Geochronology

Dating of alteration at the Radomiro Tomic porphyry copper deposit, northern Chile by the high precision (super 40) Ar/ (super 39) Ar method

by Greg Pemberton

Marcos Zentilli: http://ca.linkedin.com/in/marcoszentilli

Dating of alteration at the Radomiro Tomic Porphyry Copper Deposit, northern Chile by the high precision 40Ar/39Ar... more Dating of alteration at the Radomiro Tomic Porphyry Copper Deposit, northern Chile by the high precision 40Ar/39Ar method

Gregory Pemberton

The Radomiro Tomic (RT) porphyry copper deposit is located approximately 8-10km north of the Chuquicamata mine and 245km northeast of the port city of Antofagasta. Estimates place the deposit at over 800 million tonnes of ore with an average grade of 0.59% Cu (mostly supergene ore) and a mine life of 22 years. RT does not outcrop as the entire deposit is buried beneath Miocene piedmont gravels up to 200m in thickness and has been known only from drillcore and limited underground workings. Stripping is now underway and the mine is due to start operating in 1997 as a new separate division of CODELCO, the Chilean national copper mining company.

The deposit is hosted within an intrusive body of quartz monzodiorite composition that is approximately lower to middle Oligocene in age and was emplaced within the regional N-S trending Domeyko Fault system. This system is spatially associated with all the major porphyry deposits in northern Chile.

High precision, stepwise degassing 40Ar/39Ar dating was conducted at Dalhousie University to determine the ages of the potassic, argillic and quartz-sericite alteration assemblages at Radomiro Tomic. K-feldspar, biotite and sericite grains were hand picked from six drillcore samples representing the three hydrothermal alteration zones. Results are suggestive of potassic and argillic alteration assemblages having an average age of 32.6±0.3Ma as part of an initial hydrothermal event. As well, a younger hydrothermal event was dated to 31.8±0.3Ma from quartz-sericitic alteration minerals. Age differences among the potassic and argillic alteration minerals (K-feldspar and biotite) having high or low closure temperatures are not detectable, are indicative of rapid cooling following emplacement of the host porphyry. 39Ar degassing plateau patterns combined with X-ray diffraction analyses on the K-feldspar samples also support the scenario of a rapid decrease in temperature to less than 150oC in the order of hundreds of thousands of years, rather than millions, following each hydrothermal event.

A full suite of ancillary data was generated on the dated samples establishing the close similarity of the petrology, mineralogy and chemistry to samples of similar alteration types from Chuquicamata.

Key Words and Phrases: Radomiro Tomic, porphyry copper deposit, Chile, argon dating, geochronology, hydrothermal, alteration, potassic, argillic, quartz-sericitic, Chuquicamata

Pemberton, G.B., 1997,'The Geochronology of the Radomiro Tomic Porphyry Copper Deposit, Northern Chile', Atlantic Geology, V33, 1, pp73-74

Supervisor: M Zentilli

Geochronology of Hemphillian-Blancan aged strata, Guanajuato, Mexico, and implications for timing of the Great American Biotic Interchange

by Bart Kowallis

Flynn, J.J., Kowallis, B.J., Nuñez, C., Carranza-Castañeda, O., Miller, W.E., Swisher, C.C. III, and Lindsay, E., 2005, Geochronology of Hemphillian-Blancan strata, Guanajuato, Mexico, and implications for timing of the Great American Biotic Interchange: Journal of Geology, v. 113, p. 287-307.

Uplift along the Salt Lake segment of the Wasatch fault from apatite and zircon fission track dating in the Little Cottonwood Stock

by Bart Kowallis

Kowallis, B.J., Ferguson, J., and Jorgensen, G.J., 1990, Uplift along the Salt Lake segment of the Wasatch Fault from apatite and zircon fission track dating in the Little Cottonwood stock: Nuclear Tracks, v. 17, p. 325-329.

Results of interlaboratory comparison of fission track ages for 1992 Fission Track Workshop

by Bart Kowallis

Miller, D.S., Crowley, K.D., Dokka, R.K., Galbraith, R.F., Kowallis, B.J., and Naeser, C.W., 1994, Results of interlaboratory comparison of fission track ages for 1992 Fission Track Workshop: Nuclear Tracks, v. 21, p. 565-574.

