogene advanced argillic alteration minerals (e.g., pyrophyllite, alunite, well crystallized

kaolinite and dickite) were not observed during this study which suggests they do not occur at

the Cochise deposit or are volumetrically minor relative to widespread and intense sericite

alteration. The characteristics of alunite and kaolinite, well developed only at shallow levels, are

consistent with a supergene origin. The mineral woodhouseite, observed in one jarosite-rich

sample of breccia, has been documented in advanced argillic assemblages at other hydrothermal

deposits along with other aluminum phosphate-sulfate (APS) minerals (Stoffregen and Alpers,

1987; Seedorff et al., 2005). However, APS minerals might be stable weathering products of apatite within the acidic, sulfate-rich, and low-temperature conditions characteristic of oxidizing supergene zones of porphyry Cu systems. The textural relationship of woodhouseite and jarosite at Cochise suggests a supergene origin, and the documented occurrences of APS minerals in sedimentary rocks (Stoffregen and Alpers, 1987) also permits this interpretation. Lang et al.

(2001) state briefly that pyrophyllite, kaolinite, and alunite accompanied by APS minerals occur in the uppermost parts of the Cochise deposit, contradicting the conclusions drawn here. Future work analyzing surface and/or core samples using short-wave infrared (SWIR) reflectance spectroscopy and/or X-ray diffraction (XRD) may detect overlooked advanced argillic-altered portions of the Cochise deposit or alternatively further suggest a lack of such alteration.

Relationship of the Cochise porphyry to Lavender pit intrusions

As mentioned earlier, another disagreement from past studies is the extent to which the intrusion(s) at Cochise north of the Dividend fault correlate with the intrusions in the Lavender pit. Studies on the Warren district over the last 60 years show the Cochise area intruded by only the older quartz porphyry (Bryant and Metz, 1966; Graeme, 1981), only the younger feldspar- quartz porphyry (Bryant, 1964; Phelps Dodge Corporation, 1989), or both (Stegen et al., 2005; E.

Wright, Fluor Mining and Metals, 2007).

Based on similar mineralogy, igneous textures, and nearly identical U-Pb crystallization ages (Table 1), the Cochise porphyry most resembles the younger feldspar-quartz porphyry in the

Lavender pit. However, the similarities (% of quartz, feldspar, and biotite phenocrysts) are

somewhat generic and the two may be distinct. The geologists who studied the Lavender pit and

underground workings (Bonillas et al., 1916; Bryant, 1964; Bryant and Metz, 1966; Nye, 1968)

paid little attention to Cochise. In addition to this lack of scientific continuity, texturally-

destructive alteration of the central portion of the Cochise stock and the older quartz porphyry in

the Lavender pit obscures the original igneous characteristics.

Whether the deeper intrusion at Cochise, a medium-grained seriate biotite granodiorite, is a

deeper textural variation of the Cochise porphyry or a separate intrusion (e.g., the “Warren

porphyry” of Stegen et al., 2005) is unknown. Core was not studied sufficiently to identify

contacts or a textural continuum between the shallow and deep varieties. Because of the

distinctly coarser texture and higher biotite content (10-15%) of the deeper variant, it is likely a separate phase. Future investigations focusing specifically on correlating intrusions on both sides of the Dividend fault with the aid of immobile element lithogeochemical fingerprinting may shed light on these issues and would allow for more confident reconstruction of the magmatic- hydrothermal history of the district.

The Cochise deposit as the “roots” of Lavender pit porphyry and CRD mineralization

The genetic and spatial relationship of the Cochise deposit to the porphyry and CRD mineralization in the hanging wall of the Dividend fault has been a topic of interest by prior workers (Lang et al., 2001; Stegen et al., 2005; M.T. Einaudi, pers. comm., 2019). In general, they envisioned Cochise to be the lower roots of the SHIC and magmatic-hydrothermal system in the Lavender pit and in underground workings. Restoring movement on the Dividend fault using a 75°SW dip and a displacement of 1,200 m and un-tilting the Paleozoic strata and Bisbee Group to horizontal moves the Lavender pit SHIC and CRD-hosting Paleozoic rocks approximately 1

km above and 0.5 to 1 km southwest of Cochise (Fig. 11). The base of the 130 m-thick Bolsa

Quartzite (Ransome, 1904) would project ~200 m above Cochise; because small remnants of

Bolsa crop out southeast of Cochise in the footwall of the Dividend fault (Fig. 3) this simple reconstruction appears reasonable. This suggests that Cochise represents the deeper lateral flanks of the Lavender pit, but whether it is the actual roots is questionable. A less steep dip (50° to

60°) on the Dividend fault would restore the Lavender pit closer to directly above Cochise.

In addition to the spatial and temporal constraints (see section on structure), the following observations further support this hypothesis: 1) advanced argillic alteration, which occupies shallower portions of some porphyry systems (e.g., Sillitoe, 2010) is nonexistent or minor at

Cochise and significant in the Lavender pit (Bryant, 1964; Stegen et al., 2005), and 2) total sulfide contents are somewhat higher in the hydrolytically-altered part of the Lavender pit (15-

25%; Bryant, 1964; Stegen et al., 2005) than in the Cochise deposit (~3-15%; Stegen et al., 2005; this study). If this is accurate, the Warren district would represent a dismembered, multi-phase magmatic-hydrothermal system with surface access and drilling through nearly 2 km of the system. A thorough study involving logging core from both deposits (if available), updated mapping in the Lavender pit, geochemistry, and alteration mineral identification (XRD, SWIR) would more accurately illustrate vertical changes within this porphyry-breccia complex and the zoned alteration-mineralization patterns that have mostly been documented separately thus far.

Origin of breccias at Cochise

Breccias are common in porphyry and epithermal deposits and can be classified based on clast composition, morphology and transport distance, matrix material (rock flour vs. juvenile magmatic material vs. hydrothermal vein cement), and relationship to mineralization (pre-, syn-, to post-ore) (Sillitoe, 1985). Many ore-related breccias possess characteristics intermediate

between end-member classes so their origins and products likely form a continuum, making their

classification and interpretation difficult (Sillitoe, 1985; Seedorff et al., 2005). The most

important features of the Cochise breccias for interpreting their origin are: 1) they intruded

solidified porphyry and schist as dikes and large irregular masses, 2) they are composed of

poorly-sorted, angular to rounded, sand- to cobble-sized clasts in a matrix of rock flour with no

apparent igneous material or syn-brecciation hydrothermal cement, and 3) the matrix contains

equant to euhedral, unshattered blebs and framboidal clusters of pyrite, euhedral flakes of

secondary muscovite, is cut by through-going veins of pyrite-chalcopyrite-sericite, and is

elsewhere flooded by K-feldspar. Unlike the ~1,000 m distance of clast transportation interpreted for breccia clasts in the Lavender pit (Bryant, 1964), no stratigraphic source materials are known at Cochise to permit such an estimation.

These observations are compatible with the phreatic variety of breccias related to porphyry deposits defined by Sillitoe (1985) where magmatic heat causes a rapid expulsion of fluid that

transports material along permeable pathways. The source, or sources, of fluids for the breccias

at Bisbee and Cochise has not been evaluated. This variety is commonly late- or post-

mineralization in many porphyry systems according to Sillitoe (1985) which does not seem to be

the case for the Cochise breccias or the “intrusive” breccias of Bryant (1964) which are

sufficiently mineralized and, in the latter example, associated with CRD ore and interpreted as

pre-existing fluid pathways for CRDs. Bryant estimated that >90% of all CRD bodies were associated with these breccias; at Cochise, the majority of the resource occurs in the breccias and in the porphyry within ~250 of their contact (Fig. 9B). Schist and porphyry clasts were

transported an unknown distance and variably rounded and eventually consolidated in a matrix

of pulverized silicate minerals (quartz, feldspar, muscovite, clay) derived from the same source

as the clasts. Intact sulfides and hydrothermal muscovite grains as well as veins cutting the

matrix indicate the breccias were mineralized after they consolidated, alongside the Cochise

porphyry. Pre-brecciation mineralization is possible, but a lack of mineralized clasts (aside from

minor quartz ± molybdenite veins mentioned) suggests this was minor at most.

Copper content of sericite alteration

The Cu content of phyllic (sericite-pyrite ± quartz) alteration in porphyry Cu deposits is

variable and can be considerable (e.g., Resolution; Manske and Paul, 2002) or low-grade to

barren (Sillitoe, 2010). Within the intrusions and breccias at Cochise, chalcopyrite and lesser

bornite consistently occur in the unoxidized (non-weathered) sericitic assemblages alongside

pyrite or as blebs within it. Sericitic alteration dominates the upper ~200-400 m of the deposit,

which is where the entire supergene Cu blanket is developed. These observations indicate that

the sericitic alteration zone at Cochise originally contained sufficient hypogene Cu

mineralization to subsequently form the secondary chalcocite resource via supergene processes.

However, whether the Cu sulfides in the sericitic alteration zone were introduced by the fluids

that altered the rocks to sericite or if instead the Cu was only leached and reprecipitated from

existing K-silicate-pyrite-chalcopyrite ± bornite-molybdenite mineralization is worth

considering.

