Summary:
The Crater Lake Deposit is a large, scandium- and REE-bearing alkali igneous intrusive complex. Carbonatite and alkaline intrusive complexes (as well as their weathering products) are the primary sources of REE. Apart from REE, these rock types can also host deposits of niobium, phosphate, titanium, vermiculite, barite, fluorite, copper, calcite, and zirconium. Although these types of deposits are found throughout the world, only six are currently being mined for REE: five carbonatites (Bayan Obo, Daluxiang, Maoniuping, and Weishan deposits in China, and the Mountain Pass deposit in the USA) and one peralkaline intrusion-related deposit.
Property Geology
The Crater Lake intrusion displays a gradational contact with its host, the Mistastin rapakivi granite. Both have an A-type affinity and similar trace element composition. The Crater Lake syenites are therefore interpreted to be a late differentiate product of the Mistastin Batholith. The dominant exposed lithology (much of the intrusion is covered by a lake) is coarse- to medium-grained, massive syenite, which is mainly composed of perthitic K-feldspar and 1 to 10% by volume of interstitial ferromagnesian minerals, namely fayalite (iron chrysolite, Fe2SiO4), hedenbergite, ferro-pargasite and annite (iron-rich biotite), accompanied by accessory quartz, iron oxides (magnetite, titanium-rich magnetite, and ilmenite), zircon, fluorite, apatite and britholite (Petrella 2012). A magnetic and melanocratic unit, ferro-syenite, which commonly contains greater than 50% by volume of ferromagnesian minerals, including cumulate fayalite, hedenbergite and ferro-pargasite, occurs as large continuous to discontinuous subvertical and conical bodies, sills, narrow dikes and inclusions in the felsic syenites. The large ferro-syenite bodies are elongated and concordant to subconcordant to the main contact between the Crater Lake syenite and the Mistastin granite intrusions. These large bodies can reach up to 700 m long, up to 120 m wide, and are open at depth. Three large ferro-syenite bodies have been found on the property: TGZ, Boulder Lake and STG. Petrella (2012) interpreted the narrow ferro- syenite dikes as having formed by fractional crystallization of ferromagnesian minerals, leaving behind a residual magma that produced the felsic syenites. With continued fractional crystallization, the felsic syenites became more enriched in alkali and silica, and only became saturated with ferromagnesian at a very late stage, which explains the interstitial crystallization of the latter in the perthite-dominated syenite.
Several major radial and concentric faults are observed in the field and drill core, and have also been interpreted from magnetic data and satellite images. Most of these subvertical and (less commonly) subhorizontal structures are concentrated inwards from the contact with the Mistastin granite to within the first 800 m of the Crater Lake intrusion. Major faults are characterized by a very intense potassic alteration with local concentrations of biotite, chlorite, epidote and magnetite. Imperial’s geologists do not yet know if these faults played a role in the ferro-syenite emplacement.
The Crater Lake intrusion was interpreted by Petrella et al. (2014) to be a ring dyke complex due to the concentric lithological zonation of quartz monzonite and felsic syenite, the steep dip of the bodies toward the center of the intrusion, the presence of numerous intrusion-scale discontinuous concentric faults (interpreted from the magnetic data), and the occurrence of several late radial faults (occupied by pegmatites), all of which are characteristic features of ring complexes (e.g. Woolley, 2001; Coumans and Stix, 2016). Consistent with this interpretation, some of the Crater Lake felsic syenites feature a trachytic texture developed through the alignment of feldspar laths, indicative of flow before cooling.
Mineralization
Assay results from surface samples and from 2014-2021 drill core indicate that the different types of ferro-syenite are the main host to the scandium and REE mineralization.
At Crater Lake, scandium was enriched in the residual liquid of the parent Mistastin granite magma following extensive fractionation of feldspar, in which scandium is incompatible. This residual liquid became the Crater Lake quartz monzonite magma, which was enriched in scandium and iron. Fluorapatite, zircon, fayalite, and the cores of zoned hedenbergite crystals saturated in this magma chamber. Ring faults developed as a result of caldera collapse, and the magma and minerals were emplaced as a slurry into these faults. The ferro-syenite formed by in situ fractionation of unzoned hedenbergite crystals, magnetite and hastingsite, and their physical segregation with the previously crystallized minerals. The extremely high FeO/FeO+MgO content of the quartz monzonite liquid resulted in high partition coefficients for scandium in the hedenbergite and hastingsite, allowing scandium to be incorporated into these minerals at exceptionally high concentrations under magmatic conditions. The physical segregation of hedenbergite and hastingsite in ferro-syenite cumulate rocks through gravitational settling and/or flow differentiation spatially concentrated the Sc-bearing minerals within the intrusion, resulting in the first known scandium deposit hosted by syenite. (Beland, 2021).
The REE mineralization is contained in small primary idiomorphic zircon and hydroxyapatite crystals (identified by XRD analysis). The latter locally form aggregates that were wholly or partly replaced by britholite-(Ce). Two types of hydroxyapatite and one type of britholite-(Ce) have been identified. The first type of hydroxyapatite is magmatic and occurs as euhedral to subhedral, unzoned, transparent crystals that do not show evidence of having been altered. This type of apatite is very frequently observed in the other rock types of the intrusion. The second type of hydroxyapatite also occurs as primary, magmatic crystals but is compositionally zoned, with its core similar in composition to unzoned hydroxyapatite 1. This indicates that hydroxyapatite 2 continued to crystallize after hydroxyapatite 1. Crystals of hydroxyapatite 2 are commonly replaced in their outer parts by britholite-(Ce). Both types of hydroxyapatite commonly occur as inclusions in pyroxene, amphibole and, less commonly, fayalite.