Fluid inclusion study in different types of tin deposits associated with the Hercynian granites of western Spain

Abstract

Most of the Spanish Sn deposits are closely associated with intrusions of syn- and late kinematic Hercynian granites. A fluid inclusion study using microthermometric, Raman and crushing test analyses has been made of quartz, cassiterite, scheelite and calcite from greisens and quartz veins in the following types of deposits: (a) disseminated cassiterite in apogranites, Golpejas and Penouta; (b) stanniferous pegmatites, La Fregeneda; and (c) quartz veins and stockworks, with cassiterite, Teba, cassiterite and wolframite, San Finx, and cassiterite and scheelite, La Parrilla. The homogenization temperatures and physico-chemical characteristics of the inclusions indicate three stages of fluid circulation: The first stage is characterized by the trapping of complex CO 2-aqueous (H 2ONaClCO 2CH 4N 2H 2S), complex CO 2 (CO 2CH 4N 2H 2S) and low-salinity aqueous (H 2ONaCl) inclusions (< 10 eq.wt.% NaCl) at temperatures ranging between 500° and 250°C and pressures below 2000 bar. The second stage is represented by low-salinity aqueous inclusions (< 10 eq.wt.% NaCl), trapped at homogenization temperatures between 300° and 60°C and pressures of ∼200 bar. The third stage is characterized by mixed-salt aqueous inclusions (H 2ONaClKClCaCl 2MgCl 2), trapped at homogenization temperatures between 170° and 60°C and low pressures. The results obtained by this study indicate that, in general, with descending temperature, the chemical evolution of the mineralizing fluids is such that the fluid inclusions are characterized by a progressive increase in the total density and a progressive decrease in the salinity and the CO 2 and volatile contents. However, for the fluids trapped at lower temperatures (∼100°C), the salinity is much higher, which is presumably due to the sudden increase in Ca 2+, Mg 2+, Na + and K + salt contents.