Table Of ContentHYPOGENE ALTERATION AND MINERALIZATION IN THE DOS POBRES
PORPHYRY CU(-AU-MO) DEPOSIT, SAFFORD DISTRICT, ARIZONA:
A GOLD- AND MAGNETITE-RICH VARIANT OF
ARIZONA PORPHYRY COPPER SYSTEMS
By
Daniel Russin
_________________
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
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STATEMENT BY THE AUTHOR
This thesis 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.
Daniel Russin______________________________ ___________________
(author’s signature) (date)
APPROVAL BY RESEARCH COMMITTEE
As members of the Research Committee, we recommend that this thesis be accepted as
fulfilling the research requirement for the degree of Master of Science.
Mark D. Barton_____________________________ __________________
Major Advisor (type name) (signature) (date)
Eric Seedorff_______________________________ __________________
(type name) (signature) (date)
Jon Patchett________________________________ __________________
(type name) (signature) (date)
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Hypogene alteration and mineralization in the Dos Pobres porphyry
Cu(-Au-Mo) deposit, Safford District, Arizona: A gold- and magnetite-
rich variant of Arizona porphyry copper systems.
Daniel Russin*
Mark D. Barton
Eric Seedorff
Center for Mineral Resources, Department of Geosciences, University of Arizona,
Tucson, Arizona 85721-0077
* E-mail: [email protected]
Abstract
The Dos Pobres Cu(-Au-Mo) deposit (211 million metric tonnes sulfide resource
0.73% Cu with up to 1 ppm Au) is located in the Safford district of southeastern Arizona
and is one of several Au-bearing porphyry systems in Arizona. The deposit is centered on
Paleocene (57 Ma) quartz monzodioritic porphyry dikes that intrude and alter Late
Cretaceous (67-73 Ma) basaltic andesites. The porphyry dikes locally contain igneous
anhydrite. The volcanic rocks dip gently (10-15°) to the northeast; they and the ENE-
trending porphyry dikes are cut by a NW-striking down-to-the-west normal fault that
down-drops the southwestern portion of the Dos Pobres system by roughly 1 km. This
study focuses on the characterization and distribution of veins, hydrothermal alteration,
and ore minerals below the base of weathering, utilizing core logging and petrography
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coupled with whole-rock geochemical and electron microprobe analyses, as well as U-Pb
and Re-Os geochronology.
Hypogene veins at Dos Pobres are divided into groups based on their mineralogy,
textures, and alteration envelopes. Five early vein types have envelopes that are
dominated by biotite and/or K-feldspar. These are: hairline biotite (biotite ± magnetite ±
bornite); sugary quartz (quartz + K-feldspar ± sulfide (bornite (cid:1) chalcopyrite) ± biotite ±
anhydrite); comb quartz (inward-growing quartz + K-feldspar ± sulfide (bornite <
chalcopyrite) ± biotite); complex biotite (biotite + K-feldspar + quartz + sulfide (pyrite <
bornite < chalcopyrite) ± anhydrite); and green mica veins (biotite + sericite + K-feldspar
± sulfide (bornite (cid:1) chalcopyrite) ± anhydrite ± andalusite). Two types of veins with
chlorite ± sericite envelopes are sulfide-sericite (sulfide (chalcopyrite > pyrite)
± quartz ± sericite ± chlorite ± anhydrite) and clotty sulfide-chlorite (quartz
± sulfides (chalcopyrite > pyrite) ± chlorite ± sericite ± anhydrite). These veins cut those
with biotite and/or K-feldspar envelopes. Veins consisting of chlorite + epidote + calcite
+ quartz ± sulfide (pyrite > chalcopyrite) with chlorite + epidote + calcite envelopes are
also common and cut those with chlorite ± sericite envelopes. Zeolite veins without
alteration envelopes cut all other vein types.
Potassic alteration assemblages with pervasive biotite ± K-feldspar ±
magnetite comprise the best developed and earliest alteration suite. It is most intense in
proximal locations where quartz + K-feldspar veins locally compose upwards of 30 vol
percent of the rock. The intensity of this alteration diminishes outward and upward.
Scattered biotite + actinolite-hornblende + magnetite alteration is interpreted to represent
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the fringes of pervasive potassic alteration. Hydrolytic alteration, characterized by
sericite- and chlorite-rich replacement of feldspars and biotite is later and/or peripheral,
partially overprinting potassic alteration. Propylitic alteration (chlorite
± epidote ± calcite) forms a weakly defined zone that overprints the earlier assemblages
and shows a gradational boundary with unaltered host rock. Whole-rock geochemical
analyses indicate that the Dos Pobres rocks have unextraordinary igneous compositions
but that many of them have experienced significant metasomatic addition of
K O, whereas hydrolytic alteration is quantitatively minor.
