Summary:
The deposit model at the Eldor Property is the carbonatite-hosted REE-Nb-Ta deposit. Carbonatites are by definition igneous rocks, intrusive and extrusive, that contain more than 50% by volume carbonate minerals such as calcite, dolomite, ankerite, and less often siderite and magnesite. Intrusive carbonatites occur commonly within alkalic complexes or as isolated intrusions (sills, dikes, breccias, or small plugs) that may not be genetically related to other alkaline intrusions. Carbonatites can also be volcanic-related and occur as flow or pyroclastic rocks like the well-known active Oldoinyo Lengai volcano in Tanzania. Carbonatites are generally related to large-scale, intra-plate fractures, grabens, or rifts that correlate with periods of extension, typically Precambrian to recent in age.
Carbonatite-hosted deposits occur almost exclusively in intrusive carbonatite and may be subdivided into magmatic, replacement/veins, and residual sub-type. The Eldor Carbonatite can be classified as a magmatic sub-type, which is the same category as the St-Honore deposit in Quebec, Canada (Niobec niobium mine, Iamgold), the Mountain Pass Deposit in California, U.S.A. (REE), and the Palabora Deposit in South Africa (apatite).
The Rare earth element mineralization at Ashram is hosted primarily (essentially 100%) by monazite, rare earth fluorocarbonates (bastnäsite, parisite, and lesser synchysite) and lesser xenotime. Rare earth mineralization at Ashram is consistent throughout the deposit with minimal dilution from unmineralized fragments and clasts.
Monazite typically occurs as intergrowths with fluorite or disseminations in dolomite and less commonly as intergrowths with apatite or in association with quartz-rutile assemblages. Bastnäsite typically occurs as fibrous cavity fillings in dolomite, anhedral grains in veins associated with Y-NbTi minerals, or anhedral grains and aggregates in fluorite.
The principal heavy rare earth mineral is xenotime-(Dy) present as anhedral to subhedral crystals in association with nioboaeschynite, niobian rutile, ferrocolumbite, monazite, quartz, and mica. Xenotime is present in either pools of quartz, small veins (mm) in fluorite ferro-carbonatites, or scattered throughout the carbonatite as a trace mineral in association with disseminated monazite and/or bastnäsite.
At least two generations of monazite are present: one associated with fluorite and/or disseminated in dolomite, and a second associated with the xenotime mineral assemblage. The xenotime mineral assemblage represents a distinct (later) mineralization event from that which formed the first generation of monazite.
Grain size of the monazite is typically <10 µm to 25 µm, with bastnäsite slightly coarser at <20 µm to 50 µm. Aggregates of monazite±bastnäsite in the several-hundred micron range are present but not common. Xenotime crystals are relatively coarser grained than monazite, although still commonly less than 50 µm. Parasite-bastnäsite intergrowths, where present (BD-Zone), occur as relatively large crystals and masses ranging typically from 200 µm to 400 µm with some aggregates exceeding a millimetre. Although fine-grained, the rare earth mineralogy of the Ashram Deposit is considered simple because it contains three of the four rare earth minerals that dominate commercially production globally (monazite, bastnäsite, and xenotime). In addition, it has been demonstrated that all three rare earth bearing minerals (monazite, bastnaesite, and xenotime) liberate together and share conventional processing techniques.
The Ashram Deposit can be roughly divided into three main mineralized zones termed ‘A-Zone’, ‘B-Zone’, and ‘BD-Zone’. In general the A-Zone is central to the deposit and is rimmed by the B-Zone and BD-Zone respectively. This relationship is more prevalent along the western margin of the deposit, where the BD-Zone is in contact with an unmineralized albite amphibole phlogopitite unit, which interfingers with and transitions to a calcio-carbonatite unit. Along the eastern margin of the deposit, the B and BD zone relationship is more variable, with unmineralized calcio-carbonatite predominantly marking the contact, with the albite amphibole phlogopitite unit noticeably absent.
The A-Zone lithology is the largest unit by volume and surface footprint, and also the most highly mineralized of the Ashram Deposit (typically 1.5-3+% TREO). The unit is typically very finegrained, light to dark olive-grey, and composed of clasts of breunnerite (magnesian siderite) plus fluorite, or fluorite plus monazite, set in a complex matrix of several generations of ferrodolomite. Fluorite is typically abundant and pervasive in the zone, occurring as disseminations, blebs, patches, veins, and fracture fillings. Accessory and trace minerals include apatite, pyrite, sphalerite, magnetite, xenotime, quartz, and niobium phases (niobium rutile, nioboaeschynite, ferrocolumbite, and niobium ilmenite).
Extending from surface, within A-Zone material and central to the Ashram Deposit, is the MHREO Zone displaying an intense enrichment in the middle and heavy rare earth elements. The zone displays a marked increase in distribution for neodymium as well as the middle and heavy rare earths, including the more critical rare earths europium, terbium, dysprosium, and yttrium. MHREO as a percentage of TREO is almost double that of the overall deposit, while maintaining significant grade. The zone also contains one of the highest distributions of europium in the world.
The B-Zone unit typically comprises cataclastic ferrodolomites with coarser grain size and fewer fluorite-monazite clasts than the A-Zone. Visually, the B-Zone has a cream to grey colour with a pervasive, yet patchy, yellow-beige hue of mineralization comprising monazite, apatite, and carbonates. Patches of quartz-phlogopite are present with veins of niobian rutile, ferrocolumbite, and xenotime in association with quartz, phlogopite, and bafertisite also observed. Fluorite is present occasionally as locally abundant patches or blebs and may lend a bluish hue to the rock where present; sulphides are rare. Texturally, the B-Zone appears to be much less deformed and chaotic than the A-Zone with fewer variations in fabric present. The grade of rare earth mineralization in the B-Zone is somewhat variable compared to the A-Zone and typically ranges from 1-2+% TREO. Geochemically, the B-Zone may be classified as a magnesio-carbonatite.
The BD-Zone unit is typically cream to white in colour with orange-pink to red pervasive shades from rare earth fluorocarbonate mineralization (intergrowths of parisite-bastnäsite with lesser synchysite), and is coarser grained than the A-Zone. The BD-Zone is visually and mineralogically distinct from the A and B zones. The unit typically comprises vuggy crystalline dolomite with common to abundant rare earth fluorocarbonates (parisite intergrowths with bastnäsite) and trace to minor phlogopite, quartz, calcite, and microcline; fluorite and monazite are absent to rare. The BDzone may be highly blocky/fractured in some locales, and is more evident along the western margins of the deposit. Texturally, the BD-Zone is similar to the B-Zone and is much less deformed that the A-Zone, with occasional banding evident. Mineralization of the BD-Zone is less than that of the A and B zones, typically ranging from 0.6 – 1+% TREO, but with a distribution more enriched in Nd (>20%) as well as the middle and heavy rare earths. Geochemically, the BD-Zone may be classified as a magnesio-carbonatite.