Impact of metamorphism and deformation on pyrite and precious metals at the epithermal Haile deposit, South Carolina, USA

Abstract

The Haile deposit, South Carolina, is a high-tonnage, low-grade, pyrite-dominated, Au-Ag, low-sulfidation epithermal deposit hosted in the Carolina terrane and formed during the Neoproterozoic. It underwent greenschist facies metamorphism during the Ordovician Taconicorogeny in which sulfides, precious and critical metals endured remobilization. The degree ofphysical and chemical remobilization is characterized here with a focus on the role of pyritetexture, trace element abundance within pyrite, and the brittle-ductile behavior of pyrite during remobilization. Although remobilization of sulfides, precious and critical metals is a common occurrence in metamorphosed hydrothermal deposits, both the impact of remobilization deposit-wide and what process(es) facilitate the translocation of metals is debated. A combination of textural (microscopy), compositional (electron probe micro-analyzer, laser ablation inductively coupled mass spectrometry, secondary ion mass spectrometry), and microstructural (electron back-scattered diffraction) investigations were applied on pyrite, native gold and tellurides at the Haile deposit to decipher their behavior during metamorphismand deformation. Of the three pyrite types (Py1a, Py1b, Py2) identified, Py1a and Py1b were formed syn-genetically during the Neoproterozoic by dominantly reduced, mildly acidic to near neutral, magmatic-derived fluids at ≈ 290°C. Arsenic, precious metals and critical metals were incorporated into the pyrite structure syn-genetically (Neoproterozoic) with up to 2.49 wt.% As, 103 ppm Au, 482 ppm Sb, and 145 ppm Te. However, trace element content in these twopyrite types is highly variable with abundances a magnitude lower in Py1a and Py1b with Au grades of < 2 g/t. Py2 was formed syn-D2 or post-D2 of the Ordovician Taconic orogeny as newly crystallized, coarse, subhedral to euhedral pyrite around porous anhedral Py1b by oxidized, mildly acidic to near neutral, metamorphic fluids. Trace element contents are overall, at least a magnitude lower in Py2 compared to the older syn-genetic pyrite. During the Taconic orogeny, all three pyrite types experienced crystal-plastic and brittle deformation, albeit to different degrees: (1) low misorientation angles as a result of dislocation climb are preserved in strongly deformed Py1b and Py2; (2) higher abundance of high misorientation angles is common in most Py1a and Py1b that overprints foliation; (3) cataclastic cracks only occur in Py1b and Py2 and can have native gold and tellurides along the cracks; and (4) annealed aggregates occur in all pyrite types but are most prominent in Py2. Physical remobilization was accompanied by chemical remobilization which was driven by coupled dissolution reprecipitation reactions between porous Py1b and metamorphic fluids and resulted in the liberation of precious and critical metals. The liberation and transport of precious and critical metals was enhanced by pores and cataclastic cracks in Py1b; however, the absence of fluid pathways in Py1a prevented the liberation of these metals resulting in the absence of native gold and tellurides adjacent to Py1a. Although remobilization is a local process and occurs at the micro-scale, it impacted all ore zones at Haile and played a major role in refining and upgrading the precious metal and critical metal tenor of the deposit.