Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (18) Citing Articles via Google Scholar Google Scholar Articles by Richards, J. P. Search for Related Content GeoRef GeoRef Citation

Tectono-Magmatic Precursors for Porphyry Cu-(Mo-Au) Deposit Formation

J. P. Richards

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3 Canada

Email, Jeremy.Richards{at}UAlberta.ca

Porphyry Cu-(Mo-Au) deposits are relatively rare but reproducible products of subduction-related magmatism. No unique processes appear to be required for their formation, although additive combinations of common tectono-magmatic processes, or optimization of these processes, can affect the grade and size as well as the location of the resulting deposits. These various contributing processes are reviewed, from partial melting in the mantle wedge overlying the subducting plate, through processes of magma interaction with the lithosphere, to mechanisms for magma emplacement and volatile exsolution in the upper crust. Specific ore-forming processes, such as magmatic-hydrothermal fluid evolution, are not discussed.

Hot, hydrous, relatively oxidized, sulfur-rich mafic magmas (predominantly basalts) generated in the metasomatized mantle wedge above a subducting oceanic slab rise buoyantly to the base of the overlying crust where they stall due to density contrasts. Because these magmas are oxidized, sulfur is dominantly present as sulfate, and chalcophile elements such as Cu and Au are incompatible (i.e., they are retained in the melt). As these magmas begin to crystallize they release heat which causes partial melting of crustal rocks. Mixing between crustal- and mantle-derived magmas yields evolved (andesitic to dacitic), volatile-rich, metalliferous, hybrid magmas, which are of sufficiently low density to rise through the crust. Magma ascent is driven primarily by buoyancy forces and is dominantly a fracture-controlled phenomenon. As such, crustal stress and strain patterns play an important role in guiding the ascent of magma from the lower crust. In particular, translithospheric, orogen-parallel, strike-slip structures serve as a primary control on magma emplacement in many volcanic arcs worldwide. A feedback mechanism operates, whereby preexisting faults facilitate magma ascent, the heat from which further weakens the crust and focuses strain. Certain structural geometries, such as fault jogs, step-overs, and fault intersections, offer low-stress extensional volumes during transpressional strain. Such sites represent vertical conduits of relatively high permeability, up which magmas will preferentially ascend. Large upper crustal plutonic complexes may therefore be localized within these structural settings. Having delivered a sufficient volume of evolved, fertile arc magma to a focused position in the upper crust, magmatic fractionation, recharge, and volatile exsolution lead to the development of ore-forming magmatic-hydrothermal systems. To a first approximation, the size of the resulting deposit will be limited by the magma volume delivered to the upper crustal magma chamber. System-specific details such as magmatic-hydrothermal evolution, the nature of the country rocks, and subsequent erosional and weathering history will ultimately control the value of the deposit, but these factors fall outside the scope of this paper.

This article has been cited by other articles:

				B. H. Wilkinson and S. E. Kesler Quantitative Identification of Metallogenic Epochs and Provinces: Application to Phanerozoic Porphyry Copper Deposits Economic Geology, August 1, 2009; 104(5): 607 - 622. [Abstract] [Full Text] [PDF]

				S. J. Piercey, E. C. Chaloux, A. S. Peloquin, M. A. Hamilton, and R. A. Creaser Synvolcanic and Younger Plutonic Rocks from the Blake River Group: Implications for Regional Metallogenesis Economic Geology, September 1, 2008; 103(6): 1243 - 1268. [Abstract] [Full Text] [PDF]

				E. B. Ryskamp, J. T. Abbott, E. H. Christiansen, J. D. Keith, J. D. Vervoort, and D. G. Tingey Age and petrogenesis of volcanic and intrusive rocks in the Sulphur Spring Range, central Nevada: Comparisons with ore-associated Eocene magma systems in the Great Basin Geosphere, June 1, 2008; 4(3): 496 - 519. [Abstract] [Full Text] [PDF]

				R. H. Sillitoe Special Paper: Major Gold Deposits and Belts of the North and South American Cordillera: Distribution, Tectonomagmatic Settings, and Metallogenic Considerations Economic Geology, June 1, 2008; 103(4): 663 - 687. [Abstract] [Full Text] [PDF]

				O. P. Kreuzer, M. A. Etheridge, P. Guj, M. E. McMahon, and D. J. Holden Linking Mineral Deposit Models to Quantitative Risk Analysis and Decision-Making in Exploration Economic Geology, June 1, 2008; 103(4): 829 - 850. [Abstract] [Full Text] [PDF]

				S. E. Kesler and B. H. Wilkinson Earth's copper resources estimated from tectonic diffusion of porphyry copper deposits Geology, March 1, 2008; 36(3): 255 - 258. [Abstract] [Full Text] [PDF]

				M. Valencia-Moreno, L. Ochoa-Landin, B. Noguez-Alcantara, J. Ruiz, and E. Perez-Segura Geological and metallogenetic characteristics of the porphyry copper deposits of Mexico and their situation in the world context Geological Society of America Special Papers, January 1, 2007; 422(0): 433 - 458. [Abstract] [Full Text] [PDF]

				J. P. Richards, D. Wilkinson, and T. Ullrich Geology of the Sari Gunay Epithermal Gold Deposit, Northwest Iran Economic Geology, December 1, 2006; 101(8): 1455 - 1496. [Abstract] [Full Text] [PDF]

				D. R. Cooke, P. Hollings, and J. L. Walshe Giant Porphyry Deposits: Characteristics, Distribution, and Tectonic Controls Economic Geology, August 1, 2005; 100(5): 801 - 818. [Abstract] [Full Text] [PDF]

				D. I. Groves, K. C. Condie, R. J. Goldfarb, J. M.A. Hronsky, and R. M. Vielreicher 100th Anniversary Special Paper: Secular Changes in Global Tectonic Processes and Their Influence on the Temporal Distribution of Gold-Bearing Mineral Deposits Economic Geology, March 1, 2005; 100(2): 203 - 224. [Abstract] [Full Text] [PDF]

				C. A. Heinrich, W. Halter, M. R. Landtwing, and T. Pettke The formation of economic porphyry copper (-gold) deposits: constraints from microanalysis of fluid and melt inclusions Geological Society, London, Special Publications, January 1, 2005; 248(1): 247 - 263. [Abstract] [PDF]