Agalmatolito do Quadrilátero Ferrífero, MG

Abstract

Agalmatolite is a peraluminous rock type of industrial use composed essentially of pyrophyllite. Northwest of the Quadrilatero Ferrifero mineral province in Minas Gerais, agalmatolite occurrences and mines are distributed over an area of about 350 km 2 . There are several rock types commercially named agalmatolite and which are made up of minerals other than pyrophyllite, such as muscovite, quartz, kyanite and andalusite showing variable degrees of replacement by pyrophyllite and diaspore. Analyses by SEM/EDS, optical microscopy and X ray diffraction showed that it ispossible to distinguish pyrophyllite from muscovite by their distinct textures, pyrophyllite occurring as randomly oriented and extremely fine-grained masses, while muscovite constitutes two textural types: large flakes or fine-grained but always preferentially oriented aggregates. The country rocks, which at least in part correspond to the protolith of the agalmatolite, are metavolcanic and metasedimentary rocks interpreted as belonging to the archean Rio das Velhas greenstone belt. The metavolcanic rocks are composed of relict phenocrysts of alkali-feldspar in a metamorphic matrix of chlorite, muscovite, epidote and quartz. The metasedimentary rocks are banded iron formations, quartzites and schists with variable amounts of quartz, muscovite, kyanite, andalusite, corundum, garnet, staurolite, chloritoid, chlorite, tourmalines and rutile/leucoxene. The stable association of staurolite and chloritoid enabled to establish the temperature conditions of the regional Archean metamorphic event, which formed the country rocks and the protolith of the agalmatolite, at T~500-550 o C. The observed replacement of kyanite by andalusite indicates an episode of decreasing pressure to less the 4kbar, possibly during the regional metamorphic event. The agalmatolite was formed at T ~350-400 o C at the expenses of peraluminous metassedimentary rocks with variable amounts of kyanite, andalusite, muscovite, and quartz. Pyrophyllite was generated by hydrothermal alteration of these rocks along shear zones of possible Transamazonian age, which acted as sinks for H 2 O-rich fluids.