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Sericitic and Advanced Argillic Mineral Assemblages and Their Relationship

to Copper Mineralization, Resolution Porphyry Cu-(Mo) Deposit, Superior

District, Pinal County, Arizona

by

Alexander Raine Winant

A Prepublication Manuscript Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements

for the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

2010

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STATEMENT BY THE AUTHOR

This manuscript, prepared for publication in Economic Geology, has been submitted in partial

fulfillment of requirements for the Master of Science degree at The University of Arizona and is

deposited in the Antevs Reading Room to be made available to borrowers, as are copies of regular

theses and dissertations.

Brief quotations from this manuscript are allowable without special permission, provided that

accurate acknowledgment of the source is made. Requests for permission for extended quotation

from or reproduction of this manuscript in whole or in part may be granted by the Department of

Geosciences when the proposed use of the material is in the interests of scholarship. In all other

instances, however, permission must be obtained from the author.

__________________________________________ _____________

(authors signature) (date)

APPROVAL BY RESEARCH COMMITTEE

As members of the Research Committee, we recommend that this prepublication manuscript be

accepted as fulfilling the research requirement for the degree of Master of Science.

Dr. Eric Seedorff__________________________ _____________

Major Advisor(type name) (signature) (date)

Dr. Mark D. Barton____________________________ _____________

(type name) (signature) (date)

Dr. Frank K. Mazdab _________________________ _____________

(type name) (signature) (date)

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Sericitic and Advanced Argillic Mineral Assemblages and

Their Relationship to Copper Mineralization,

Resolution Porphyry Cu-(Mo) Deposit, Superior District,

Pinal County, Arizona

Alexander R. Winant and Eric Seedorff

Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona

1040 East Fourth Street, Tucson, Arizona 85721-0077

Hamish R. Martin

Resolution Copper Company, 47206 N. Magma Shaft #9 Road, Superior, Arizona 85273

Frank K. Mazdab and Mark D. Barton

Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona

1040 East Fourth Street, Tucson, Arizona 85721-0077

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Abstract

The Resolution deposit is a giant, deep, high-grade deposit in the Laramide porphyry

copper province of Arizona that is currently being developed. This study focuses on the features

at Resolution that formed from acidic hydrothermal fluids (including sericitic and advanced

argillic alteration types) that are well developed in the upper part of the system. The distribution

of alteration-mineralization features are illustrated along two, roughly perpendicular fences of

drill holes that were logged with concurrent mineral identifications made with a PIMA

infrared spectrometer and ultraviolet light and supplemented with subsequent reflected and

transmitted light petrographic observations. Hydrothermal minerals formed during intense

hydrolytic alteration at Resolution commonly are related to multiple superimposed, crosscutting

events. Though showing some degree of stratigraphic control, particularly at deep levels, the

distribution of hydrothermal minerals and mineral assemblages shows only weak degrees of

structural control at the deposit scale.

The intermediate sulfidation opaque assemblages containing chalcopyrite characterize

the many hydrothermal mineral assemblages that formed potassic alteration of igneous rocks,

skarn, and calc-silicate hornfels, which are best developed outside the region of this study.

Earlier sericitically altered rocks contain pyrite chalcopyrite, but later sericitic and advanced

argillic assemblages contain higher sulfidation state opaque assemblages, such as pyrite + bornite

chalcocite with kaolinite, dickite, and topaz, with lesser alunite, pyrophyllite, and zunyite.

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Veins with assemblages characteristic of advanced argillic alteration consistently offset veins

associated with sericitic alteration. Most of the advanced argillic assemblages at Resolution

formed at relatively low temperatures, stable with kaolinite and dickite.

Resolution contains fairly high levels of fluorine. The most important fluorine-bearing

minerals are biotite (~3-4 wt% F), topaz (~11-12 wt% F), fluorite (~49 wt% F), and sericite (~1

wt% F), although other fluorine-bearing phases also are locally present (e.g., zunyite, 6-7 wt% F).

Topaz formed at Resolution during advanced argillic alteration and the mineral has a relatively

fluorine-poor composition (XF-tpz ~0.6), as is topaz from other base-metal lode deposits such as

Butte, in contrast to topaz in those porphyry deposits in which a more fluorine-rich topaz occurs

in sericitic and potassic assemblages.

Resolution is a relatively arsenic-poor system, in strong contrast to the nearby Magma

vein system. The deeper part of the ore body, where potassic alteration dominates, is nearly

arsenic-free, whereas the upper part of the copper ore body is arsenic-bearing. Although enargite

has been observed petrographically, arsenic occurring in solid solution in other sulfides (e.g.,

arsenic-bearing pyrite) may be responsible for many of the local spikes in arsenic content at

Resolution.

Introduction

Intense hydrolytic alteration of the sericitic and advanced argillic types, though known

also from other types of hydrothermal ore deposits, occurs commonly in three related types of

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magmatic-hydrothermal ore deposits, porphyry deposits, base-metal lode deposits, and

acid-sulfate or high-sulfidation epithermal deposits (Meyer and Hemley, 1967; Hemley et al.,

1980; Einaudi, 1982; Arribas, 1995; Seedorff et al., 2005a). Intense hydrolytic alteration,

regardless of deposit type, can be pervasive or can be confined to structures or stratigraphic units;

rocks exhibiting intense hydrolytic alteration can be barren to highly mineralized. Where

mineralized, high- to very-high sulfidation state opaque minerals commonly are associated with

advanced argillic alteration of silicate minerals (Meyer and Hemley, 1967; Einaudi, 1982;

Einaudi et al., 2003).

Intense hydrolytic alteration is characteristic of shallower levels of certain porphyry

systems (e.g., Red Mountain, Arizona; Resolution, Arizona; El Salvador, Chile; Central deposit,

Oyu Tolgoi, Mongolia) and base-metal lode deposits (e.g., Bisbee, Arizona), though sericitic and

advanced argillic alteration can persist to deep levels, as at Butte, Montana (Bryant, 1968; Meyer

et al., 1968; Corn, 1975; Gustafson and Hunt, 1975; Bodnar and Beane, 1980; Hedenquist and

Lowenstern, 1994; Reed and Meyer, 1999; Watanabe and Hedenquist, 2001; Manske and Paul,

2002; Khashgerel et al., 2009). For the high-sulfidation epithermal deposits, links to porphyry

systems are well established in certain cases (e.g., Lepanto- Far Southeast in the Philippines) but

to date are lacking in many other districts (e.g., Goldfield, Nevada, and Yanacocha, Peru)

(Einaudi, 1982; Arribas et al., 1995; Harvey et al., 1999; Sillitoe and Hedenquist, 2003).

Likewise, it is not necessarily clear whether fluids that formed intense hydrolytic alteration

represent evolution of fluids that produced potassic alteration at earlier stages or whether they

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represent a temporally distinct hydrothermal system (e.g., Meyer et al., 1968; Brimhall and

Ghiorso, 1983).

