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Midnite

Washington, USA

Main commodities: U
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The Midnite hardrock uranium mine is located approximately 60 km NW. of Spokane, in Washington state, USA, near the confluence of the Columbia and Spokane rivers on the Spokane Indian Reservation. Historically this has been the second largest hardrock uranium mine in the US.

The oldest country rocks within the area are the 900 to 6100 m thick Mesoproterozoic Togo Formation which locally comprises around 1000 m of meta-carbonates overlain by metapelites and intruded by a series of Proterozoic amphibolite sills. The Togo Formation is part of the Deer Group and may correlate with the upper part of the Belt Supergroup Wallace Formation to the east. The mine area lies on the overturned west limb of an antiform, with tight local drag folds.

The Deer Group was folded and metamorphosed to greenschist facies during the Jurassic to early Cretaceous and intruded by a Cretaceous hornblende granodiorite mass some 6 km to the east of the Midnite mine. Within the mine area, the sequence is intruded by the post-deformation, slightly younger batholith of tan to pink, medium to coarsely crystalline porphyritic quartz monzonite of the Loon Lake Granite, characterised by distinctive 2 to 5 cm K feldspar phenocrysts. The texture of quartz-monzonite varies widely within 50 m of the contact from the normal porphyry to coarse, fine and equigranular varieties, including pegmatitic and aplitic phases with both sharp and gradational margins.

Following erosion, all of these rocks are overlain by andesites and dacites of the 51 Ma Eocene Sanpoil Volcanics and intruded by dykes of very fine grained hornblende dacite.

Within the vicinity of the porphyry intrusive in the mine area, the pelitic lithologies of the Togo Formation have been metamorphosed to quartz-graphite white mica phyllites, schists and hornfelses, commonly containing coarse pyrite, pseudomorphed after tabular pyrrhotite. Within ~30 m of the contact, andalusite and biotite are developed within the hornfels. Calcareous rocks within the contact aureole of the quartz-monzonite porphyry are commonly fine-grained siliceous quartz-calcite-diopside and epidote hornfels and lensoid medium to coarse grained garnet-tremolite-wollastonite-vesuvianite and phlogopite skarn.

The uranium ore of the Midnite mine is essentially confined to meta-sedimentary rocks along the western margin of a 1.5 km wide roof-pendant in the roof of the quartz-monzonite. Within this pendant, the host sediments are around 200 m thick over gently dipping intrusive contacts, although in detail, the contact is complex, possibly reflecting pre-intrusion faulting of the meta-sediments.

Both reduced and oxidised uranium mineralisation is contained within shear zones, in fracture zones hosted by mica-hornfels, as disseminations following the foliations of muscovite schists and along montmorillonite-rich layers in calc-silicate rocks. The quartz-monzonite intrusive however, is essentially barren.

The ore zone occurs as grossly tabular 600 x 250 x 30 m volume. In detail, the ore zone is thicker where it occupies depressions or troughs in the intrusive contact, but has an overall flat upper contact, truncated by spikes and ridges in the roof of the quartz-monzonite intrusive. The distribution and shape of the body is also influenced by 'hydrologic barriers' within the host.

Secondary, oxidised U minerals, principally autunite and meta-autunite, are found above a fluctuating water table. Below the water table the mineralised zone occurs as a zone of partially oxidised sooty and compact uraninite and coffinite with associated sulphide (pyrite, marcasite, hisingerite, sphalerite, chalcopyrite and bornite mineralisation. The uraninite and coffinite occurs as: i). intact colloform masses; ii). mineralogically zoned fracture fillings; iii). complex intergrowths; and iv). micro-breccias with uraninite clasts in a coffinite matrix. Botryoidal masses of marcasite and shrinkage cracks in the uraninite imply colloidal precipitation in the formation of the primary U ore. The mineralisation has associated zones of argillic alteration that contain kaolinite, illite, montmorillonite and interspersed adularia.

The most recent source geological information used to prepare this summary was dated: 1991.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Ludwig K R, Nash J T and Naeser C W,  1981 - U-Pb isotope systematics and age of uranium mineralization, Midnite Mine, Washington: in    Econ. Geol.   v76 pp 89-110


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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