The geology and isotope geochemistry of the talc deposits of Puebla de Lillo (Cantabrian zone, northern Spain)

The talc deposits of Puebla de Lillo (Cantabrian zone, Variscan belt of Ibe ria) are hosted by hydrothermal dolostones replacing Carboniferous limeston es and are generally located adjacent to Ordovician quartzites. The talc or ebodies occur as metasomatic replacement zones of the dolostones near fault s or lithologic contacts and, to a lesser extent, as replacements of the ne arby quartzites or slates. These rocks are affected by a very low grade reg ional metamorphism but occur in a highly deformed setting with abundant thr usts 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 region al barren dolostones. The third, a white coarse-grained dolomite (III) is d irectly associated with the talc. The final episode is characterized by lat e-stage vuggy dolomite (IV), calcite, and quartz. Fluid inclusion studies in dolomite and quartz suggest that the dolomitizin g fluids were aqueous NaCl-CaCl2 brines of variable salinities (0-23 wt % N aCl equiv) and a low CO2 content (0.003-0.01 X-CO2). Thermodynamic consider ations, fluid inclusion data, and comparison to regional studies suggest th at dolomite (events I and II) formed at temperatures between 100 degrees an d 280 degrees C:, the talc-forming event (III) was between 280 degrees and 405 degrees C, and the postore carbonates and quartz (IV) precipitated betw een 65 degrees and 170 degrees C. Fluid pressure during the hydrothermal ev ent was low, probably between 165 and 450 bars. The delta(18)O value of the dolomite reflects the interaction of an infiltr ating O-18-depleted fluid with the country-rock limestones having delta(18) O(SMOW) = 22.5 to 33.2 per mil and leading to a concomitant lowering of the average delta(18)O value to 19.9 +/- 4.0 (dolostone I), 17.2 +/- 6.2 (dolo stone II), and 16.8 +/- 1.9 per mil (dolomite III) during the different dol omitization events. The talc replacing the dolostone has a monotonous delta (18)O signature of 10.7 to 12.7 per mil and a delta D-SMOW composition of - 64 to -62 per mil. However, the talc replacing quartzites, slates, or impur e dolostones has more variable and lower delta(18)O values (8.1, 9.0, and 1 1.5 parts per thousand, respectively). The late dolomite IV has a delta(18) O composition of 14.7 +/- 3 per mil, whereas the associated quartz has a de lta(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 min erals. The combined delta(18)O-Sr-87/Sr-86 data support a complex hydrothermal evo lution involving the mixing of two fluids. Fluids related to the dolomitiza tion of the limestones were dominated by deep brines equilibrated with the underlying siliciclastic rocks (delta(18)O > 5-7 parts per thousand; Sr-87/ Sr-86 greater than or equal to 0.7090). They gradually mixed with a meteori c fluid with incomplete equilibration with the basement. The hydrothermal f luid during talc formation had an isotopic signature of delta(18)O = 5.0 to 7.9 per mil and delta D close to -70 per mil, whereas the postore hydrothe rmal fluids were mostly meteoric in origin (delta(18)O < 0 parts per thousa nd; Sr-87/Sr-86 greater than or equal to 0.7086). The delta(13)C values of all hydrothermal carbonates investigated (-2 to +4.9 parts per thousand) ar e indicative of carbon being buffered by the host limestones, in agreement with the observed low X-CO2 of the fluid. The isotopic composition of sulfur in pyrite (delta(34)S(CDT); -3.8 to +10. 2 parts per thousand) also indicates derivation from two different sources, the host limestones containing sulfides with high delta(34)S formed by the abiogenic reduction of seawater sulfates and siliciclastic rocks from the basement having isotopically light sulfur of bacteriogenic origin. The talc mineralization of Puebla de Lillo resulted from a regional scale e pisodic hydrothermal activity related to early Alpine wrench faulting. The mineralization is located along tensional structures adjacent to the deep-s eated Cofinal strike-slip fault, a major structure of the Cantabrian zone. These extensional faults acted as conduits for large amounts of hydrotherma l fluids. Upwelling silica and Mg-rich brines reacted with the carbonate ro cks, producing the dolomitization and superimposed talc mineralization. The critical factor in the formation of the talc instead of die regional dolos tones (+stable quartz) was a local increase in temperature. This localized high heat flow cannot be solely explained by normal geothermal gradients an d suggests the existence of a deep igneous intrusion near the fault, which provided heat that drove the hydrothermal system. The role of the nearby qu artzites was probably to saturate the hydrothermal fluids in silica as they were most likely SiO2 undersaturated due to heating after equilibration wi th the basement.