Rocks exhibiting intense hydrolytic alteration commonly represent a special challenge in

identifying mineral assemblages, defined as a group of minerals that appear to be stable together

at the mesoscopic scale and to have formed contemporaneously (e.g., Seedorff et al., 2005a). In

many cases, the hydrothermal minerals clearly are related to multiple superimposed, crosscutting

events, yet the identity of the products of each event may be difficult to determine at the hand

specimen scale. Moreover, the silicate minerals commonly are light colored, fine-grained, and

difficult to identify with the naked eye or hand lens and in some cases petrographically, such as

distinguishing between sericite and pyrophyllite. Even where the minerals can be determined by

with aid of infrared spectrometers and X-ray diffraction, the textural relationships generally are

lost at the spatial scales of such determinations, i.e., the minerals identified may have formed in

multiple events, so the nature of the mineral assemblage remains uncertain. For these reasons,

the identification of mineral assemblages within areas of intense hydrolytic alteration commonly

is avoided or not deemed possible (e.g., Khashgerel et al., 2006), thereby limiting the types of

geochemical or genetic conclusions that might be drawn.

This study was conducted at the Resolution deposit in Arizona. The study focuses on the

upper part of the Resolution system where sericitic and advanced argillic assemblages are

prevalent, building on work by Manske and Paul (2002), Ballantyne et al. (2003), Schwarz

(2007), and the geologic staff at Resolution, especially on previous work by Troutman (2001)

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and Harrison (2007) on sericitic and advanced argillic alteration. The purposes of this study are

to document the distribution, abundance, and compositions of associated hydrothermal minerals,

to attempt to define the mineral assemblages that constitute sericitic and advanced argillic

alteration, to determine the lateral and vertical changes in abundance of sericitic and advanced

argillic alteration,, and to document the relative ages of associated veins. We show that acidic

hydrothermal fluids at Resolution formed a variety of vein types and mineral assemblages,

though some uncertainty remains in defining assemblages. Most assemblages at Resolution are

of the sericitic and advanced argillic types, but they include some assemblages that are

transitional between those two types. Most of the advanced argillic assemblages formed at

relatively low temperatures, stable with kaolinite and dickite. The Resolution deposit contains

fairly high levels of fluorine (Schwarz, 2007), and we document that fluorine occurs mainly in

topaz, sheet silicate minerals, and fluorite and that the onset of topaz deposition occurred during

advanced argillic alteration. Resolution is a relatively arsenic-poor system (e.g., Fig. 15 of

Manske and Paul, 2002), in strong contrast to the nearby Magma vein system, and we show that

local spikes in arsenic content at Resolution probably occur mostly where arsenic occurs as a

minor component in other sulfides (e.g., arsenic-bearing pyrite), rather than as occurrences of

discrete arsenic minerals, such as tennantite or enargite.

After reviewing the geologic setting of the district, this paper will illustrate the

distribution of key metals and hydrothermal minerals, document the advanced argillic and

sericitic assemblages and their relative ages as documented during core logging of two,

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approximately orthogonal fences of drill holes, with concurrent use of a PIMA infrared

spectrometer and ultraviolet light and subsequent reflected and transmitted light petrographic

observations to aid mineral identification. Elemental distributions and mineral compositions that

were determined by electron microprobe and the scanning electron microscope, especially of

arsenic- and fluorine-bearing minerals, further constrain the geochemical environment of

hydrothermal fluids. The silicate components of the assemblages are classified into alteration

types using activity ratio diagrams, and the sulfide-oxide component of the assemblages are

classified by sulfidation state, to assess the degree of correlation between advanced argillic

alteration and high-sulfidation state mineral assemblages. The geochemical stabilities of

successive mineral assemblages are used to define the possible evolutionary paths of fluids. The

results have potential applications to exploration, production planning, milling, and smelting.

Geologic Setting

The Resolution deposit is located in the Superior (Pioneer) district, Pinal County, Arizona,

north of Tucson and east of Phoenix (Fig. 1). Porphyry-related deposits in the Superior district

formed within the Late Cretaceous to early Tertiary Laramide arc, which has been variably

dismembered and tilted by mid- to late Tertiary normal faulting (Titley, 1982; Wilkins and

Heidrick, 1995; Lang and Titley, 1998; Maher, 2008; Seedorff et al., 2008; Stavast et al., 2008).

The geology of the Superior district has been mapped and described by Peterson (1969),

Hammer and Peterson (1968), and Manske and Paul (2002), and is summarized here. The

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Proterozoic Pinal Schist forms the local basement of the district and adjacent areas and is

overlain by Proterozoic strata of the Apache Group, which consists in ascending order of the

Pioneer Formation, the Dripping Spring Quartzite, the Mescal Limestone, and locally by basaltic

lava flows. The Apache Group was intruded at 1.1 Ga by a series of diabase sills, which also

intrude the underlying Pinal Schist as sheets of similar orientation. The Apache Group is capped

by the Proterozoic Troy Quartzite. The Proterozoic strata are overlain disconformably by >800 m

of Paleozoic carbonate and clastic rocks that now dip east at 35 to 40. The Paleozoic

stratigraphic section includes the Cambrian Bolsa Quartzite, Devonian Martin Formation,

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

sedimentary and intermediate volcanic and volcaniclastic rocks, correlated regionally with

quartzites of the Pinkard Formation and with the Williamson Canyon Volcanics, respectively, are

preserved inside a down-faulted structural block that includes the Resolution deposit. The

Mesozoic, Paleozoic, and Proterozoic rocks are intruded by felsic porphyry dikes and sills,

perhaps soon after periods of thrust, normal, and strike-slip faulting (Manske and Paul, 2002).

Laramide rocks have proven to be a geochronologic challenge to date accurately, but the

Resolution center probably includes rocks formed at ~63 Ma, and other porphyries in the district

may be as old as ~69 Ma (Ballantyne et al., 2003; Seedorff et al., 2005b).

Pre-Tertiary rocks are unconformably overlain by the Whitetail Conglomerate, an

east-dipping growth sequence deposited in a half-graben that constituted the Whitetail

sedimentary basin. The basal unconformity and the lowest beds exposed in this sequence dip at

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least 25 to the east, with strata higher in the sequence dipping less steeply. The 25 dip likely

represent the minimum amount of post-ore eastward tilting of the deposit, although the actual

amount of tilting is uncertain. The Whitetail Conglomerate is overlain by the Apache Leap Tuff,

which is a welded ash-flow tuffdated at 18.6 Ma that can exceed 400 m in thickness and dips 10

to 15 to the east (Peterson, 1969, 1979; Ferguson et al., 1998; McIntosh and Ferguson, 1998).

Until the last decade, the Superior district was known primarily for production from the

Magma vein and from related mantos that replace selected beds in the Paleozoic carbonate

sequence (Short et al., 1943; Gustafson, 1961; Hammer and Peterson, 1968; Paul and Knight,

1995; Friehauf, 1998; Pareja, 1998), and the Magma vein and mantos have similarities to other

base-metal lode deposits, such as the Butte, Montana, and Cerro de Pasco, Peru (Einaudi, 1982).

The Magma vein and mantos occur north of the town of Superior and extend eastward under the

Apache Leap Tuff toward the #9 Shaft. The Resolution deposit occurs beneath the Apache Leap

Tuff, largely south and east of the Magma vein (Manske and Paul, 2002; Ballantyne et al., 2003;

Schwarz, 2007; Fig. 1). Manske and Paul (2002), among others, have argued that the Magma

vein and Resolution deposit are distinct magmatic-hydrothermal systems, but the relationship

between the two centers, Magma and Resolution, remains controversial.

The Resolution Deposit

The Resolution deposit is a major porphyry copper deposit first discovered in the

mid-1990s (Manske and Paul, 2002; Paul and Manske, 2005). The known extent of the deposit

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and its development toward becoming a mine have been enhanced more recently by Resolution

Copper Mining LLC (Ballantyne et al, 2003; Schwarz, 2007; Anonymous, 2010), a joint venture

between Resolution Copper Company (55%), a subsidiary of Rio Tinto plc, and BHP Copper, Inc.

