Mineralogy and geochemistry of the micas from Santa Elena and El Peñón pegmatites, Pampean Pegmatite Province, northwestern Argentina.

Abstract

 This study describes the physical and optical properties, polytypism, chemical composition and paragenesis of a Li-Al-bearing mica suite from the northernmost pegmatite field of Pampean Pegmatite Province. The micas are from two possible cogenetic rare-element pegmatites: Santa Elena, a complex-type, petalite subtype pegmatite, and El Peñón, a beryl-type, beryl-columbite-phosphate subtype pegmatite. The micas are muscovite, lithian muscovite, mixed forms and lepidolite. The muscovite s.I. has 2 M, structure and chemistry with comparatively low contents of Mn, Li, Rb, Cs, Ti and Y; one specimen could be classified as a 'rose muscovite'. The lepidolite and mixed forms have 2 M, dominant polytypism with some scarce X-ray reflections of 1 M or 2 M, polytypes. They are higher in SiO₂, MnO, CaO, Li₂O, Rb₂O, Cs, Ti, Y and P. Li₂O ranges between 3,71 and 4,73%, and Rb₂O from 0,29 to 2,53%. The geochemical evolution of the micas, illustrated by K/Rb versus lithium, manganeso, cesium, thallium and ytrium diagrams, shows a differentiation trend similar to micas from other LCT pegmatite fieids. The inferred crystallization path has evolved approximately around 2-3 kbar and 350-400ºC, in the stability field of spodumene, in a H₂O-saturated environment, under increasing HF, KF and LiF activities.