THE NATURE OF GEOMAGNETIC ANOMALIES IN METAMORPHOSED CHROMITE-BEARING DUNITES: A CASE STUDY OF THE SOUTHERN KLYUCHEVSKOY COMPLEX, (Central Urals)
T.A. Sherendo1, A.G. Vdovin1, P.S. Martyshko1, V.Ya. Mitrofanov2, A.V. Alekseev3, D.A. Zamyatin3, V.A. Vazhenin4, L.A. Pamyatnykh4
1Institute of Geophysics, Ural Branch of the Russian Academy of Sciences, ul. Amundsena 100, Yekaterinburg, 620016, Russia
2Institute of Metallurgy, Ural Branch of the Russian Academy of Sciences, ul. Amundsena 101, Yekaterinburg, 620016, Russia
3A.N. Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, per. Pochtovyi 7, Yekaterinburg, 620075, Russia
4Ural Federal University, Institute of Natural Sciences, ul. Lenina 51,Yekaterinburg, 620083, Russia
Keywords: Fe-Cr-spinel, magnetization carrier, magnetic cluster, microscale phase, chromite mineralization, geomagnetic anomaly, dunite-harzburgite metamorphic complex
Abstract
Chromite mineralization in metamorphosed dunites from the southern Klyuchevskoy dunite-harzburgite ultramafic complex (Central Urals) has been investigated using geomagnetic surveys along with laboratory studies of ore-forming and accessory spinels of the same genetic type. Magnetization in the study area is carried mainly by accessory Fe-Cr-spinel of a variable Fe2+(Cr2- x Fe x 3+)O4 composition. Metamorphism caused changes in element contents and in both crystal and magnetic structure of the primary nonmagnetic accessory spinel, unlike the almost fresh ore-forming spinel. Thus, orebodies stand against their host rocks, which is a prerequisite for the use of geomagnetic surveys for exploration of podiform chromite deposits in dunite-harzburgite complexes. Ground magnetic surveys at a test site composed of faulted rocks bearing disseminated chromite mineralization in metamorphosed dunites resolved a chromite ore zone and a fault block boundary showing up as geomagnetic anomalies. Laboratory studies using high technologies (thermomagnetic analysis at 4 to 1000 K, as well as magnetic resonance and magnetic force spectroscopy) revealed, for the first time, magnetic clusters (superparamagnetic phases) in primary nonmagnetic accessory spinel, which are responsible for the magnetic properties of the host rocks. Microscale variations in Cr-spinel correlate with the geomagnetic anomalies recorded by field surveys at the test site.
DOI: http://dx.doi.org/10.1016/j.rgg.2015.02.008
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