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Strong remanent magnetization in pyrrhotite: A structurally controlled example from the Paleoproterozoic Tanami orogenic gold province, northern Australia


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Direen, NG and Pfeiffer, KM and Schmidt, PW (2008) Strong remanent magnetization in pyrrhotite: A structurally controlled example from the Paleoproterozoic Tanami orogenic gold province, northern Australia. Precambrian Research, 165 (1-2). pp. 96-106. ISSN 0301-9268

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Rocks associated with an orogenic gold system in the Paleoproterozoic Granites/Tanami Inlier of northern
Australia exhibit strong natural remanent magnetism, identifiable in regional aeromagnetic data.
Petrographical analysis indicates the two dominant magnetic minerals in the rocks of this region are
multidomain magnetite and monoclinic pyrrhotite. Chemical analysis using an electron microprobe has
determined that the magnetite and pyrrhotite are stoichiometrically pure, without significant elemental
substitution. Alternating field demagnetization, Curie Temperature, and hysteresis analysis indicates that
the remanently magnetized phase is monoclinic pyrrhotite with low magnetic susceptibility. In contrast,
multidomain magnetites lack remanence and have high magnetic susceptibilities.
Pyrrhotite is preserved in the rock mass along with other sulfide minerals as millimeter-scale veins
parallel to regionally developed, penetrative mylonitic shear bands (c-planes) formed during sulfide
remobilization in thewaning, cooling stages of hydrothermal Au deposition. In contrast, magnetite grains
exhibit porphyroblastic textures, consistent with formation during prograde to peakmetamorphism, indicating
formation prior to pyrrhotite, earlier in the orogenic cycle. Because of the lowCurie Temperature (ca.
325 ◦C) of pyrrhotite, it is therefore likely that remanent magnetization effects observed in aeromagnetic
images of the Tanami Inlier, are associated with structures intimately related to the late stages of Au mineralization
during retrograde greenschistmetamorphism after the peak thermal point of the orogenic cycle.

Item Type: Article
Keywords: Gold Magnetite Magnetics Paleoproterozoic Pyrrhotite Tanami
Journal or Publication Title: Precambrian Research
Page Range: pp. 96-106
ISSN: 0301-9268
Identification Number - DOI: 10.1016/j.precamres.2008.05.007
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Date Deposited: 02 Jun 2009 05:21
Last Modified: 18 Nov 2014 04:00
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