Arizona Geological Society


Upcoming events

    • 04 Apr 2017
    • 6:00 PM - 9:00 PM
    • Sheraton, 5151 E Grant Rd. (& Rosemont), Tucson AZ 85712

    Sponsored by:  Layne Christensen Company

    The Nature and Origins of Internal Zonation within Granitic Pegmatites

    by David London

    Abstract: A comprehensive understanding of the complex internal zonation of pegmatites has eluded petrologists for over a century. Explaining the origins of the zonation represents a challenge to our understanding of igneous and hydrothermal processes, and our ability to discern them.

    Liquidus Undercooling. Liquidus undercooling of hydrous and flux-bearing (but not exotic) granitic liquids by ~ 200oC is the single most important step in the formation of zoned pegmatites. The crystallization of pegmatites commences at ~ 450o-500oC, ~ 200oC below the liquidus temperature of granitic melts. In response to undercooling, the first-formed zones exhibit anisotropy as unidirectional solidification inward, graphic intergrowths, and sequential mineralogical assemblages.

    Feldspar and Quartz. The most common and prominent manifestation of zonation entails feldspathic outer zones and quartz-rich cores. This zonation arises from differences in the Gibbs Free Energies of crystallization (Ḡi,liquid → Ḡi,crystal, where i is a component of the melt with crystalline stoichiometry) in the highly undercooled state of pegmatite-forming melts. At 500oC and 200 MPa, the energy released by the crystallization of (typical) plagioclase (Ab85An15) is - 26839 J/m, that of alkali feldspar (Or70Ab30) is -26760 J/m, and of quartz (8 oxygen basis) is - 18586 J/m. Consequently, the greater energy release from feldspars favors their crystallization over quartz at the start, and the commensurate quartz component missing in the outer zones is deposited sequentially in the interior portions of pegmatite bodies.

    Zonation of Alkali Feldspar and Plagioclase. When crystallization begins with a plagioclase or a K-feldspar assemblage along one margin of a melt body, the complementary assemblage nucleates on the opposite side of the melt body. The chemical potential gradients caused by the initial assemblage appear at the far end of the melt column because that is the finite boundary condition for the diffusive supply of ions to the crystallization front. Far-field chemical diffusion leads to spatial segregation of plagioclase and K-feldspar.

    Oscillating Zonation and Chemical Fractionation. At 450o-550oC, the concentrations of components that are excluded from the first crystalline assemblage build up in a boundary layer of melt adjacent to the crystallization front. Boundary layer pile-up has two consequences for zonation: (1) the crystallization front is alternately saturated in multiple mineral assemblages, or (2) constitutional zone refining leads to an accumulation of excluded components in the boundary layer liquid until the bulk melt has been exhausted, whereupon the mineralogy changes from ordinary to exotic. The formation of coarse-grained pegmatitic texture, and eventually gems, results more from process (2) than from (1).

    Bio: David London obtained his B.A. in geology (1975) at Wesleyan University, Connecticut, after which he worked as a field geologist for the U.S. Geological Survey from 1975-1976. He received his M.S. (1979) and Ph.D. (1981) in geology from Arizona State University. Following a postdoctoral research fellowship at the Geophysical Laboratory of the Carnegie Institution of Washington (1981-1982), London joined the faculty of the School of Geology and Geophysics at the University of Oklahoma, where he is the Stubbeman-Drace Presidential Professor, the Norman R. Gelphman Professor of Geology, and director of the University’s electron microprobe lab. London also is the chair and managing editor of the “Pegmatite Interest Group” of the Mineralogical Society of America. He is the author of the book Pegmatites, which was published in 2008 as Canadian Mineralogist Special Publication 10, and he is the namesake of the mineral londonite, isometric CsAl4Be4[B11Be]O28 (Can. Mineral. 39: 747-755).

