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Elk Creek |
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Nebraska, USA |
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Nb REE Ti
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Super Porphyry Cu and Au
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The Elk Creek niobium-titanium-rare earth element bearing carbonatite deposit is located ~8 km south of Tecumseh, Johnson County, southeastern Nebraska, USA (#Location: 40° 16' 0"N, 96° 11' 10"W).
The Elk Creek carbonatite is a 6 to 8 km diameter, ~500 Ma alkaline complex that intrudes Palaeoproterozoic gneissic country rocks and is buried beneath ~200 m of Upper Carboniferous (Pennsylvanian) marine clastic sedimentary cover. It was emplaced in the southwestern section of the NE-SW trending, 1.1 Ga, Midcontinent Rift System, near the intersection with the northern limit of the near parallel NNE-SSW aligned, ~300 Ma, fault controlled Nemaha Uplift. In aeromagnetic data, this intersection is manifested by a sinistral offset of the distinct regional magnetic response of the Mid-continental Rift Zone.
The Elk Creek carbonatite was first indicated in a 1963-64 regional aeromagnetic survey flown by the USGS over SE Nebraska on 3.2 km spaced east-west lines and 300 m altitude. In 1970, a reconnaissance gravity geophysical survey of SE Nebraska by the Conservation and Survey Division of the University of Nebraska-Lincoln outlined a positive gravity response coincident with the magnetic anomaly detected by the USGS. The geophysical gravity and a concurrent magnetic survey outlined a near-circular anomaly that was ~7 km in diameter. Analysis of the geophysical data modelled a cylindrical mass of indefinite vertical length, and a radius of 1675 m. In 1971, the Nebraska Geological Survey, with some support from the United States Bureau of Mines, drilled a hole to test the anomalies. This hole passed through 191 m of marine sedimentary rocks, into a carbonatite that persisted to the end of the hole at 290 m (Brookins et al., 1975). From ~1973, the Molybdenum Company of America (Molycorp) and briefly in 1974, Cominco American, investigated different parts of the anomalous area, although the former continued investigations until ~1986. In May 2010, Quantum Rare Earth Developments Corp. acquired the mineral rights to the Project and undertook further drilling from 2011. In March 3, 2013, Quantum changed its name to NioCorp Developments Ltd. and continued delineation and development drilling from 2014. The proposed Elk Creek Project is expected to produce Ferroniobium (FeNb), Scandium Trioxide (Sc2O3) and Titanium Dioxide (TiO2) over the 36 year operating life of an underground mine.
The Precambrian basement in this part of Nebraska is predominantly composed of granite, diorite, basalt, anorthosite, gneiss, schist and clastic sedimentary rocks, representing a series of Palaeoproterozoic island arcs sutured onto the Archaean craton. The structural trend of these suture zones influenced the development of both the Mid-continental Rift and Nemaha Uplift. The Midcontinent Rift System (or Keweenawan Rift) is characterised by mafic igneous intrusive and extrusive rocks with associated red and reduced sedimentary suites that form a >2000 km long x ~55 km wide belt. That belt is exposed in the Lake Superior Region, but extends southwards under cover through the states of Michigan, Wisconsin, Minnesota, Iowa, Nebraska and into Kansas (Carlson, 1992). The Nemaha Uplift is a narrow belt that extends southward from southeastern Nebraska and across Kansas, sub-parallel to the eastern margin of the midcontinent rift system (King, 1969). Complex fault zones and steeply dipping units are found along the northern and eastern margins of the uplift. These regional NNE to NE striking faults are locally cut by NW trending structures, including the Central Plains mega shear (or Central Missouri Fault) to the north, and the Oklahoma mega shear to the south (McBee, 2003). The Elk Creek Carbonatite body intruded near to the axis of the Nemaha uplift and is of a similar age to a cluster of carbonatites north of Lake Superior that are dated in the range of 560 to 580 Ma (Woolley, 1989; Erdosh, 1979). The carbonatite is located south of the boundary between the 1.84 Ga Penokean Orogen and the 1.78 Ga Dawes Terrane of the Central Plains Orogen (Carlson and Treves, 2005). Regional geophysical data and drilling have also outlined kimberlitic intrusives in northern Kansas to the SW of the Elk Creek Carbonatite. These kimberlites are younger, having been emplaced along the same rift system during the Cretaceous (Berendsen and Weis, 2001). The Paleozoic cover sequence is dominated by ~200 m of predominantly flat-lying Upper Carboniferous marine sequences composed of carbonates, sandstones and shales. Eastern Nebraska was glaciated several times throughout the early Pleistocene (Wayne, 1981), resulting in the deposition of ~50 m of unconsolidated till.