(45%), a subsidiary of BHP Billiton Ltd. The top of the ore body is ~1.5 km below the surface,

and Resolution Copper Mining LLC plans to use a panel cave method to mine the deposit

beginning in the year 2020. At this time, the deposit is known only from drill core obtained from

holes that are primarily greater than 2 km in length, steeply plunging, and irregularly spaced (e.g.,

Anonymous, 2008). Resolution Copper Mining LLC reported in March 2010 that the deposit has

an Inferred Mineral Resource of 1.624 billion tonnes at a grade of 1.47 per cent copper and 0.037

per cent molybdenum (Anonymous, 2010).

The Resolution deposit is geologically distinctive for several reasons (Manske and Paul,

2002; Schwarz, 2007), including: (1) High hypogene copper grades occur in a variety of

environments, principally as chalcopyrite in diabase, calc-silicate rocks, and intermediate

volcanic rocks, which tend to occur at relatively deep (pre-tilt) levels, but also as bornite

digenite in rocks that tend to occur at shallower levels. (2) Large volumes of rock are affected by

moderate to intense hydrolytic alteration of the sericitic and advanced argillic types. (3) Bornite,

rather than chalcopyrite, is abundant in intense sericitic alteration. (4) Enargite is rare to absent in

advanced argillic alteration, as is tennantite in sericitic alteration, in marked contrast to the

nearby Magma vein.

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Methods

At the project site, the senior author relogged >2,500 m of core from eight drill holes to

supplement existing drill logs and multi-element assays, with a focus on attempting to identify

the hydrothermal mineral assemblages (i.e., silicate, sulfides, and other minerals) in vein fillings

and alteration envelopes and the crosscutting relationships between partially superimposed

events. The holes selected for logging are oriented along two crossing sections, with data

projected onto sections oriented at azimuths of approximately 100 and 180 (Fig. 1), which are

referred to as nominally east-west and north-south sections, respectively.

Geologic logging included using a hand lens to observe and record sulfide and silicate

volume percent estimates, to estimate abundances of alteration minerals, and to measure veins

(angle to core axis, abundances, widths). Data simultaneously were collected with the PIMA

(Portable Infrared Mineral Analyzer) short-wave infrared spectrometer during every logging

interval, and samples of drill core frequently also were observed under ultraviolet (UV)

illumination to check for the presence of hydrothermal topaz, which strongly fluoresces

blue-white under short-wave UV (Marsh, 2002). Although the PIMA spectrometer

occasionally gives spurious information (e.g., indicating presence of stilbite where none is

present), previous work by Troutman (2001) at Resolution that linked visual core logging and

PIMA spectrometer analysis in the field with petrography and X-ray diffraction analysis in the

laboratory demonstrated the overall utility of using the PIMA spectrometer to aid in

identifying the fine-grained minerals that are typical of sericitic and advanced argillic alteration

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at this deposit. In this study, 355 samples were analyzed using the PIMA and short-wave UV.

The various geologic and geochemical observations and measurements subsequently were

plotted on the two cross sections using Microsoft Excel, using color-coded data points to portray

intensity or grade, with the purpose of examining spatial distributions and correlations. The most

instructive features were imported into a drafting program, where they were manually contoured,

as described further in a later section.

In laboratories at the University of Arizona, transmitted and reflected light petrography

was carried out on 70 polished thin sections, and these data were added to the cross sections and

incorporated into tables. A Scanning Electron Microscope (SEM) and an electron microprobe

were used to confirm the identities of minerals, as well as to confirm presence of and/or to

quantity the abundance of certain elements, such as arsenic and fluorine. A CAMECA SX 50

electron microprobe was used to obtain quantitative compositions of biotite, sericite, topaz, and

clay minerals using routine methods and standards.

Distribution of Rock Types, Alteration, and Selected Elements

Geologic cross sections and assays from the Resolution project provide the geologic and

geochemical framework of the deposit and a basis for interpreting data collected in this study.

The distribution of rock types and alteration zones are taken directly from the block model that

was developed by Resolution geologists, which is based on drill holes throughout the deposit,

rectified in three dimensions. The understanding of the geology continues to be improved by

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continued study and analysis and by drilling additional holes (e.g., Anonymous, 2008, 2010),

The present understanding of the distribution of rock types within the deposit generally follows

that of Ballantyne et al. (2003), but the structural history of the district remains uncertain. There

are both pre-ore and post-ore faults in the district with large displacement, but the faults present

within the cross sections have relatively modest offsets and are thought to be largely pre-ore in

age.

Multi-element geochemical data from drill holes in the two selected cross sections were

contoured by hand, attempting to be consistent with existing geologic observations (e.g., degree

of lithologic control of mineralization). Nonetheless, these contours are non-unique

interpretations and are subject to considerable uncertainty, given the current spacing and

orientation of drill holes.

Two cross sections are shown here (Fig. 1). The deposit is elongate in an east-west

direction, and the east-west cross section illustrates at least some of the effects of eastward,

post-ore tilting of the deposit. Additional holes drilled farther east and to greater depths may be

required to describe the full geometry and size of the system. The north-south section, in contrast,

is a slice through the system that largely obscures the effects of tilting by movement on normal

faults and seemingly displays the full north-south extent of the Resolution system.

Distribution of rock types

Figure 2 shows the distribution of rock types at Resolution in both cross sections.

Beginning from the bottom of the cross sections, the first ~ 600 m consists mostly of schist

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overlain by stratified sedimentary rocks, consisting mostly of Proterozoic Pinal Schist, quartzite,

and limestone (mostly converted to skarn), with sills and sheets of diabase, Paleozoic quartzite

and limestone, and Cretaceous quartzite (Fig. 2). A 600- to 700-meter thick Cretaceous

volcaniclastic unit that dips to the northeast overlies this sequence of units. Two main types of

porphyry stocks and dikes intrude the Cretaceous volcaniclastic unit and older rocks, including a

porphyry stock on the eastern side of the east-west section (Fig. 2). The pre-Tertiary units, none

of which is post-ore in age, are overlain by Tertiary post-ore units, the Whitetail Conglomerate

and overlying Apache Leap Tuff (Fig. 2).

Distribution of alteration zones

The Resolution block model includes a field for the dominant alteration type or zone (Fig.

3). The model describes six alteration zones, the first five of which are hypogene in origin:

chlorite epidote calcite adularia (referred to as propylitic), quartz + sericite + pyrite

(sericitic), biotite K-feldspar chlorite anhydrite (potassic), quartz + sericite + pyrite

overprinting biotite K-feldspar (sericitic/potassic), dickite pyrophyllite alunite zunyite

andalusite (advanced argillic), and the supergene hematitic leached cap (Fig. 3). In this

simplified view, the alteration in sedimentary rocks is assigned the same type as those of nearby

igneous rocks.

The base of the cross sections is composed of potassically altered rocks or potassically

altered rocks with a sericitic overprint (Fig. 3); these are confined mainly to Proterozoic schist,

diabase, and lesser Mescal Limestone. Rocks altered to sericitic and advanced argillic alteration

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for copper, arsenic, iron, and sulfur.

