Big easy prospect: a well-preserved low-sulfidation epithermal Au-Ag system in the late neoproterozoic, Newfoundland, Canada

Junco Rojas, Andrés Eduardo (2022) Big easy prospect: a well-preserved low-sulfidation epithermal Au-Ag system in the late neoproterozoic, Newfoundland, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Big Easy prospect is a low-sulfidation epithermal gold occurrence of Neoproterozoic age in the Burin-Bonavista region of Newfoundland, Canada. Mineralization is predominantly hosted in epiclastic rocks of the Musgravetown Group. This well-preserved system lies within the tectonostratigraphic Avalon Zone and is dated as Late Neoproterozoic (ca. 573 Ma). Previous work suggests that the most prospective period for epithermal deposits within the Neoproterozoic Avalon Zone is from ca. 620 Ma until the nominal end of the Neoproterozoic at 541 Ma. This period encompassed three episodes of arc magmatism, as recognized in the Burin Peninsula region: ca. 620 Ma (e.g., Broad Island Group), ca. 585 – 570 Ma (Marystown and Musgravetown Groups), and ca. 565 Ma (Long Harbour Group). Big Easy appears to have been associated with the 585 – 570 Ma episode, as were a number of volcanic/volcaniclastic-hosted occurrences in this region. The mineralization at Big Easy is subdivided here into four different styles of discrete Au- and Ag-bearing veins, containing characteristic ore mineral assemblages: Type I) Electrum dominant (High Au) – (pyrite + electrum ± sphalerite); Type II) High Au:Ag – (pyrite + electrum + native silver + naumannite + acanthite ± aguilarite ± chalcopyrite ± uytenbogaardtite); Type III) Ginguro (High Ag:Au) – (native silver + acanthite + electrum ± pyrite ± aguilarite ± chalcopyrite ± freibergite-tetrahedrite,); and Type IV) Molybdenite-rich (molybdenite + native silver + acanthite ± pyrite ± chalcopyrite ± sphalerite). Gold is mainly deported in type I and II veins as electrum observable in grains attached to, or enclosed by, pyrite crystals, or as rare uytenbogaardtite (Ag3AuS2). Geological and numerical models using existing drill hole data exhibit a strong spatial correlation between gold, silver, molybdenum, and arsenic distributions, and illustrate the predominant ambient hydrothermal alteration zone present in the preserved epithermal system. Strong correlations from PCA analysis, Pearson correlation values, and drill hole correlation analysis are observed for Au-Ag, but also for Mo-As. Mo and As in minor concentrations may be contained within pyrite in veins and pyritized selvages directly associated with Au-Ag mineralization. However, the more likely interpretation is that they are a signature of the extensive silicification and intense pyritization evident in the host epiclastic rocks, which predates the ore mineralization. Fluid inclusion microthermometry and SEM imaging were used to characterize the mineralogical and textural features of each vein style, as well as the estimated fluid temperatures and salinities (from 124.7 °C to 321.15 °C, and from 0.18 to 2.74 wt.% NaCl eq., respectively). Low-sulfidation Au-Ag deposits are a significant source of these metals globally. Big Easy, and other occurrences in the Avalon Zone, are a testament that mineral exploration for preserved epithermal deposits hosted in pre-Cenozoic rocks should not be neglected. New insights documented in this thesis will be useful for future exploration programs in the area. Big Easy represents a highly prospective epithermal system within the Avalon Zone and is one of the most important examples of this deposit type recognized in the Burin-Bonavista region to date.

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