The leached cap (the surficial footprint of oxidized hypogene sulfides) and the zone of

sericitic alteration both extend laterally beyond the chalcocite blanket (Fig. 9), indicating that the

peripheral sericitic zone was not appreciably mineralized. This means the sericitic alteration zone

varied laterally with respect to Cu content. The few drill holes sufficiently deep to test both the

potassic and sericitic zones (the 1975-76 Occidental CCSC-series) indicate that the Cu contents of both zones are approximately equivalent. Potassic alteration was noted on the lowest benches

of the Lavender pit (Lang et al., 2001), and because these rocks would structurally restore to

approximately 1 km above and slightly to the side of the Cochise deposit, it is probable that

potassic alteration once extended through surficial exposures at Cochise prior to intense

hydrolytic overprinting. All of this suggests that sericitic alteration reprecipitated, but only

remobilized over short distances, Cu mineralization introduced previously by potassic alteration.

No compelling textural or geochemical data are available to substantiate this.

As an alternative thought experiment, considering the need to source metals for the CRDs, and considering the low hypogene Cu grades of hydrolytically-altered rocks at Cochise (~0.15%

Cu) and the porphyry intrusions in the Lavender pit (~0.1% Cu with small zones 0.3-0.5%;

Stegen et al., 2005; unpublished Freeport data), partial leaching and remobilization of protore

hypogene Cu during sericitic and advanced argillic alteration could be a source. Using the simple

reconstruction of the SHIC (Fig. 11), the hydrolytic alteration zone had approximate aerial and

vertical extents of >1 million m2 and at least 1.2 km, respectively. This volume of 1.2 billion m3 or ~3.2 billion tons of altered rock (at a density of 2,700 kg / m3) would need to have contributed

an average of 0.1% Cu from hypogene hydrolytic leaching to move 3.2 Mt of Cu into the 53 Mt

of CRD ore averaging 6% Cu. For example, a decrease in grade from 0.25% Cu to 0.15% Cu by

sericitic alteration of K-silicate-altered and mineralized rock could be a possible source of the

0.1% Cu needed. This scenario has not been tested rigorously at Bisbee, but there are examples

of hypogene remobilization of metals from K-silicate-altered porphyry deposits into high-grade

veins via hydrolytic alteration backed by geologic observations and experimental data (e.g.,

Butte, Montana; Brimhall, 1980). Deeper drilling at Cochise may reveal that earlier K-feldspar-

stable alteration assemblages contain higher concentrations of Cu, Zn Pb, and precious metals than sericitic assemblages to support this scenario. Telescoping of alteration zones atop one

42 another via episodic intrusions and fluid pulses, as has been suggested at Bisbee (Bryant, 1964;

Bryant and Metz, 1966; Nye, 1968), would create a mechanism for such hypogene leaching as fluids of varying temperature, oxidation state, and acidity were superimposed on existing mineralogic assemblages.

Estimation of supergene enrichment factor and volume of leached rock

In addition to metallurgical (SX-EW) and environmental (no smelting or tailings) advantages, the supergene zones of porphyry Cu deposits typically contain Cu concentrations 2 to 10 times that of the underlying hypogene mineralization (Titley and Marozas, 1995). To estimate the enrichment factor at Cochise (see Fig. 12), some generalizations and assumptions must be made. Copper grades in the supergene zone range from 0.2% to over 2% within permeable structures but the resource averages 0.46% Cu. Grades in the pyrite-chalcopyrite ± bornite zone beneath enrichment range from 0.13 to 0.21 % and average 0.15% Cu (Stegen et al.,

2005; Fluor Mining and Metals, 2007). These figures indicate an enrichment factor ranging from

1.5 to >10X, with an average of 3X (from 0.15% to 0.46%) for the 262 million metric ton resource. Assuming a rock density of 2,700 kg / m3, the chalcocite resource comprises ~100 million m3 of mineralized rock. For this volume of 3X-enriched rock, the Cu content of ~200 million m3, or a column around 220 m thick, of mineralized rock averaging 0.15% Cu was leached, remobilized, and added to the protore grades of the supergene resource. This assumes extremely efficient leaching of Cu during supergene oxidation. This is probably a fair assumption considering the high pyrite content (5-15%), poor acid-neutralization capacity of quartz-sericite rocks, very low Cu grades in the leached cap (~100-400 ppm Cu), and essentially no chrysocolla or Cu carbonates in the leached cap. The thickness of the leached cap varies from

80 m to essentially zero (Fig. 4), indicating that a substantial amount, likely the majority (~200

m), of the leached material that contributed to the secondary enrichment blanket at Cochise has been eroded away. Clasts of mineralized Jurassic porphyry clasts in the Glance have already been described. Because the published resource represents only a portion of the total mineralized rock, a calculation of all supergene and hypogene material would involve greater volumes of rock at lower grades. Also, any lateral movement of fluids which are important hydrologic factors to consider for supergene and exotic systems were ignored in this simple exercise.

DISCUSSION

Comparison to other supergene porphyry Cu deposits

Porphyry Cu deposits occur in many premier metallogenic provinces around the world and

display much variability with respect to tonnage, grade, and hypogene and supergene

characteristics. A sample of individual deposits and provinces are briefly described below to put

into context the important features of the Cochise deposit described so far.

Southwestern North America

The Jurassic Cochise deposit is smaller than most Laramide supergene porphyry Cu deposits

in southwestern North America (Leveille and Stegen, 2012), but is quite similar with respect to

enrichment factor, hypogene and supergene Cu grades, blanket thickness, and supergene

alteration products (Titley and Marozas, 1995, Table 1). Mineral Park, Ray, Silver Bell and

Morenci in Arizona, Santa Rita in New Mexico, and La Caridad in Mexico all have hypogene Cu

grades of 0.07 – 0.3% with enriched grades of 0.34% to 0.85%. In these examples, hypogene

alteration beneath enrichment consist of variable amounts of orthoclase and biotite but

ubiquitous sericite, and supergene alunite is common. Using the hypogene Cu grades (assuming

entirely chalcopyrite) and pyrite:chalcopyrite ratios from Titley and Marozas (1995, Table 1)

these other deposits contained ~2 to >5% hypogene pyrite, indicating that Cochise (5-15%

pyrite) is on the relatively high end of the pyrite spectrum for porphyry systems in this region.

Portions of the Lavender pit have hypogene Cu grades of ~0.1% and pyrite content of 15-25%

(Bryant, 1964; Stegen et al., 2005), indicating an even higher pyrite content and higher

pyrite:chalcopyrite ratio than Cochise. Previous authors (e.g., Blanchard, 1968; Anderson, 1982)

have used empirical and geochemical evidence to demonstrate the need for high pyrite content

and moderate to high pyrite:Cu sulfide ratios to efficiently oxidize and remobilize Cu from

sulfide deposits during supergene alteration. It is clear that the porphyry systems at Bisbee and

Cochise attain those conditions. The low hypogene grades, lateral extent of mineralization

(although increased by dissection of the SHIC by the Dividend fault), and erosion were likely the

factors confining the volume and grade of secondary enrichment in the Warren district.

Northwestern North America

In British Columbia, Yukon, and Alaska, Mesozoic porphyry deposits are common. Many of the porphyry deposits formed in late-Paleozoic to Early Jurassic island arcs and oceanic sedimentary basins that were accreted as terranes onto western Canada in the Jurassic (Logan and Mihalynuk, 2014). Numerous alkalic Cu-Au porphyry deposits, including Afton, Mount

Polley, and Galore Creek, as well as calc-alkalic Cu-Mo porphyry deposits including Highland

Valley and Gibraltar, were formed within these arcs near the Triassic-Jurassic boundary. In stark contrast to southwestern North America, the Cretaceous and younger porphyry deposits in B.C. are dwarfed in number and economic importance by those formed between ~195 – 210 Ma. To explain the concentration of metal endowment of that period, Logan and Mihalynuk (2014) argue that slab tears at the margins of subducted arcs or ocean rides produce the partial melts suitable for the formation of the deposits.

Because of unfavorable climatic conditions (permafrost, cold temperatures) and glacial erosion, most porphyry Cu deposits in the region lack significant supergene enrichment

(McMillan et al., 1995; Young et al., 1997). In contrast, supergene-enriched deposits occur predominantly in orogenic belts with arid, temperate, or tropical climates (Titley and Marozas,

1995; Sillitoe, 2005). In addition, many of the Triassic to Jurassic alkalic porphyry Cu ± Au deposits of British Columbia (discussed again below) contain relatively weak hydrolytic alteration and low pyrite (Bissig and Cooke, 2014), which are necessary precursors for efficient supergene enrichment. In addition, carbonate in sulfide-bearing veins is documented in many alkalic porphyry deposits (e.g., Afton and Mount Polley, British Columbia, McMillan et al.,

1995; Cadia district, Australia, Harris et al., 2020) which would neutralize acid generated by the oxidation of sulfide minerals.

An important exception is the Late Cretaceous Casino porphyry Cu-Au-Mo deposit in

Yukon of western Canada. It has a considerable supergene zone and overlying leached cap, permitted by the higher pyrite content and hydrolytic alteration typical of calc-alkaline porphyry deposits as well as the unglaciated nature of the region (Allan et al., 2013). The Big Onion porphyry Cu-Mo prospect of British Columbia (Wojdak and Stock, 1995) also has a well- developed sericite-pyrite halo which allowed supergene oxidation and formation of chalcocite and covellite. For both of these examples, the degree of enrichment is still low.

