Grants Uranium Region - Gallup, Churchrock, Smith Lake, Ambrosia Lake, Laguna, Paguate, Jackpile

New Mexico, USA

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The Grants Uranium Region of central-western New Mexico, USA, comprises an arcuate, WNW to NW trending, approximately 175 km long belt of more than 200 sandstone hosted uranium and thorium mines and occurrences.  Mining areas within this region, eastward from the Arizona border, are Gallup, Churchrock, Smith Lake, Ambrosia Lake, Grants and Paguate-Jackpile, grouped into three mining districts: Gallup, Grants and Laguna.

This belt of deposits is located within the south-eastern segment of the Colorado Plateau, a generally oval shaped area of some 600 km east-west by 750 km north-south of more stable, thicker cratonic crust, bounded by Late Cretaceous to Tertiary foreland thrust belts, and the younger Rio Grande rift zone to the south-east in New Mexico.

The plateau is largely covered by sequences deposited prior to the Cretaceous to Tertiary Laramide Orogeny. These comprise Palaeozoic and Mesozoic platform sediments deposited within basins on the western margin of the Interior Platform, and intra-cratonic deposits within foreland basins. The Grants Uranium Region is located near the southern margin of the San Juan Basin in the south-eastern Colorado Plateau.

The Mesozoic sequence on the Colorado Plateau was deposited in a series of interconnected basins on the margin of the Interior Platform, including the interconnected San Juan, Henry Mountains, Kaiparowits and Black Mesa basins. The sequence commences with a lower, widespread early to middle Triassic reddish-brown mudstone-siltstone that thickens westwards from the eastern borders of the two states. To the east it is fluvial, but becomes inter-tidal over much of the Colorado Plateau, becoming more marine with grey calcareous siltstone, fine sandstone, limestone, gypsiferous shale and gypsum members in the west where it reaches 300 to 700 m in thickness in eastern and south-eastern Nevada. It is unconformably overlain by regional upper Triassic fluvial channel fill sandstones and conglomerates at the base of the Chinle Formation. The upper sections of the Chinle Formation are made up of several other members with different colour, grain size and composition which overlie these deposits, ranging from grey to green to purple to red mudstones, siltstones and sandstones, with bentonitic clays and volcanic ash beds. The Chinle Formation ranges from 150 to 300 m in thickness, averaging 250 m (Baars, 1988). These are unconformably overlain by lower Jurassic massive brown aeolian sandstones and lesser lacustrine sediments of the Glen Canyon Group.

Subsequent deposition was largely within the series of foreland basins being formed to the east of the emerging contractional Cordilleran Orogen, commencing from the middle Jurassic with 30 to 350 m of mudstone, sandstone, limestone and gypsum of the Lower San Rafael Group, thickest adjacent to the north-south 'Utah-Idaho Trough' to the immediate west of the Sevier-Cordilleran Foreland Thrust Front. Marine limestone and mudstone occur to the west, flanked farther east and south-east by sabkha mudstones and gypsum lenses, with red aeolian sandstones further to the east in the lower sections. This was overlain by the Middle San Rafael Group, a 200 to 250 m sandstone member, comprising flat bedded sabkha facies siltstone and sandstone to the north-west and south to south-east, while aeolian cross-bedded sandstones predominate further east. Above this, following a brief erosional break was the Upper San Rafael Group, composed of 30 to 150 m of sediments, commencing with marine glauconitic greenish-grey sandstone and scarce limestone, followed by red mudstones and siltstone with minor gypsum lenses, grading eastward into grey sandstones. These are in turn unconformably overlain by the late Jurassic Morrison Formation of up to 150 m or more of fluvial sandstone or conglomerate and thin mudstone beds of the basal Tidwell and overlying Salt Wash Members, followed by smectitic mudstone, thin bentonitic clays and some sandstone of the upper Morrison Brushy Creek-Fiftymile-Recapture Members that are equivalents from different sub-basins. Unconformably overlying Lower Cretaceous sediments comprised fluvial sandstone and over-bank mudstones (Baars, et al., 1988).

Most uranium production has come from the Westwater Canyon Member of the Morrison Formation, (equivalent to the upper Recapture Members) while significant amounts are also hosted by the Jackpile sandstone at the top of the Brushy Basin Member of the same formation. The Westwater Canyon Member and Jackpile sandstone are units of fine to coarse grained, aeolian and fluvial sandstone interstratified with thin layers of overbank, mudflat and lacustrine mudstone. These sandstones were deposited on an alluvial plain by braided-streams along the southern margin of the San Juan Basin. The Westwater Canyon Member is predominantly reddish, although near uranium deposits it is pale grey, while the overlying Brushy Basin mudstones are mainly light greenish to grey-green. The Jackpile sandstone is light grey. The pale grey, ore-bearing sections of these sandstones contain abundant fossilised plant remains, some of which are carbonised or silicified (Shawe et al., 1991).