    • 22 Apr 2017
    • 7:30 AM - 4:00 PM
    • Oro Valley - Tucson area attendees will meet at 7:30 AM at the parking lot of the Walmat Store located at 2150 East Tangerine Road.
    • 11

    The Laramide-age Chilito Porphyry Copper Deposit, its host rocks, and styles of mineralization, Dripping Springs Mtns., Gila Co., Arizona

    Field Trip Leader: Don Applebee, Asarco LLC

    Acknowledgements - The Arizona Geological Society thanks ASARCO LLC for granting us permission to visit their Chilito property and providing geologists to lead the tour.

    Field Trip Description - AGS Members and their guests are invited to visit Asarco's Chilito porphyry copper deposit in Gila County, Arizona. 

    The Chilito deposit is situated in the central Dripping Springs Mountains and is one of the last largely undeveloped deposits remaining in the prolific porphyry copper province of southeastern Arizona. The planned program will include an examination of skeleton core and stops at significant geologic outcrops in the vicinity of the deposit.

    Tour Date/Time -  Saturday. April 22, 2017, 7:30 AM. 

    Carpool Locations - Due to limited space, carpooling is encouraged.  The field trip will require 4-wheel drive vehicles,  no exceptions.   It is encouraged that participants arrange beforehand to share rides with those, who go on the trip with their 4-WD vehicles.

    Tucson area attendees will assemble at 7:30 AM in Oro Valley at the Walmart parking lot located at 2150 East Tangerine Road, Oro Valley, Az 85737.  (Note: please park in the outer areas, not immediately adjacent to Walmart).  We will leave parking lot promptly at 7:45 AM, so don't be late

    Carpool locations will be established from Phoenix area once participant interest is known. Participant contact information will be sent in advance so carpool arrangements can be made prior to the trip.

    Route - Tucson based participants will travel north from Oro Valley, along Highway 77, through Oracle Junction and then northeast  to Winkleman, Arizona. The approximate travel time is 1 hour 15 minutes.

    Attendees traveling from outside the Tucson area can join the group at Winkleman Shell Station (located at the junction of Route 77 and Route 177) at 9:00 AM.

    Group size - Limit 60: Preference will be given to AGS members. A backup list will be maintained in case of cancellations. Please let us know in advance if you need to cancel in order to allow others to participate from the backup list.

    Once the limit of 60 has been reached, you can sign up on the wait list.

    Meals - Sandwiches, cookies, bottle water and sodas will be provided for lunch.

    Level of Difficulty - The level of difficulty for the trip is normal.  Temperature are expected to be in pleasant warm range from 60 to 80 degrees F.  Elevation is 1,000 to 1,200 meters (3,000 to 3,500 feet).  Bring a light jacket if it is a windy or rainy day.  Sunscreen lotion and a good hat to protect the head from too much UV are strongly recommended.  Bring sturdy field boots.  Eye protection is required, to protect yourself not only from your own hammering away on rocks, but also to shield you from rock chips flying of the adjacent geologist's pick.  Hand protection is also encouraged (work gloves).

    Field Trip Stops - All of the stops except for the last one will be essentially road cuts or places where people only have to walk 50 yards on easy trails.

    The last stop will be optional and will require a 2/3 mile hike  (one way) with change in elevation of approximately 300 feet.  It will involve scrambling for more intrepid souls, but also is designed for those who only want to walk a few dozen yards or so.  The goal will be outcrops of porphyry  with stock work down at the bottom of the canyon, access by walking down (and also back up) on washed out drill roads.  This is a bit harder to get to that I don't think everyone will want to do - at the same time there are plenty of things to look at right on the road.

    This will will be a fairly long stop for those who want to do little scrambling, so at this last point of the trip, so some might just want to hit the road and head home.

    General Information -The Chilito deposit is a small to medium size porphyry copper deposit located in Gila County, Arizona about 4.5 miles north of Hayden on the southwestern flank of the Dripping Springs Mountains.  Primary access to the property is via 4 miles of dirt road that leaves Arizona State Highway 177 about two miles west of Hayden and follows Keystone and Schneider gulches for four more miles to the mine gate.  Another dirt road, just west across the wash from the Chilito road, provides access to the 79 Mine, the Overland Mine and the Brick prospects.

