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Low-Temperature Formation of Pyrrhotite from Magnetite Pyrite: Evidence from Low-Grade Metamorphism and Implications for Magnetite Preservation during Deep Burial

GILLETT, STEPHEN L., University of Nevada, Reno, NV

Early magnetite in sedimentary rocks must be preserved to retain a paleomagnetic signature; however, reactions such as Fe(3)O(4) + 3 FeS(2) + "CH(4)" (organic matter) = 6 "FeS" (pyrr.) + CO(2) + 2 H(2)O tend to the right for temperatures less than approximately 200 degrees C. Such low temperatures strongly imply that magnetite can be destroyed merely by deep burial.

Such pyrrhotite formation is illustrated by very low-grade metamorphism in a contact aureole surrounding the Jurassic Notch Peak stock in the central House Range of western Utah. The stock invades a Cambrian sedimentary sequence containing miogeoclinal limestones with intercalated siltstones. Limestones in the aureole yield a scattered, two-polarity remagnetization, residing in pyrrhotite, which extends into rocks that appear unmetamorphosed (temperatures approximately 250 degrees C). The pyrrhotite could not have been precipitated from externally derived fluids because oxygen isotopic data from the limestones show no influence of magmatic or phreatic water; hence, it must have formed in situ, as in the above equation.

Outside the aureole, a characteristic magnetization is preserved that apparently reflects late Paleozoic remagnetization and that probably resides in authigenic magnetite. Hence, it appears that this relatively late magnetite was in turn destroyed by modest reheating from the pluton.

In general, thermal remagnetization of sedimentary rocks is probably rare; chemical changes probably have a much more profound effect on the paleomagnetic signatures.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)