Stratiform Chromite Deposit Model: Chapter E in Mineral Deposit Models for Resource Assessment: Usgs Scientific Investigations Report 2010-5070-E

ISBN: 1288850565

Author: Schulte, Ruth F

Condition: New

A new descriptive stratiform chromite deposit model was prepared which will provide a framework for understanding the characteristics of stratiform chromite deposits worldwide. Previous stratiform chromite deposit models developed by the U.S. Geological Survey (USGS) have been referred to as Bushveld chromium, because the Bushveld Complex in South Africa is the only stratified, mafic-ultramafic intrusion presently mined for chromite and is the most intensely researched. As part of the on-going effort by the USGS Mineral Resources Program to update existing deposit models for the upcoming national mineral resource assessment, this revised stratiform chromite deposit model includes new data on the geological, mineralogical, geophysical, and geochemical attributes of stratiform chromite deposits worldwide. This model will be a valuable tool in future chromite resource and environmental assessments and supplement previously published models used for mineral resource evaluation. Stratiform chromite deposits are found throughout the world, but the chromitite seams of the Bushveld Complex, South Africa, are the largest and most intensely researched. The chromite ore is located primarily in massive chromitite seams and, less abundantly, in disseminated chromite-bearing layers, both of which occur in the ultramafic section of large, layered mafic-ultramafic stratiform complexes. These mafic-ultramafic intrusions mainly formed in stable cratonic settings or during rift-related events during the Archean or early Proterozoic, although exceptions exist. The chromitite seams are cyclic in nature as well as laterally contiguous throughout the entire intrusion. Gangue minerals include olivine, pyroxenes (orthopyroxene and clinopyroxene), plagioclase, sulfides (pyrite, chalcopyrite, pyrrhotite, pentlandite, bornite), platinum group metals (mainly laurite, cooperite, braggite), and alteration minerals. A few deposits also contain rutile and ilmenite. The alteration phases include ser