Paper No. 2
Presentation Time: 8:15 AM
ORIGIN OF BIF-LIKE JASPILITE IN THE 2.72 GA HUNTER MINE GROUP, ABITIBI GREENSTONE BELT, QUEBEC BY METASOMATIC REPLACEMENT OF FELSIC VOLCANIC ROCKS
Precambrian chert and banded iron formation (BIF) primarily occur as marine chemical precipitates. However, chert is also known to form from hydrothermal alteration, in which case it replaces preexisting rocks, or is deposited as massive quartz. The 2.72 Ga Hunter Mine Group (HMG), Abitibi Greenstone Belt, Quebec, Canada, represents a collapsed subaqueous caldera accompanied by volcanogenic massive sulfide (VMS) deposits. We divided the lithologic components of the HMG into three groups that include a range of hydrothermally altered rhyolites, hyaloclastites, pyroclastics, and chert ± magnetite-rich rocks, whose origin may be sedimentary or result from the replacement of host volcanics. The least altered volcanic rocks (Group 1) have textures that include euhedral phenocrysts, hyaloclasts, and fiamme. Compositionally, they are characterized by flat REE slopes, PrSN/YbSN ≤ 1, Eu/Eu*SN < 1, La/La*SN < 1, and Y/Ho values 24-34. Group 2 volcanic rocks preserve phenocrysts and/or matrix material, but have been partly silicified, texturally evident by the development of chert lenses. Group 2 samples have a positive overall slope PrSN/YbSN < 1, Eu/Eu*SN > 1, La/La*SN > 1, and Y/Ho values 23.9-34, which is atypical of the host volcanic rocks, but similar to seawater. The most altered samples (Group 3) consist of chert ± iron-rich rocks, with remnant (locally very faint) clasts, fiamme, and microbrecciated textures. Chemically these rocks resemble precipitated chemical sediments with a positive slope, PrSN/YbSN < 1, Eu/Eu*SN > 1, La/La*SN > 1, and Y/Ho values from 25-55. Mineralogically and compositionally, Group 3 rocks resemble primary BIF. However, remnant volcanic textures can be traced through Groups 2 and 3 and show a progressive compositional replacement from rhyolite, to rhyolite with patchworks of chert, to chert ± iron. Pumice fragments, hyaloclasts and glass shards have been compositionally replaced to chert. Original glassy matrix has been replaced to chlorite, chert, iron-oxides, and iron-sulfides. Combined textural and chemical data suggests the chert ± iron-rich rocks of the HMG likely formed through replacement via metasomatic interactions with primary glassy, felsic igneous rocks, tuffs, and volcaniclastic material, and is consistent with alteration seen within associated Abitibi VMS deposits.