Contrasting coronas: microscale fluid variation deduced from monazite breakdown products in altered metavolcanic rocks associated with the Grangesberg apatite-iron oxide ore, Bergslagen, Sweden

Three different types of secondary coronas developed around monazite-(Ce) were discovered in altered metavolcanic rocks closely associated with the Palaeoproterozoic apatite-iron oxide ore deposit in Grangesberg, Sweden. All three types of reaction coronas include fluorapatite that is either rimmed by allanite-(Ce), REE-fluorocarbonate(s), or hingganite-(Y). The latter mineral has not been previously observed among monazite breakdown products. A unique feature of the described reaction coronas around monazite is their spatial proximity to each other, not exceeding a few hundreds of micrometres. We infer that the observed, strongly contrasting monazite breakdown assemblages highlight the presence of a heterogeneous fluid that mediated these microscale decomposition reactions. Thus, it is emphasized that metasomatic fluid variability in natural systems may often be too large to be predicted and reproduced experimentally.

Automated summary

Three types of secondary coronas were found around monazite in altered metavolcanic rocks associated with the Palaeoproterozoic apatite-iron oxide ore deposit. These coronas contain unique mineral assemblages and are spatially close to each other.