South America

The Cenozoic Andean arc of Chile, Peru, and Argentina is the most prolific porphyry Cu province on Earth (Sillitoe, 2012) and hosts the giant deposits El Teniente (94 Mt Cu),

Chuquicamata (66 Mt Cu), Rio Blanco-Los Bronces (57 Mt Cu), La Escondida (32 Mt Cu), and

El Salvador (11 Mt Cu) (Cooke et al., 2005). The most productive periods in southern Peru and

northern Chile were the Paleocene (~50-60 Ma) and Eocene to early Oligocene (~43-32 Ma) and

in central Chile and adjacent Argentina during the Miocene-Pliocene (10-6 Ma) (Mpodozis and

Cornejo, 2012). Deposits formed in areas with thickened crust and long-lived magmatic activity,

commonly near major structures but in varied local stress regimes. Supergene activity

commenced after uplift in the middle and late Cenozoic in a semi-arid to arid climate alongside

deposition of pediment gravels and local exotic Cu deposition (Sillitoe, 2005). Differential uplift

of porphyry systems and the alteration-mineralization characteristics of portions of those systems

exposed to weathering are first-order controls on ensuing supergene development. Deposits

young eastward, and many of the youngest systems have the least mature supergene profiles. The

Andean arc in Ecuador and Columbia contains important porphyry Cu and porphyry Au deposits,

including the Jurassic belt in Ecuador (Gendall et al., 2000), but their supergene zones are

immature and relatively unimportant due to the high modern erosion rates and shallow water

tables of tropical to subtropical climates. Glaciated regions in the high Andes are likewise

unfavorable for the preservation of supergene enrichment analogous to some northern latitude

deposits.

The Toki porphyry Cu-Mo cluster (Rivera et al., 2009), located southwest of the

Chuquicamata district in the Chilean Atacama desert, presents a variety of supergene

characteristics among its individual mineralized centers which illustrates important controls on

supergene development and provides context for the features of the Cochise deposit and Warren

district. Supergene ore comprises 6 Mt of the total 22 Mt of the Toki cluster’s contained Cu.

Supergene zones of the Genoveva and Quetana deposits contain chrysocolla, malachite, Cu-Mn

oxides, Cu-bearing clays, goethite and scarce hematite. These deposits are hosted in relatively reactive rocks including tonalite, andesite, and biotitic metavolcanics and do not have well-

developed sericitic halos or pyrite content. The Toki mineralized center is cut by abundant late

pyrite veins and consequently developed a thicker supergene blanket and an immature zone of

mixed chalcocite-covellite and hypogene sulfides. Enrichment factors are low and at Genoveva

the oxide zone has a lower grade than that of the protore. The moderate amounts of goethite and

scarce hematite in the oxidized zones of the Toki cluster deposits differs starkly from the

hematite-dominated, Cu-poor leached caps in the Warren district. Comparing these mineralized

centers to porphyry deposits with high pyrite content and intense hydrolytic alteration such as

Cochise, Morenci, and La Caridad (Titley and Marozas, 1995), demonstrates the Eh-pH

dependence of supergene oxidation processes and products on host rock lithology, hypogene

alteration, and hypogene sulfide content.

The paucity of Jurassic porphyry deposits in southwestern North America

Jurassic magmatism along the western United States and Mexico was an important aspect of

Mesozoic subduction and orogenesis, but associated porphyry deposits are rare and economically

subordinate to deposits in the region that were formed in the Laramide arc. To illustrate this

disproportionality in Arizona, a GIS analysis of the 1:1,000,000 scale map of Arizona by Richard

et al. (2000) indicates that Triassic to Jurassic felsic to intermediate igneous rocks cover 926 km2

(67 % intrusive) compared to 5,183 km2 (75% intrusive) for Late-Cretaceous to Paleocene

(Laramide). Accordingly, Triassic and Jurassic igneous rocks represent 15% of this total but the

single Jurassic porphyry deposit at Bisbee accounts for only 2.3% of the state’s Cu endowment

(Leveille and Stegen, 2012, Table 1). The situation is similar in Mexico, where only 1 of 61

porphyry deposits (or at least suspected porphyry-style systems) is Jurassic (El Arco) and the

remainder are Late Cretaceous or younger (Valencia-Moreno et al., 2007).

Exceptions to the low Cu productivity of the Jurassic in addition to Bisbee, El Arco, and

Courtland-Gleeson, include the Yerington porphyry, skarn, and IOCG district that formed at

~168 Ma (Dilles et al., 2000), and the Royston, Crow Springs, and Gilbert porphyry systems farther south that formed at ~200 Ma (Seedorff, 1991; Barton et al., 2011) in Nevada. These

instances prove that ore-forming metallogenic conditions were met multiple times during

Jurassic subduction and magmatism in western North America. What was special about these

plutons compared to the multitudes of barren intrusions?

Many authors have contemplated porphyry deposit genesis with a focus on general tectonic,

structural, magmatic, and hydrothermal controls (Titley and Beane, 1981; Tosdal and Richards,

2001) as well as specific provinces including western North America (Lang et al., 2001; Barton

et al., 2011), British Columbia (Logan and Mihalynuk; 2014), eastern Australia (Blevin and

Chappell, 1992) and South Korea and Japan (Sillitoe, 2018). Favorable magma compositions and

stress regimes of the upper crust into which chambers are formed seem to be fundamental

considerations that ultimately can be traced back to tectonic controls such as subduction rate,

slab angle, perturbations during subduction events, and the thickness and composition of the

overlying crust.

Because the Jurassic arc developed within a similar crustal environment as the Laramide in

southeastern Arizona, New Mexico, and northwestern Mexico (except for in areas with suspected

allochthonous Jurassic rocks; Tosdal et al., 1989), a difference in crustal composition is not a

compelling explanation. Jurassic igneous rocks are chemically more diverse than those of the

Laramide with respect to alumina saturation and alkalinity, yet many Jurassic magmas were

oxidized (magnetite ± titanite) and hydrous hornblende- and biotite-bearing granites and

granodiorites (Tosdal et al., 1989; Barton et al., 2011), which are the typical characteristics of

49 intrusions related to porphyry Cu mineralization (e.g., Blevin and Chappell, 1992; Seedorff et al.,

2005). Because about one-third of Jurassic igneous rocks are volcanic, erosion of nearly all potential porphyry deposits as the principal explanation is doubtful.

Tosdal and Richards (2001) stress the importance of a confluence of relatively common geologic processes for porphyry genesis; if only one key ingredient is missing, productive hydrothermal systems will be absent or subeconomic. Considering that magmas of broadly appropriate compositions were delivered to the upper crusts during the Jurassic and much of this crustal section is still preserved (Lang et al., 2001; Barton et al., 2011), perhaps the specific tectonic environment and consequent inappropriate stress regime of the middle and upper crust may be the best explanation for the dearth of porphyry Cu deposits in the region. Crustal conditions necessary for the magmatic modification processes (assimilation, fractionation) and the accumulation of upper-crustal chambers likely to concentrate and release hydrothermal fluids may have been absent which hindered porphyry development throughout most of the Jurassic arc. The negative relationship between deposits and contemporaneous extensional / contractional structures, the equant geometries of many mineralizing intrusions, and vein orientations around many porphyries led Tosdal and Richards (2001) to conclude that porphyry deposits tend to form in neutral stress regimes which are likely infrequent and short-lived time periods in active arcs.

The transition from transpressional to largely extensional stress regimes from the Triassic to

Early Jurassic (Tosdal et al., 1989; Barton et al., 2011) may have been the narrow window when an area of relatively low-stress upper crust was intruded by magmas which formed a shallow magma chamber and a mineralizing hydrothermal system. Subsequent reverse faults near Bisbee

(Ransome, 1904; Hayes and Landis, 1964; Davis, 1979; Favorito and Seedorff, 2021) and post- mineral normal faulting, especially the Dividend fault, may have been important factors in the

50 preservation of such a system. Like most geologic phenomena, a combination of these factors is almost surely responsible for the observed metallogenic endowment of a province, and more information will be needed to weigh the influence of each individual control on ore formation.

Exploration implications

Leached cap appraisal techniques were developed by many geologists over the 20th century

(e.g., Emmons, 1917; Locke, 1926; Blanchard, 1968; Anderson, 1982; Titley and Marozas,

1995) who utilized empirical observations as well as geochemical and mineralogical experimental data. Original hypogene characteristics can be reasonably well-predicted using fracture density and the abundance and proportion of hematite, goethite, and jarosite. Even the simple limonite map from this study (Fig. 9A) broadly identifies the underlying enrichment zone based on the strength and character of hematite in the leached cap, similar to findings at Silver

Bell, Arizona (Lopez and Titley, 1995). Anderson (1982) demonstrates that deposits with sufficient total sulfide and high pyrite:chalcopyrite ratios have abundant hematite and jarosite but subordinate goethite, all of which fit observations at Cochise.

The strength of hydrolytic alteration (Fig. 9B), which changes from nearly imperceptible halos of slightly coarser muscovite around weathered sulfide veins in the Pinal Schist on the deposit margins to nearly complete replacement of the porphyry and breccia towards the deposit center, would also serve (intuitively) as a vector towards the best Cu mineralization. Faults may also be used as exploration tools because they concentrate supergene fluids and may provide the

“best case scenario” Cu grades if drilled. Areas with exposed jarosite indicate proximity to the sulfide-stable zone, although strong jarosite was exposed in multiple areas where the underlying grades were low, implying sufficient hypogene pyrite and a source of potassium to fix jarosite but little Cu sulfide. The environment suitable to form jarosite and supergene alunite occurs both

51 above and to the sides of the deposit, so alunite and jarosite on their own may not be useful guides once the leached cap is identified.

