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

Fernando Tornos^1,, Carmen Galindo², José Luis Crespo³ and Baruch F. Spiro⁴

¹ Instituto Geológico y Minero de España, c/Azafranal 48, E–37001 Salamanca, Spain
² Departmento de Petrología y Geoquímica, Universidad Complutense de Madrid, E–28040 Madrid, Spain
³ SIEMCALSA, c/Incas, 5, E–47008 Valladolid, Spain
⁴ Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom

E-mail address: f.tornos{at}igme.es

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 ¹⁸O values of the garnet and clinopyroxene in the prograde skarn are indicative of ¹⁸O_fluid compositions near 9.5–10.9. The compositions of clinoamphibole and biotite of the retrograde skarn suggest equivalent ¹⁸O_fluid values (9.1–12.0). These heavy ¹⁸O 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 (⁸⁷Sr/⁸⁶Sr in the range 0.7117–0.7119; Nd_300Ma between –9.4 and –8.5), garnet–pyroxene skarn (⁸⁷Sr/⁸⁶Sr in the range 0.7119–0.7128; Nd_300Ma between –8.5 and –8.7) and plagioclase-rich skarn (⁸⁷Sr/⁸⁶Sr in the range 0.7124–0.7129; 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 ⁸⁷Sr values (⁸⁷Sr/⁸⁶Sr > 0.7112–0.7149) and intermediate Nd signatures (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 (⁸⁷Sr/⁸⁶Sr in the range 0.7108–0.7110; 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.

Keywords: scheelite, isotope geochemistry, skarn, leucogranite, Spanish Central System, Los Santos deposit, Spain.