The Rio dos Remédios Group comprises a supracrustal sequence that occupies the base of the Espinhaço Supergroup, São Francisco Craton, Brazil. Its basal formation, Novo Horizonte, crops out in the Paramirim region mainly as metavolcanic rocks that represent one of the fewer occurrences of blue quartz phenocrysts in South America. Their mineralogy consists of quartz and K-feldspar phenocrysts, whereas biotite, muscovite, fluorite, allanite, chlorite, sericite, zircon, and opaque phases occur immersed in a quartz-feldspar-rich groundmass. Such heterogeneous composition is also supported by x-ray diffraction and chemical data. Electron probe microanalysis in some samples revealed the presence of two distinct groups of biotite (magmatic and neoformed), in addition to the presence of iron-rich white mica and almost pure orthoclase feldspar. Our data suggest that the studied metavolcanic rocks have maintained their magmatic characteristics, which were progressively overprinted by hydrothermal fluids and ductile-to-brittle deformation. The magmatic mineralogy is akin to strongly peraluminous and alkaline magmas, common in anorogenic settings -a fertile site for the origin of blue quartz-bearing rocks worldwide.

Rock forming blue quartz occurrences are reported on all continents, but little is known about the specific causes for the coloration, and even less about its geological significance. The present study investigates the occurrence hosted by the metarhyolites of the Rio dos Remédios Group in order to determine the potential causes of the blue coloration and gain insight into the geological significance of the occurrence in the wider context of Brazilian geology. The optical observations have shown that the blue color is likely the result of the Rayleigh scattering of light. The color is stable even at temperatures of 800°C. The FT-IR investigations have not detected the presence of inclusions capable of scattering light. The electron microscopy has shown that there are significant amounts of Fe and Ti in the system, clearly concentrated as Ti/Fe oxides hosted by the matrix, and potentially as light scattering nanometric inclusions. This type of inclusions would likely be syngenetic with the host quartz. The alternation between the blue and colorless areas could be the result of chemical fluctuations in the melt, which may have resulted in disruptions of the crystallization of the light scattering inclusions. A potential metallogenic correlation with other blue quartz occurrences from the Paramirim Aulacogen and Arai Rift is suggested.