If Cochise were eroded ~400 m, areas of the stock lacking pervasive sericitic alteration would be exposed to weathering. The low pyrite content (~1-2%) of the orthoclase-chlorite zone and the acid-neutralizing ability of feldspar and any siderite present would restrict efficient leaching and enrichment. Inexperienced explorers might drill these areas first based on higher residual Cu content of rocks and soils and the presence of chrysocolla or Cu oxides at the surface. To their initial disappointment, they would drill hundreds of meters grading around

0.2% Cu or less, where adjacent areas subjected to sericite-pyrite alteration would have stronger development of limonite and lower Cu contents in the leached cap but enriched secondary sulfides beneath.

Because the majority of easily identified, near-surface deposits have been discovered and developed in mature districts around the world, exploration increasingly relies on subtle geochemical prospecting techniques, geophysics, and deep drilling. The discovery of the high hypogene-grade (~1.5% Cu) Resolution porphyry Cu deposit in Arizona in the 1990’s (Manske and Paul, 2002) and its incremental development since then (Hehnke et al., 2012) demonstrates the economic potential, albeit rare, of bulk-tonnage hypogene underground-mineable porphyry deposits. However, considering the hypogene grades of porphyry Cu deposits rarely exceed 0.5%

Cu (e.g., Leveille and Stegen, 2012), perhaps targeting buried supergene deposits whose grades are commonly 0.5 to >1% Cu (Titley and Marozas, 1995) will prove to be fruitful. If so, the recognition and appraisal of leached cappings will continue to be a useful exploration technique and not a lost art of the 20th century. Taking into account the geophysical and geochemical signatures of a buried leached cap as well as the topographic and paleo-climatic characteristics

beneath the overburden in question gives the explorer different perspectives than when targeting

a large, high-grade hypogene deposit.

CONCLUSIONS

The Cochise deposit and Warren mining district are an important resource to Arizona. The

district’s rich mineral potential was recognized over 100 years ago and past production of Cu,

Au, and other metals was the foundation of Phelps Dodge Corporation. Past and ongoing study

of the SHIC and associated carbonate replacement, breccia-hosted massive sulfide, and

supergene-enriched porphyry mineralization has furthered concepts in economic geology and

also the broader topics of supergene environments, multiphase breccia-porphyry complexes, and

Jurassic metallogeny, magmatism, and tectonics in southwestern North America. The

“discovery” of Cochise in the 1970s is attributable to exploration geologists’ willingness to

reconsider subeconomic mineralization in light of changes in extractive technologies and

economic conditions.

This current review is far from comprehensive but attempts to capture the salient geologic features of the Cochise deposit not only for its own sake but also to serve as context for the greater Warren district and a framework for future studies. The hypothesis that the Cochise block in the hanging wall of the Dividend is the lower flank of the Lavender pit and CRDs is plausible based on structural interpretations and alteration-mineralization characteristics. Thus, the district is a natural laboratory not only because of the intact supergene profile at Cochise but also the opportunity to study a composite intrusion-breccia complex with diverse alteration and mineralization styles exposed at various depth intervals.

The potential for current and future studies of the Warren district to inform exploration endeavors is exciting and could include metal zonation in CRDs and their spatial connection with

53 larger porphyry systems, the mineralogy and chemistry of the leached cap and hypogene mica alteration to vector towards productive parts of porphyry Cu systems, and the use of intra- mineral breccia dikes as guides to ore and possibly sampling media of blind mineralization.

FIGURES AND TABLES

Figure 1 - Location of the Cochise deposit and Warren (Bisbee) mining district. The 3

Jurassic porphyry Cu systems discussed in this paper as well as the more numerous Laramide porphyry deposits are both indicated. Porphyry deposit locations from Singer et al. (2008).

Figure 2 - Geology and location of the southern Mule Mountains, southeastern Arizona. The extent of historic underground workings which targeted high-grade ore bodies in carbonate rocks is shown in purple. Note that the underground workings ended abruptly at the Dividend fault which juxtaposed Paleozoic rocks with Proterozoic schist and roughly bisects the Sacramento

Hill intrusive complex (SHIC). The Cochise deposit is hosted within the northeastern portion of the SHIC in the footwall of the Dividend fault. Geology is simplified after Hayes and Landis

(1964). See text for description of geology.

Figure 3 - Geologic map of the Sacramento Hill intrusive complex (SHIC) which is the source of mineralization in the Warren mining district. Section traces A-A’ and B-B’ correspond to cross sections in Figure 4. See text for description of rock units and structural features.

Geology north of the Dividend fault is from this study; south of the fault by E. Wright (Fluor

Mining and Metals, 2007).

Figure 4 - Cross sections A-A’ and B-B’ through the SHIC, Lavender pit, and Cochise areas. For unit abbreviations and the location of the sections, refer to Figure 3. The supergene chalcocite blanket (red dashed line) and geology were interpreted from surface mapping from this study, drill data, and cross sections by Phelps Dodge Corporation (1989) and Stegen et al.

(2005). The numbers over drill traces represent average Cu % for the interval either above

(leached cap), within, or below the chalcocite zone, separated at the intersection of the drill hole trace and the red dashed line; parentheses indicate hypogene sulfides inferred from a drop in grade and, where available, low acid-soluble Cu values. Y-axis values are meters above sea level. Features represented below the extent of drilling at Cochise are highly speculative.

Figure 5 – Photographs of important rock types at the surface of the Cochise deposit. A)

Outcrop of foliated quartz-muscovite Pinal Schist from distal part of Cochise system with wavy quartz vein cutting primary foliation. B) Slab of Cochise porphyry collected at surface with phenocrysts of sericitized feldspar, quartz, and minor chloritized to sericitized biotite in an aphanitic groundmass of quartz and sericite. C) Boulder of heterolithic breccia with 1-15 cm subangular to rounded clasts of schist, porphyry, and quartzose material. D) Breccia rich in quartz granules in a fine groundmass of rock flour altered to sericite. Foliations in the breccia matrix are accentuated by heavy jarosite; note intact quartz vein on the left side of the slab. E)

Close up of 5C to show pyrite veins cutting across the clasts and matrix of the breccia boulder.

Figure 6 – Photographs and photomicrographs of sericitic alteration at Cochise. A)

Photomicrograph (crossed polars) of core sample CCSC-5-841m with quartz-sericite vein cutting granular quartz and secondary orthoclase in the groundmass of the deeper porphyry intrusion. B)

BC-40-49m: Crossed polar and reflected light views of an intact framboidal pyrite cluster intergrown with muscovite (sericite) laths in the pulverized rock flour matrix (quartz-sericite ± clay) of a breccia; evidence for post-brecciation growth of sericite and pyrite. C) Outcrop of intense, texturally-destructive sericitic alteration of Cochise porphyry (?) with stockwork veins of quartz and ~10% disseminated pyrite. D) Outcrop of intense pervasive pyrite-sericite-altered heterolithic breccia; has distinct clasts of quartz and a subrounded schist fragment with preserved schistosity still visible. E) Core sample CCSC-5-915m (crossed polars) showing tourmaline associated with quartz-orthoclase alteration engulfed and replaced by secondary sericite. F) BC-

30-119m: Cochise porphyry (or deeper porphyry variety) with pervasive green sericite replacing feldspar and biotite phenocrysts and a vein of pyrite with a 2 cm white sericite halo.

Figure 7 – Photographs and photomicrographs of deeper alteration styles in intrusive rocks at Cochise. A) CCSC-5-453m: medium-grained seriate biotite granodiorite(?) with multiple sets of quartz-orthoclase veins cut by pyrite-chalcopyrite veinlets with sericite halos; feldspars and the groundmass are altered to sericite, whereas biotite phenocrysts are altered to chlorite except when immediate adjacent to pyrite-(sericite) veinlets. B) CCSC-5-841m: Seriate biotite granodiorite with pervasive chlorite and a quartz-(orthoclase)-tourmaline vein; small veinlets of sericite-pyrite ± pyrite cutting the bottom left corner of the photo. C) Hand lens view of secondary tourmaline (dark greenish black) and siderite (amber to yellow) near quartz-orthoclase vein. Tourmaline forms both anhedral aggregates and radiating fans and siderite forms

intergranular with quartz and tourmaline. D) CCSC-3-916m: Photomicrograph (crossed polars) showing tourmaline fan and intergranular siderite in deep, quartz-orthoclase-altered biotite granodiorite.

Figure 8 – Representative examples of primary (hypogene) and secondary (supergene)

mineralization at Cochise. A) CCSC-5-359m: Pinal schist with secondary orthoclase replaced by

a pod of quartz-sericite and chalcopyrite. For this photomicrograph, tansmitted light polarizers

were crossed but upper analyzer for reflected light was deviated 10° from extinction to allow

enough light show chalcopyrite (yellow minerals forming rim of pod). B) pyrite vein in the

biotite granodiorite in CCSC-5-448m with numberous 5-50 µm blebs of chalcopyrite. Elsewhere, similar blebs of bornite and minor galena occur within pyrite grains in sericitic assemblages. C)

Hand lens view of sample BC-60-124m showing pyrite-chalcopyrite vein cutting Cochise porphyry with incipient replacement by chalcocite and lesser covellite. D) BC-40-49m (reflected

67 light) is interpreted as a fault zone in the breccia unit. Fragmental pyrite grains, often lineated along the foliation planes of the fault breccia, are coated and partially replaced by chalcocite.

This Cu assay for this interval was substantially higher grade than adjacent intervals, another indication of a fault zone (concentrating supergene enrichment fluids). Pyrite in the hydrothermal breccia is normally intact and disseminted throughout the breccia matrix, not lineated.

