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