    Miami Copper, Inspiration and Bear Creek explored the Chilito deposit in the 1950s and 1960s. Bear Creek acquired what became roughly two-thirds of the deposit and Inspiration the other third.  Bear Creek’s portion went to Kennecott, then to ASARCO in 1986 as part of the Ray Mine acquisition.  Freeport owned Inspiration’s ground in the southeast sector of the deposit. In 2014 ASARCO purchased Freeport’s other third of the deposit. The deposit is interesting in that two mineralized shells are reported: a shallow shell near the surface and a deeper one, hosted by diabase 1500 feet deep. A reserve study, done by ASARCO in 1990 and restricted to the upper shell, indicated a sulfide resource of 85.7 million tons with a cutoff grade of 0.30% Cu and a 1.52:1 stripping ratio. Various other estimates have been made by Kennecott including both sulfide and leachable resources but the data for these estimates are considered to be questionable (see the reports by Borrok, Johns and Graybeal). This was based on only fifty-six holes that been drilled in the deposit up to that point, with questionable recoveries, and there appears to be good potential for increasing both the grade and size of the reserve within the property as it now exists plus excellent potential for the discovery of new deposits in adjacent areas, especially to the west-southwest toward the 79 Mine and Overland Mines. 

    Following the completion of a four year drilling program in 2011, Independent Mining Consultants prepared a new ore reserve model during 2013.  This model included the upper zone and just barely reaches the upper limit of the deeper zone.  The bottom of the block model has an elevation of 610 meters (2,001 feet).  This study places the Chilito ore reserves at approximately 96 million tonnes containing 0.462% Cu at a cut-off grade of 0.27% TCu.

    The Chilito deposit lies between the Ray Mine to the northwest and the Christmas Mine to the southeast along an extensive northwest trending structural zone that extends all the way to Superior.  Numerous small lead-zinc-copper-gold-silver mines and prospects occur along this trend.


    Banks, Norman and Krieger, Medora (1977): Geologic Map of the Hayden Quadrangle, Pinal and Gila Counties, Arizona; U.S.G.S Map GQ-1391; US Geological Survey

    Borrok, David (Sept., 1999): Review of the Chilito porphyry copper prospect; unpublished company report, ASARCO Incorporated, 13  pages.

    Eastlick, J.T. (1968): Geology of the Christmas mine and vicinity, Banner Mining District, Arizona in Ridge, J. D., ed. Ore Deposits of the United States, 1933- 1967; New York, Am. Inst. Mining, Metall., and Petrol. Eng., p1191-1210.

    Einaudi, Marco (1982): Description of skarns associated with porphyry copper plutons, southwestern North America in Advances in geology of the porphyry copper deposits, southwestern North America; Titley, S. R., Editor; University of Arizona Press, Tucson, Arizona, pages 139-183.

    Graybeal, Fred, (undated): Chilito Porphyry Copper Deposit, Gila County, AZ:  unpublished company memorandum, ASARCO Incorporated, 1  page.

    Keith, Stanley B. (1972): Mineralogy and paragenesis of the 79 Mine lead-zinc-copper deposit; Mineralogical Record, Nov.-Dec., 1972, pages 247-264.

    Kiersch, George A.  (1949): Structural control and mineralization at the Seventy-nine Mine, Gila County, Arizona; Economic Geology V. 44, pages 24-39.

    Koski, R. and Cook, D. (1982): Geology of the Christmas porphyry copper deposit, Gila County, Arizona in Advances in geology of the porphyry copper deposits, southwestern North America; Titley, S. R., Editor; University of Arizona Press, Tucson, Arizona, pages 353 – 373.

    McClave, M. (2007): Geology of the Chilito Mine, unpublished company report, ASARCO LLC, 15 pages.

    Taylor, S.A. and Babcock, R.C. (1963): Chilito Examination Progress Map; unpublished  company map, Bear Creek Mining Company.