Other uranium mineralisation is hosted by the Jurassic Todilto Limestone Member of the Wanakah Formation, a limestone-gypsum unit that overlies the Entrada Sandstone, 50 to 180 m below the Morrison Formation. Minor production has come from the Entrada Sandstone and the unconformably overlying Upper Cretaceous Dakota Sandstone.

The main ore-bearing units were deposited contemporaneously with the formation of a series of gentle east-trending folds on the southern margin of the San Juan Basin. Deposition of the Dakota Sandstone in the Upper Cretaceous followed uplift and erosion of the previous sediments. Further faulting and folding was followed by the late Tertiary Mt Taylor volcanic pile (Shawe et al., 1991).

Three types of sandstone uranium deposits are recognized in the Grants Uranium Region, namely: i). primary tabular, blanket-like or "trend", ii). redistributed roll-front, post-fault, secondary, fault-related, stacked, and iii). remnant deposits.

The primary or "trend" deposits of the Westwater Canyon Member and Jackpile sandstone are generally flat lying and manto-like, elongated parallel to the sediment deposition trends, occurring singly or in clusters, and occasionally as stacked lenses. The individual bodies may be up to 1 km long, more than 100 m wide, but generally no more than 5 m thick, and commonly grade into roll-front concentrations. The tabular bodies display splits, due to the presence of less permeable lenses and interbeds within the host sandstone (Shawe et al., 1991).

Ore is generally a uniformly dark grey colour (due to the presence of uraniferous carbonaceous material) within feature-less sandstone, streaked and mottled where lithological permeability variations have influenced the flow of mineralising waters. Uranium grades show similar variations. The primary uranium minerals are coffinite and uraninite, with associated pyrite, marcasite, calcite, jordisite, ilsemannite abd ferrosilicate. Mo and Se zoning is evident within the tabular deposits of the Grants Uranium Region, being concentrated at or near the upper and lower margins of the ore lenses (Shawe et al., 1991).

Primary deposits within the Todilto Limestone Member were controlled by gentle to tight and irregular folds and associated local brecciation, producing bodies up to 450 m long parallel to the fold axes containing small bodies of up to 20 t U3O8 at grades of around 0.2% U3O8. Un-oxidised deposits in this unit carry uraninite, pyrite, fluorite and barite (Shawe et al., 1991).

The redistributed roll-front and fault related accumulations were formed after the deposition of the Dakota Sandstone, when faulting allowed the ingress of meteoric waters (Shawe et al., 1991).

The remnant deposits are found on the south-western edge of the Grants Uranium Region, having been left as un-oxidised remnants of primary "trend" style mineralisation after the encroachment of young oxidising waters (Shawe et al., 1991).

The unuaul Woodrow Mine in the Laguna District exploits a vertical, 10 m diameter breccia collapse pipe (15 m stratigraphic collapse) within the Jackpile sandstone which was mined to a depth of 60 m and produced ore with a grade of 1.26% U
3O8, 0.05% V2O5 (Shawe et al., 1991).

Mineralisation in the Smith Lake District, for example, is largely concentrated in the Mariano Lake and Ruby 1 deposits which are hosted by the 25 to 55 m thick Brushy Basin Member fluvial sandstones of the Upper Jurassic Morrison Formation.   The orebodies, which are offset by Laramide Faults, are enriched in amorphous organic material introduced to the sandstone subsequent to deposition.   Mineralisation has been dated at 6 Ma.   The Mariano Lake and Ruby 1 deposits occur as flat lying, tabular, en echelon layers of uranium mineralised rock commonly elongated in a NW-SE direction sub-parallel to the depositional trend of the host.   Locally ore transgresses bedding and is crudely crescent-shaped in cross section.   Ore layers commonly occur adjacent to intercalated mudstone lenses, or clay clast conglomerates.   The ore lenses which are restricted to reduced sandstones are often found suspended within barren oxidised sandstone.   Individual lenses may be 500 m to 1500 m long and 50 to 100 m wide and be from 0.5 to 3 m thick.  Ore contains >0.05% U
3O8 (Fishman, et al., 1985).

More than 155 000 t of U
3O8 were produced between 1948-2002 (including 85 180 t U3O8, 3560 t V2O5from 1948 to 1970) from the Grants Uranium Region deposits, accounting for 97% of the total production in New Mexico and more than 30% of the total production in the United States. Grades averaged 0.22% U3O8, 0.14% V2O5 (McLemore, 2007, Shawe, et al., 1991).

Approximately 100 000 t U
3O8 remain as resources in the San Juan Basin at a USD 30.00 price (USGS).

The most recent source geological information used to prepare this summary was dated: 2005.    
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:
Fishman N S, Reynolds R L, Robertson J F  1985 - Uranium mineralization in the Smith Lake district of the Grants Uranium region, New Mexico: in    Econ. Geol.   v80 pp 1348-1364
Sanford R F  1994 - A quantitative model of ground-water flow during formation of tabular sandstone Uranium deposits: in    Econ. Geol.   v 89 pp 341-360

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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