Figure 9 – Limonite map and simple hydrolytic or “phyllic” alteration map of the Cochise deposit. A) Limonite distribution in leached cap overlying the Cochise deposit. The important part of the secondary chalcocite resource underlies moderate to strong hematite-dominated limonite. Jarosite-rich limonite overlies supergene sulfide zones of pyrite often coated by chalcocite ± covellite which is only exposed along road cuts and trenches. Drill holes (orange circles) are symbolized as “foot percent Cu” calculated by the combined length of intervals

≥0.2% Cu multiplied by the average grade for that interval (e.g., 100 feet averaging 1% Cu would be 100 ft% Cu). The intervals are not necessarily continuous and Cu concentrations are total Cu, not acid-soluble. This method also does not differentiate short, high grade intervals from lengthy, low grade intervals, but illustrates the distribution of significant Cu mineralization.

B) Generalized surficial hypogene alteration at the Cochise deposit. The Cochise stock is altered to moderate pervasive sericite-pyrite ± chlorite and to more intense quartz-sericite-pyrite at its southwestern margin near larger masses of hydrothermal breccias. Pinal Schist is weakly altered away from the porphyry-breccia complex and hosts oxidized sulfide veins with cryptic sericite halos. Alteration continues south of the Dividend fault and highway but was not mapped as a part of this study.

Figure 10 – Photos of important features of the leached cap at Cochise. A) Slabbed hand

sample of pervasively sericitized Cochise porphyry collected at the surface with relict sulfide

veins now oxidized to hematite; further from the vein the limonite is dominated by jarosite. B)

Outcrop of steeply-dipping structure with concentrated amounts of “fluffy” or light and crusty

hematite, possibly indicative of an oxidized fault or fracture zone formerly hosting chalcocite.

Outcrops like these are common above the chalcocite resource. C) Exposure near an abandoned

mill site showing transition from the leached cap into the underlying sulfide zone in Pinal Schist.

Geologists for scale. D) Photomicrograph of typical occurrence of alunite in the supergene zone.

Alunite is granular (elsewhere, plumose), monomineralic and lacks a halo of any sort. Veins like

this are common in all lithologies in the leached cap and into the mixed supergene-hypogene

sulfide zone. E) Core showing typical transition from hematite-dominated limonite in the upper

portions of the leached cap downward to jarosite-dominated. Depending on the thickness of the leached cap, the hematite > goethite zone can persists for up to 80 meters before reaching the jarosite zone above the supergene sulfide interface. Core is HQ diameter.

Figure 11 – Simple reconstruction of cross section B-B’ (Fig. 4) using average estimations for the dip and displacement on the Dividend fault (refer to segment on structure). The dismembered section was rotated 15° E to account for the apparent dip of the Paleozoic strata and Bisbee Group. These parameters show the Cochise deposit as the lower flank, but probably not the direct roots, of the SHIC and hydrothermal system exposed in the hanging wall of the

Dividend fault. Greater displacement and/or a lower dip angle would shift the Lavender pit area further to the northeast and closer to directly above the Cochise deposit.

Figure 12 – Estimation of the enrichment factor at Cochise using the average Cu grade of the leached cap, ore zone, and underlying hypogene zone, and the approximate dimensions of the chalcocite resource. See text for details. To the right, downhole Cu grades of one of the highest- grade drill holes is shown as an example of the distribution of Cu through the leached cap- supergene-hypogene profile.

Table 1 – Compilation of rock type classifications and descriptions from previous work and this study.

APPENDIX A - EXPANDED GEOLOGY, MINERALIZATION, AND HISTORY OF THE WARREN MINING DISTRICT

Unlike the Cochise deposit specifically, the greater Warren district has received a plethora of published geologic attention over the last ~120 years. These efforts are summarized here to place the Cochise deposit in its geologic context. Ransome (1904) provided the first

comprehensive description of the geology and ore deposits in the Warren district which, in many

respects, remains the most thorough and useful to this day. Later work by Bonillas et al. (1916),

Bryant (1964; 1968), Bryant and Metz (1966), and Nye (1968) documented the petrography and

alteration of the intrusions and breccias comprising the Sacramento Hill Intrusive Complex

(SHIC), the structural environment, and the variety of ore deposits in the district. Their distilled

findings are compiled in Table 1. Although the detailed and quality work of geologists during the

20th century elucidate much about the district, it is clear that many questions remain regarding

the relative timing of intrusive and brecciation events as well as their relationship to mineralizing

fluids.

Proterozoic Pinal Schist

Pinal Schist was the name given originally to basement quartz-muscovite schists around the

Globe-Miami mining district (Ransome, 1904). These metamorphoses siltstones, wackes,

sandstones, and lesser volcanic rocks of predominantly greenschist-facies mineral assemblages

(Meijer, 2014) form the ~1.65-1.75 Ga Mazatzal accretionary province forming the basement of

parts of Arizona, New Mexico, and Colorado (Karlstrom and Bowring, 1988). In the Mule

Mountains, the schist underlies Paleozoic and younger sedimentary rocks and was intruded by

granites and felsic porphyritic stocks, dikes, and sills in the Early Jurassic. Schistosity is

commonly well-developed but can be cryptic in more massive facies. Microscopic investigations

show mineral content to be predominantly quartz and fine muscovite with accessory tourmaline,

garnet, zircon, chlorite, and magnetite or ilmenite (Ransome, 1904; Meijer, 2014).

Phanerozoic sedimentary rocks

Paleozoic calcareous and siliciclastic rocks attain an aggregate thickness of ~1,500 to 1,800 m in the Mule Mountains (Ransome, 1904). The oldest sedimentary rocks resting unconformably on the Pinal Schist are the 1) Cambrian Bolsa Quartzite with a basal conglomerate, overlain in-

turn by 2) the silty, thinly-laminated and cherty Cambrian Abrigo Limestone, 3) the dark-gray,

impure, and variably dolomitic Devonian Martin Limestone, 4) the thick-bedded, cliff-forming

Mississippian Escabrosa Limestone, and 5) the fossiliferous and moderately-thick-bedded

Pennsylvanian to lower Permian Naco Group limestones. Detailed descriptions of the thickness,

structure, lithology, and fossil content of these strata are provided by Ransome (1904), Bonillas et al. (1916) and Nye (1968). All of these carbonate-rich units are important hosts to CRD and lesser skarn ore in the Warren district (Bryant, 1964).

The Bisbee Group rests unconformably on a Late Jurassic erosional surface of Proterozoic,

Mural Limestone, and Cintura Formation. The total thickness of the Bisbee Group is estimated to approach or exceed 1,500 m, depending on the extremely variable local thickness of the Glance

Conglomerate (Ransome, 1904; Hayes and Landis, 1964).

Features of the basal Glance Conglomerate provide key insight into the timing of

mineralization in the district, dynamics of post-mineral faulting, and erosional and supergene

enrichment history. The inconsistency of the Glance’s stratigraphic thickness and clast

composition on either side of the Dividend Fault suggests substantial variation in pre-

depositional topography with respect to that structure. Northeast of the Dividend Fault, the

footwall block hosting the Cochise deposit, the Glance is fairly homogeneous and consistently 0

to 20 m thick, indicating deposition on a low-relief erosional surface of exposed Pinal Schist and

a portion of the mineralized SHIC. Southwest of the Dividend, the Glance contains clasts of

SHIC and Paleozoic sedimentary rocks and its thickness ranges wildly from 0 to over 1,000 m

thick, indicating substantial paleo-relief (Ransome, 1904; Bonillas et al., 1916; Bryant and Metz,

1966). Rounded clasts of gossanous material with chalcocite (Nye, 1968; Stegen et al., 2005)

indicate exposure, oxidation, and some degree of supergene enrichment of the SHIC during the

Middle to Late Jurassic.

Deposited on topography filled and flattened by the Glance (Ransome, 1904) are the three

other formations of the Bisbee Group, including the Morita Formation calcareous siltstones and

sandstones, the Mural Limestone which forms the pronounced cliffs northeast of Bisbee, and the

Cintura Formation which is lithologically similar to the Morita. The largest exposure of the

Cintura is on the eastern slopes of the Mule Mountains where it is eventually concealed beneath

Quaternary alluvium.

The Sacramento Hill intrusive complex (SHIC)

Disseminated and massive replacement base and precious metal mineralization in the

Warren district formed within and around the ~1.5 km wide Sacramento Hill Intrusive Complex

(SHIC). Ransome (1904) noted the obvious spatial relationship of the SHIC to mineralization

79 and Bonillas et al. (1916, p. 300) differentiated the “granite porphyry” from the “monzonite porphyry” based on the abundance of quartz phenocrysts and the preservation or destruction of feldspar and biotite phenocrysts. Bryant (1964) also recognized two separate intrusions as well as two distinct breccia variants in the open pits and underground workings. The southwest portion of the SHIC in the hanging wall of the Dividend Fault was studied intensely compared to the exposures at Cochise, but because mineralization in the two structural blocks is linked, the geology in and adjacent to the Lavender pit serves as crucial context for understanding the geology of the Cochise deposit itself and is summarized below.

