Mineralization

The mineralization of the Nechalacho deposit is generally sub-horizontal and has been

traced for over 1500 metres (Sheard et al., 2012). The primary elements of interest are:

light rare earth elements (LREE) contained in allanite, monazite, bastnaesite and synchysite; heavy rare earth elements (HREE), niobium and tantalum contained in fergusonite; niobium contained in ferrocolumbite; and HREE, niobium, tantalum and

zirconium contained in zircon (Pinckston and Smith, 1995; Ciuculescu et al., 2013). The

proportion of HREE to LREE increases from the top of the deposit to the bottom. HREE account for 30% of the total REE in the high grade basin within the bottom of the basal

zone (Ciuculescu et al., 2013). Sheard et al. (2012) described the progression from the hanging wall sodalite cumulates, down through the upper zone which contains coarse- grained nepheline aegirine syenites locally enriched with zircon silicates, to the basal zone which is comprised of a foyaitic syenite within a zone of altered eudialyte cumulates. This sequence of rocks was intensely altered by sodium and iron rich hydrothermal fluids which resulted in magnetite and hematite as a few of the major

alteration minerals (Sheard et al., 2012). The alteration zone extends from the surface

down to approximately 80 metres depth, with some regions extending as far down as 200

metres (Ciuculescu et al., 2013). The original mineralogy of the deposit consisted of a

rooftop sodalite, enclosing layers of aegirine nepheline syenite with cumulate zircon, followed by more syenite layering and finally cumulate inferred eudialyte. Extensive alteration by what is thought to have been a fluorine bearing hydrothermal fluid resulted in the breakdown of eudialyte to secondary minerals that contain HREE, such as zircon and fergusonite, and leaving behind alteration minerals such as magnetite, which has a highly anomalous magnetic signature. As the fluid altered the zircon cumulates, REE were leeched from the cores of the crystals and deposited along fractures and crystal boundaries. LREE were remobilized further and form the high degree of LREE in the

upper portion of the deposit (Sheard et al., 2012).

The goal of this study is to use the knowledge collected in the exploration process, including previous geological research and geophysical survey data, to further delineate the boundaries of the upper and lower zones of mineralization. The initial steps in the process, reported here, involve inverting magnetic field data to produce a magnetic susceptibility model and constraining it with magnetic susceptibility measurements

collected from boreholes as boundary conditions. Future steps will incorporate

gravitational field data to further constrain the model and enhance important lithological boundaries. The anticipated end result is a model that improves on existing interpretation of the lateral and vertical extents of the deposit.

2.3

Data

The data used in this study are airborne total magnetic field data obtained by Natural

Resources Canada (NRCan, 2011). The survey was flown by Fugro GeoServices Ltd. at

an altitude of 100 metres above ground level with a traverse line spacing of 250 metres

and covered a total area of approximately 250 square kilometres (Figure 2.4). The

survey collected total magnetic field data with a fixed-wing aircraft, using a single cell cesium vapour magnetic system. When examining the survey result on a regional basis, the east-northeast trending signature of the Hearne dyke swarms is apparent, as well as a highly magnetic northwest trending anomaly presumed to be related to the Indin dyke

swarm (Mumford and Cousens, 2014). Full-tensor airborne gradiometry and gravity

anomaly surveys were collected of the same area by NRCan and will play a larger role in future work.

There has also been extensive subsurface exploration through the drilling of boreholes in the area. Magnetic susceptibility measurements were collected from the cores of over 400 holes on the Thor Lake property. These measurements were taken directly on the core with a handheld KT-9 instrument at 1 metre intervals by Avalon Rare Metals (Avalon Rare Metals, 2014).

A region of interest was defined based on the drilling pattern. The resulting Thor Lake inversion region is a 12.8 square kilometre area that is centered on the Thor Lake Syenite, chosen for its proximity to a prominent magnetic high and the large number of boreholes

concentrated in this area (Figure 2.4).

Figure 2.4: Map of the residual magnetic field from the aeromagnetic survey completed by NRCan (2011) of the Blatchford Lake Complex. The eastern large positive elliptical anomaly is the Thor Lake deposit while the western elongated anomaly is associated with the Caribou Lake gabbro that hosts Cu-Ni-PGE

mineralization. Units are in nanoTeslas. The inset plot is an enlarged in view of the inversion area with drill-holes plotted in black. Drillhole locations obtained from Avalon Rare Metals (2014).

It is expected that the source of this large magnetic signal is the abundance of alteration minerals such as magnetite and hematite that are associated with REE mineralization within the Nechalacho deposit. It is important to note that the anomalies, while indicative of REE deposits, are not caused by the REE but the mineralogy of the intrusions that are

associated with them (Verplanck and Van Gosen, 2011). Therefore regions of high magnetic susceptibility will likely be associated with regions where primary minerals such a eudialyte and aegirine have been altered to magnetite. In the case of the

Nechalacho deposit, this occurs mainly within the basal zone, thus anomalous densities and magnetic susceptibilities are expected where the REE mineralization is present in

larger concentrations (Sheard et al., 2012).