Assessing graphite precipitation mechanisms of the Albany graphite deposit

Abstract

Graphite is recognized as a 'critical raw material' due to its strategic importance for diverse industries (e.g., steel, automobile, clean technologies). The Albany graphite deposit, located west of Hearst in Thunder Bay, is a fluid-derived, igneous-hosted deposit. The morphology of the Albany deposit is unique because of fine-grained graphite occurring within two large breccia pipes. Albany graphite deposit is located in the south of the Nagagami Alkali Complex, north of the Gravel River fault. The objective of this study is to determine the mineralogy and alteration assemblages (non-weathering) using mineral compositions and textural associations and evaluate the alteration assemblages to determine whether they play any role in graphite precipitation. In the Albany deposit, graphite typically occurs as elongated, lath-shaped, and plate-like crystals (<0.05 mm in width and ranges from 0.1 to 1.5 mm in length). Crystals are typically characterized as randomly oriented and showing both intergranular and intragranular textures. Graphite is mainly distributed along grain boundaries in clast components (intragranular), whereas it shows intergranular texture in the matrix. The abundance of graphite in the matrix (<15%) is higher than that in the clast (<10%). Three sub-groups of graphite, namely clast, matrix, and alteration assemblages were examined. The matrix assemblage is comprised of lithic fragments derived from the clasts. Clasts are mainly comprising varying proportions of graphite, plagioclase, potassium feldspar, quartz, biotite, amphibole, and chlorite, while the matrix is mainly comprised of equigranular graphite, plagioclase, potassium feldspar, quartz, and biotite. Hydrothermal alteration phases present biotite, sericite, calcite, and chlorite. Biotite and sericite are determined as pre-graphitization alterations due to their textural relationship with graphite. Calcite forms partial and rim replacement of plagioclase in the matrix, while chlorite replaces biotite and alters plagioclase along grain boundaries. Petrographic observations reveal that both calcite and chlorite alterations are directly associated with graphite in the Albany deposit. Calcite and chlorite, which constitute at the propylitic alteration assemblage, developed coevally with graphite. [...]