    Contact Info - If you have additional questions about the Spring Field Trip contact Don Applebee at (520) 247-8577 or

    • 02 May 2017
    • 6:00 PM - 9:00 PM
    • Sheraton, 5151 E Grant Rd. (& Rosemont), Tucson AZ 85712

    Sponsored by:  Available

    America's Water Crisis and What to Do About It

    By Robert Glennon




Past events

07 Mar 2017 Derek J. Thorkelson presents The Precambrian Tectonic Connection between Yukon and Arizona
07 Feb 2017 Daniel Hummer presents Mineral ecology and evolution: Using large datasets to tell the story of the co-evolution of Earth and life
03 Jan 2017 David A. Sawyer presents Dating Geologic Time in the Cretaceous: Integrating Biostratigraphy, Isotope Geochronology, and Astrochronology in Sedimentary Deposits of the Cretaceous Western Interior Seaway
06 Dec 2016 John W. Ewert presents USGS Responses to Some Volcanic Crises and Eruptions Around the World
01 Nov 2016 Alexander Schauss presents Minerals, Trace Elements and Human Health
22 Oct 2016 Fall Field Trip - The Peach Spring Tuff and the Silver Creek Caldera, Northwestern Arizona
04 Oct 2016 Hamish Martin Presents Geology of the Resolution Cu-Mo Deposit, Superior Arizona
06 Sep 2016 Keith R. Long Presents No Bonanza from Cheap Oil
02 Aug 2016 Lee Allison Presents The Future of State Geological Surveys: the Arizona Case Study
05 Jul 2016 Don Applebee Presents Genesis of the Chilito Porphyry Copper Deposit
07 Jun 2016 Robert Hildebrand Presents Collisions, Slab Failure Magmatism and the Development of Cordilleran Batholiths
03 May 2016 Peter Modreski will present "Pegmatites: Mineralogy, Gemstones, Economic Geology, and maybe not quite the same Giant-Crystal Rocks you always thought they were"
30 Apr 2016 Spring Field Trip - An Introduction to the Pinal Schist in Southeastern Arizona
05 Apr 2016 Jordon Bright Presents Looking for an Ocean in the Desert, the Enigmatic Bouse Formation
01 Mar 2016 Peter R. Johnson Presents Tectonics and Mineral Deposits of the Arabian-Nubian Shield
02 Feb 2016 Dr. Karen Wenrich Presents The Ga-Ge Rich Apex Mine, Utah - A Tsumeb, Namibia Analogue
05 Jan 2016 Sarah Baxter Presents Calc-silicate Alteration and Ore Characterization, ASARCO Mission Complex, Pima County, Arizona
01 Dec 2015 Peter Smith Presents The Latest News from Mars
14 Nov 2015 Fall Field Trip - Northern Plomosa Mountains and Bouse Formation in Blythe Basin
03 Nov 2015 Karen Kelley presents The Giant Concealed Pebble Cu-Au-Mo Porphyry Deposit, Southwest Alaska
06 Oct 2015 Caleb King presents Eocene Hydrothermal Systems and Contrasting Hydrothermal Alteration in the Battle Mountain District, Nevada
01 Sep 2015 Carl Bowser presents The Genesis of the Kramer Borax Deposit, Rogers Lake, Mojave Desert, CA:
04 Aug 2015 Dan Lynch presents Volcanoes in the Back Yard
07 Jul 2015 Erik Melchiorre presents The Complex Geological History Recorded by Arizona Placer Deposits:
02 Jun 2015 Jan C. Rasmussen Presents - Arizona Mineralization through Geologic Time
05 May 2015 Gordon Haxel Presents - Alpine peridotite in the desert - Arizona's Laramide subduction complex
02 May 2015 Spring Field Trip - Oak Creek - Mormon Lake Graben
18 Apr 2015 Third Annual Arizona Geological Society Doug Shakel Student Poster Event
03 Mar 2015 Apollo 17 Astronaut and Former Senator Harrison H. Schmitt presents A Geological Visit to a Valley on the Moon
