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

Joice Dias de Moraes; Pedro Cordeiro; Eduardo Abrah&#227;o Filho; Juliana Rezende Oliveira; Carlos Victor Rios da Silva Filho

doi:10.1016/j.gsf.2020.11.018

Volume 13 Issue 5

Sep. 2022

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Joice Dias de Moraes, Pedro Cordeiro, Eduardo Abrahão Filho, Juliana Rezende Oliveira, Carlos Victor Rios da Silva Filho. Metamorphic disturbances of magnetite chemistry and the Sm-Nd isotopic system of reworked Archean iron formations from NE Brazil[J]. Geoscience Frontiers, 2022, 13(5): 101131. doi: 10.1016/j.gsf.2020.11.018

Citation:

Joice Dias de Moraes, Pedro Cordeiro, Eduardo Abrahão Filho, Juliana Rezende Oliveira, Carlos Victor Rios da Silva Filho. Metamorphic disturbances of magnetite chemistry and the Sm-Nd isotopic system of reworked Archean iron formations from NE Brazil[J]. Geoscience Frontiers, 2022, 13(5): 101131. doi: 10.1016/j.gsf.2020.11.018

Citation:

Joice Dias de Moraes, Pedro Cordeiro, Eduardo Abrahão Filho, Juliana Rezende Oliveira, Carlos Victor Rios da Silva Filho. Metamorphic disturbances of magnetite chemistry and the Sm-Nd isotopic system of reworked Archean iron formations from NE Brazil[J]. Geoscience Frontiers, 2022, 13(5): 101131. doi: 10.1016/j.gsf.2020.11.018

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

doi: 10.1016/j.gsf.2020.11.018

a. Instituto de Geociências, Universidade de Brasília (UnB), 70910-900 Brasília, DF, Brazil;
b. Department of Mining Engineering, Pontifical Catholic University of Chile, 7820436 Santiago, Chile

Funds:

This work was supported by the National Council for the Improvement of Higher Education (CAPES) and the Brazilian Council for Research and Technological Development (CNPQ), which granted an MSc scholarship to the first author. The University of Brasí

lia is gratefully acknowledged for fieldwork support and access to laboratory facilities. We are also grateful for the editorial handling of our manuscript by Nick Roberts and the contribution of three anonymous reviewers who made themselves available to help us improve our manuscript in the middle of a global pandemic.

Received Date: 2020-05-30
Accepted Date: 2020-11-26
Rev Recd Date: 2020-09-30
Publish Date: 2020-12-30

Abstract

Abstract

Iron formations are valuable archives of sedimentary conditions and post-depositional events. However, geochemical proxies commonly used to determine genetic characteristics can be variably modified during metamorphism and deformation, hampering their use as records of regional geological events. This work focuses on strongly reworked magnetite-quartz-rich rocks from the São José do Campestre Massif, one of the oldest fragments of preserved crust in South America. The genetic classification of these magnetite-quartz-rich rocks is not straightforward because primary assemblages and textures were variably modified by granulite facies metamorphism during a regional Paleoproterozoic migmatization event. To address genetic ambiguities, we analyzed their magnetite and pyroxene chemistry, whole-rock geochemistry, and Sm-Nd isotopes. Magnetite chemistry indicates that pyroxene-poor iron formations (Type B) are low in trace elements such as Ti, Al, V, and Mn, suggesting a chemical similarity to iron formations elsewhere. In contrast, magnetites from pyroxene-enriched Type A iron formations 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 Neoproterozoic sources. Therefore, our geochemical tools point out to heterogenous signatures of these magnetite-quartz rocks and proxies compatible with both low and high-temperature conditions and age of deposition spanning sources from the Archean to the Neoproterozoic. We interpret that the studied São José do Campestre magnetite-quartz rocks represent Archean iron formations with original magnetite chemistry and isotopic signatures variably modified by metamorphism and by at least one deformation-related hydrothermal event. These results contrast with similar examples from China and Greenland where iron formations either preserved the magnetite chemistry or the primary isotopic signatures. Our study indicates that metamorphism can selectively affect chemical proxies used to study iron formations and undermine the genetic classification of iron ores. Thus, these proxies should be carefully applied in the interpretation of syn-depositional environments of polydeformed belts.
- Iron formation,
- Borborema Province,
- Sm-Nd isotopes,
- Magnetite chemistry,
- Archean

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References(46)

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