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Hypogene sulfides are abundant and systematically distributed beneath the base of
oxidation (depth ~350 m). Early bornite (± chalcocite ± digenite) dominates the deep
core of the deposit coincident with the most intense K-silicate alteration and quartz veins.
There is a transition from these through bornite-chalcopyrite assemblages into
chalcopyrite-dominated veins. The chalcopyrite-bearing veins typically cut bornite-rich
veins and surround the bornite-dominated core but are also associated with K-silicate
alteration. Pyrite (± chalcopyrite)-bearing veins have hydrolytic envelopes and are most
abundant in a ring-shaped zone surrounding the core. Molybdenite is rare but is most
commonly associated with chalcopyrite in deep, flanking veins with associated hydrolytic
alteration. Gold occurs as tiny (mostly <10 μm) grains of electrum (~10-15 wt % Ag) and
sylvanite in early bornite. Silver is also present in early bornite, occurring as electrum,
hessite, sylvanite, and a silver sulfide mineral (argentite?). Silver also occurs as hessite
and sparse argentiferous galena (up to ~2.5 wt % Ag) that commonly rim and crosscut all
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Cu-Fe sulfides. Supergene bornite, chalcocite, and rare covellite locally replace hypogene
sulfides beneath the oxide zone.
Dos Pobres is unusual among Arizona porphyry deposits in that it is relatively
gold- and bornite-rich, and magnetite-bearing and is intimately associated with relatively
mafic (quartz monzodiorite - low silica granodiorite) porphyry dikes. Conversely, it has
considerably less pyrite and acid alteration than most other Arizona porphyry deposits.
These features are like those in many other Au-rich porphyry systems, and they have also
stimulated comparisons with andesite-hosted iron-oxide(-Cu-Au) (IOCG) systems such
as Candelaria.
At Dos Pobres, as in most Cu-Au-Mo porphyry deposits, Cu-Fe sulfides are
deposited with voluminous early quartz veins, magnetite is widespread but minor in
abundance, bornite is the dominant early ore mineral, deposition of pyrite postdates most
deposition of Cu, and there is a close spatial and temporal association of mineralization
with relatively mafic porphyry intrusions – all features consistent with introduction and
cooling of magmatic fluids. These characteristics differ profoundly from andesite-hosted
IOCG deposits, which typically have abundant Fe-oxides (tens of percent), REE
enrichment, voluminous Na-Ca and/or K-Ca alteration, comparatively sparse quartz
veins, and generally late copper mineralization.
Introduction
The Dos Pobres porphyry Cu(-Au-Mo) deposit is located in the North American
Laramide porphyry belt, which extends from western Mexico into Arizona and New
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Mexico. This region is richly endowed with porphyry Cu-(Mo) deposits, yet Dos Pobres,
one of at least four porphyry deposits in the Safford district (Langton and Williams,
1982), represents one of the few porphyry Cu(-Au-Mo) deposits, a distinctive family of
porphyry deposits that are commonly associated with relatively mafic intrusive rocks
(Seedorff et al., 2005), that has been documented to date. The spatial and temporal
association of Dos Pobres with comparatively Au-poor porphyry Cu-Mo deposits in the
region and perhaps in the Safford district poses questions about comparisons between
these systems and the processes that form them. Many hypotheses have been presented,
including differences in composition and/or thickness of underlying crust (Hollister,
1975), differences in emplacement depth or wall-rock properties (Kesler, 1973), vertical
zoning of Cu and Au coupled with different erosion levels (Titley, 1982), composition of
mineralizing intrusions (Kesler, 1973; Sillitoe, 1979), and other complex magmatic
geochemical factors (Sillitoe, 2000). As more of these deposits have been discovered and
described, it is apparent that these systems show differences among themselves and that
none of these hypotheses is sufficient – thus there is the need for continued systematic
study of the deposits themselves and for their comparison with others.
This study, which was sponsored by Phelps Dodge Exploration (now Freeport-
McMoRan Copper and Gold), was undertaken with the twin goals of characterizing the
mineralogy and distribution of precious metals in the hypogene mineralization and
developing insight into the possible reasons for the unusual metal endowments in this
atypically (for Arizona) Au-rich porphyry copper system.
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Dos Pobres shows strong similarities to many other porphyry Cu(-Au-Mo)
deposits worldwide, such as the predominance of early potassic (secondary biotite
± K-feldspar) alteration that carries most Cu and has associated precious metals (cf.