Copper: Copper grades exhibit a strong control by host rock lithology (Anonymous,

2008). The highest copper grades (>3 wt%) flare out from the upper volumes of porphyry into

Cretaceous volcaniclastic rocks and in beds of Proterozoic Mescal Limestone and sheets of

diabase (Fig. 4). Copper grades of 1 to 3 wt% can occur in most lithologies with the exception of

Proterozoic Pinal Schist. The highest copper grades are observed mostly in advanced argillic

altered rocks, although there are significant volumes of rock interpreted as sericitic,

sericitic/potassic, and potassic alteration in the RCC alteration block model. Cretaceous and

Proterozoic quartzites generally have lower copper grades than adjacent rock units.

Arsenic: As noted by previous workers (e.g., Fig. 15 of Manske and Paul, 2002; Schwarz,

2007), Resolution is a relatively arsenic-poor system. Troutman (2001), Manske and Paul (2002),

and Harrison (2007) did not report observing either enargite or tennantite. These minerals also

were not observed in this study, although petrographers for Resolution have identified enargite in

thin section. Manske and Paul (2002) note that it is uncommon for arsenic levels to exceed 100

ppm, even in areas with abundant chalcocite/digenite with copper grades >1 percent, and fewer

than 2% of the intervals in the present Resolution assay database exceed 300 ppm As. Arsenic

assays, nonetheless, do display a systematic spatial distribution. In this study, arsenic assays from

drill holes in the two cross sections considered herein are contoured. The upper part of the copper

ore body is arsenic-bearing in both cross sections (cf. Fig. 5 and Fig. 4), whereas the deeper part

of the ore body, where potassic alteration dominates (Fig. 4), is nearly arsenic-free (Fig. 5).

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Representative samples from intervals with elevated arsenic levels that were logged in

this study subsequently were examined in reflected light and with a Scanning Electron

Microscope (SEM) to assess the mineralogic host of arsenic. Neither enargite nor tennantite was

detected; rather, rocks with high arsenic levels showed a relatively uniform distribution of the

element in other sulfide minerals, such that arsenic-bearing pyrite is probably the source of most

local spikes in arsenic abundance [Fig. 6].

Iron: Iron contents can reflect both primary iron contents (e.g., high in diabase and low in

quartzite) and hydrothermal modifications, especially by metasomatic addition of iron during

hydrothermal alteration-mineralization, mostly as iron and copper-iron sulfide minerals.

Nonetheless, the highest contents of iron (Fig. 7) largely coincide with regions of advanced

argillic and sericitic alteration, extending downward into the underlying area of potassic

alteration (Fig. 3). Iron is enriched, however, on the northern part of the north-south section (Fig.

7B), where advanced argillic alteration is better developed (Fig. 3B). The distribution of iron is

even more asymmetric on the east-west section (Fig. 7A, where the highest iron contents also

occur within advanced argillic alteration, and the contours of iron abundance either truncate

upward and eastward against the tilted Tertiary erosion surface and/or iron contents diminish

eastward into a body of relatively silicic porphyry. The highest iron contents (>10wt%) occur

almost exclusively in Cretaceous volcaniclastic rocks and Mescal Limestone For comparison,

relatively fresh diabase contains ~6-10 wt% Fe, as 8-11 wt% FeO and 1-3 wt% Fe2O3 (e.g.,

Wrucke, 1989).

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contoured, with qualitative to semi-quantitative contour intervals chosen based on the resolution

of the data. Contours are shown with solid lines where interpolated between the senior authors

observations in logged portions of drill holes; dashed lines represent inferred extensions of those

contours based on data other than those drill logs.

Clay: The term clay here refers only to the aluminum silicates kaolinite, dickite, and

halloysite, identified principally with the PIMA spectrometer. Although montmorillonite

(smectite) is also a clay mineral, it is discussed separately below.

Although kaolinite occurs within the leached cap and there is probably all or in part of

supergene origin (e.g., Troutman, 2001), most of the kaolin group minerals logged at Resolution

occur with pyrite and other sulfides, where the clay minerals are interpreted to be of hypogene

origin (Fig. 16D). The occurrence of kaolin group minerals at Resolution includes white kaolinite

in strongly silicified zones containing bornite chalcocite and massive translucent pale green

dickite with pyrite, bornite, and chalcocite (Troutman, 2001; Manske and Paul, 2002). The kaolin

group minerals are present mostly in two general areas: (1) extending out from porphyry through

Cretaceous volcaniclastic rocks, and (2) stratabound occurrences within certain sedimentary

units in the lower part of the cross sections (Figs. 2 and 9). The regions with the most abundant

kaolin group minerals coincide with regions of the RCC block model that are assigned to the

advanced argillic alteration zone but extending into the sericitic zone, whereas moderate

abundances of clay extend downward into the potassic zone and outward into the propylitic zone

(Figs. 3 and 9). Microprobe analyses of 11 kaolin group minerals contain ~0.2 to 1.1 wt% F;

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representative analyses are shown in Table 1.

Sericite:PIMA measurements identified 170 samples containing muscovite and 104

samples containing illite (out of a total 253 measurements), supported by petrographic

observations (Fig. 16D,E). These minerals were grouped together for the purposes of describing

sericitically altered rock. High sericite values are observed across the two cross sections (Fig. 10);

rocks containing such values include Cretaceous volcaniclastic rocks, Cretaceous quartzites,

diabase, Mescal limestone, Pinal Schist, and quartzite (Fig. 2). High sericite values are contoured

primarily in areas that the RCC block model assigns to the advanced argillic, sericitic, and

sericitic/potassic alteration zones (Fig. 3).

The electron microprobe was used to analyze 16 grains of sericite, and representative

analyses are reported in Table 1. The compositions vary widely between 2.9 and 9.4 wt% K2O

with negligible Na2O, and none of the analyzed grains is a dioctahedral mica. The mean K2O

content of 6.2 wt%, which corresponds to a mean occupancy of the A site of only ~60%,

implying that the grains have non-muscovite components such as illite (e.g., Bailey, 1984;

Brigatti and Guggenheim, 2002). The major-element compositions of sericite from Resolution

are thus distinct from those of sericite analyzed from some other porphyry systems, in which the

A site generally is almost fully occupied (e.g., Koloula, Guadalcanal, Chivas, 1978; San

Manuel-Kalamazoo, Arizona, Guilbert and Schafer, 1979; Santa Rita, New Mexico, Parry et al.,

1984; Henderson, Colorado, Seedorff and Einaudi, 2004a). The analyzed sericite grains also

contain ~0.5 to 1.7 wt. percent F, which overlaps with but extends to higher levels than observed

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for clay.

Topaz: Topaz occurs widely across certain parts of the deposit (Fig. 11), and suspected

occurrences can be easily confirmed with the assistance of short-wave UV light, with the

PIMA spectrometer, and petrographically (Fig. 16G). As noted by Marsh (2002), topaz occurs

most commonly in alteration envelopes on pyrite veins. Topaz is in equilibrium with bornite and

chalcocite and has not been observed in equilibrium with chalcopyrite.

High and moderate topaz values are observed in Cretaceous volcaniclastic rocks,

porphyry, Cretaceous quartzite, and Proterozoic limestone, quartzite and diabase (Figs. 2 and 11).

About half of the topaz identified in this study occurs in rocks assigned to the advanced argillic

zone in the RCC block model, and the remainder occurs in the sericitic zone (Fig. 3).

