Arizona Geological Society
2025 Speaker Series
Tuesday, 30 September 2025 | 5:30 - 8:00 PM
(Please note change in meeting date)
Location: Hexagon Mining Division Office
40 East Congress Street, Suite 150, Tucson, Arizona 85701
Parking: On the street or parking garage (Old Pueblo Parking)
Social Hour with Sandwiches from Beyond Bread (5:30-6:30 PM), Presentation (6:35 PM)
For those planning to attend the event, please register by 6:00 PM on Sunday, September 28, 2025
Livestream URL Pending
.png "Hexagon")
The Arizona Geological Society thanks Hexagon
for generously providing the venue and drinks
Peraluminous Copper-Oxide Greisen Deposits
Stanley B. Keith1 and Jan C. Rasmussen2,
1 President, MagmaChem Research Institute; 2Jan Rasmussen Consulting
Abstract: Characterization of copper deposits in Pinal County led to the recognition of a new type of copper deposit that is economically significant: Peraluminous Copper-Oxide Greisen Deposits. This new type of copper deposit typically forms in middle crustal settings (typically 10 to 15 km deep) associated with peraluminous biotite granodiorite to biotite-muscovite granites. A more detailed explanation of the deposit type is presented in the forthcoming Arizona Mineral Districts, Volume 5, Pinal County (Rasmussen and Keith, 2025) and a summary is provided below.
The main copper ore type of the copper-oxide greisen deposits is primary chrysocolla, with iron oxide mainly as specular hematite (and/or magnetite). The paucity of sulfur (which typically makes pyrite in upper crustal, more familiar, metaluminous porphyry copper deposits) results in iron oxide as the main primary iron mineral that coexists with primary chrysocolla and other minor copper oxide minerals, such as malachite and azurite. The primary specular hematite/magnetite readily oxidizes to red, earth hematite. As a result, peraluminous copper-oxide greisen deposits have commonly been confused with secondary copper and iron oxide caps over primary sulfur-rich chalcopyrite-pyrite in more classic porphyry copper deposits. The peraluminous copper-oxide greisen deposits lack leached caps and their underlying chalcocite blankets, due to their low pyrite content and low acid-generation potential during supergene oxidation.
Unlike the upper crustal metaluminous porphyry copper deposits, the peraluminous greisen deposits were not oxidized until after their uplift in the Miocene at the end of the Galiuro orogeny (as defined by Keith and Wilt, 1985). Emplacement ages of peraluminous copper-oxide greisen deposits (at 58 - 43 Ma) are slightly younger than the typical metaluminous, southern Arizona porphyry copper deposits (at 68 - 55 Ma).
These peraluminous copper-oxide greisen deposits can be economically significant, as shown by the giant Gunnison-Burro-South Star copper oxide system that forms a low-temperature skarn halo developed in lower Paleozoic rocks adjacent to the north and east contacts of the 55 Ma Texas Canyon peraluminous pluton. Peraluminous magma chemistry is defined as where molecular Al2O3/CaO+Na2O+K2O ratios are greater than 1, typically over 1.1. Their peraluminous (aluminum-rich and low sulfur) composition results in extensive, deep-seated greisens (quartz-muscovite ± hematite or magnetite alteration) that is superimposed on early quartz-orthoclase-magnetite alteration.
A mass balance study by Fitzpatrick (2021) of the 7 km by 11 km by 2 km deep, Texas Canyon pluton at 21 ppm copper, showed that the amount of expelled copper could have amounted to 8 million tons copper, of which about 3 million tons has been drill-indicated or mined from the Johnson-Gunnison copper deposits. The large size of the Gunnison and related deposits may partly be explained by the large size of the spatially and temporally associated peraluminous Texas Canyon pluton.
Other significant production from copper-oxide greisen deposits has come from the Carlota, Zonia?, and Little Hills mines. The Coronation deposit in La Paz County and Red Hills, Durham-Suizo, and North Star deposits in Pinal County are the same type of deposit and are excellent prospects.
One distinguishing feature of this deposit type is the possible presence of primary chrysocolla, which may have formed as a non-crystalline, high-temperature, globular colloid. Another possible distinguishing supergene feature is the presence of a unique assemblage of glycolate minerals (lazaraskeite, rasmussenite, and others) formed from the oxidation of chrysocolla at Pusch Ridge in the Santa Catalina Mountains.
The tectonic setting of these peraluminous copper-oxide greisen deposits appears to be restricted to lower plate windows that are northeast of the leading edge of the Maricopa thrust system, as originally defined by Keith (1983) and Keith and Wilt (1985, 1986).
Figure 1. Geologic map of Pinal County and surrounding area showing inferred lower plate domains in red. (Source: modified from Richard et al. (2000). Red stars indicate wells that intersected the lower plate. WT= White Tanks, E-SM = Estrella-South Mountain, S = Sacaton, CG = Casa Grande Mtns., P = Picacho Mtns., T = Tortilla Mtns., PM = Pinal Mtns., TO = Tortolita Mtns., SC = Santa Catalina Mtns., ST-P = Santa Teresa – Pinaleno Mtns.
Our current understanding is that lower plate windows consist of more frequent and larger areas of crystalline rocks than the areas referred to as metamorphic core complexes in the literature. These areas are characterized by reduced radiometric dates that reflect extensive uplift from mid-crustal levels during the middle Tertiary between about 28 and 18 Ma.
