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Gold upgrading in metamorphosed massive sulfide ore deposits: Direct evidence from laser-ablation–inductively coupled plasma–mass spectrometry analysis of invisible gold

¹ Institut für Geowissenschaften, Universität Tübingen, Wilhelmstrasse 56, D-72074 Tübingen, Germany
² Mineralogisches Institut, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
³ Institut für Geowissenschaften, Universität Tübingen, Wilhelmstrasse 56, D-72074 Tübingen, Germany
⁴ Boliden Mineral AB, SE-93681 Boliden, Sweden

The metamorphosed Boliden volcanic-hosted massive sulfide (VHMS) ore deposit, northern Sweden, is characterized by an abundance of massive arsenopyrite ore and unusually high gold enrichments (up to 300–600 g/t) in crosscutting veins and fracture zones. The ore textures observed record progressive recrystallization and porphyroblast growth during meta morphism. We conducted a systematic laser-ablation–inductively coupled plasma–mass spectrom etry (LA-ICP-MS) study on the gold distribution in fine-grained massive arseno-pyrite and pyrite ore, partially recrystallized arsenopyrite ore, and both arsenopyrite and pyrite porphyroblasts found in gold-rich veins. The gold concentrations in massive arseno-pyrite are highest (30–50 ppm on average) and systematically decrease to 0–2.7 ppm with increasing degree of recrystallization. By contrast, gold concentrations in pyrite ore are much lower and do not exceed 0.2 ppm. The LA-ICP-MS data, coupled with textural data, provide the first direct conclusive evidence that gold is progressively liberated from massive sulfide ores during metamorphism and reprecipitated in veins and other low-strain sites. We suggest that such upgrading is the principal mechanism responsible for significant gold enrichment observed in many metamorphosed VHMS deposits worldwide.

Key Words: massive sulfides • arsenopyrite • metamorphic upgrading • invisible gold

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