The Elk Creek Carbonatite comprises an elliptical magmatic body that has a NW elongated axis, perpendicular to the strike of the Midcontinent Rift System, and is located near the northern extremity of the Nemaha uplift (Burchett, 1982; Carlson, 1992). It intrudes gneissic Palaeoproterozoic country rocks of variable composition, and is predominantly composed of dolomite, calcite and ankerite, with lesser chlorite, barite, phlogopite, pyrochlore, serpentine, fluorite, sulphides and quartz (Xu, 1996). Biotite from within the carbonatite returned a K-Ar age of 544 ±7 Ma in the Cambrian (USGS Isotope Laboratory; Paterman, 1985). Additional K-Ar biotite dates of 464 ±5 and 484 ±5 Ma, respectively are both Ordovician (Ghazi, Georgia State University), whilst U-Pb zircon dates from the carbonatite range from 540 ±14 to 480 ±20 Ma (Farmer et al., 2013), all of which are younger than the Midcontinent Rift. It is therefore inferred that the carbonatite was emplaced at ~500 Ma (Xu, 1996), controlled by structures that influenced development of the subsequent Nemaha Uplift, but predated deposition of the ~299 to 318 Ma Upper Carboniferous sedimentary cover sequence. The contact between the carbonatite body and the Carboniferous sedimentary rocks, as seen in drill core, is sheared but is also oxidised, interpreted to represent a structurally reactivated palaeosol. Both rock types appear to have been affected by at least one main brittle-ductile deformation event, resulting in the formation of fault structures. Microstructures, including sub-vertical and sub-horizontal tension veins, together with related sheared veins and fault planes displaying sub-vertical and sub-horizontal slickensides in drill core are indications of extensional and oblique to strike-slip faulting. At least five main sets of brittle faults variably cut through the overlying Carboniferous rocks and the carbonatite boundary which appears to be tectonic.
Apatite beforsite is the volumetrically dominant lithology within the carbonatite complex, followed by undifferentiated mafic rocks (including apparent dykes and sills), syenite, beforsite-breccia, barite-beforsite and a small body of magnetite-beforsite. Beforsite is a carbonatite lithology in which dolomite is a primary mineral. Niobium mineralisation, which typically carries ~1% Nb, is primarily associated with the mineral pyrochlore, which is, in turn, associated with the small-volume magnetite-beforsite mass. Rare earth mineralisation, which is typically 1% REE, is primarily associated with the barite-beforsite lithology. The carbonatite complex lithologies have been classified into two main units, Massive Carbonatite that includes Dolomite carbonatite; Apatite-bearing dolomite carbonatite and Pyrochlore-bearing carbonatite; Apatite-dolomite carbonatite; Hematite-dolomite carbonatite; and Magnetite-dolomite carbonatite; and Brecciated Carbonate divided into the following, based on the silicate content which includes Altered basalt; Altered lamprophyre; and Altered syenite. Since 2014, this has been simplified to the following: Dolomite carbonatite; Dolomite carbonatite Breccia; Hematite-dolomite carbonatite; Magnetite-dolomite carbonatite; Magnetite-dolomite carbonatite Breccia. The spatial and temporal relations between the various lithologies are generally complex and poorly appreciated, although on petrological grounds, a three-stage paragenetic sequence has been proposed as follows (Xu, 1996): i). A magmatic stage that includes emplacement of most of the carbonatite lithologies and syenite, as well as mafic rocks which cut at least some of the other lithologies; ii). a deformation and recrystallisation stage, characterised by breccias and recrystallisation of dolomite and apatite, and iii). an alteration stage or stages, principally characterised by widespread oxidation. Alteration is observed throughout the Elk Creek drill core, and is locally very intense.
