Analysis of evaporate mounds as a complement to fluid-inclusion thermometric data: Case studies from granitic environments in Nova Scotia and Peru

The bulk composition of fluid inclusions is generally determined by using the temperature of ice melting (i.e., freezing-point depression of H2O-NaCl) and relating this to the ice-melting curve in the two-component system H2O-NaCl. In reality, natural systems are more complex. Therefore, the bulk composition is referred to as equivalent wt.% NaCl to account for the presence of other, generally undetermined, cations (e.g., K, Ca, Fe, Mn, Mg). Inferences of a more complex chemical composition are generally made on the basis of the depression of the eutectic temperature for the H2O-NaCl system (-21.2degreesC), the other common solutes being CaCl2, MgCl2, and FeCl2. Such an approach is qualitative at best, and the inherent problem of metastability invites caution. An alternative, relatively simple and cost-effective method among the many analytical methods available, is to artificially decrepitate fluid inclusions and analyze the resulting evaporate mounds with an electron microprobe equipped with an imaging system. To illustrate the potential of the method, three granitic environments, in Nova Scotia and in Peru, were chosen, two mineralized (Sn, Cu, Zn) and one barren (tourmaline pegmatite). Standard fluid-inclusion thermometric data have been integrated with compositions determined from evaporate mounds in the same suite of samples. The following points emerge: (1) the inclusions are invariably multi-component, being dominated by Na-K-Ca, but in consistently different proportion that reflect protracted fluid:rock interaction and mixing of reservoirs; (2) the low first-melting temperatures (i.e., <-50degreesC) do not unequivocally indicate the presence of CaCl2 in solution; (3) high concentrations of Fe and Mn are common within the mineralized systems examined, in addition to lesser amounts of Sr, Ba, P and Zn, and (4) the two methods complement each other, together providing a much more comprehensive account of the nature of the fluid and its evolution in space and time.