03 Feb 2015 Don Yurewicz Presents Assessing Unconventional (Continuous) Hydrocarbon Resource Plays
06 Jan 2015 Arend Meijer presents: Sulfide-rich Proterozoic Mafic Rocks and Arizona Porphyry Copper Deposits - A Connection?
02 Dec 2014 Victor R. Baker: Megafloods on Earth, Mars, and Beyond
15 Nov 2014 Fall Field Trip - Debris Flows Shape the Sabino Canyon Landscape - look out below!
04 Nov 2014 Isabel F. Barton: Historical Development & Current State of Geological Research in the Central African Copperbelt
07 Oct 2014 Apollo 17 Astronaut and Former Senator Harrison H. Schmitt presents A Geological Visit to a Valley on the Moon
02 Sep 2014 Lewis Land presents Evaluation of Groundwater Residence Time in a Karstic Aquifer System
05 Aug 2014 Jamie Molaro presents Thermal Stress Weathering in the Inner Solar System
01 Jul 2014 Jim Leenhouts presents Surface-water/groundwater Interactions in Arizona
03 Jun 2014 Arend Meijer presents Pinal Schist of So. Arizona--A Paleoproterozoic Fore-Arc Complex
06 May 2014 John C. Lacy presents The Genesis of Mining Law
26 Apr 2014 Spring Field Trip - Geology of the Christmas Porphyry Copper Deposit
24 Apr 2014 Second Annual Arizona Geological Society Doug Shakel Student Poster Event
01 Apr 2014 Eric Seedorff presents, Structural Dismemberment of a Porphyry Molybdenum System, Spruce Mountain District, Northeastern Nevada
04 Mar 2014 John Dreier presents, Copper Deposits of the Coast Ranges of Chile; A trip through time, space, and ore deposit nomenclature
04 Feb 2014 Ralph Stegen presents The Morenci Porphyry Cu-Mo Deposit, Greenlee County, Arizona: A Geologic Summary with Emphasis on Hypogene and Supergene Mineralization
07 Jan 2014 Steve Castor presents, Mountain Pass and other North American Rare Earth Element Deposits
03 Dec 2013 Stephen Jackson, USGS, Looking forward from the past: Ecological impacts of climate change through the lens of history
05 Nov 2013 Malcolm Siegel, PhD, MPH, LJS Consulting, Inc and School of Medicine, University of New Mexico, Albuquerque, NM. 87047: Uranium Mining in the American Southwest: Can Medical Geologists Ask the Right Questions?
04 Oct 2013 Field Trip - H. Wilson Sundt Generating Station
01 Oct 2013 Mark Logsdon, Principal Geochemist, Geochimica, Inc.: What Does “Perpetual” Management and Treatment Mean? Toward a Framework for Determining an Appropriate Period-of-Performance for Management of Reactive, Sulfide-Bearing Mine Wastes
01 Oct 2013 Mark J. Logsdon, Geochimica, Inc., Does acid-rock drainage lead to waste-rock instability? Geological, hydrological, and geochemical framework for the Questa Mine
03 Sep 2013 Pete Reiners, UA, Geosciences Dept., Geochronology of secondary Fe & Mn oxides in bedrock
30 Aug 2013 Field Trip - University of Arizona Tree-Ring Research Laboratory
06 Aug 2013 Bill Stavast, Freeport-McMoRan Copper & Gold, The Safford Mine: What we have learned since production began
02 Jul 2013 Lukas Zurcher, USGS, presents: "Tectono-magmatic evolution of the Central Tethys Region"
04 Jun 2013 Steve Van Nort presents, "Gold Fever! The BRE-X/Busang Story
07 May 2013 Federal lands and mineral resources: Colorado Plateau uranium deposits and the Sonoran Desert Heritage
18 Apr 2013 First Annual Arizona Geological Society Doug Shakel Student Poster Event

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