The origin of the Atshan talc deposit in the Hamata area, Eastern Desert, Egypt: A geochemical and mineralogical study

The Atshan talc mine is one of several talc deposits in the Hamata area of southeastern Egypt. Our detailed geochemical and mineralogical investigatio n suggests that precursors to the talc orebodies were impure dolomitic lime stones locally intercalated with elastic sediments. The extremely low conce ntrations of trace elements, including the REE, and the low and variable Al concentrations in these rocks are inconsistent with igneous protoliths. Th e magnesium needed to form the talc orebodies was derived from the breakdow n of pre-existing carbonates. This mode of origin is comparable to that in other carbonate-hosted talc deposits on the global scale, such as the Raben wald and Lassing deposits in Austria. Rocks at the Atshan mine have been su bjected to at least two episodes of metamorphism contact and regional. Mine ral assemblages (a) enstatite + cordierite + hercynite (granulite facies) a nd (b) enstatite + clinopyroxene (pyroxene hornfels facies) probably crysta llized during emplacement of the Reiidi grey granite (contact metamorphism) , and represent relict high-temperature assemblages in the elastic fragment s. Temperatures above 600 degrees C are suggested for the enstatite-cordier ite pair, and a minimum range of temperatures between 500 degrees and 600 d egrees C was estimated for the pyroxene hornfels. The serpentine + talc + t remolite + chlorite assemblage replaced the carbonates during regional meta morphism and associated faulting and shearing. The maximum temperature for this mineral assemblage is limited to <500 degrees C, and the X(CO2) of the fluid is limited to <0.2 by the ubiquitous presence of serpentine. Stable reactions in the system of CaO - MgO - SiO2 - H2O - CO2 demonstrate that se rpentine and tremolite at the Atshan deposit could have formed during progr ade reactions, and talc, during retrograde reactions. Small lenses of massi ve and disseminated sulfide (pyrite, pyrrhotite, sphalerite, chalcopyrite) within the talc orebodies may represent sulfide segregation in the original sediments prior to metamorphism. Some of the pyrite and pyrrhotite grains are fragmented and rimmed by talc, suggesting that they were present prior to talc mineralization. Chalcopyrite was probably remobilized, and it occur s along tremolite cleavages and fractures.