Possible secondary apatite fission track age standard from altered volcanic ash beds in the Middle Jurassic Carmel Formation, southwestern Utah

by Bart Kowallis

Kowallis, B.J., Christiansen, E.H., Everett, B.H., Crowley, K.D., Naeser, C.W., Miller, D.S., and Deino, A.L., 1994, Possible secondary apatite fission track age standard from altered volcanic ash beds in the Middle Jurassic Carmel Formation, southwestern Utah: Nuclear Tracks, v. 21, p. 519-524.

Newly recognized Cedar Mountain Formation in Salina Canyon, Sevier County, Utah

by Bart Kowallis

Willis, G.C., and Kowallis, B.J., 1988, Newly recognized Cedar Mountain Formation in Salina Canyon Sevier County, Utah: Brigham Young University Geology Studies, v. 35, p.57-61.

Fission track ages from volcanic rocks in southwestern Utah and southeastern Nevada

by Bart Kowallis

Kowallis, B.J., and Best, M.G., 1990, Fission track ages from volcanic rocks in southwestern Utah and southeastern Nevada: Isochron/West, n. 55, p. 24-27.

U-Pb dating of zircons from the Salt Wash Member of the Morrison Formation, central Utah

by Bart Kowallis

Bradshaw, R.W. and Kowallis, B.J., 2009, U-Pb dating of zircons from the Salt Wash Member of the Morrison Formation, central Utah: GSA Abstracts with Programs, v. 41, n. 7, p. 125.

Fission-track and single-crystal 40Ar/39Ar laser-fusion ages from volcanic ash layers in fossil-bearing Pliocene sediments in central Mexico

by Bart Kowallis

Kowallis, B.J., Swisher, C.C., III, Carranza-Castañeda, Miller, W.E., and Tingey, D.G., 1998, Fission-track and single-crystal 40Ar/39Ar laser-fusion ages from volcanic ash layers in fossil-bearing Pliocene sediments in central Mexico: Revista Mexicana de Ciencias Geológicas, v. 15, p. 157-160.

Fission-track dating of volcanically derived sedimentary rocks

by Bart Kowallis

Kowallis, B.J., Heaton, J.S., and Bringhurst, K., 1986, Fission-track dating of volcanically derived sedimentary rocks: Geology, v. 14, p. 19-22.

The record of Middle Jurassic volcanism in the Carmel and Temple Cap Formations of southwestern Utah

by Bart Kowallis

Kowallis, B.J., Christiansen, E.H., Deino, A.L., Zhang, C., and Everett, B.H., 2001, The record of Middle Jurassic volcanism in the Carmel and Temple Cap Formations of southwestern Utah: Geological Society of America Bulletin, v. 113, p. 373-387.

The Bishop Conglomerate ash beds, south flank of the Uinta Mountains, Utah: Are they pyroclastic fall beds from the Oligocene ignimbrites of western Utah and eastern Nevada?

by Bart Kowallis

Kowallis, B.J., Christiansen, E.H., Balls, E., Heizler, M.T., and Sprinkel, D.A., 2005, The Bishop Conglomerate ash beds, south flank of the Uinta Mountains, Utah: Are they pyroclastic fall beds from the Oligocene ignimbrites of western Utah and eastern Nevada?, in Dehler, C., Pederson, J., Sprinkel, D. and Kowallis, B. (eds.), Geology of the Uinta Mountains: Utah Geological Association Publication 33, p. 131-145.

The long-term burial and exhumation history of basement blocks in the footwall of the Wasatch fault, Utah

by Bart Kowallis

Nelson, S.T., Harris, R.A., Kowallis, B.J., Dorais, M., Constenius, K.N., Heizler, M., and Barnett, D., 2009, The long-term burial and exhumation history of basement blocks in the footwall of the Wasatch fault, Utah: Rocky Mountain Geology, v. 44, p. 103-119.

Cenozoic Soldiers Pass volcanic field, Central Utah—implications for the transition to extension-related magmatism in the Basin and Range Province

by Bart Kowallis

Christiansen, E.H., Baxter, N., Ward, T.P., Zobell, E., Chandler, M.R., Dorais, M.J., Kowallis, B.J., and Clark, D.L., 2007, Cenozoic Soldiers Pass volcanic field, Central Utah—implications for the transition to extension-related magmatism in the Basin and Range Province: Utah Geological Association Publication 36, p. 123-142.

New U-Pb zircon ages from an ash bed in the Brushy Basin Member of the Morrison Formation near Hanksville, Utah

by Bart Kowallis

Kowallis, B.J., Britt, B.B., Greenhalgh, B.W., and Sprinkel, D.A., 2007, New U-Pb ages from an ash bed in the Brushy Basin Member of the Morrison Formation near Hanksville, Utah: Utah Geological Association Publication 36, p. 75-80.