Six topaz grains from Resolution were analyzed by electron microprobe; representative

analyses are shown in Table 2. Topaz is uniformly relatively fluorine-poor, containing 11-12 wt

percent F. This is equivalent to a mole fraction of fluor-topaz in topaz solid solution (XF-Topaz) of

0.580.64. As discussed further in a later section, the fluorine content of topaz helps to

constrain the geochemical environment of its formation (e.g., Barton, 1982; Seedorff, 1986;

Seedorff and Einaudi, 2004b), and XF-Topaz) of ~0.6 is indicative of forming in an advanced

argillic alteration environment.

Pyrophyllite and andalusite: In this study, only three occurrences of pyrophyllite,

confirmed by PIMA spectrometer, were documented on the two cross sections that were

studied (Fig. 12). All occurrences of pyrophyllite occur as intergrowths with alunite, chalcocite,

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and bornite and are present in the same region (Fig. 12) where topaz is present in moderate or

high levels (Fig. 11). Three isolated occurrences of pyrophyllite, with alunite, topaz, and

kaolinite, also were detected in a large-scale infrared spectral reflectance study commissioned by

RCC that used the HyLogger technique in two drill holes (Huntington and Yang, 2009). The

relative paucity of pyrophyllite (Fig. 12) compared to clay (Fig. 9) at Resolution that was

observed in this study is consistent with the earlier observations (Troutman, 2001; Manske and

Paul, 2002; Harrison, 2007).

Andalusite has not been documented at Resolution by earlier workers (Troutman, 2001;

Manske and Paul, 2002; Harrison, 2007; Schwarz, 2007). Andalusite was not observed in core

logging or petrographic examinations made in this study, but several samples submitted by

Resolution geologists for petrographic description have reported local occurrences of andalusite.

Phases indicative of quartz-undersaturated conditions, such as diaspore and corundum (Hemley

et al., 1980), also were not observed.

Alunite: High alunite values occur in the Cretaceous volcaniclastic rocks, although there

are also rare occurrences in Cretaceous quartzites, porphyry, and Proterozoic Mescal Limestone,

diabase, and quartzite (Figs. 2, 12, 16F). High alunite values occur in rocks assigned to both the

advanced argillic and sericitic zone of the RCC block model (Figs. 3 and 12), but RCC has

observed alunite at deep levels of the deposit (e.g., the bottom of hole RES-2A) in rocks assigned

to the zone with sericitic alteration superimposed on potassic alteration. Harrison (2007)

observed several instances of alunite nucleating around cores of APS minerals (see below).

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APS minerals: Although not observed in this study, aluminum-phosphate-sulfate (APS)

minerals were identified in felsic protoliths by Harrison (2007) using the scanning electron

microscope. Harrison (2007) observed that the minerals occureither with an irregular, ragged

texture in kaolinite, form cores of alunite, or occur as dendritic rapidly cooled masses and zoned

fragments in alunite vein matrix that may be pseudomorphs of precursor apatite. Electron

microprobe analyses by Harrison (2007) revealed APS compositions intermediate between the

end-members hinsdalite, woodhouseite, and svanbergite, with no evidence for the weilerite

component, as arsenic occurred below the detection limit.

Zunyite: Seven occurrence of zunyite were recorded in this study at the locations plotted

on Figure 12, coinciding with areas of the cross sections that contain moderate and high levels of

topaz. Zunyite also is reported by Troutman (2001) and Manske and Paul (2002). Two electron

microprobe analyses of zunyite, shown in Table 2, contain 6 to 7 wt percent F and more than 2

wt percent Cl.

Biotite, chlorite, and fluorite: In this study, core generally was not logged in areas with

well-developed potassic or propylitic alteration, where biotite and chlorite are abundant.

Nonetheless, several potassically altered specimens were collected, and hydrothermal biotite was

analyzed by electron microprobe (Table 1). The analyzed biotite grains are phlogopitic and

contain 3 to 4 wt. percent F. Though it was not observed in rocks with advanced argillic

alteration, fluorite (~49 wt% F) is also a common gangue mineral in the Resolution deposit

(Schwarz, 2007).

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Montmorillonite: The PIMA spectrometer identified the presence of montmorillonite

as frequently as the other clay minerals and sericite, including in feldspar sites that were soft but

not necessarily pyrite-bearing. Abundances of montmorillonite were not compiled in this study

given the lack of evidence for association with advanced argillic or sericitic alteration. Although

it is possible that this PIMA determination is spurious, an alternative is that montmorillonite

was formed as a late, weak hydrolytic overprint at low temperatures, postdating deposition of all

or most sulfides.

Pyrite: Abundances of sulfide minerals were estimated visually during logging of core

and are summarized in terms of relative abundances (Fig. 13). Large volumes of rock contain

high pyrite abundances, which are observed within rocks with advanced argillic alteration and, to

a lesser degree, with sericitically altered rocks (Figs. 3 and 13).

Where pyrite occurs with other sulfides, petrographic observations of sulfide textures

suggest that pyrite was deposited earliest relative to other sulfides (Fig. 16A-C). Pyrite is

commonly observed as angular, brecciated grains and as rounded grains cemented in

bornite-chalcocite-digenite or chalcopyrite-bornite.

Bornite and chalcocite: Supergene chalcocite occurs only locally within the leached cap

(Manske and Paul, 2002). Hypogene examples of both chalcocite and digenite are well

documented at Resolution (e.g., Manske and Paul, 2002, p. 212). Chalcocite and digenite

commonly are intergrown with one another and with bornite (Fig. 16B,C); in addition, there are

uncommon occurrences of hypogene covellite (Harrison, 2007), which was not logged in this

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study. Indeed, electron microprobe analyses by Harrison (2007) reveal the presence of a variety

of Cu-S phases of various stoichiometries, as well as various compositions of bornite. As noted

by earlier workers (Troutman, 2001; Manske and Paul, 2002; Harrison, 2007), petrographic

evidence suggests that bornite commonly was precipitated around earlier grains of pyrite and

fills cracks in and locally partially replaces pyrite (Fig. 16A-C). Chalcocite appears to be stable

with pyrite, though not necessarily coprecipitated with it (Fig. 16B,C).

In this study, the term chalcocite may be used for any of the Cu-S phases. Given the

common intimate intergrowth of bornite and chalcocite, visual estimates of abundance were

recorded for the sum of bornite and chalcocite (Fig. 14). The highest bornite + chalcocite values

are observed within advanced argillic altered regions and to a lesser extent in rocks assigned to

the sericitic and potassic/sericitic zones in the RCC alteration block model. Resolution geologists

have observed that the highest concentrations of bornite occur at a fairly distinct level at an

elevation of ~ -500m, which is near the base of sericitic alteration with 7-14 vol% pyrite but

outside the highest zone of pyrite. In contrast, the most abundant chalcocite tends to occur at or

slightly above the region of most abundant bornite in the most pyritic rocks.

Chalcopyrite: As shown in Figure 15, high values of chalcopyrite are observed in

advanced argillic, sericitic, and potassic/sericitic altered zones of the RCC alteration block model.

The highest chalcopyrite values are observed consistently in Proterozoic diabase and Mescal

Limestone and more locally in Cretaceous volcaniclastic rocks and porphyry, generally where

altered to potassic assemblages and skarn, which were not the focus of this study. Most of the

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other occurrences of chalcopyrite are associated with pyritic veins with sericitic alteration

envelopes. No occurrences of chalcopyrite were observed that are unambiguously associated

with advanced argillic assemblages.

