Mineral chemistry and nd isotope geochemistry of apatite in orogenic Au-associated gabbros, Baie Verte, Newfoundland

Gümüş, Fatma (2023) Mineral chemistry and nd isotope geochemistry of apatite in orogenic Au-associated gabbros, Baie Verte, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Orogenic Au deposits on the Baie Verte Peninsula, Newfoundland Appalachians, are structurally controlled and primarily hosted by Lower Ordovician volcanic cover sequences (Snooks Arm Group) and ophiolitic rocks. Gabbro-hosted orogenic Au can be found in three locations: the Stog’er Tight (642,000 t @ 3.02 g/t Au indicated, and 53,000 t @ 5.63 g/t Au inferred resources), the Argyle (436,800 t @ 2.53 g/t Au indicated and 500 t @ 2.77 g/t Au inferred resources) deposits, and the Animal Pond prospect. All these deposits are located in the hanging wall of the Scrape thrust fault typically within 100s of meters of the surface trace of the fault. Gold mineralization in all three areas is associated with coarse-grained, pegmatoidal gabbros that are variably deformed and were metasomatically altered by orogenic Au-related hydrothermal fluids. Gold is also associated with hydrothermally modified apatite, hydrothermal zircon, monazite ((Ce,La)PO4) and xenotime (YPO4). Phosphate minerals are widely distributed throughout the Au-related hydrothermal alteration zones. The deposition of gold occurred concurrent with the dissolution and reprecipitation in apatite, which caused the leaching of REE from apatite and the precipitation of coeval hydrothermal monazite and xenotime. All apatite in fresh and altered rocks are subhedral to euhedral and have homogeneous backscattered electron images. In contrast, cathodoluminescence (CL) images for apatite in unaltered rocks displays yellow to yellow-green luminescence, whereas apatite in altered rocks displays a dark green to gray luminescence. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry data for apatite in altered rocks show they are depleted in Mn, Cl, LREE (low La/YbN, La/SmN), Th, and U, and enriched in Sr and and have higher Sr/Y compared to apatite in the fresh rocks. This is compatible with the altered apatites having experienced fluid aided REE-Mn-Cl-Th-U remobilization and subsequent growth of hydrothermal monazite and lesser xenotime inclusions. Despite variable elemental concentrations, the Nd isotope signatures of fresh and altered apatite are similar in all apatite grains with εNd(t)= +2.34-6.60 vs. =+2.80-6.96, respectively. This study highlights that the combination of textural studies, CL imaging, and chemical composition of apatite can be used to determine igneous vs apatite influenced by Au-related hydrothermal alteration and that apatite can be modified by hydrothermal fluids during fluid-rock interaction.

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