The Geology and Isotope Geochemistry of the Talc Deposits ofPuebla de Lillo (Cantabrian Zone, Northern Spain)

Abstract

The talc deposits of Puebla de Lillo (Cantabrian zone, Variscan belt of Iberia) are hosted by hydrothermal dolostones replacing Carboniferous limestones and are generally located adjacent to Ordovician quartzites. The talc orebodies occur as metasomatic replacement zones of the dolostones near faults or lithologic contacts and, to a lesser extent, as replacements of the nearby quartzites or slates. These rocks are affected by a very low grade regional metamorphism but occur in a highly deformed setting with abundant thrusts and faults. Dolostones similar to those hosting the talc deposits are widespread in the Cantabrian zone; most of them are barren, although a few host epithermal sulfide mineralization. Four hydrothermal events are recognized. The first two formed gray (I) and beige (II) hydrothermal dolostones (± quartz), very similar to the regional barren dolostones. The third, a white coarse-grained dolomite (III) is directly associated with the talc. The final episode is characterized by late-stage vuggy dolomite (IV), calcite, and quartz. Fluid inclusion studies in dolomite and quartz suggest that the dolomitizing fluids were aqueous NaCl-CaCl 2 brines of variable salinities (0-23 wt % NaCl equiv) and a low CO 2 content (0.003-0.01 X CO2). Thermodynamic considerations, fluid inclusion data, and comparison to regional studies suggest that dolomite (events I and II) formed at temperatures between 100° and 280°C, the talc-forming event (III) was between 280° and 405°C, and the postore carbonates and quartz (IV) precipitated between 65° and 170°C. Fluid pressure during the hydrothermal event was low, probably between 165 and 450 bars. The δ 18 O value of the dolomite reflects the interaction of an infiltrating 18 O-depleted fluid with the country-rock limestones having δ 18 O SMOW = 22.5 to 33.2 per mil and leading to a concomitant lowering of the average δ 18 O value to 19.9 ± 4.0 (dolostone I), 17.2 ± 6.2 (dolostone II), and 16.8 ± 1.9 per mil (dolomite III) during the different dolomitization events. The talc replacing the dolostone has a monotonous δ 18 O signature of 10.7 to 12.7 per mil and a δD SMOW composition of-64 to-62 per mil. However, the talc replacing quartzites, slates, or impure dolostones has more variable and lower δ 18 O values (8.1, 9.0, and 11.5‰, respectively). The late dolomite IV has a δ 18 O composition of 14.7 ± 3 per mil, whereas the associated quartz has a δ 18 O of 13.0 to 15.7 per mil. The oxygen isotope results are indicative of a systematic oxygen isotope disequilibrium between the hydrothermal minerals.