Concentration of platinum-group elements by magmatic fluids in layered intrusions

Duke University, Department of Geology, Durham, NC, United States
University of Washington, United States

Evidence leading to a model which supports the formation of Pt-Pd reefs in a layered intrusion by the action of magmatic fluids is presented, as well as some of the constraints imposed on such a model. The Stillwater Complex is used as an example of this model. Within the Stillwater Complex, no laterally continuous sulfide-enriched cumulates occur above the lowermost parts of the Ultramafic series until one reaches the platiniferous J-M reef. This sulfide-poor stratigraphic interval beneath the J-M reef is characterized by unusually Cl-rich apatite. Although reported sulfide modes and whole-rock sulfur contents in the cumulates below the J-M reef are approximately an order of magnitude lower than that expected for a sulfide-saturated magma; sulfide saturation prior to the formation of the J-M reef is suggested by relatively high Pd and Pt concentrations reported in the cumulates below the J-M reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals. Hence it is suggested that the footwall cumulates to the J-M reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter-cumulus liquid. Sporadic sulfide-bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur. These observations, as well as the association of the J-M reef at the first significant stratigraphic break in the Cl/(Cl + F) ratio of apatite and the relatively high solubility of Pt and Pd as aqueous Cl complexes, are consistent with the formation of the J-M reef by exsolution of Cl- and volatile-rich fluids from footwall intercumulus liquids which leached and transported ore components (both the PGE and S) upward to the level of the reef.Using published whole-rock platinum-group element (PGE) concentrations for the Ultramafic and Lower Banded series, the cumulates below the J-M reef must have been enriched by a factor of, at most, 1.24 for Pt and 1.75 for Pd if these rocks were to be the source for the metals in the J-M reef. Model Raleigh fractionation calculations suggest that the cumulates beneath the J-M reef could have had the required metal concentrations if, during the crystallization of a parent magma containing no more than 15 ppb of either metal, the average bulk solid/liquid distribution coefficients for Pt and Pd were no more than 5; these values are consistent with assumption of sulfide saturation prior to the formation of the J-M reef.During solidification of intercumulus liquid, the separation and loss of a Cl-rich fluid will deplete the residual assemblage of S and PGE. If 0.1 to 0.5 wt percent fluid containing 5 percent S evolved during solidification of the footwall cumulates (the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid-saturated intercumulus liquid), the fluid/liquid distribution coefficients for Pt and Pd would need to be almost as large as the (sulfide liquid)/(silicate liquid) distribution coefficients to remove significant amounts of Pt and Pd. Much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing. The latter possibility is consistent with the low S contents of the footwall cumulates beneath the J-M reef. Numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the PGE and S during the degassing of intercumulus silicate liquids, the PGE being enriched at a sulfide-dissolution front as upward migrating sulfide-undersaturated fluids resorb cumulus sulfides. Zones of PGE and S enrichment can occur where degassing fronts encounter stratigraphic physical-chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids.

This record provided courtesy of AGI/GeoRef.

This article has been cited by other articles:

				A. J. Naldrett, A. Wilson, J. Kinnaird, and G. Chunnett PGE Tenor and Metal Ratios within and below the Merensky Reef, Bushveld Complex: Implications for its Genesis J. Petrology, April 20, 2009; (2009) egp015v1. [Abstract] [Full Text] [PDF]

				J. E. Mungall and A. J. Naldrett Ore Deposits of the Platinum-Group Elements Elements, August 1, 2008; 4(4): 253 - 258. [Abstract] [Full Text] [PDF]

				J. J. Hanley, J. E. Mungall, T. Pettke, E. T. C. Spooner, and C. J. Bray Fluid and Halide Melt Inclusions of Magmatic Origin in the Ultramafic and Lower Banded Series, Stillwater Complex, Montana, USA J. Petrology, June 1, 2008; 49(6): 1133 - 1160. [Abstract] [Full Text] [PDF]

				B. Godel and S.-J. Barnes IMAGE ANALYSIS AND COMPOSITION OF PLATINUM-GROUP MINERALS IN THE J-M REEF, STILLWATER COMPLEX Economic Geology, May 1, 2008; 103(3): 637 - 651. [Abstract] [Full Text] [PDF]

				J. D. Greenough and B. J. Fryer TRACE-ELEMENT EVIDENCE FOR VOLATILE-INFLUENCED DIFFERENTIATION IN A FLOW OF ALKALI BASALT, PENG HU ISLAND, TAIWAN Can Mineral, April 1, 2008; 46(2): 305 - 315. [Abstract] [Full Text] [PDF]

				A. WILSON and G. CHUNNETT Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa J. Petrology, December 1, 2006; 47(12): 2369 - 2403. [Abstract] [Full Text] [PDF]

				C. Li and E. M. Ripley FORMATION OF Pt Fe ALLOY BY DESULFURIZATION OF Pt Pd SULFIDE IN THE J M REEF OF THE STILLWATER COMPLEX, MONTANA Can Mineral, August 1, 2006; 44(4): 895 - 903. [Abstract] [Full Text] [PDF]

				A. Kerr and A. M. Leitch Self-Destructive Sulfide Segregation Systems and the Formation of High-Grade Magmatic Ore Deposits Economic Geology, March 1, 2005; 100(2): 311 - 332. [Abstract] [Full Text] [PDF]

				A. E. Boudreau and D. M. Hoatson HALOGEN VARIATIONS IN THE PALEOPROTEROZOIC LAYERED MAFIC-ULTRAMAFIC INTRUSIONS OF EAST KIMBERLEY, WESTERN AUSTRALIA: IMPLICATIONS FOR PLATINUM GROUP ELEMENT MINERALIZATION Economic Geology, August 1, 2004; 99(5): 1015 - 1026. [Abstract] [Full Text] [PDF]