U‐Pb and 40Ar/39Ar Ages for a Tephra Lens in the Middle Jurassic Page Sandstone: First Direct Isotopic Dating of a Mesozoic Eolianite on the Colorado Plateau

by Bart Kowallis

Dickinson, W.R., Stair, K.N., Gehrels, G.E., Peters, L., Kowallis, B.J., Blakey, R.C., Amar, J.R., and Greenhalgh, B.W., 2010, U-Pb and 40Ar/39Ar ages for a tephra lens in the Middle Jurassic Page Sandstone: First direct isotopic dating of a Mesozoic eolianite on the Colorado Plateau: Journal of Geology, v. 118, p. 215-221.

Age of the Brushy Basin Member of the Morrison Formation, Colorado Plateau, western USA

by Bart Kowallis

Kowallis, B.J., Christiansen, E.H., and Deino, A., 1991, Age of the Brushy Basin Member of the Morrison Formation, Colorado Plateau, western USA: Cretaceous Research, v. 12, p. 483-493.

Age of the Cenomanian-Turonian boundary in the Western Interior of the United States

by Bart Kowallis

Kowallis, B.J., Christiansen, E.H., Deino, A.L., Kunk, M.J., Heaman, L.M., 1995, Age of the Cenomanian-Turonian boundary in the Western Interior of the United States: Cretaceous Research, v. 16, p. 109-129.

High-precision 40Ar/39Ar geochronology and the advent of North America's Late Cretaceous terrestrial fauna

by Bart Kowallis

Cifelli, R.L., Kirkland, J.I., Weil, A., Deino, A.L., and Kowallis, B.J., 1997, High-precision 40Ar/39Ar geochronology and the advent of North America’s Late Cretaceous terrestrial fauna: Proceedings of the National Academy of Science, v. 94, p. 11163-11167.

The Cambrian Metamorphic History of Tasmania

by Reia M. Chmielowski

This is my PhD research, completed in 2009 at the University of Tasmania.

The Tyennan Orogeny produced low to medium-grade metamorphic rocks distributed across the western third of Tasmania.... more The Tyennan Orogeny produced low to medium-grade metamorphic rocks distributed across the western third of Tasmania. Chemical U-Th-Pb monazite dating reveals that the peak episode of metamorphism took place in the Cambrian, with a weighted mean age for all units analysed of 505 ± 1 Ma. However, variations in the results by region range from ~ 511 to ~ 497 Ma. The pelitic schists of the Franklin Metamorphic Complex contain garnet porphyroblasts which record a rapid, nearly isothermal, pressure increase; the garnet cores formed at ~ 600o C, 6,000 bars and the rims at ~ 700o C, 14,000 bars at 511 ± 3 Ma. Likewise, the eclogite from the same region records a change from ~ 550o C, 6,250 bars to ~ 650o C, 19,000 bars. The whiteschist, which was obtained from the opposite side of a major local fault, formed garnet cores at ~ 545o C, 19,600 bars; its garnet rims and matrix minerals formed at 506 ± 5 Ma after an increase in temperature of at least 30-90o C. All of these units show evidence of very rapid isothermal exhumation. Other Franklin Metamorphic Complex fault blocks record P/T for peak conditions at ~ 570o C, 8,600 bars (Mt. Mary), and ~ 700o C, 11,400 bars (Raglan Range). The Forth Metamorphic Complex achieved peak metamorphism at 509 ± 7 Ma, at conditions of 670o C, 16,900 bars, and the nearby Settlers Schist gives results of 513 ± 8 Ma. The garnet porphyroblasts of the Port Davey Metamorphic Complex record a single episode of metamorphism which took place at 505 ± 2 Ma at ~ 550 to 570o C and ~ 6,000 bars during which a dehydration event resulted in both a change of garnet composition and texture. The regional geology indicates metamorphism predated post-collisional extension and associated eruption of the Mount Read volcanics at 506 to 500 Ma. Most of the monazite dating is consistent with this observation. However, the Mersey River Metamorphic Complex gives very consistent results of 497 ± 3 Ma. This is problematical, as it would have been at depth undergoing metamorphism after that extension took place. This could be the result of an unknown analytical problem, but the Mersey River monazite grains are indistinguishable chemically from monazite in the other units, and this sample has undergone repeated analysis. Alternatively, this sample reflects a different metamorphic event than that recorded in all other samples studied.