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

Abstract

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 δ18O values of the garnet and clinopyroxene in the prograde skarn are indicative of δ18Ofluid compositions near 9.5–10.9‰. The compositions of clinoamphibole and biotite of the retrograde skarn suggest equivalent δ18Ofluid values (9.1–12.0‰). These heavy δ18O 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; eNd300Ma between −9.4 and −8.5), garnet–pyroxene skarn (87Sr/86Sr in the range 0.7119–0.7128; eNd300Ma between −8.5 and −8.7) and plagioclase-rich skarn (87Sr/86Sr in the range 0.7124–0.7129; eNd300Ma 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 eNd signatures (eNd300Ma 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; eNd300Ma = −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.