The Thor Lake rare metals deposit is hosted by the peralkaline Blachford Lake intrusion, an Aphebian-age ring complex emplaced in Archean-age supracrustal rocks of the Yellowknife Supergroup. The principal rock types in the intrusion are syenites, granites and gabbros and associated pegmatitic phases hosting rare metal mineralization. The key rock units in the vicinity of the mineralization are the Grace Lake Granite, the Thor Lake Syenite and an unnamed nepheline-sodalite syenite. The Grace Lake Granite surrounds the Thor Lake Syenite with the two separated by the enigmatic "Rim Syenite". It forms a distinct semi-circular ridge, locally termed the rim syenite that can be traced for a distance of about eight kilometers and is thought to be a ring dyke. In outcrop, Thor Lake syenite is seen to transition to Grace Lake granite with the appearance of quartz on the solidus in an otherwise felspathic rock. Thus the Grace Lake Granite and Thor Lake Syenite are believed to be closely related intrusives. The host of the Nechalacho mineralization, the nepheline-sodalite syenite, is within and below the Thor Lake Syenite, and exposed locally in the northwest part of the Thor Lake Syenite.
Five distinct zones or deposits of rare metal mineralization have been identified as being of potential economic interest: the Nechalacho deposit (Lake Zone) and smaller North T, South T, S and R Zones.
The Nechalacho deposit is currently top priority due to its large size and enrichment in REE. It also contains significant yttrium, tantalum, niobium, gallium and zirconium mineralization. Nechalacho is particularly notable for its enrichment in the more valuable HREEs such as europium, terbium and dysprosium, relative to LREEs such as lanthanum and cerium.
The nepheline-sodalite syenite that hosts the Nechalacho deposit has the following key distinctive features which contrast it to the Thor Lake syenite and Grace Lake granite:
- It has a distinct chemical composition showing undersaturation in quartz., with nepheline and sodalite variously as rock-forming minerals.
- It has cumulate layering.
- It contains agpaitic zircono-silicates including eudialyte.
- It is the host to the Nechalacho zirconium-niobium-tantalum-rare earth mineralization.
This syenite is only exposed at surface in a window through the Thor Lake Syenite in the area encompassing Long Lake to Thor Lake. It is believed to dip underneath that Thor Lake syenite in all directions. Also, the Nechalacho deposit mineralization, which occurs in the top, or apex, of the syenite, is also present in throughout this window through the Thor Lake syenite. For the sake of convenience at this time, this unnamed syenite will be referred to in this report as the "Ore (Nechalacho) Nepheline Sodalite Syenite".
The Nechalacho deposit is a tabular hydrothermal alteration zone extending typically from surface to depths of 200 to 250 metres, characterized by alternating sub-horizontal layers of relatively high and lower grade REE mineralization. HREE are present in the Nechalacho deposit in fergusonite ((Y,HREE)NbO4) and zircon (ZrSiO4), whereas the LREE are present in bastnaesite, synchysite, allanite and monazite. Niobium and tantalum are hosted in columbite as well as fergusonite.
There is a gradual increase in HREE from surface to depth with the lowermost subhorizontal layer, which is also the most laterally continuous, being referred to as the Basal Zone. Thus typical proportions of HREO relative to TREO in upper zones can be 7 to 10% but in the Basal Zone averaging over 20% and reaching as high as 50% in individual samples. There is also a tendency for the Basal Zone, which undulates to some extent, to increase in HREO with depth.
The Ore (Nechalacho) nepheline sodalite syenite consists of a layered series of increasingly peralkaline rocks with depth. A consistent downward progression is observed from hanging wall sodalite cumulates, through coarse grained to pegmatitic nepheline aegirine syenites which are locally enriched in zirconosilicates, to foayaitic syenite with a broad zone of altered eudialyte cumulates (referred to above as the Basal Zone). This upper sequence is strongly to intensely hydrothermally altered by various Na and Fe fluids. Pre-existing zircon-silicates are completely replaced by zircon, allanite, bastnaesite, fergusonite and other minerals. Below the Basal Zone cumulates, alteration decreases relatively quickly, with relict primary mineralogy and textures increasingly preserved. Aegirine and nephelline-bearing syenites and foyaitic syenites progress downward to sodalite foyaites and naujaite. Drilling has not extended beyond this sodalite lithology to date. Minerals related to agpaitic magmatism identified from this lower unaltered sequence include eudialyte, catapleite, analcime, and possibly mosandrite.
The North T and South T deposits contain beryllium and potentially recoverable REE, yttrium and niobium. The predominant beryllium mineral in the North T deposit is phenacite, a rare high-grade beryllium silicate mineral with an average BeO content of 24% (more than double the beryllium content of the more common beryllium mineral beryl). The R and S Zones are of interest mainly for yttrium and REE. Avalon is currently not conducting any work on the North T, South T, R or S Zones at this time.
ZDROJ: Avalon Rare Metals Inc.