				W. P. Meurer, F. A. Hellstrom, and D. T. Claeson THE RELATIONSHIP BETWEEN CHLORAPATITE AND PGE-RICH CUMULATES IN LAYERED INTRUSIONS: THE KLAPPSJO GABBRO, NORTH-CENTRAL SWEDEN, AS A CASE STUDY Can Mineral, April 1, 2004; 42(2): 279 - 289. [Abstract] [Full Text] [PDF]

				A. E. Boudreau PALLADIUM, A PROGRAM TO MODEL THE CHROMATOGRAPHIC SEPARATION OF THE PLATINUM-GROUP ELEMENTS, BASE METALS AND SULFUR IN A SOLIDIFYING PILE OF IGNEOUS CRYSTALS Can Mineral, April 1, 2004; 42(2): 393 - 403. [Abstract] [Full Text] [PDF]

				Y.-R. Park, E. M. Ripley, J. D. Miller Jr, C. Li, J. Mariga, and P. Shafer Stable Isotopic Constraints on Fluid-Rock Interaction and Cu-PGE-S Redistribution in the Sonju Lake Intrusion, Minnesota Economic Geology, March 1, 2004; 99(2): 325 - 338. [Abstract] [Full Text] [PDF]

				J. E. Mungall and J. M. Brenan EXPERIMENTAL EVIDENCE FOR THE CHALCOPHILE BEHAVIOR OF THE HALOGENS Can Mineral, February 1, 2003; 41(1): 207 - 220. [Abstract] [Full Text] [PDF]

				F. Gervilla and K. Kojonen THE PLATINUM-GROUP MINERALS IN THE UPPER SECTION OF THE KEIVITSANSARVI Ni Cu PGE DEPOSIT, NORTHERN FINLAND Can Mineral, April 1, 2002; 40(2): 377 - 394. [Abstract] [Full Text] [PDF]

				W.D. Maier, S.-J. Barnes, and M. J. van der Merwe Platinum-group elements in the Pyroxenite Marker, Bushveld Complex: implications for the formation of the Main Zone South African Journal of Geology, December 1, 2001; 104(4): 301 - 308. [Abstract] [Full Text] [PDF]

				A. H. Wilson and M. D. Prendergast Platinum-Group Element Mineralisation in the Great Dyke, Zimbabwe, and its Relationship to Magma Evolution and Magma Chamber Structure South African Journal of Geology, December 1, 2001; 104(4): 319 - 342. [Abstract] [Full Text] [PDF]

				A. H. WILSON Compositional and Lithological Controls on the PGE-Bearing Sulphide Zones in the Selukwe Subchamber, Great Dyke: a Combined Equilibrium-Rayleigh Fractionation Model J. Petrology, October 1, 2001; 42(10): 1845 - 1867. [Abstract] [Full Text] [PDF]

				T. Moreno, T. MORENO, W. GIBBONS, H. M. PRICHARD, and R. LUNAR Platiniferous chromitite and the tectonic setting of ultramafic rocks in Cabo Ortegal, NW Spain Journal of the Geological Society, July 1, 2001; 158(4): 601 - 614. [Abstract] [Full Text] [PDF]

				C. HARRIS and J. B. CHAUMBA Crustal Contamination and Fluid-Rock Interaction during the Formation of the Platreef, Northern Limb of the Bushveld Complex, South Africa J. Petrology, July 1, 2001; 42(7): 1321 - 1347. [Abstract] [Full Text] [PDF]

				C. Li, C. Li, W. D. Maier, and S. A. de Waal The Role of Magma Mixing in the Genesis of PGE Mineralization in the Bushveld Complex: Thermodynamic Calculations and New Interpretations Economic Geology, May 1, 2001; 96(3): 653 - 662. [Abstract] [Full Text] [PDF]

				V. A. Glebovitsky, V. S. Semenov, B. V. Belyatsky, E. V. Koptev-Dvornikov, N. F. Pchelintseva, B. S. Kireev, and A. B. Koltsov THE STRUCTURE OF THE LUKKULAISVAARA INTRUSION, OULANKA GROUP, NORTHERN KARELIA: PETROLOGICAL IMPLICATIONS Can Mineral, April 1, 2001; 39(2): 607 - 637. [Abstract] [Full Text] [PDF]

				D. Pirrie, D. PIRRIE, M. R. POWER, J. C. O. ANDERSEN, and A. R. BUTCHER Platinum-group mineralization in the Tertiary Igneous Province: new data from Mull and Skye, Scottish Inner Hebrides, UK Geological Magazine, November 1, 2000; 137(6): 651 - 658. [Abstract] [Full Text] [PDF]

				C. C. WILLMORE, A. E. BOUDREAU, and F. J. KRUGER The Halogen Geochemistry of the Bushveld Complex, Republic of South Africa: Implications for Chalcophile Element Distribution in the Lower and Critical Zones J. Petrology, October 1, 2000; 41(10): 1517 - 1539. [Abstract] [Full Text] [PDF]

				M. D. Prendergast Layering and Precious Metals Mineralization in the ;Rincon del Tigre Complex, Eastern Bolivia Economic Geology, January 1, 2000; 95(1): 113 - 130. [Abstract] [Full Text] [PDF]

				W. D. Maier and S.-J. Barnes Platinum-Group Elements in Silicate Rocks of the Lower, Critical and Main Zones at Union Section, Western Bushveld Complex J. Petrology, November 1, 1999; 40(11): 1647 - 1671. [Abstract] [Full Text] [PDF]

				A. Boudreau Fluid Fluxing of Cumulates: the J-M Reef and Associated Rocks of the Stillwater Complex, Montana J. Petrology, May 1, 1999; 40(5): 755 - 772. [Abstract] [Full Text] [PDF]