Douglas Hall

Re-equilibration Processes of Magnetite from Iron Skarn Deposits

including iron skarn, IOCG (iron oxide-copper-gold), Kiruna-type, BIF (banded iron formation), and magmatic Fe-Ti oxide deposits (Dupuis and Beaudoin, 2011). Magnetite typically hosts a large variety of trace elements that are largely dependent on its formation environments and thus can be used as an indicator for the genesis of this mineral and associated ore deposits (Dupuis and Beaudoin, 2011; Nadoll et al., 2014). However, recent studies have shown that magnetite could be re-equilibrated with subsequent hydrothermal fluids, forming secondary magnetite domains during the re-equilibration processes (Hu et al., 2014). Such processes would significantly modify the trace element composition of magnetite, making some of the existing trace-element discriminative diagrams (e.g., the Ti+V versus Ca+Al+Mn diagram; Dupuis and Beaudoin, 2011) problematic in deciphering the origin and formation environment of magnetite and associated ore deposits. In this study, new textual and compositional data of magnetite from nine iron skarn deposits ranging in age from Mesoproterozoic to Mesozoic are presented to further document the re-equilibration reactions (dissolution-reprecipitation processes – DRP). A total of 11 samples were collected from 9 iron skarn deposits. Four samples were collected from the Iron Crown (GR-94107-1 and GR-94-107-2), Merry Window (GR-94-115), and Paxton (GR-94-44) deposits from British Columbia, Canada. Another four samples were taken from the Grenville-age Forsyth (X-DL-1B and X-DL-1D) and Bristol (X-DL-2A) deposits from Quebec and the Marmora deposit (X-DL-3A) from Ontario. Sample OF177 was selected from the Terezia Mica deposit, Romania. The remaining two samples were collected from the Daye (12-79) and Chengchao (CC100) iron deposits, eastern China. Magnetite is the dominant mineral (>80 vol. %) in most of the samples, coexisting variably with minor amounts of diopside, garnet, epidote, phlogopite, and chlorite. The sample CC100, which comes from a prograde skarn vein hosted within the ore-related granitic intrusion in the Chengchao iron deposit, contains much less magnetite (5%) that coexists with abundant diopside and garnet. The occurrence, morphology and paragenesis of magnetite within the samples were initially characterized optically. The morphological and textural features of magnetite were then investigated using a JEOL 6400 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer. The electron probe microanalysis (EPMA) and elemental X-ray mapping were conducted using a JEOL JXA-733 Superprobe at the University of New Brunswick, Canada. Both the oxy-exsolution and dissolution – reprecipitation textures have been recognized in magnetite from the studied samples. Exsolution lamellae of Fe-AlTioxides, including ulvospinel, hercynite, and corundum, were documented in samples DL-1D, GR-94-107-1, 1279, CC100, and X-DL-3A. Within DL-1D, orientated exsolution lamellae of ulvospinel are widespread and closely related to corundum and hercynite. In contrast, magnetite from the rest iron deposits only contains ulvospinel as exsolution. In sample GR-94-107-1, magnetite grains exhibit distinct core-rim textures, with the core being rich in texturally equilibrated ulvospinel and the rim being free of ulvospinel. Similar textures have previously been observed in magnetite from magmatic FeTi iron deposits (Dupuis and Beaudoin, 2011). Magnetite from the Marmoration Fe skarn deposit (Sample X-DL3A) either exhibits a core-rim texture or is homogeneous with triple junction texture. Similarly, the core contains numerous ulvospinel inclusions, whereas the rims are homogeneous with no mineral inclusions. The SEM-BSE images reveal that magnetite from the HU Hao, David LENTZ, LI Jianwei and Douglas HALL, 2014. Re-equilibration Processes of Magnetite from Iron Skarn Deposits. Acta Geologica Sinica (English Edition), 88(supp. 2): 354-356.