Metamorphic disturbances of magnetite chemistry and the Sm-Nd isotopic system of reworked Archean iron formations from NE Brazil

Abstract

Iron formations are valuable archives of sedimentary conditions and post-depositional events. However, geochemicalproxies commonly used to determine genetic characteristics can be variably modified during metamorphismand deformation, hampering their use as records of regional geological events. This work focuses onstrongly reworked magnetite-quartz-rich rocks from the São José do Campestre Massif, one of the oldest fragmentsof preserved crust in South America. The genetic classification of these magnetite-quartz-rich rocks is notstraightforward because primary assemblages and textures were variably modified by granulite facies metamorphismduring a regional Paleoproterozoic migmatization event. To address genetic ambiguities, we analyzed theirmagnetite and pyroxene chemistry, whole-rock geochemistry, and Sm-Nd isotopes. Magnetite chemistry indicatesthat pyroxene-poor iron formations (Type B) are low in trace elements such as Ti, Al, V, and Mn, suggesting achemical similarity to iron formations elsewhere. In contrast, magnetites from pyroxene-enriched Type A ironformations are rich in trace elements and more akin to magnetite crystallized from higher temperature systems,such as skarn and IOCG. The 147Sm/144Nd of these rocks show substantial variation even at the outcrop scale,indicating a locally-controlled, highly heterogeneous mixture of Archean, Paleoproterozoic, and Neoproterozoicsources. Therefore, our geochemical tools point out to heterogenous signatures of these magnetite-quartz rocksand proxies compatible with both low and high-temperature conditions and age of deposition spanning sourcesfrom the Archean to the Neoproterozoic. We interpret that the studied São José do Campestre magnetite-quartzrocks represent Archean iron formations with original magnetite chemistry and isotopic signatures variably modifiedby metamorphism and by at least one deformation-related hydrothermal event. These results contrast withsimilar examples from China and Greenland where iron formations either preserved the magnetite chemistry orthe primary isotopic signatures. Our study indicates that metamorphism can selectively affect chemical proxiesused to study iron formations and undermine the genetic classification of iron ores. Thus, these proxies should becarefully applied in the interpretation of syn-depositional environments of polydeformed belts.