Magnetite Chemistry As A Provenance Indicator In Archean Metamorphosed Sedimentary Rocks

Abstract

Abstract: Conventional provenance studies of high grade (e.g., amphibolite facies) metamorphic sandstone are limited due to intense recrystallization, destruction of original minerals, and growth of new minerals. Magnetite is a heavy mineral common in a wide range of sedimentary rocks. It can originate from a wide range of rock sources from high-temperature to low-temperature magmatic, metamorphic, or hydrothermal environments. Laser ablation (LA)-ICP-MS and electron microprobe analytical techniques allowed determination of a wide range of trace elements at sub ppm levels in magnetite to identify the provenance of two Archean sedimentary formations in Eeyou Istchee Baie-James, Quebec, Canada. Results were plotted on multi-element diagrams. The most compatible elements in magnetite were identified as the most discriminant elements to differentiate four potential different rock sources for the Magin Formation. Overlap between the spectra of the Magin and Keyano formations substantiate the existence of a common source. Interbedded conglomerate magnetite-bearing clasts were examined to determine potential source-rocks. Mafic clasts apparently do not contain magnetite. Detrital magnetites analyzed have low Ti values that would support either felsic (to intermediate) plutonic or a hydrothermal source. The trace-element spectra of detrital magnetite from the sandstone beds were compared to magnetite spectra from conglomerate clasts of banded iron formation and felsic to intermediate plutonic rocks. Only magnetites from felsic plutonic rocks have a signature similar to those from the sandstone. To further substantiate a felsic plutonic source, a detailed study of magnetite chemistry from ten felsic and intermediate regional plutons from a regional databank allowed us to recognize that each of these plutons has a particular signature on the basis of wide variations in Ni and Cr. From this data base, four individual plutons were identified as potential sources for the Magin Formation. Recently published magnetite discrimination diagrams proposed for mineral deposits can be applied to sedimentary magnetite provenance. However, our database shows the importance of identifying low-Ti magnetite from felsic rocks since these could be misidentified as low-Ti magnetite from hydrothermal mineral deposits. Thus, we propose improvement to these diagrams by differentiating magnetite from felsic magmatic (low Ni/Cr) and hydrothermal (high Ni/Cr) sources before applying the mineral-deposit discrimination diagrams.