George A. O'Reilly

The Sangster Lake and Larrys River Plutons are small (<36 km2.), strongly peraluminous granitoids which intrude, at 375Ma, the Cambro-Ordovician Meguma Group metasedimentary rocks in the eastern Meguma Zone. Thermal metamorphic mineral assemblages related to emplacement of the plutons post-date regional metamorphic mineral assemblages in the adjacent metasediments. A deformation fabric related to a dextral east-west trending shear zone (363Ma) overprints both plutons, in particular, the Sangster Lake Pluton. Within the Sangster Lake Pluton the juxtaposition of concentric dome-like structures with large blocks of metasediment trapped around the peripheries suggest that the pluton is exposed at its uppermost roof zone.

The plutons consist predominantly of textural varieties of monzogranite, varying from megacrystic and biotite-rich to equigranular and muscovite-rich. The monozogranites are intruded by much smaller minor intrusions and dykes of leucomonozogranite, leucogranite and pegmatite. One of these intrusions, the Beans Lake Leucomonozogranite, has extensively metasomatized contact zones with sheeted quartz vein zones and associated greisens.

The Sangster Lake Pluton has an airborne gamma-ray spectrometric response that is anomalous in equivalent uranium (eU) and the equivalent uranium/equivalent thorium ratio (eU/eTh). The Larrys River Pluton has an average response, typical of most of the granitoids of the Meguma Zone. Within the Sangster Lake Pluton the increase of airborne gamma-ray spectrometric response correlates well with areas that display features indicative of increasing intensity of post- crystallization hydrothermal alteration. For example, there was widespread albitization of plagioclase and development of secondary muscovite, apatite, Cl-apatite and Fe-U-phosphate. The alteration occurs in two modes: (a) pervasive, widespread replacement of the primary minerals throughout much of the eastern portion of the pluton; and (b) fracture-controlled intense alteration restricted to contact zones of late-stage, highly evolved intrusions.

Whole-rock geochemistry indicates that the hydrothermally altered areas are characterized by an increase of the Na₂O/K₂O ratio, and an increase in P₂O₅ levels. There is also a general enrichment in such elements as U, Sn, Rb, Li, F, Be, and B and decrease of Sr, Th, Ba, TiO2 and CaO. The geochemical trends are a result of a combination of magmatic and postmagmatic processes with the latter being most important in the enrichment of U within the rocks. Levels of rare earth elements (REE) and d18O are comparable to levels reported from other Meguma Zone granitoids and are consistent with interaction of the rocks with a fluid phase at some time during their evolution. A smooth and continuous decreasing trend of REE with decreasing Zr, Th, TiO₂ and P₂O₅ is interpreted to be the result of fluid-melt interaction. Therefore, the extremely low REE levels in the hydrothermally altered rocks is inherited from the original, unaltered rock and the REE- rich accessory minerals were unaffected by the alteration.

The coincidence of textural, mineralogical and geochemical characteristics indicative fo hydrothermal alteration in these rocks with increasing gamma-ray radiometric response demonstrates that such surveys can detect plutons that have undergone similar processes. The association of post-magmatic alteration with granophile element mineralization is well established; thus radiometric surveys can be a useful exploration tool for mineralization other than uranium.

Keywords:
Pages: 312
Supervisor: Gunter Muecke