MINERAL TEXTURES AND HYDROTHERMAL REACTIONS IN OROGENIC GOLD-BEARING VEINS, CENTRAL NEWFOUNDLAND: IMPLICATIONS FOR MOBILITY OF GOLD AND CRITICAL ELEMENTS

Crustal-scale fault splays in central Newfoundland host Early Devonian orogenic gold mineralization. Mineral textures in structurally controlled, multiphase, quartz-rich vein sets indicate that precious metal and copper-iron sulfide mineralization is associated with telluride, selenide, and bismuthide compounds. The metalloids that comprise these compounds (e.g. Te, Se, Bi) are becoming increasingly important because their recovery as mining by-products may be critical to sustaining global-scale supply chains (i.e. critical elements).

Backscatter electron imaging reveals that native gold and silver, as well as electrum (Au,Ag), occur with secondary goethite (FeO(OH)), malachite (Cu₂CO₃(OH)₂), bornite (Cu₅FeS₄), fischesserite (Ag₃AuSe₂), acanthite (Ag₂S), lenaite (AgFeS₂), hessite (Ag₂Te), bismuth metal, and Sb-Cu-bearing siderite, all of which overgrow and replace primary hydrothermal assemblages consisting of quartz, pyrite (FeS₂), chalcopyrite (CuFeS₂), tourmaline, and rutile. Pervasive overgrowth of goethite along the edges and within fractures of pyrite grains indicates that brittle microstructures accommodated oxidation and hydration of pyrite by the following reaction: 2pyrite + 9O₂ + 2H₂O + 6e^- = 2goethite + 2H⁺ + 4SO₄^2-. Micro-fractures in pyrite also facilitated metasomatic reactions involving metals and metalloids. Chalcopyrite growth on goethite can be explained by the following dehydration reaction involving metasomatism of Cu⁺: pyrite + goethite + 2Cu⁺ + 20H⁺ + 2SO₄^2- + 18e^- = 2chalcopyrite + 10H₂O. Mobilization and re-mineralization of precious metals and critical elements were likely influenced by desulfidation reactions such as: 1) acanthite + Au⁺ + 3Ag⁺ + Te^2- + 2Se^2- = fischesserite + hessite + S^2-; and/or 2) acanthite + 2Au⁺ + 4e^- = 2electrum + S^2-. Mineral textures also support closed system, sulfur-conserving alteration reactions such as: chalcopyrite + 2Cu₂S (chalcocite) = bornite.

Alteration mantles on sulfide grains are comprised of precious metal- and critical element-bearing sulfosalts with complex textures that are consistent with dissolution of mineral grains and mobilization and re-precipitation of elements. Such reaction mechanisms suggest that local supergene alteration of hydrothermal veins may have contributed to late mobilization and mineralization of elements. This research demonstrates that post-emplacement alteration of structurally controlled veins can play a key role in concentrating precious metals and critical elements in orogenic gold settings.