HYDROTHERMAL EVENTS IN THE VALLE-DE-TENA (SPANISH WESTERN PYRENEES) AS EVIDENCED BY FLUID INCLUSIONS AND TRACE-ELEMENT DISTRIBUTION FROM FLUORITE DEPOSITS

This paper provides further evidences from fluorite and calcite geochemistry and from fluid inclusion studies for the origin of the Valle de Tena fluorite deposits. The Valle de Tena, Spanish Western Pyrenees, contains two kinds of fluorite mineralizations: (1) vein-type deposits located in the vicinity of the Panticosa granite and enclosed by Devonian carbonate facies; and (2) carbonate-hosted mineralizations on highly silicified Lower Carboniferous limestones. Based on geological evidences the latter deposits can be referred to as Permo-Triassic. Microthermometrical measurements on two-phase, water-rich inclusions with high liquid to vapor ratios yield homogenization temperatures ranging from 100 degrees to 200 degrees C for the two deposit-types. Salinities range from 4.0 to 11.6 eq wt% NaCl for carbonate-hosted deposits and from 12.5 to 21.3 eq wt% NaCl for vein mineralizations. REE contents are variable and highest abundances are found in the fluorite from Lanuza and Tebarray (green fluorite),These later fluorite mineralizations have primary REE patterns with positive Eu anomalies which possibly reflect the alteration of feldspar. The REE distribution patterns of fluorites and calcites from the carbonate-hosted mineralizations are different from those occurring as veins; the decrease of LREE in the former resulted in roof-shaped REE patterns. The negative Eu anomalies as well as the trace-element distribution allow us to deduce that the fluid from which these later Ca-minerals were formed was probably equilibrated with the sedimentary country rocks. The fluid inclusion, geochemical, geologic and preliminary stable isotope data support two temporally distinct hydrothermal systems accounting for the fluorite formation and suggest a sedimentary basin source for the ore fluids, High-salinity fluids (basement brines) are involved in the formation of the vein-type deposits, whereas the subsequent carbonate-hosted mineralizations were formed by mobilization of the fluorite veins by formation waters. The most likely fluorite deposition mechanism is cooling with minor contribution of fluid-rock interaction. In such context, magmatic activity is considered as heat source, The above mineralization events possibly span a time from Permian to Triassic which is in close agreement with the timing of hydrothermal processes documented throughout the Hercynian Europe.