Microanalytical approaches to the study of hydrothermal alteration in epithermal and orogenic AU-AG deposits in Newfoundland

Arbiol González, Carlos (2022) Microanalytical approaches to the study of hydrothermal alteration in epithermal and orogenic AU-AG deposits in Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Island of Newfoundland (Canada) is an amalgamation of terranes defining the northernmost extent of the Appalachian orogen in North America. A subset of these terranes host epithermal and orogenic gold occurrences of Neoproterozoic and Silurian-Devonian age, respectively. Well-preserved examples of high- and low- sulfidation epithermal gold mineralization and associated phyllosilicate-rich alteration occur in the Neoproterozoic portion of the Avalon Zone. High-sulfidation epithermal occurrences, including the Hickey´s Pond prospect and the Hope Brook deposit, are characterized by the phyllosilicates Fe muscovite (2M1 polytypism), paragonite, Al clinochlore (ripidolite with IIb polytypism), pyrophyllite, kaolinite group minerals, and mixed layer clay phases, which formed at relatively high temperatures (200-300 ºC). Low-sulfidation epithermal occurrences, such as the Heritage prospect, feature a phyllosilicate assemblage defined by Fe-Mg muscovite (high Tschermak substitution and 2M₂ polytypism), Al clinochlore (brunsvigite with IIb polytypism), and mixed layered clay phases, which formed at lower temperatures (100-200 ºC). Raman microspectroscopy proved highly effective in characterizing alunite group minerals (alunite and natroalunite endmembers), pyrophyllite, white mica (muscovite/paragonite and phengite), and Fe-Mg chlorite (clinochlore and chamosite end-members) in these examples. The sensitivity of the white mica Raman band between 1000 cm⁻¹ and 1150 cm⁻¹ to changes in Si⁴⁺ and Al³⁺ in tetrahedral coordination revealed that compositional variations due to Tschermak substitution (muscovite-paragonite versus phengite(celadonite)) can be effectively detected by Raman microspectroscopy. The position of the main chlorite Si-O-Si Raman band shifts from >680 cm⁻¹ for Mg-rich chlorite to <670 cm⁻¹ for Fe-rich chlorite, permitting the estimation of the Fe²⁺ content in chlorite. The Baie Verte Peninsula (Dunnage Zone) hosts abundant orogenic gold mineralization with formation of hydrothermal rutile at distances of 10s of m from the auriferous veins. The distal-to-proximal evolution of Fe-Ti phases consists of (Ti)-magnetite - ilmenite - titanite - rutile, showing a progression from Fe- to Ti-rich phases. Vein-proximal rutile from the essentially barren Animal Pond prospect contains high concentrations of Fe and Mn, while vein-proximal rutile from the auriferous Stog’er Tight and Argyle deposits contains higher concentrations of Nb, Ta, Sn, and, in particular, Sb (80-100 ppm). The Fe-Mn-poor, Sb-rich composition of rutile formed proximal to auriferous veins has potential application to gold exploration in the Baie Verte region and elsewhere.

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