From a geophysical study of magnetic and gravity data over the deposit, Drenth (2014) concluded the following: The vertical gravity gradient tensor component and gravity highs over the carbonatite are explained by a large density contrast with the gneissic country rocks. Aeromagnetic anomalies are primarily attributed to mafic rocks, with a smaller contribution from the magnetite beforsite, a dense and strongly magnetised lithology that hosts niobium mineralisation in this complex. Geophysical anomalies suggest a significant volume of dense and strongly magnetic rocks exist at depth below the deepest drillholes. These rocks are considered to likely represent more magnetite beforsite and thus niobium mineralisation. Alternatively, they could reflect another lithology that is dense and strongly magnetised. Aeromagnetic lineaments are considered to represent faults, and suspected fault trends which pass through the area where magnetite beforsite occurs, suggesting that faulting played a role in localising that particular rock type. Other high-resolution airborne gravity gradient anomalies with probable sources within the carbonatite are hypothesised to represent variations of alteration. Whilst the REE mineralisation is primarily associated with barite beforsite, this lithology’s physical properties are similar to most of the other rock types within the carbonatite, and cannot be readily isolated using geophysics (Drenth, 2014).
The deposit within the carbonatite contains niobium, titanium and scandium mineralisation as well as REE and barium. Three mineralised domains have been defined within the magnetic (hematite) dolomite carbonatite: i). High grade niobium, titanium and scandium mineralisation, with >1% Nb2O5 and width of >10 m, which occurs over a volume with a strike length of ~750 x 400 m width x 800 m down dip below the unconformity. The high grade Nb2O5/TiO2 domain is composed of sixteen individual zones, each of which has a slightly different orientation, but generally follows the overall 100 to 130° azimuth trend with a dip of 32 to 52°. ii). A high grade Sc domain which consists of seven individual zones, each with a slightly different orientation, but generally following a 105 to 125° azimuth trend and 41 to 47° dip. iii). Low grade domain, which comprises one ~830 x 500 x 850 m zone with an azimuth of 120° and dip of 74°, based on an ~0.3% Nb2O5 cutoff.
Niobium mineralisation is fine-grained, with ~77% of the Nb occurring in the mineral pyrochlore, while the balance is found in an Fe-Ti-Nb oxide mineral of varying composition. Within the mineralised carbonatite, the maximum Nb2O5 grade is 4.472%, and the mean is 0.518%. TiO2 is typically proportional to the niobium grades within a range of 3:1 to 4:1 within the core of the deposit. The scandium mineralisation does not directly correlate with niobium but does increase in grade with respect to niobium at low grades, whilst at higher grades of >75 ppm Sc, it is also associated with
higher concentrations of CaO, Mgo, Th, U, Pb and As.
REE mineralisation within the carbonatite occurs within the four minerals: Bastnäsite (Ce,La,Y)CO3F, Parisite Ca(Ce,La)2(CO3)3F2, Synchysite Ca(Ce,La)(CO3)2F and Monazite (Ce,La)PO4.
A drill log of a high grade REE intersection recorded by Molycorp (1986) reads as follows (after Nordmin Engineering Ltd Technical Report, 2019):
"Barite beforsite is the predominant lithology from 149.4 to 304.8 m. It contains xenoliths of syenite, older mafic rocks and apatite beforsite I, and is intruded by younger mafic rocks. Intervals of 33 m between 149.4 to 274.3 m contain 2.13% to 2.75% Lanthanide oxides. An 18.3 m interval between 179.8 and 198.1 m contains 3.89% Lanthanide oxides. The highest grade mineralisation intersected was 3 m @ 4.72% Lanthanide oxides at 155.4 to 158.5 m. Lanthanide minerals occur as radial patches and random aggregates of needles, irregular patches and vein-like aggregates. The aggregates occur with and without quartz. The aggregates appear as light-grey patches in reddish-brown, hematite-altered beforsite. Although individual lanthanide mineral grains are in the micrometre size range, aggregates of lanthanide minerals range from 0.23 to 8 mm in maximum dimension. Monazite and bastnäsite have been identified in the aggregates, and spectra show Ce > La."