Veins and crosscutting relationships

Mineralogy of veins: Many types of hypogene veins were observed and logged, and

photographs of key examples are illustrated in Figure 17. In some cases, the vein filling and the

alteration envelope are distinct; in other cases, they are not, especially where the outer edges of

the envelopes cannot be determined because of a high density of superimposed veins. Similar

veins were grouped into a smaller number of types (Table 3).

Crosscutting relationships: Crosscutting relationships between veins were recorded and

documented during logging, with attempts to avoid potentially deceiving exposures. Figure 18

shows photographs of representative observed crosscutting relationships between veins, and the

petrographic relationships also provide paragenetic constraints (Fig. 16). Table 3 provides a

matrix tabulation of all observations from this study, which focused on a part of the deposit with

abundant sericitic and advanced argillic alteration and thus lacks exposures of many higher

temperature or earlier veins. The table shows that the general paragenesis of veins in the deposit

from older to younger is quartz - molybdenite veins (which may lack alteration envelopes but

generally occur in areas of potassic alteration), through veins related to sericitic alteration,

transitional sericitic-advanced argillic assemblages (e.g., Fig. 16H), and uncommon pyrophyllite-

and alunite-bearing assemblages, and finally kaolinitic clay-bearing advanced argillic

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assemblages (Fig. 16D). The data are consistent with the observations of Marsh (2002) that

topaz-bearing veins probably formed broadly contemporaneous with pyrophyllite and kaolinite.

There are a few observations to the lower left of the diagonal of the matrix in Table 3, which is

the field of possible reversals in the vein paragenesis. (See Seedorff and Einaudi, 2004a, for

explanation of terminology regarding reversals and normal and anomalous types of crosscutting

relationships.) Because the detailed paragenesis of veins is only partially constrained by the

relatively limited number of observations in Table 3, the apparent reversals that involve different

veins within the same silicate alteration type (e.g., those advanced argillic veins in the lower

right part of the matrix) probably are not significant. The other apparent reversals are isolated

observations that could represent spurious observations because of cryptic, unrecognized

deceiving exposures or may represent local reversals. In any case, no definitive evidence has

been provided to date for multiple mineralizing events or major reversals in the sequence of

crosscutting veins (Troutman, 2001; Harrison, 2007; A. Schwarz, oral comm., 2007).

Orientations of veins: A variety of data exist regarding vein orientations at Resolution.

The staff geologists collect extensive data on vein orientation based on measurements on

oriented core their correlation with down hole data. One such tool, Wellcad, is a core orientation

program that collects both bore hole televiewer and acoustic resonance imaging data. The

program collects copious amounts of information regarding vein fracture and fault orientations.

An RCC internal technical report (Trout, 2009) that summarizes the vein database reports an

overall vein trend of N30E; the earliest, quartzmolybdenum veins and veins within the

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porphyry are reported to have a northwesterly strike; chalcopyrite veins strike west; and

chalcocite- and bornite-bearing veins follow a N30E trend. In an earlier study, Troutman (2001)

notes a dominant strike of N55E with southeasterly dips in the northern part of the deposit and

northwesterly dips in the southern part of the deposit for veins associated with sericitic and

advanced argillic alteration.

In this study, vein orientations to core axis were measured (Table 4). Because the portions

of the holes logged in this study are nearly vertical, 90 minus the angle to core axis is the

approximate dip of the vein. These data show that the dips on sericitic veins vary widely,

whereas for veins associated with advanced argillic alteration the orientations are more

systematic, with the angle to core axis generally varying from 0 to 30 degrees, i.e., the present

dip of the veins (i.e., after tilting associated with normal faulting) is steep at ~60-90. The data of

Troutman (2001) and the Wellcad database suggest that these veins have northeasterly strikes.

Interpretations

Characterization and classification of assemblages into silicate alteration types

As in many deposits where advanced argillic alteration is developed, the assignment of

alteration products at Resolution to hydrothermal mineral assemblages is complicated by the

widespread evidence for superposition of events, the difficulty in some cases of relating

alteration features to particular veins or fluid channels, and the fine-grained nature of many of

the products of hydrolytic alteration. In spite of hundreds of PIMA and UV light determinations,

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supplemented by petrography and electron microprobe analyses, the ability to identify

hydrothermal mineral assemblages with certainty continuously down a given drill hole remains

impossible nonetheless, it is possible to do this discontinuously in many drill holes. The

information gathered from such intervals, as illustrated in Figure 19, can be synthesized to

construct a list of the various mineral assemblages produced by acidic fluids at Resolution (Table

5), even if the assignments are less certain than they might be in other deposits or other parts of

the Resolution deposit, and to assign them to silicate alteration types using the criteria of

Seedorff et al. (2005a). Interpretations rely heavily on the most definitive cases, such as

compelling cases for coprecipitated minerals filling veins (Fig. 17) and petrographic textural

evidence observed here (Fig. 16) and documented in other works (e.g., Troutman, 2001; Harrison,

2007). Given the difficulty of this effort, Table 5 also offers guidance as to the confidence in each

proposed assemblage and an interpretation of the relative abundance of each assemblage. Most

of the assemblages observed (Table 5) either (1) are of the sericitic type (Figs. 16E, 19D,G), (2)

are transitional from sericitic to advanced argillic types (Figs. 16H,19E), or (3) are of the

advanced argillic type (Figs. 16F,G, 19A-C,H).

The intensity of advanced argillic alteration at Resolution is largely governed by the

abundance of kaolinite and dickite (Fig. 9), given the relative lack of pyrophyllite (Fig. 12) and

rarity of andalusite. Among other aluminous minerals, such as topaz (Fig. 11), alunite (Fig. 12),

and zunyite (Fig. 12), that can be present in advanced argillic alteration, only topaz is fairly

abundant at Resolution. The geochemical stabilities of these minerals do not require that the

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former three minerals form only under advanced argillic conditions (e.g., Barton, 1982; Seedorff,

1986; Seedorff et al., 2005a), and some of these minerals clearly have formed in other deposits in

the sericitic or potassic environments (e.g., Seedorff and Einaudi, 2004a, b). At Resolution, hand

specimen and petrographic observations indicate that topaz in many cases clearly formed in

equilibrium with kaolinite but not with K-feldspar or sericite, although certain quartz-topaz

occurrences are not diagnostic. Nonetheless, the fluorine-poor nature of all topaz grains analyzed

(XF-Topaz of ~0.6) coupled with phase relations (e.g., Barton, 1982; Seedorff and Einaudi 2004b)

indicate that topaz at Resolution probably formed mostly in the advanced argillic environment.

The close association of alunite with pyrophyllite at Resolution also indicates that the few alunite

occurrences known from Resolution also formed in the advanced argillic environment.

Relationship between silicate alteration assemblages and opaque assemblages

At Resolution, the deeper part of the ore body characterized by potassic alteration (e.g.,

Manske and Paul, 2002) contains opaque assemblages of chalcopyrite, chalcopyrite + magnetite,

and chalcopyrite + pyrite, i.e. intermediate sulfidation state assemblages (Einaudi et al., 2003).

