Banded quartz veinlets at the Miocene Cerro Maricunga (Fenix) gold deposit, Maricunga gold belt, North Chile

Banded quartz veinlets (BQVs) are typical of the gold deposits in the Maricunga belt, but have not been described in detail so far. Here we document BQVs in the Cerro Maricunga (Fenix) gold deposit, located in the root zone of the now largely eroded Miocene Ojo de Maricunga volcano. The BQVs are the principal gold carrier and are characterized by white-black/grey banding at the millimeter-scale. The darker tones of the veinlet bands are caused by abundant single-phase vapor fluid inclusions with equally abundant microcrystalline magnetite. The gold-bearing BQVs, with common symmetric five- and three-part banding and variants thereof, show distinctive dendritic textures (feathery, colloform, flamboyant, ghost sphere and moss-like), made up by abundant sub-micron to micron-sized vapor fluid inclusions plus nano- and microcrystalline magnetite in siliceous matrix. The mineralized BQVs are macroscopically similar to barren BQVs, the latter having bands due to dispersed dark phyllosilicates (possibly vermiculite after biotite) in the porous quartz matrix. The magnetite nanocrystals likely were physically transported with silica colloids and gold nanoparticles from greater depth by a vapor-dominant fluid as result of vigorous boiling/flashing. The porous nature of the BQVs, due to dehydration of silica gel, is key to efficient gold recovery in ore processing.