GEOCHEMISTRY AND ORIGIN OF CALCIC TUNGSTEN-BEARING SKARNS, LOS SANTOS, CENTRAL IBERIAN ZONE, SPAIN

摘要

The mesozonal, calcic and reduced Los Santos skarn, in the Central Iberian Zone, is a low-tonnage, high-grade tungsten deposit that occurs as discontinuous stratabound lenses enriched in scheelite along the contact between Variscan granodiorite- monzogranite and early Cambrian calcic and dolomitic marble and calc-silicate and pelitic hornfelses. The delta~(18)O values of the garnet and clinopyroxene in the prograde skarn are indicative of delta~(18)O_(fluid) compositions near 9.5-10.9 per thousand. The compositions of clinoamphibole and biotite of the retrograde skarn suggest equivalent delta~(18)O_(fluid) values (9.1-12.0 per thousand). These heavy delta~(18)O values indicate that the mineralizing fluids equilibrated with deep crustal rocks, with no significant input of surficial waters, even during the formation of the retrograde skarn. The Sr and Nd radiogenic isotope data for the scheelite (~(87)Sr/~(86)Sr in the range 0.7117-0.7119; epsilon Nd_(300Ma) between -9.4 and -8.5), garnet-pyroxene skarn (~(87)Sr/~(86)Sr in the range 0.7119-0.7128; epsilon Nd_(300Ma) between -8.5 and -8.7) and plagioclase-rich skarn (~(87)Sr/~(86)Sr in the range 0.7124-0.7129; epsilon Nd_(300Ma) between -9.7 and -7.0) trace the complex evolution of hydrothermal fluids circulating through deep perigranitic systems. These signatures, as well as the REE contents, suggest equilibration of an external fluid with the host metasedimentary sequence. The regional geology and the Nd isotopes indicate that the ultimate source of the hydrothermal fluids and the tungsten and fluorine is not the adjacent barren granodiorite-tonalite nor the host metamorphic rocks, but rather an unexposed granitic pluton geochemically equivalent to the (biotite +- tourmaline)-bearing, fine-grained leucogranite dikes that crop out nearby. These are characterized by high ~(87)Sr values (~(87)Sr/~(86)Sr > 0.7112-0.7149) and intermediate eNd signatures (epsilon Nd_(300Ma) in the range -5.6 to -4.6) and are similar to those hosting perigranitic W-(Sn) mineralization in other areas of the Variscan Belt. Thus, the geochemical data show that the magmatic fluids extensively interacted with the host aluminosilicate rocks but were not able to precipitate the scheelite and fluorite until the reaction with carbonate rocks. Scheelite from nearby regionally stratabound orebodies have isotopic signatures (~(87)Sr/~(86)Sr in the range 0.7108-0.7110; epsilon Nd_(300Ma) = -8.6) similar to those of the Los Santos skarn, strongly suggesting that they are also of Variscan perigranitic origin and not exhalative or synmetamorphic, as had been previously proposed.