Published Ore Reserves and Mineral Resources as at May 10, 2022 (NioCorp Superalloy Materials News Release, 22 May 2022) were:
Probable Ore Reserves 36.656 Mt @ 0.811% Nb2O5, 2.92% TiO2, 70.2 ppm Sc,
containing - 197 278 t of Nb2O5 for 170 409 t of recoverable Nb; 1 071 072 t of contained TiO2 and 431 793 t of payable TiO2; 2573 t of contained Sc and 3677 t of payable Sc2O3.
Indicated Mineral Resources at USD 180/t NSR Cut-off -
188.8 Mt @ 0.51% Nb2O5, 2.24% TiO2, 60.06 ppm Sc, 0.2774% LREO, 0.0579% HREO, and 0.3353% TREO;
Light Rare Earths: 0.0773% La2O3, 0.1335% Ce2O3, 0.0143% Pr2O3, 0.0524% Nd2O3,
Heavy Rare Earths: 0.0129% Sm2O3, 0.0046% Eu2O3, 0.0110% Gd2O3, 0.0012% Tb2O3, 0.0048% Dy2O3, 0.0007% Ho2O3, 0.0015% Er2O3, 0.0002% Tm2O3, 0.0010% Yb2O3, 0.0001% Lu2O3, 0.0199% Y2O3.
containing - 0.9703 Mt of Nb2O5, 4.221 Mt of TiO2, 11 337 t of Sc; 0.5236 Mt of LREO, 0.1093 Mt of HREO and 0.6329 Mt of TREO;
Light Rare Earths: 145 800 t of La2O3, 251 900 t of Ce2O3, 26 900 t of Pr2O3, 98 900 t of Nd2O3;
Heavy Rare Earths: : 24 300 t of Sm2O3, 8600 t of Eu2O3, 20 800 t of Gd2O3, 2300 t of Tb2O3, 9100 t of Dy2O3, 1300 t of Ho2O3, 2900 t of Er2O3, 300 t of Tm2O3, 1900 t of Yb2O3, 300 t of Lu2O3, 37 600 t of Y2O3.
Inferred Mineral Resources at USD 180/t NSR Cut-off -
180 Mt @ 0.39% Nb2O5, 1.92% TiO2, 52.28 ppm Sc, 0.3257% LREO, 0.0512% HREO, and 0.3769% TREO;
Light Rare Earths: 0.0943% La2O3, 0.1576% Ce2O3, 0.0163% Pr2O3, 0.0575% Nd2O3,
Heavy Rare Earths: 0.0116% Sm2O3, 0.0038% Eu2O3, 0.0090% Gd2O3, 0.0010% Tb2O3, 0.0042% Dy2O3, 0.0006% Ho2O3, 0.0014% Er2O3, 0.0002% Tm2O3, 0.0010% Yb2O3, 0.0001% Lu2O3, 0.0182% Y2O3.
containing - 0.4266 Mt of Nb2O5, 2.082 Mt of TiO2, 5660.2 t of Sc; 0.3526 Mt of LREO, 0.0555 Mt of HREO and 0.4082 Mt of TREO;
Light Rare Earths: 102 100 t of La2O3, 170 600 t of Ce2O3, 17 700 t of Pr2O3, 62 200 t of Nd2O3;
Heavy Rare Earths: : 12 600 t of Sm2O3, 4100 t of Eu2O3, 9 800 t of Gd2O3, 1100 t of Tb2O3, 4600 t of Dy2O3, 700 t of Ho2O3, 1500 t of Er2O3, 200 t of Tm2O3, 1100 t of Yb2O3, 100 t of Lu2O3, 19 700 t of Y2O3.
This summary is largely drawn from: Brown, A., Dougherty, C., Winters, D., Larochelle, E., Menard, G., Kuntz, G., St. Onge, J.-F., Tinucci, J., Sames, J., Willow, M.A., Romaniuk, O. and Harton, S., 2019 - Feasibility Study, Elk Creek Superalloy Materials Project, Nebraska; An NI 43-101 Technical Report prepared by Nordmin Engineering Ltd. for NioCorp Developments Ltd., 596p.
The most recent source geological information used to prepare this decription was dated: 2019.
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.
Elk Creek
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Selected References:
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Drenth, B.J., 2014 - Geophysical expression of a buried niobium and rare earth element deposit: The Elk Creek carbonatite, Nebraska, USA: in Interpretation, v.2, pp. SJ169-SJ179. dx.doi.org/10.1190/INT-2014-0002.1
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