As shown in Table 5, a sericitic assemblage with relatively low abundance is characterized by the

intermediate sulfidation state opaque assemblage of chalcopyrite + pyrite. The most abundant

sericitic assemblages, however, are high-sulfidation state assemblages with pyrite, bornite, and

chalcocite (Fig. 19D,G), confirming earlier conclusions of Troutman (2001) and Manske and

Paul (2002) regarding Resolution but differing from observations in many other porphyry

systems (e.g., Einaudi, 1982). High-sulfidation state assemblages persist through transitional

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advanced argillic-sericitic and advanced argillic assemblages (Table 5). Indeed, three polished

thin sections in this study (e.g., Fig. 19D) contain bornitechalcocite with sericite but lack

kaolinite, topaz, and/or alunite; nonetheless, the majority of polished thin sections contain

bornite-chalcocite stable with clay and/or topaz. The highest copper grades (>3%) coincide

generally with high abundances of bornite-chalcocite-associated exclusively with zones of

intense advanced argillic alteration and high pyrite content (Figs. 4, 7, 9, 11, 13, 14). As noted

above, enargite is uncommon in high-sulfidation mineral assemblages, as local spikes in arsenic

content correspond primarily with occurrences of arsenic-rich pyrite (Fig. 6). The very-high

sulfidation state assemblage covellite + pyrite, also associated with advanced argillic alteration,

has been reported locally at Resolution (e.g., Harrison, 2007) but was not observed in this study.

Evolutionary paths of fluids

The succession of mineral assemblages (Table 5) with time as documented by

crosscutting relationships (Fig. 18, Table 3), supplemented by petrographic observations (Fig.

16), can be used to deduce evolutionary paths of hydrothermal fluids that can be displayed as

paths across phase diagrams. Although some portions of a path may represent progressive

evolution of a single batch of hydrothermal fluid as it reacts with wall rock, new inputs of fluid,

perhaps varying compositions, may also occur with time.

The sequence of opaque assemblages (Table 5) documented above constitutes an

evolutionary path of increasing sulfidation state with time in the region of the Resolution deposit

examined in this study. This path probably corresponds to a segment of the gently

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upward-inclined arc of the looping path on a T-fS2 diagram commonly observed in

porphyry-related systems that attain high and very high sulfidation states (e.g., Einaudi et al.,

2003).

Changes in the acidity of the fluid with decreasing temperature are illustrated

schematically in Figure 20. Without better constraints on temperature (e.g., from fluid inclusion

data), a precise path cannot be shown, but the sequence of silicate phases is nonetheless

indicative of a general trajectory based on relative ages of assemblages. The sequence of veins

and mineral assemblages (Tables 3 and 5) indicate that earlier, presumably hotter, fluids were

stable with K-feldspar and/or biotite, producing potassic alteration. Through time, probably

associated with a decline in temperature, the fluid became stable with sericite, although the

potassium-poor compositions of sericite at Resolution (Table 1) suggest that much of this sericite

was produced at fairly low temperatures, approaching or within the nominal stability of illite (Fig.

20). The abundance of kaolinite-dickite and the rarity of pyrophyllite suggest that the path may

have left the sericite field and entered the field of aluminosilicate minerals at fairly low

temperatures (~300C) in the vicinity of the pyrophyllite-kaolinite boundary (Fig. 20), coinciding

with the transition from sericitic to advanced argillic alteration. The absence of observations of

diaspore and corundum and the silicification commonly associated with kaolinite (e.g., Troutman,

2001) indicate that the fluid probably was quartz-saturated during advanced argillic alteration at

Resolution (see also Hemley et al., 1980). If the PIMA identifications of montmorillonite and

the suggestion that they might represent a late sulfide-absent alteration product are valid (see

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above), then the fluid, after producing abundant kaolinite, may finally have become less acid at

very late stages (Fig. 20). The path can be compared with other possible paths that might produce

advanced argillic alteration somewhere during their evolution (e.g., Fig. 12 of Seedorff et al,

2005a).

The presence of topaz and its composition offers further insight into the geochemical

evolution of the fluid, as displayed on diagrams involving temperature and activities of K+

and F-

(Figs. 21, 22). Topaz is commonly observed in association with kaolinite/dickite (Table 5); hence,

the general path of the fluid on these diagrams interpreted to have traversed from the sericite or

muscovite field (without topaz) to a position straddling the topaz-kaolinite boundary while topaz

was deposited. The fluorine-poor compositions of topaz solid solution (Table 2) are consistent

with formation of topaz during advanced argillic alteration at relatively low temperatures of

~300C (Figs. 21, 22). The suggestion that at least some of the kaolinite and dickite postdate

topaz (with kaolinite) indicates that the path then headed into the kaolinite-only field (Fig. 22),

perhaps then veering toward higher values of K+/H+ at late stages (if montmorillonite formed

late, see above; Fig. 22). The paths of Figures 21 and 22 are markedly different than those that

produced topaz during potassic and sericitic alteration at Henderson (e.g., Figs. 9, 10, 12 of

Seedorff and Einaudi, 2004b).

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Discussion

Geometry and characterization of sericitic and advanced alteration

The distribution of sericitic and advanced argillic alteration in porphyry deposits is

variable (e.g., Fig. 10 of Seedorff et al., 2005). In some deposits, advanced argillic alteration

largely is barren (e.g., the lithocap of Sillitoe, 2010), whereas in many systems it can be well

mineralized (e.g., Einaudi et al., 2003; Seedorff et al., 2005). Likewise, there also can be a

general vertical progression from high to low temperature advanced argillic alteration from

deeper to shallow levels, i.e., andalusite to pyrophyllite to kaolinite (e.g., Gustafson and Hunt,

1975; Watanabe and Hedenquist, 2001).

Rocks exhibiting sericitic alteration and advanced argillic alteration at Resolution

generally are mineralized and commonly exhibit high grades and thus do not constitute a barren

lithocap at the preserved levels. Although some uncertainty remains because the top of the

Resolution system has been eroded, the preserved and drilled portion of the pattern suggests that

advanced argillic alteration is commonly enveloped in three dimensions by a thick rind of

sericitic alteration, although in places advanced argillic alteration extended downward into

potassic alteration and may have extended as a pipe or funnel locally upward through the

sericitic rind at levels above the Tertiary erosion surface (Figs. 3, 9, 10). One could regard the

preserved part of Resolution to be the root of a zone of advanced argillic alteration, to the

extent that some rocks affected by advanced argillic alteration have been eroded, yet it is notable

that the preserved root is overwhelming dominated by kaolinite and dickite, rather than either

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pyrophyllite or andalusite. Moreover, the relatively potassium-poor compositions of sericite at

Resolution (Table 1) indicates that the uneroded part of the Resolution system that was the

subject of this study also is not the root of sericitic alteration, but rather probably its uppermost

branches. Hence, it seems unlikely that much of the top of the system has been eroded.

Resolution represents an interesting variant on many possible geometries of hydrolytic

alteration in porphyry systems.

The sericitic to advanced argillic transition

Although the identities, distributions, and relative ages of mineral assemblages is crucial

to understanding the geochemical environment of alteration-mineralization and the dynamics of

hydrothermal systems (e.g., Seedorff et al., 2005a), rocks exhibiting hydrolytic alteration

commonly represent a special challenge in identifying mineral assemblages and in many cases is

been regarded as virtually impossible (e.g., Khashgerel et al., 2006). The difficulty is that the

silicate minerals commonly are fine-grained and light colored and difficult to identify with the

naked eye, hand lens, or in some cases even petrographically, such as distinguishing between

sericite and pyrophyllite. Other techniques, such as infrared spectrometers and X-ray diffraction,

aid in mineral identification, but the scale resolution of such determinations commonly results in

the loss of the textural relationships necessary to establish that the minerals formed

contemporaneously in apparent equilibrium.

