A global magnetotelluric graphite type locality: Multi-decade, multi-scale studies of the Eyre Peninsula anomaly

Abstract

A major electrically conducting crustal structure has been imaged since the 1980 s using widely-spaced long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) sites across the Archean to Paleoproterozoic Eyre Peninsula, South Australia. The structure extends from the southern continental margin, trending SW-NE for at least 300 km. To constrain depth and lateral extent of the anomaly a transect of 39 broadband MT sites were collected with ∼ 2 km separation across lower Eyre Peninsula. Two-dimensional inversion images a low-resistivity conductor at depth 2–10 km, 20 km wide and dipping ∼ 10° to the east, spatially correlated with a topographic high. The conductor is bound on the eastern margin by the transpressional Kalinjala Shear Zone along the Paleoproterozoic Donington Suite, and on the western margin by the Archean Sleaford Complex. The low resistivity structure, with conductance of 20,000 S occurs in Paleoproterozoic carbonaceous marine sediments of the Darke Peak and Cleve groups that were thickened to amphibolite-grade metamorphic depths of at least 15 km and 550 ⁰C during the Kimban Orogeny (1730–1690 Ma), converting carbon to flake graphite in metapelites. Tight folding and exhumation of amphibolite-grade metapelites during the Kimban Orogeny, and dextral strain along the Kalinjala Shear Zone resulted in a laterally continuous electrical crustal conductor which outcrops as economic-grade graphite deposits adjacent to Kalinjala Shear Zone mylonites. In addition, a ∼ 40 km wide lower crustal conductor of integrated conductance 2000 S is imaged west from the Kalinjala Shear Zone. This lower crustal conductor may also be graphitic and represents deeply buried metapelites that have not been exhumed. We argue that the Paleoproterozoic Eyre Peninsula Anomaly results from graphitisation of sediments with significantly enhanced amount of organic carbon compared to any other time in Earth’s history, during the ∼ 2 Ga Lomagundi–Jatuli Event, which is associated with the great oxidation event.