The geochemistry of albitisation in the Nechalacho layered alkaline suite, Northwest Territories, Canada

The Nechalacho Layered Alkaline Suite (NLAS), which hosts the Nechalacho REE deposit containing a measured and indicated resource of 121.26 Mt grading 1.50% REE2O3 (0.25% HREE2O3, including Y2O3), 2.56% ZrO2, 0.34% Nb2O5 and 0.03% Ta2O5, and located in the Slave Craton, NWT, is pervasively altered by secondary albite. Albitisation was most severe along the southern and western margins of the NLAS, overprinting all rock types and mineralised zones, and forming a porous, pink rock (>50% pink, bladed albite, <10% ferromagnesian minerals), i.e., albitite, which contains hydrothermal REE and HFSE minerals e.g.: Bastnäsite-(Ce), monazite-(Ce), columbite and zircon. The albitite underwent brittle deformation; the matrices of the resulting breccias as well as pore space between secondary albite crystals contain an intergrowth of quartz and fluorite and host the REE and HFSE mineralization. Secondary albite also contains REE-, Nb-, Y-, Zr-, Ti- and U-bearing solid micro-inclusions. Conditions of albitisation were constrained from phase relationships assuming 4.0k bar (Mumford 2013) and 367°C (Feng 2014) where fO2 is at the H-M buffer = -29.3, aNa+ = 2.80 and aK+ = 0.14, pH is between 5.6 and 6.0 and logaSi02 is between -2.2 and -1.4. The δ18O values calculated at 367°C for the hydrothermal fluid in equilibrium with quartz (NLAS) range from 8.8 to 11.4‰, 8.8 to 16.1‰ in secondary albite and 6.7 to 12.2‰ in secondary microcline. Along with the presence of Ca-bearing minerals in the albitised rocks, not present in the magmatic rocks, the isotopic values suggest an external fluid source.The NLAS is the youngest and only silica-undersaturated intrusion of the Blachford Lake Complex. Discrete hydrothermal satellite deposits are hosted within the Grace Lake Granite (GLG) and Thor Lake Syenite units peripheral to the NLAS, the largest of these deposits being the T-Zone (1.1 million tons grading 0.71 wt. % REE2O3, 0.48 wt. % BeO and 0.53 wt. % Nb2O5). Fenitisation of the GLG and TLS is evident in hand samples within 2 km of the NLAS manifested by the replacement of K-feldspar and riebeckite by albite and magnetite, and, within 100m of the NLAS, in discontinuous veinlets composed of fluorite, calcite and hematite (Type B veins) surrounded by haloes of albite, aegirine and fluorite. Another vein type (Type C) affects the TLS close to the NLAS, and is calcite-dominant, up to several cm wide, with alteration haloes greater than a metre thick, in which all ferromagnesian minerals and quartz have been removed or replaced by calcite, and primary K-feldspar has been albitised. It is proposed that formation waters in equilibrium with the GLG were drawn into the younger NLAS intrusion by convection, thereby shifting the two-feldspar phase boundary to be consistent with fluid compositions in the field of stability of albite. This promoted albitisation. Another effect of drawing fluids through the GLG and warming them was to dissolve quartz due to its increased solubility with increasing temperature, resulting in episyenitisation and veining (Type C). This also created pore spaces that were filled by albite. Exsolved magmatic fluids from the cooling NLAS were saturated in REE, preferentially mobilising the LREE over the HREE, which were later precipitated in pore space within the albitite.

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