The crystalline complexes do not contain classical porphyry copper deposits, despite their extensive exploration (for example Red Hills in Pinal County that did not find a chalcocite blanket). The lower plate crystalline complexes do not contain volcanic or sedimentary rocks in depositional contact with the crystalline rocks. Compared to the metamorphic grade of upper plate crystalline metasedimentary rocks that achieve lower to middle greenschist facies, lower plate metasedimentary rocks have experienced upper greenschist to lower and mid-amphibolite grades.
The source of copper for peraluminous copper-oxide greisen deposits relates to the chemistry of the associated peraluminous plutons. These plutons were formed by hydrous water flooding of the lower to mid-crust during flat subduction in the latest Laramide. These deposits were formed after deposition of the more mafic, metaluminous, hornblende-bearing plutonic sequences during intermediate angle subduction in the middle Laramide. As such, the peraluminous plutons acquired their copper from minimum melt, sialic plutonic to metasedimentary sources in the lower to middle crust, under middle to upper amphibolite conditions. In contrast, metaluminous plutons acquired their copper from mafic, gabbroic melts developed by hydrous melting of hanging wall, layered asthenospheric mantle then in the hanging wall of the flattening subducting Farallon plate.
At present, 29 known or possible peraluminous copper-oxide greisen deposits have been recognized throughout Arizona. These peraluminous copper-oxide greisen deposits occur in a northwest-trending belt of lower plate windows northeast of the Late Laramide Maricopa thrust from the Parker region to the Safford region. We consider this new peraluminous copper-oxide greisen deposit type to be worthy of region regional grass-roots exploration with potential for significant copper economics.
Bios: Stanley B. Keith has over 45 years of exploration experience focusing on ideas, exploration, and discovery of mineral and energy resources. His hands-on geological and mineralogical knowledge, incredible memory, and extensive international experience include most mineral deposit types.
Stan began his career in the early 1970s with a mapping project in the Dripping Spring Mountains supervised by Dr. John Guilbert that led to a paper detailing the mineralogy of the 79 Mine. Stan continued mapping in the Tortilla Mountains and Galiuro Mountains for AMAX and Kennecott in the Ray region where he recognized an empirical relationship between mineral deposits and the alkalinity of associated igneous rocks. Stan was hired as a field and research geologist by the Arizona Geological Survey in 1978, then was hired in the early 1980s by Exxon to research the geology and mining districts in the western U.S.A.
In 1983, Stan founded MagmaChem Exploration Inc. and directed the development of the magma-metal series classification, while working on numerous exploration and research projects for both mineral and energy exploration companies. Stan is a prolific writer and speaker and has co-authored hundreds of technical reports and publications, including most recently co-authorship of Mineralogy of Arizona, 4th edition (Grant et al., 2022). His marathon three-day, magma-metal series workshops and field trips have been presented to major companies in the mining and oil industries.
Beginning in 2000, Stan and colleagues began applying the MagmaChem model to oil and gas, which led to concepts of the serpentosphere (at the Moho), the hydrothermal origin of oil, and a reconsideration of the origin of the Kupferschiefer deposits as a result of mud volcanism. These concepts are explained in podcasts on the MagmaChem Research Institute’s website. Stan was recently honored by having a new mineral, stankeithite, a manganese tellurite from the Moctezuma mine, Mexico, named after him. Throughout his career, Stan continually returns to the reality of the field, testing ideas through geologic mapping. Stan is a University of Arizona alumnus and received a B.S. degree in Philosophy in 1971 and an M.S. degree in Geology in 1975 with a paper on variable dip subduction in Geology (Keith, 1972).
Jan C. Rasmussen, formerly Jan Carol Wilt, earned an M.S. in stratigraphy in 1969 and a Ph.D. in economic geology from the University of Arizona in 1993 under the direction of Dr. John Guilbert and Dr. Mary Poulton. Jan has co-authored 18 books or open-file reports and numerous articles on Arizona geology and in 2022 co-authored Mineralogy of Arizona, 4th edition published by the University of Arizona Press.
Jan received the Individual GEM award in 2010 from the Society of Mining, Metallurgy, and Exploration for her work as Curator of the Arizona Mining and Mineral Museum in Phoenix to educate people about the importance of minerals and mining in their lives. Early in her career, Jan was Associate Curator of the University of Arizona Mineral Museum. Throughout her career, Jan has taught Physical, Historical, and Environmental Geology part time at Pima Community College in Tucson, Austin Community College in Texas, and Cochise College at Ft. Huachuca, Arizona.
Jan has over 45 years of experience as an economic geologist. She worked for the Arizona Geological Survey on coal, oil and gas, molybdenum, and uranium. She later worked for Woodward-Clyde on the Yucca Mountain project in Nevada and then worked for SRK Consulting on permitting documents for the closure of BHP’s San Manuel Mine and more recently on permits for Capstone’s Pinto Valley Mine and BHP’s Miami-Globe area closed mines. Jan has also consulted for MagmaChem Exploration projects on hydrothermal oil in the North Sea for major Norwegian oil companies and has co-authored articles on the Kupferschiefer copper deposits in Europe. Jan was honored in 2024 by having a new mineral, Rasmussenite, a glycolate from the Santa Catalina Mountains, named after her.
Jan and Stan are currently working on a series of county books published by Amazon on “Arizona Mineral Districts” with vol. 1 Cochise and Santa Cruz Counties, vol. 2 Pima County, vol. 3 La Paz and Yuma Counties, and vol. 4 Graham and Greenlee Counties already available from Amazon and vol. 5 Pinal County to be published in the next few months.
Hexagon Mining Division Office - 40 East Congress Street,
Suite 150, Tucson, Arizona 85701