This study represents one of the few attempts to determine mineral assemblages in areas

of intense hydrolytic alteration and their relative ages (cf., Lipske and Dilles, 2000; Khashgerel

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et al., 2009). At Resolution, numerous advanced argillic assemblages are present (Table 5),

though most contain kaolinite or dickite, and topaz is abundant. The predominance of kaolinite

contrasts with deposits such as Butte and El Salvador, where andalusite and pyrophyllite are

much more common (e.g., Meyer et al., 1968; Howard, 1972; Brimhall, 1977; Gustafson and

Hunt, 1975; Watanabe and Hedenquist, 2001; Field et al, 2005; Rusk et al., 2008).

Presence of topaz and other fluorine-bearing minerals

Topaz forms in a wide range of geochemical environments (e.g., Barton, 1982). In certain

porphyry molybdenum and tungsten systems, it forms during potassic and sericitic alteration, but

in porphyry copper deposits it forms almost exclusively during advanced argillic alteration

(Seedorff, 1986; Seedorff and Einaudi, 2004b).

Metallurgical tests indicate that Resolution is a relatively fluorine-rich deposit (Schwarz,

2007), although the spatial distribution of fluorine at Resolution is not well known because

fluorine is not routinely assayed. Microprobe analyses conducted in this study (Table 2) indicate

that the most important fluorine-bearing minerals throughout the deposit likely are biotite (~3-4

wt% F), topaz (~11-12 wt% F), fluorite (~49 wt% F), and sericite (~1 wt% F), and other

fluorine-bearing phases also are locally present (e.g., zunyite, 6-7 wt% F). As in other porphyry

copper systems, topaz at Resolution formed during advanced argillic alteration but at relatively

low temperatures with kaolinite, consistent with its relatively fluorine-poor composition (XF-Topaz

= 0.580.64).

The location and mineralogic host of fluorine in porphyry systems has potential practical

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implications in processing and environmental storage (e.g., Pangum et al., 1997; Sutter, 2002).

Arsenic abundance and mineralogy in porphyry-related systems

For those porphyry systems that contain considerable arsenic, arsenic is generally present

in intermediate sulfidation state assemblages as tennantite and in high-sulfidation state

assemblages as enargite and luzonite, which tend to be associated with advanced argillic

alteration (e.g., Meyer et al., 1968; Einaudi, 1982; Einaudi et al., 2003).

Even though the nearby Magma vein, containing both tennantite and enargite (Gustafson,

1961; Hammer and Peterson, 1968), is notably rich in arsenic, Resolution is relatively

arsenic-poor in spite of widespread, intense advanced argillic alteration (e.g., Manske and Paul,

2002), a distinction shared with Oyu Tolgoi (Khashgerel et al., 2008, 2009). The upper part of

the Resolution ore body is arsenic-bearing, but arsenic occurs most in solid solution in other

sulfides (e.g., arsenic-bearing pyrite) rather than as enargite.

The low arsenic contents of ores have metallurgical and environmental benefits to the

project, but geochemical controls on arsenic content and mineralogy are not well understood. An

elevated oxidation state of the fluid has been suggested as one possible cause (R. Beane, quoted

in Manske and Paul, 2002).

Relationship between silicate alteration types and sulfidation state of sulfides

Coexisting minerals, mainly the opaque sulfide and oxide minerals, can be used to

constrain the sulfidation state in which they formed (e.g., Barton, 1970; Barton and Skinner,

1967, 1979; Einaudi et al., 2003), and there is a long-recognized tendency for mineral

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assemblages containing silicate minerals characteristic of advanced argillic alteration to contain

sulfide minerals characteristic of high and very high sulfidation states, because reactions

involving sulfur species that lead to higher sulfidation states also generate acid (e.g., Meyer and

Hemley, 1967; Einaudi, 1982).

At the Resolution deposit, most of the sericitic assemblages and all of the advanced

argillic assemblages contain high sulfidation state opaque assemblage (Table 5). There is no a

priori reason why coupled reactions involving sulfur species should cause the transition from

sericitic to advanced argillic to coincide precisely with the transition from intermediate to high

sulfidation states.

Resolution is thus an exception to the general rule that high sulfidation-state minerals

are deposited only during advanced argillic alteration, for which Chuquicamata is another

possible example. Advanced argillic alteration is not present (or at least not yet reported) at

exposed and drilled levels of Chuquicamata, in spite of the fact that high-sulfidation state sulfide

minerals, including enargite, are widespread and vertically extensive, seemingly cogenetic with

sericitic alteration (Ossandn et al., 2001).

Source of high copper grades

The search for genetic understanding of controls on metal grades are an enduring theme

of economic geology, and Resolution is distinctive for having high hypogene copper grades in a

variety of alteration types (Manske and Paul, 2002; Schwarz, 2007). Telescoping of

alteration-mineralization events is also offered as one possible explanation for porphyry deposits

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with higher hypogene grades (e.g., Sillitoe, 2010). The compositions of certain host rocks, such

as carbonate rocks and diabase, also may have locally enhanced metal deposition (Fig. 4), but

many porphyry deposits in the region are hosted by identical units yet have half the grade of

Resolution, as noted by Manske and Paul (2002).

Although some of the highest grades in the Resolution deposit occur in areas of advanced

argillic alteration with abundant digenite (Figs. 4, 14) and superposition of later high-sulfidation

on earlier intermediate sulfidation assemblages locally increased copper grades and probably

added to the size of the ore body, the fact remains that areas of the deposit containing only

intermediate sulfidation assemblages also exhibit high hypogene grades (Figs. 4, 15). These

observations suggest that the principal control on high hypogene grades at Resolution may be

high fluxes of copper-bearing hydrothermal fluid, which would have been dictated by conditions

in the underlying magma chamber, rather than by conditions at the site of metal deposition.

Resolution offers considerable opportunity for future work to yield a better understanding

of whether or how hydrolytic alteration redistributed or added copper to the ore body.

Conclusions

Resolution is a geologically and economically significant example of porphyry copper

mineralization with extensive development of both advanced argillic alteration and

high-sulfidation state opaque assemblages. This study represents one of the few attempts to

determine mineral assemblages in areas of intense hydrolytic alteration and their relative ages.

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The deposit is an exception to the general rule that high sulfidation-state minerals are

deposited only during advanced argillic alteration, as sericitic assemblages contain

high-sulfidation state opaque assemblages. Numerous advanced argillic assemblages are present;

most contain kaolinite or dickite, in contrast to deposits where andalusite and pyrophyllite are

much more common. Resolution is a relatively fluorine-rich deposit, and topaz, formed during

advanced argillic alteration, is abundant and predictably exhibits fluorine-poor compositions.

Acknowledgments

Support for this project was generously provided by Resolution Copper Mining LLC and

by Science Foundation Arizona through the UA Lowell Institute for Mineral Resources. We are

grateful for the opportunity to build on the work of Resolution geologists and earlier workers.

During the early stages of the project, Adam Schwarz provided invaluable assistance in geologic

logging, and Bill Hart provided invaluable aid by making the geologic and geochemical database

more useful to us. We thank Ken Domanik for technical assistance with electron microprobe

analyses. We acknowledge also benefitting, directly and indirectly over many years, from the

observations and insights on the geology of the Superior district from Geoff Ballantyne, Marco

Einaudi, Don Hammer, Kurt Friehauf, David Maher, Scott Manske, Tim Marsh, Alex Paul, and

Sandra Troutman.

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