Previous publications by Bonillas et al. (1916), Bryant (1964, 1968), Bryant and Metz

(1966), Lang et al. (2001), and Stegen et al. (2005) mostly converge on a sequence of igneous and hydrothermal events to explain the geologic features of the district, but significant disagreements between publications hinders a full understanding of its geologic history. Most important of these discrepancies, which are expanded upon in following sections and can be seen in Table 1, are: 1) which intrusion(s) described in the Lavender pit correlate with intrusion(s) of the Cochise deposit, 2) the presence or absence of advanced argillic alteration in the older quartz porphyry, younger quartz-feldspar porphyry, and Cochise porphyry, 3) which alteration events accompanied copper mineralization, and 4) the relative timing between formation of the massive sulfide CRDs and that of the “intrusion” breccia associated with the older quartz porphyry.

Apparently, some of the conclusions reached by Bryant (1964; 1968) regarding the paragenesis of intrusions, alteration, and mineralization were more recently called into question by that author himself before passing away (R.J Stegen, pers. commun., 2021). This study was not designed to rectify any of these issues, however this synthesis may serve as a starting point for future attempts at their resolution.

Older quartz porphyry

The oldest intrusion of the SHIC (U-Pb zircon age of 198.9±1.6 Ma; Lang et al., 2001) is the older quartz porphyry or Sacramento porphyry which forms Sacramento Hill on the north side of the Lavender pit. In this intrusion, sparse (~5%) embayed quartz phenocrysts and accessory rutile-apatite-zircon are the only preserved igneous minerals after intense texturally-destructive silicification and pyritization (15-25% pyrite). According to Bryant and Metz (1966) and Lang et al. (2001), advanced argillic minerals pyrophyllite, dickite, and alunite are important constituents in addition to sericite. Bryant (1964, p. 42-43) stained numerous micaceous-looking samples of altered older quartz porphyry for potassium, and the lack thereof suggested that pyrophyllite was much more abundant than muscovite (sericite) or illite. Ten of these samples were also analyzed using X-ray diffraction (XRD) in 1958 by Phelps Dodge personnel which supported Bryant’s results. However, Stegen et al. (2005) interpreted the XRD data differently and argued that advanced argillic alteration is restricted to the “intrusion breccia” at the bottom of the Lavender pit, and that quartz-sericite-pyrite alone defines the alteration of the older quartz porphyry. What is agreed upon is intense silicification, hydrolytic alteration, and pyritization of this unit, which may have occurred coeval with Cu-poor massive silica-pyrite replacement bodies in the limestones (Bryant and Metz, 1966; Einaudi, 1982). Geologic maps by Bryant and Metz (1966),

Lang et al. (2001), and an unpublished Phelps Dodge map by Ernie Wright (Fluor Mining and

Metals, 2007) indicate the presence of older quartz porphyry in the footwall of the Dividend

Fault at the Cochise area, whereas maps by Bryant (1964), a Phelps Dodge map from 1988, and mapping from this study show the intrusion(s) at Cochise to be distinct from the preceding descriptions of the older quartz porphyry and more petrographically similar to the younger feldspar-quartz porphyry regarding both mineralogy and alteration.

Younger feldspar-quartz porphyry

The youngest mineralized intrusion (U-Pb zircon age of 199.9±0.8 Ma; Lang et al., 2001) was named the younger feldspar-quartz porphyry (or something similar—Table 1). It forms masses and dikes at the eastern end of the SHIC in the Lavender pit, occurs as sills and dikes in

Paleozoic rocks encountered in underground workings, and cuts the older quartz porphyry and

the contact / “intrusion” breccia described below (Bonillas et al., 1916; Bryant, 1964). This

intrusive phase has phenocrysts of embayed quartz, sericitized feldspar, and chloritized biotite in a groundmass of quartz and sericite (Bryant, 1964; Stegen et al., 2005). It lacks silicification and contains minor (~1%) pyrite and weak chalcopyrite, leading Bryant and Metz (1966) to believe

the intense silicification and pyritization of the older quartz porphyry and quartz-pyrite

replacements of limestone occurred prior to the emplacement of the younger feldspar-quartz

porphyry. Lang et al. (2001, p. 60) mention advanced argillic alteration (pyrophyllite-kaolinite-

alunite) in this unit, but other publications (Bryant, 1964, 1968; Bryant and Metz, 1966; Stegen

et al., 2005) emphasize a lack of advanced argillic alteration and silicification. The contradicting

statements “alunite and dickite, but not diaspore, have been observed in the Lavender feldspar-

quartz porphyry” (Bryant, 1964, p. 45) and “alunite and dickite have not been recognized in this

[feldspar-quartz porphyry] rock” (Bryant and Metz, 1966, p. 196) may be simple writing errors,

or could be due to supergene alteration and ambiguous phrasing. To be sure, igneous textures

were preserved and pervasive sericite and weak disseminated pyrite are widespread. Most

authors consider this unit to be pre-main stage Cu mineralization because of its crosscutting

relationships, common association with CRDs (e.g., Nye, 1968), and itself being variably

mineralized as noted in underground exposures (Bonillas et al., 1916; Bryant, 1964).

Descriptions of the porphyry intrusion(s) at Cochise from this study and others (Table 1)

most closely resemble those of this younger feldspar-quartz porphyry. The two intrusions contain 82

notable differences in pyrite content (5-15% at Cochise vs. 1-2% in the younger feldspar-quartz porphyry in the Lavender pit), but both units have similar phenocryst mineralogy, size, and abundance, so the two intrusions may be genetically related. The Cochise porphyry was studied during drilling campaigns by Phelps Dodge personnel and has a U-Pb zircon age of 200.0 ± 0.8

Ma (Lang et al., 2001).

Another possible intrusion encountered during deep drilling named the Warren porphyry has an age of 201.4 ± 0.8 Ma and was considered to be the deeper equivalent of the younger feldspar-quartz porphyry (Lang et al., 2001). Stegen et al. (2005) believed the Warren porphyry to be a distinct stock-like mass beneath the Lavender pit which may connect with a biotite-quartz monzonite dike in the Cochise area.

Contact / “intrusion” breccia

A heterolithic breccia at the southern and western margins of the older quartz porphyry in the Lavender pit was named the contact breccia by Bonillas et al. (1916) or “intrusion” breccia in subsequent studies (not to be confused with the “intrusive” breccia, described below). The contact between the older quartz porphyry and Paleozoic wall rocks is sheared and locally mylonitic with angular to subrounded fragments of schist, limestone, older quartz porphyry, and silicious pyrite, all in a clastic non-igneous matrix (Bonillas et al., 1916; Bryant, 1964; Bryant and Metz, 1966). It is intensely silicified and pyritized similar to the older quartz porphyry and has isolated, mostly unoxidized lenses of massive pyrite-bornite that were mined underground in the early 1900’s and in the Sacramento pit mined from 1919-1929 (Stegen et al., 2005).

The genesis and timing of the contact / “intrusion” breccia relative to key mineralization events are unsettled. The unit has been interpreted as protoclastic, formed during the emplacement of the mostly crystalline older quartz porphyry stock (Bonillas et al., 1916; Bryant,

1964; Stegen et al., 2005). However, Bryant and Metz (1966) offer evidence that conflicts with

this interpretation, such as the non-igneous matrix, roundness of fragments, and heterogeneity

and apparent transport distance of the clasts. Bryant and Metz (1966) emphasize that the location

of this breccia at an intrusive contact and the “contorted swirling flow structure” (p. 197) are the

key criteria for its genetic interpretation and for distinguishing it from the hydrothermal /

“intrusive” breccias, which are described below. Considering that the contact / “intrusion”

breccia shares many characteristics with the hydrothermal / “intrusive” breccias (Table 1),

perhaps it shares a similar hydrothermal origin, was preferentially developed along the margins of the older quartz porphyry intrusion after to the introduction of widespread silica-pyrite, and was subsequently deformed and sheared by faulting at this contact. This proposal would address those concerns by Bryant and Metz (1966) and explain its geometry, its containing quartz-pyrite fragments, and the abundance of sheared or mylonitized pyrite within the breccia.

As another point of controversy, the high-grade fragments of massive pyrite-bornite (not low-grade quartz-pyrite) in the contact / “intrusion” breccia were interpreted by Stegen et al.

(2005, p. 9) to be clasts of previously-formed manto (CRD) mineralization based on similar mineral assemblages to the Campbell ore body and the “strong consistent trend of mantos with stratigraphic control” (p. 9). These claims refute two observations by Bonillas et al. (1916) which state the sulfidic bodies were “not confined to any definite horizon” (p. 328) and these were “the only type of ore in the camp in which chalcopyrite is not an important constituent” (p. 327).

Bryant (1964; 1968) and Bryant and Metz (1966) considered CRD formation to have occurred after the formation of the contact / “intrusion” breccia, and that limestone clasts in the breccias were replaced in-situ after brecciation. The almost ubiquitous association of hydrothermal /

“intrusive” breccias (which cut the contact / “intrusion” breccia) with CRDs led those authors to

conclude that they acted as passageways for fluids during the main stages of copper

mineralization, consistent with paragenesis judgments by Bonillas et al. (1916) and Nye (1968).

Bryant (1964, p. 104) acknowledged the temptation to interpret the massive copper ore

fragments in the breccia to have formed prior to brecciation, but after studying polished slabs

states they show “preferential replacement of limestone by the ore after, not before,

consolidation of the breccia”. However, apparently Bryant questioned these conclusions later in

his career (R.J Stegen, pers. commun., 2021). Unless there were multiple CRD-forming events, at least one of these timing assertions is incorrect. It is clear from these geologic descriptions that that the contact / “intrusion” breccia is an enigmatic and fascinating feature that would benefit from further study. Because access to the open pits and underground workings are increasingly restricted, this geologic problem, as well as others, may prevail.

