SERPENTINE MINERALOGY OF THE TAMARACK MAGMATIC NI-CU-CO-PGM SULFIDE DEPOSIT, MINNESOTA

The Tamarack magmatic Ni-Cu-Co-PGM sulfide deposit is hosted by the 1106 Ma Tamarack Intrusive Complex, which is composed of peridotite, feldspathic peridotite, feldspathic pyroxenite, melatroctolite and melagabbro that intruded metagraywackes and slates of the Paleoproterozoic Thomson Formation. Mineralized rock ranges from disseminated to semi-massive and massive sulfide with indicated resources of 8.5 Mt averaging 1.73 % Ni, 0.92 % Cu, 0.05 % Co, 0.34 g/t Pt, 0.21 g/t Pd, and 0.17 g/t Au. After ore deposition, significant hydrothermal alteration and serpentinization of the host rocks affected the character of the gangue minerals.

Thin sections and metallurgical reject material (tailings) were examined using optical microscopy, Raman spectroscopy, and X-ray diffraction to characterize the nature and extent of serpentinization. Serpentinization ranges from partial to nearly complete replacement of mafic silicates such as olivine and pyroxene. Secondary minerals include serpentine, chlorite, talc, clay and carbonate. Raman spectroscopy has proven to be especially effective in distinguishing among the various serpentine minerals. Lizardite and minor chrysotile are the only serpentine minerals that have been identified. Lizardite is predominant as rims on olivine grains and as replacements along cleavages and fractures in olivine and pyroxene. Chrysotile forms vein fillings in small fractures and occurs as cores of “isotropic serpentine” in replaced olivine grains.

The abundance of serpentine minerals opens the possibility of enhanced carbon capture during ore processing and tailings disposal. Serpentine minerals are more readily carbonated compared to olivine and pyroxene. Magnesium carbonate minerals, such as hydromagnesite and nesquehonite, are known to form passively in serpentine-rich tailings. These mineralogic transformations will enhance the possibility that the proposed mine could have a carbon-neutral impact, in addition to aiding in dust management.