Publication Type:Book Chapter
Source:Targeted Geoscience Initiative 4; Canadian nickel-copper-platinum group elements-chromium ore systems; fertility, pathfinders, new and revised models, Geological Survey of Canada, Calgary, AB, Canada, p.87-102 (2015)
Keywords:Archean, assimilation, Canada, Canadian Shield, chemostratigraphy, chromite ores, copper ores, eastern canada, genesis, geochemistry, igneous rocks, intrusions, KOMATIITE, lithogeochemistry, magmas, Magnetite, metal ores, metals, mineral deposits, genesis, Mineral exploration, mineralization, Neoarchean, nickel ores, North America, Ontario, oxides, platinum group, Precambrian, stratiform deposits, superior province, Trace elements, volcanic rocks, whole rock
The Black Thor intrusive complex (BTIC) contains a conduit-hosted, stratiform Cr-Ni-Cu-(PGE) deposit with a very large amount of chromite for an intrusion of its size. Most conduit-hosted stratiform deposits are Archean, formed from komatiitic magmas containing approximately 3000 ppm Cr (sub 2) O (sub 3) , and are typically saturated in chromite. The fundamental problem in understanding the genesis of the BTIC deposit and other deposits of this type is explaining how such large quantities of chromite crystallized from a magma that normally crystallizes only small amounts chromite and normally have a chromite:olivine abundance ratio of 1:60. Current genetic models, such as in situ crystallization (by oxidation, pressure increase, magma mixing, and/or wholesale assimilation of felsic rocks or iron formation) or physical transportation of chromite slurries do not provide a wholly satisfactory explanation for the high abundance of chromite in this type of deposit. We are testing an alternative model: partial assimilation (as opposed to wholesale assimilation) of local oxide-silicate-facies iron formation by a Cr-rich magma. As low-Mg komatiite is saturated in chromite, the magma may dissolve the silicate component (quartz/chert and Fe-silicate minerals) of the iron formation, but would be unable to dissolve the oxide component (magnetite). Through interaction with the high-temperature (1400 degrees C) Cr-rich magma, the fine-grained magnetite could be upgraded via diffusion to chromite during transportation within the conduit. This upgrading is similar to the upgrading of barren sulphide xenomelts that has been proposed for Ni-Cu-(PGE) deposits.
GeoRef, Copyright 2018, American Geological Institute.<br/>2016-108629<br/>Black Thor Complex<br/>McFaulds Lake greenstone belt<br/>partial assimilation