Hydrothermal / “intrusive” breccias

Another breccia type that is both widespread and intimately associated with mineralization was termed “intrusive” breccia by Bryant (1964; 1968) and Bryant and Metz (1966) and was interpreted as hydrothermal in origin. This type cuts all other units of the SHIC and occurs as dikes, sills, thin films, and pipes with clasts of subangular to well-rounded (sometimes spherical) schist, limestone, quartzite, porphyritic igneous rock, massive siliceous pyrite, and ptygmatic quartz vein material. Fragments range in size from microscopic to blocks over 30 m in diameter with matrix material of rock flour and smaller fragments of the same materials as the clasts, variably cemented together with calcite, quartz, pyrite, and copper sulfides (Bryant, 1964). These breccias at Bisbee were classified by Sillitoe (1985) as phreatic or hydromagmatic breccias, which share similarities to the “pebble dikes” documented at Tintic, Utah (Farmin, 1934). Bryant

(1968) maintained the term “intrusive breccia” to illustrate its geometries and inferred mode of

emplacement, but breccias with these textures and morphologies are not uncommon in

epithermal and porphyry environments (Sillitoe, 1985). Some clasts were transported upward on the order of 1,000 m. Clasts are aligned horizontally to vertically with orientations parallel to or

perpendicular to contacts with wall rocks, recording a turbulent and dynamic transport history

(Bryant, 1964; Bryant and Metz, 1966). During the formation of the breccias, alteration and

sulfide mineralization was weak, but because the dikes are almost invariably associated spatially

with ore deposits in the district and they were useful guides to ores, the breccias were interpreted

as pathways for mineralizing fluids (Bryant, 1964), or at a minimum their emplacement was

guided by the same structural pathways that subsequently controlled ore fluids (Nye, 1968).

Bryant (1968) considered many possible origins for the breccias and concluded they were

hydrothermal in origin and formed during the waning stages of magmatic activity of the SHIC.

Descriptions of the breccias at Cochise (this study) differ from these “intrusive” breccias only in

minor detail and were formed by similar processes.

Other intrusions in the Mule Mountains

Two intrusions that post-date mineralization are worth mentioning. Thin dikes (<5 m) of approximate andesitic composition cut the SHIC locally. They are fine grained and equigranular to porphyritic, variably altered to clay-chlorite-iron oxides, and gave an age of 190.8 ± 0.5 Ma from an exposure at Cochise area (Lang et al., 2001). The 175 Ma Juniper Flat granite forms a large elongate stock north of Tombstone Canyon associated with dikes along Escabrosa Ridge west of Bisbee. It is the largest intrusion in the Mule Mountains (Hayes and Landis, 1964) and experienced minor hydrothermal alteration (Lang et al., 2001). Ransome (1904) described the intrusion as phaneritic, fine- to medium-grained, roughly equigranular, and composed of orthoclase, quartz, oligoclase, biotite, and accessory tourmaline, muscovite, apatite, zircon, and

magnetite or ilmenite. Based on mineralogy and major element geochemistry, that author

classified the Juniper Flat intrusion as an alkali granite and its fine-grained or aplitic occurrences alaskite.

Structure

The Mule Mountains are divided into two differing geologic blocks on either side of the northwest-southeast-trending Tombstone Canyon, along which Arizona Highway 80 runs. The northeastern block consists of Bisbee Group sedimentary rocks resting on Proterozoic schist and the Cochise portion of the SHIC. The southwestern side of Tombstone Canyon contains densely- faulted Paleozoic sedimentary rocks sitting atop windows of schist cut by Jurassic dikes and sills.

Thus, Jurassic intrusions are exposed on both sides of the canyon, but the preserved and exposed sedimentary sections are markedly different. Paleozoic beds dip gently or moderately away from the Juniper Flat granite, suggested by Bryant and Metz (1966) to indicate anticlinal doming related to the emplacement of this intrusion; 10-30° E to NE dips on post-intrusion Bisbee Group strata (Hayes and Landis, 1964) suggest alternative mechanisms, however. In the Warren mining district, Paleozoic rocks typically dip to the east 25° to 35° (Stegen et al., 2005).

The Dividend normal fault runs parallel to Tombstone Canyon and is the most relevant structure to the currently study because of its considerable displacement, dissection of the mineralized system, and influence on Jurassic sedimentation and supergene history.

Measurements on the dip range from nearly vertical (Ransome, 1904), 60-70° S-SE (Bryant,

1964; Bryant and Metz, 1966), to an even wider range of ~55° S to vertical (Bonillas et al., 1916;

Stegen et al., 2005). Normal displacement is constrained by Paleozoic sedimentary rocks on either side of the fault but varies along the strike of the fault from 600m up to 1,500m (Bryant,

1964; Stegen et al., 2005). The difference in throw has been attributed to additional displacement

on parallel synthetic faults, not to the tipping out or tapering displacement on the Dividend Fault

itself (Bryant and Metz, 1966; Nye, 1968). These synthetic faults mapped in the Lavender pit

down-dropped limestone-hosted ore bodies tens to hundreds of meters (Bryant and Metz, 1966).

In the vicinity of the Cochise deposit and Lavender pit, a displacement of ~1,000 to 1,200 m is

appropriate. The trace of the Dividend Fault is lost in Proterozoic schist to the west and in

Cretaceous Morita Formation and Quaternary alluvium to the east (Ransome, 1904; Stegen et al.,

2005).

The timing of movement of the Dividend Fault has been discussed previously by numerous

workers and is important to understanding both the overall geology of the Mule Mountains as

well as its specific implications for mineralization of the district. Ransome (1904, p. 62) believed the majority of movement on the Dividend Fault preceded the emplacement of the Jurassic porphyry, but there was renewed movement after deposition of the Glance Conglomerate, evidenced by offsetting of the Glance along Mule Gulch. The footwall was eroded flat and stripped of its Paleozoic cover whereas Paleozoic sedimentary rocks in the down-dropped hanging wall were eroded into hilly topography and subsequently filled by the Glance. Pre-

Glance movement is strongly indicated by the consistent 0 to 20 m thickness and predominance of schist clasts of the Glance to the northeast of the Dividend Fault versus the extreme variation in thickness (from 0 to 1,000 m) and locally-derived clast lithology in Glance to the southwest

(Bryant and Metz, 1966). These observations, in conjunction with the presence of iron-oxide-rich

clasts of porphyry in the Glance, indicates substantial movement after the assemblage of the

SHIC and mineralization.

Other major notable northwest-trending normal faults include the Abrigo, Gold Hill, and

Bisbee West faults (Ransome, 1904). Faults of lesser offset but still high importance strike

northeast and are conspicuous in the Cretaceous beds northeast of Bisbee, as well as in Paleozoic beds to the south of Bisbee, including the Quarry and Czar faults (Ransome, 1904). Northeast- trending faults in the hanging wall of the Dividend Fault are numerous in this area and are commonly associated with ore bodies in the Paleozoic limestones (Bryant and Metz, 1966).

Approximately 10 km southeast of Bisbee (Fig. 2), a reverse fault that crops out near Gold hill has a moderate southwesterly dip and places limestone of the Naco Group on top of Bisbee

Group strata. The reverse fault in turn is cut and offset by a more steeply southwest-dipping normal fault (Ransome, 1904; Hayes and Landis, 1964). In the vicinity of the reverse fault, the

Bisbee Group beds are commonly folded with steep to overturned dips, and the fault displays moderate- to steep cutoff angles with bedding and displays a footwall syncline that extends for at least 10 km (Hayes and Landis, 1964, cross section D-D’).

Carbonate replacement deposits (CRDs)

At the Cochise deposit, Paleozoic sedimentary rocks and thus any potential skarn or carbonate replacement deposits have been eroded away. However, considering ~70% of the district’s Cu production and nearly all of its Pb, Zn, Ag, and Au production came from CRDs, a review of these deposits in the Warren district is justified. The ores first mined near Bisbee were from oxidized and enriched massive sulfide CRDs formed in Paleozoic rocks, mainly the

Devonian Martin and lower Mississippian Escabrosa formations and, to a lesser extent, certain horizons in the the Cambrian Abrigo Formation and Pennsylvanian Naco Group (Ransome,

1904; Bonillas et al., 1916). Crudely stratabound tabular mantos and discordant chimneys formed near the intersections of favorable beds and high-angle faults (Bryant, 1964). Mineralized porphyritic intrusions and hydrothermal breccias were commonly found near CRDs, but Nye

(1968) emphasized that exceptions to this suggested that pre-existing structures served as the

pathways for both the permeable breccias and the subsequent hydrothermal fluids. Altered

porphyry dikes and sills adjacent to ore bodies were themselves rarely sufficiently mineralized to

be economic in the early days, and underground cross-cuts were stopped when large bodies of porphyry were encountered (Ransome, 1904). Breccias and fault zones also enhanced oxidation and enrichment processes and thus were effective guides to both hypogene and supergene ore.

Individual deposits ranged from a few hundred tons to over 1 million tons (Bryant, 1964). In plan view these ore bodies form a semi-circle radiating from the SHIC.

Hypogene minerals in the CRDs are zoned from cores of siliceous pyrite outward to copper sulfides, to lead-zinc sulfides, and outermost specular hematite (Bryant, 1964; Schumer, 2017).