Gustafson and Hunt, 1975; Sillitoe, 1979; Seedorff et al., 2005), lack of well-developed
quartz + sericite alteration (Sillitoe, 1979), and relatively low molybdenum content. As
the other deposits in the Safford district become more completely explored and described,
the district may present an uncommon opportunity to compare and contrast different
deposits that formed in identical host rocks and closely in time. The data presented in this
study will add to the understanding of vein and alteration types and mineralization style
present in Cu(-Au-Mo) deposits.
This study presents data on the host and intrusive rocks and their geochemistry
and U-Pb geochronology, alteration types and distribution, vein types (and their
distribution, paragenetic sequence, and Re-Os geochronology), and sulfide mineralogy
and distribution. Selected intervals of drill core were logged to enable the construction of
an ENE-trending cross section through the deposit and a plan map on the 2,000 ft (600
m) level; selected samples from these intervals were made into polished thin sections.
These results compile to give a detailed picture that shows that Dos Pobres resembles
aspects of other porphyry Cu-Au deposits worldwide, notably in the southwest Pacific,
such as Grasberg, Papua, Indonesia, and Panguna, Bougainville, Papua New Guinea
(Rubin and Kyle, 1997; Fountain, 1972), Far Southeast, Philippines (Hedenquist et al.,
1998), and Goonumbla and Endeavour, New South Wales, Australia (Lickfold et al.,
2003; Wilson et al., 2003).
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Location
Dos Pobres is located in southeastern Arizona ~50 km southwest of the Morenci
district (Fig. 1A), near the boundary between the Basin and Range province of southern
and western Arizona and the Colorado Plateau to the northeast. Dos Pobres is located in
the Safford (Lone Star) district, which contains a NW-SE oriented group of porphyry
deposits located northeast of the town of Safford, in the northwest-trending Gila Range
(Fig. 1B). Dos Pobres is situated at the northwestern end of the district; the other deposits
include San Juan, Lone Star, and Sanchez, as well as several small prospects. The Lone
Star deposit is situated beneath the spine of the Gila Range, whereas Dos Pobres, San
Juan, and Sanchez crop out in the southwestern foothills.
Exploration and Development History
Copper mineralization in the Safford (Lone Star) district has been known for over
one hundred years. The San Juan, Lone Star, and Sanchez deposits were claimed in the
late 1800s, and some small-scale exploration and mining took place at these three
deposits in the late 19th and early 20th centuries. By about 1920, exploration and
production had largely ceased (Robinson and Cook, 1966).
Porphyry copper exploration of the district by many companies resumed in the
late 1940s and was most active until the late 1960s. The Dos Pobres deposit was
discovered by Phelps Dodge in 1958 (Langton and Williams, 1982). A project in the late
1970s to evaluate bulk underground mining of the deposit was abandoned due to
geotechnical problems and other factors, such as copper price, water availability, and
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project economics) (J. E. Gerwe, pers. comm., 2006). The Lone Star deposit, which was
discovered by Bear Creek Mining Co. (a Kennecott subsidiary) in the mid-1950s and
studied as a potential in situ leaching project (Robinson and Cook, 1966; Anonymous,
1973; D’Andrea et al., 1974), was acquired by Phelps Dodge in 1986. Drilling resumed at
Dos Pobres in early 1990, this time focused on the supergene mineralization; by 1991, a
leachable resource was delineated. Phelps Dodge purchased the nearby San Juan deposit
in 1992, in 1994 initiated permitting to put both Dos Pobres and San Juan into production
as a single leach operation, and then in 1995 purchased the Sanchez deposit. Freeport
McMoRan acquired Phelps Dodge in 2007.
Production from the leachable part of the Dos Pobres deposit commenced in late
2007, and the combined Dos Pobres-San Juan leach operation at the end of 2007 carried a
proven and probable ore reserve of 549 million metric tonnes at 0.36% Cu (weighted
average of crushed and run-of-mine leach ore, FMI 2007 10-K Report). The sulfide
portion of the Dos Pobres deposit contains 211 million metric tonnes of mineralized
material at an average grade of 0.73% Cu (FMI 2007 10-K Report). There are no
published figures for the sulfide portion of the San Juan deposit. FMI is actively
exploring the Lone Star deposit and currently considers that the deposit contains 1,451
metric tonnes of leachable mineralized material at an average grade of 0.38% Cu (FMI
2007 10-K Report). FMI does not currently quote a size or grade of the sulfide portion of
the Lone Star deposit, but Williams and Forrester (1995) published a sulfide resource of
~4,300 million metric tonnes at an average grade of 0.47% Cu. Sanchez contains 209
million metric tonnes of mineralized material at an average grade of 0.29% Cu (FMI
Description:Gold occurs as tiny (mostly
