Controls on ore metal ratios in granite-related ore systems: an experimental and computational approach

Earth and Environmental Science Transactions of the Royal Society of Edinburgh - Tập 83 Số 1-2 - Trang 317-326 - 1992
Philip A. Candela1
1Philip A. Candela, Laboratory for Mineral Deposit Research, Department of Geology,University of Maryland, College Park, Maryland, 20742-4211, U.S.A

Tóm tắt

ABSTRACTSize and composition (bulk metal ratios) of magmatic hydrothermal mineral deposits are affected by a number of chemical and physical processes including the nature of the source region and mode of emplacement. At shallow levels, rising plumes of vapour bubbles + melt, and the advection of water through interconnected vapour bubbles, allows access of the magmatic aqueous phase to the upper reaches of a magma chamber. These processes are operative at shallow levels where low water solubility and high molar volume for water make these processes more efficient.Partitioning experiments suggest that oxygen fugacity-dependent crystal/melt partitioning of ore metals leads to different efficiencies of removal of Cu, W, and Mo from silicate melts into ore-forming aqueous fluids. For example, the Mo/W ratio in magmatic hydrothermal deposits should increase as the oxygen fugacity of the magma increases. Further, Cu should behave as a crystal-compatible element in H2O-undersaturated, sulfide-saturated felsic magmas with fO2 NNO + 1 due to the strong partitioning of Cu from the melt into pyrrhotite.Cycling of oxidised, hydrated, sulfidised and Cl-enriched oceanic crust into mantle can give rise to magmas that contain S but are oxidised (≥NNO). The combination of high oxidation state, relatively hydrous but shallow conditions and a high Cl/H2O ratio leads to saturation with respect to H2O early during crystallisation, and loss of a large proportion of magmatic Cu to the aqueous phase. Ores formed from these oxidised magmas also possess high Mo/W ratios due to the effect of oxygen fugacity on the sequestering of Mo vs W.In less oxidised magmas, Cu and Mo are partitioned into sulfides and Ti-bearing phases, respectively, resulting in lower efficiencies of removal of Cu and Mo from melts into aqueous fluids. Further, the partitioning of W into crystallising phases is reduced, producing a more efficient removal of W into ore-forming fluids. This ultimately leads to mineral deposits with higher W/(Mo + Cu) ratios relative to deposits associated with more oxidised systems. Silicic, high-F magmas with fO2 = NNO can be found in tensional environments (e.g. rocks associated with the Climax-type deposits of the Colorado Mineral Belt). High HF/H2O activity ratios in the source regions yield melts that evolve an aqueous phase late during crystallisation, leading to relatively low ratios of compatible/incompatible elements in the melt at H2O saturation.

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Tài liệu tham khảo

10.1029/JB093iB06p06503

van Middelaar, 1990, Ore-bearing granitic systems; Petrogenesis and mineralizing processes, GEOL SOC AM SPEC PAPER, 246, 21

10.2113/gsecongeo.81.1.1

10.2113/gsecongeo.83.2.266

Dingwell, 1988, The structures and properties of fluorine-rich magmas: a review of experimental studies, Recent advances in the geology of granite-related mineral deposits, 39, 1

Barton, 1988, Metamorphism and crustal evolution, western conterminous United States, 7, 110

Andiambololona, 1978, Répartition et comportment des éléments de transition dans les roches volcaniques. I. cuivre et zinc, BULL B.R.G.M, 2, 121

10.2113/gsecongeo.85.3.633

10.1139/e70-098

10.1038/340199a0

10.1016/0377-0273(84)90057-X

10.1007/BF00372365

10.1016/0016-7037(84)90257-6

Carten, 1988, Comparison of field-based studies of the Henderson porphyry molybdenum deposit, Colorado, with experimental and theoretical models of porphyry systems, Recent advances in the geology of granite-related mineral deposits, 39, 351

10.1130/0016-7606(1988)100<0912:MAAADO>2.3.CO;2

10.2113/gsecongeo.80.1.148

10.1038/323598a0

Piccoli, 1991, Spatial variations in model halogen concentrations in magmas based on apatite chemistry: examples from natural systems, GEOL SOC AM ABSTR PROG, 23, 5

10.1130/0091-7613(1983)11<99:ISITFC>2.0.CO;2

Candela, 1989, Magmatic ore-forming fluids: thermodynamic and mass transfer calculations of metal concentrations, Ore Deposits Associated with Magmas, Reviews in Economic Geology, 4, 203

Barton, 1990, The nature and origin of Cordilleran magmatism: Boulder Colorado, GEOL SOC AM MEM, 174, 283

10.1038/339613a0

10.1016/0016-7037(87)90057-3

10.2113/gsecongeo.82.7.1827

Webster, 1990, Ore-bearing granitic systems; Petrogenesis and mineralizing processes, GEOL SOC AM SPEC PAP, 246, 21

10.2113/gsecongeo.85.1.99

10.1093/petrology/31.5.1071

10.1093/petrology/28.5.781

10.1007/BF01160186

10.1029/JB089iB12p10161

Brandeis, 1987, Magmatic processes: physiochemical principles, 307

10.1007/978-1-4612-4928-3_10

Candela, 1989, Felsic magmas, volatiles, and metallogenesis, Ore Deposits Associated with Magmas, Reviews in Economic Geology, 4, 223

Candela, 1991, Physics of Aqueous Phase evolution in Plutonic environments, AM MINERAL, 76, 1081

10.1016/0012-821X(81)90220-X

Haffty, 1972, Release and migration of molybdenum during the primary crystallization of peralkaline silicic volcanic rocks, ECON GEOL, 70, 857

Jaeger, 1968, Basalts, 503

Lehmann, 1990, Metallogeny of Tin

Lynton, 1990, Experimental determination of copper partitioning between pyrrhotite and high silica rhyolite, GEOL SOC AM ABST PROG, 22, 181

10.1007/BF00373674

10.2475/ajs.282.6.808

10.1038/333397a0

10.1130/0016-7606(1988)100<1720:CCSARI>2.3.CO;2

10.2475/ajs.277.8.937

10.1093/petrology/20.3.421

Peck, 1977, AM J SCI, 277, 415

Sato, 1982, Chaacteristics of tungsten skarns in Japan, two constrasting types, Proc. Symp. W Geol. Jiangxi, China, 1981, 203

Taylor, 1988, Degassing of rhyolitic magmas: Hydrogen isotope evidence and implications for magmatic-hydrothermal ore deposits, Recent Advances in the Geology of Granite-Related Mineral Deposits, 39, 33

Titley, 1981, Economic Geology 75th Anniversary Volume

10.1016/0016-7037(89)90382-7

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Earth and Environmental Science Transactions of the Royal Society of Edinburgh

Tập 83 Số 1-2

317-326

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