The pilot knob magnetite deposit in the Proterozoic St. Francois Mountains Terrane,southeast Missouri,USA: A magmatic and hydrothermal replacement iron deposit |
| |
| Institution: | 1. Earth Science, University of Central Missouri, Warrensburg, MO 64093 USA;2. ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart 7001 Australia;1. University of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, T6G 2E3, Canada;2. Helmholtz Centre Potsdam—GFZ German Research Centre for Geosciences, Telegrafenberg B121, Potsdam 14473, Germany;3. Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany;4. School of Environment and Technology, University of Brighton, Cockcroft Building, Lewes Road, Brighton BN2 4GJ, UK;5. Department of Geological Sciences, University of Texas, Austin, TX 78712, USA;6. Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E2, Canada;1. State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. Institut für Mineralogie, Leibniz Universität Hannover, 30167 Hannover, Germany;3. Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Section 4.2, Telegrafenberg, Potsdam, D–14473, Germany;4. Perm State University, Geological Department, Bukireva 15, 614990 Perm, Russia;5. Geological Survey of Norway, Leiv Eirikssons vei 39, N-7040 Trondheim, Norway |
| |
| Abstract: | The Pilot Knob magnetite deposit is located in southeast Missouri within the 1380–1480 Ma St. Francois Mountains terrane rhyolitic/trachytic volcanic rocks. The deposit is tabular, dips about 45°, and is sill-like in nature, being approximately parallel to the bedding in the host tuffs. The deposit was uncovered by erosion and exposed to weathering in the late Proterozoic, and is overlain in angular unconformity by the Cambrian Lamotte Sandstone. This Proterozoic weathering cycle apparently had little effect on the deposit with only the updip edge being converted to hematite. Textural and mineralogical features of the deposit suggest a combined magmatic and hydrothermal replacement origin. The magnetite-rich ores that make up the bulk of the deposit are interpreted as having crystallized from an iron-rich magma, and a surrounding envelope of lower- to moderate-grade ores where magnetite has clearly replaced the tuffaceous host rocks are interpreted as hydrothermal in origin. After the development of the higher-grade magnetite ores and the envelope of lower- to moderate-grade ores, late hydrothermal minerals were deposited as cross-cutting veins and breccia fill. The two most abundant minerals in the higher-grade portions of the deposit are magnetite and albitic plagioclase, and petrologically the higher-grade ores could be described as a magnetite sodic syenite. The most abundant gangue mineral within the lower-grade impregnated envelope of ores around the higher-grade ores is K-feldspar, apparently relict from the rhyolites/trachytes. Thin lenses within the higher-grade ores contain calcite as a matrix mineral to the magnetite and are considered to indicate carbonatitic affinity. Rare earth elements are elevated in one of five whole rock chemical analyses of the ores and the REE-bearing mineral ferriallanite has been identified. Minor portions of the deposit, below the Proterozoic weathering cap, consist of high-grade hematite ores having equilibrium textures. At depth, the deposit is intruded by the Shepherd Mountain gabbro, a 120 m thick, near-horizontal dike, which resulted in minor contact metamorphism of the ore. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