This zoning from Cu-Au to Pb-Zn to Mn-Fe is also apparent at the district scale. Manganese masses in the limestones consisted of psilomelane and braunite formed outboard from Pb-Zn deposits and were mined in several places at the surface (Bonillas et al, 1916).

Ransome (1904) proposed the following paragenesis: 1) silica+pyrite+calc-silicate replacement of limestones ± chalcopyrite-sphalerite, 2) a distinct period of chalcopyrite and sphalerite deposition, and 3) secondary products of weathering and oxidation including chalcocite, copper carbonates and oxides and limonite. Bryant and Metz (1966) observed large bodies of siliceous pyrite beneath productive ore bodies and also concluded that these marked an early event of low-grade silica and pyrite introduction. Paragenetic observations by Bonillas et al. (1916) support these interpretations.

Oxidized ore bodies contained chalcocite, native copper, cuprite, malachite, azurite, chrysocolla, brochantite, and tenorite. Of these, chalcocite was by far the most economically

important ore mineral of the district. Chalcocite commonly forms rims on pyrite grains, large

envelopes around pyritic masses, and as sooty powder (not dissimilar from the supergene-

enriched porphyry deposits of the district). Much of the oxidized ore is associated with limonite

and clay which constituted the dominant gangue mineralogy of many oxidized ore bodies

(Ransome, 1904). Unlike the supergene porphyry environment, enrichment of CRDs was

attributed to the removal of uneconomic elements, not by the transportation and concentration of

Cu itself (Bryant, 1964). The depth of oxidation of the CRDs is highly variable; along certain

faults, oxidation has been shown to extent over 300 m lower than in adjacent areas (Bonillas et

al, 1916; Bryant, 1964). The renowned copper carbonates unveiled in the Warren district became

some of the most exquisite specimens of their type ever collected. Ransome (1904, p. 125)

described: “The walls of these caves were covered with velvety moss-green malachite, and sparkled with the blue crystals of azurite, while from the roofs hung translucent stalactitic draperies of calcite, delicately banded and tinted with the salts of copper”.

Alteration of limestone

Skarn is uncommon in this district and mineralization was rarely associated with secondary calc-silicates. Rather, minor recrystallization of carbonate rocks is the only perceptible change to carbonate rocks surrounding most CRDs (Bryant, 1964). Mineralogical and geochemical investigations by Schumer (2017) indicate temperatures of ~325-450°C for CRD formation and the evolution of oxidized fluids towards lower temperatures and higher sulfidation states. Within

~300 m of the SHIC, minor amounts of fine-grained secondary calc-silicates including tremolite, diopside, garnet, vesuvianite (idocrase), and chlorite were observed (Ransome, 1904). Bonillas et al. (1916) explain that high-temperature anhydrous skarn minerals including garnet and pyroxene are rare and never associated with ore, whereas hydrous amphiboles are more common. Einaudi

(1982) suggested that, because these calc-silicates are most common in impure shaly limestones, they are probably entirely metamorphic (reaction skarn) in origin. The lack of andradite-

91 salite±magnetite-chalcopyrite skarn at Bisbee, which is commonly associated with potassium- silicate alteration of other porphyry systems, led Einaudi to suggest that potassic alteration did not occur at exposed portions of the hydrothermal system in the Lavender pit. Since then, potassic alteration has been observed on the lowest benches of the pit and within deep drill holes

(Lang et al., 2001), so this link would be worth investigating further.

Disseminated Cu mineralization in the Lavender and Sacramento pits

The remaining ~30% of Cu produced in the district came open pit mining of relatively low- grade, supergene-enriched chalcocite mineralization. Mineralization in the Lavender pit was discontinuous and erratic and much of the secondary chalcocite blanket was restricted to permeable masses of hydrothermal breccias and porphyry intrusions. Bryant and Metz (1966) recognized four ore types which correlate with their host rock. The older quartz porphyry contains 15-25% pyrite but low Cu sulfide content. Silicification led to poor permeability and enrichment, so the unit was typically waste except in highly fractured zones which allowed concentration of supergene fluids. The contact / “intrusion” breccia at the southern margin of the older quartz porphyry was mined by both underground and open-pit methods for its isolated lenses of massive Cu sulfide which commonly graded outward into disseminated ore. In the younger feldspar-quartz porphyry, copper grades only exceeded 1% in intensely fractured zones which hosted sooty chalcocite replacing pyrite and chalcopyrite grains typically associated with argillic alteration of suspected supergene origin. Much of the enrichment blanket occurred within and adjacent to dikes and masses of hydrothermal / “intrusive” breccia where chalcocite replaced disseminated hypogene sulfides in the breccia matrix as well as sulfides deposited along fractures in the clasts and matrix. To illustrate the erratic distribution of Cu and the need for selective mining methods, Bryant and Metz (1966) stated that “there was seldom a bank shot that

did not contain ore, leach ore, and waste”. Ore types where chalcocite partially replaced and

coated pyrite resulted in concentrates of relatively low grade, averaging ~12% Cu.

The chalcocite blanket mined in the Sacramento pit from 1919 to 1929 and in the Lavender pit from 1954 to 1974 ranged from 15-120 m thick and continues into the Cochise deposit undisrupted by the Dividend fault (Cook, 1994; Stegen et al., 2005). The vertical transition from

leached capping to enriched ore is abrupt, and this surface undulates conformably to paleo- topography on which the Glance Conglomerate was deposited (Bryant and Metz, 1966).

Alteration in the Lavender pit

The relationship of Cu mineralization to widespread hydrolytic alteration of the SHIC was

recognized by Ransome (1904) and Bonillas et al. (1916) who documented texturally-destructive

recrystallization of quartz phenocrysts to aggregates of quartz and the total replacement of

feldspars to sericite, kaolinite, and pyrite. Subsequent studies distinguished sericitic, advanced

argillic, and deep potassic alteration assemblages, although their distribution and relationship to

mineralization are contested (Table 1; Bryant, 1964; Bryant and Metz, 1966; Lang et al., 2001;

Stegen et al., 2005). These assemblages are described by lithology above.

Hydrothermal muscovite Ar-Ar dates by Lang et al. (2001) from older quartz porphyry and

Cochise porphyry showed Ar loss and yielded minimum ages of 191.1 ± 0.8 Ma and ~172-173

Ma, respectively. Zircon U-Pb ages of the SHIC intrusions (~200 ± 2Ma) and a post-mineral andesite dike (190.8 ± 0.5 Ma) by Lang et al. (2001) more confidently bracket the age of porphyry mineralization. Petrographic (not microthermometric) investigations of fluid inclusions by those authors suggest low-temperature (<300°C) and low-salinity, liquid- and vapor-rich inclusions for advanced argillic alteration, dominantly liquid-rich inclusions of low to moderate

salinity for sericitic alteration, and high salinity and vapor-rich inclusions in potassic

assemblages suggesting phase immiscibility.

According to Bryant (1964, 1968), Bryant and Metz (1966), and Nye (1968) the assemblage

of the SHIC and the relative timing of hydrothermal events were: 1) intrusion of the older quartz

porphyry and contemporaneous(?) formation of the contact / “intrusion” breccia at its margin, 2)

silicification and pyritization of these units simultaneous with silica-pyrite replacements in the

limestones (also see Einaudi, 1982), 3) intrusion of the younger feldspar-quartz porphyry, 4)

formation of the hydrothermal / “intrusive” breccias simultaneous with or slightly preceding the

major phase of Cu sulfide mineralization, including the formation of the massive Cu-Pb-Zn

CRDs. Evidence for this includes cross-cutting relationships of porphyry intrusions, clast

lithology of the two breccia types, the lack of silicification and relative paucity (~1%) of pyrite

in the younger feldspar-quartz porphyry compared to the silicified older quartz porphyry (15-

25% pyrite), and the almost invariable association of mineralization with hydrothermal /

“intrusive” breccias. As mentioned above, Stegen et al. (2005) argue that formation of the CRDs

predated intrusion of older quartz porphyry and its contact / “intrusion” breccia because of the

presence and stratigraphic regularity of manto (CRD) lenses in said breccia. Lang et al. (2001)

infer multiple vertically-zoned hydrothermal events and thus their view of the time-space relationship of alteration-mineralization is more complex. They also argue that the main chalcopyrite-forming event accompanied advanced argillic alteration and that sericitic alteration contains only trace primary Cu mineralization. The overprint of supergene alteration further

obscures the entire situation.

Geologic and geochronologic evidence indicates at least two periods of enrichment in the

Lavender pit and Cochise deposit. Clasts of porphyritic SHIC rocks and oxidized mineralized

material, including turquois, in the Glance Conglomerate (Bryant and Metz, 1966; Stegen et al.,

2005) requires erosion and oxidation of the SHIC during the Middle to Late Jurassic. Continual

deposition of the Bisbee Group buried the district through at least the mid-Cretaceous but

subsequent uplift and erosion exposed the system to weathering by the Miocene. Evidence for

late Cenozoic enrichment includes the roughly conformable geometry of the chalcocite blanket

to current topography, the lack of blanket offset by the Dividend fault, and K-Ar ages of 9.08 ±

0.22 Ma and 3.5 ± 0.33 Ma on supergene alunite and jarosite, respectively (Cook, 1994; Cook

and Porter, 2005). Reddish to maroon hematitic horizons in the leached caps of the Lavender pit

and Cochise deposit may also suggest supergene cycles during both the Jurassic and late

Cenozoic, referring to researchers who have shown that hematitic horizons indicate the oxidation of former chalcocite blankets (Blanchard, 1968; Anderson, 1982).

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