The Russell Gold Deposit, Carolina Slate Belt, North Carolina

摘要

Gold deposits have been mined in the Carolina slate belt from the early 1800s to recent times, with most of the production from large mines in South Carolina. The Russell mine, one of the larger producers in North Carolina, is located in the central Uwharrie Mountains, and produced over 470 kg of gold. Ore grades averaged about 3.4 grams per tonne (g/t), with higher-grade zones reported. The Russell deposit is interpreted to be a sediment-hosted, gold-rich, base-metal poor, volcanogenic massive sulfide deposit in which gold was remobi-lized, in part, during Ordovician metamorphism. The ore was deposited syngenetically with laminated silt-stones of the late Proterozoic Tillery Formation that have been metamorphosed to a lower greenschist facies. The Tillery Formation regionally overlies subaerial to shallow marine rhyolitic volcanic and volcaniclastic rocks of the Uwharrie Formation and underlies the marine volcanic and sedimentary rocks of the Cid Formation. Recent mapping has shown that a rhyolitic dome near the Russell mine was extruded during the deposition of the lower part of the Tillery Formation, at about the same time as ore deposition. Relict mafic rock fragments present in the ore zones suggest contemporaneous bimodal (rhyolite-basalt) volcanism. The maximum formation age of the Russell deposit is younger than 558 Ma, which is similar to that of the larger, well known Brewer, Haile, and Ridgeway deposits of South Carolina. Gold was mined from at least six zones that are parallel to the regional metamorphic foliation. These strongly deformed zones consist of northeast-trending folds, high-angle reverse faults, and asymmetric doubly plunging folds overturned to the southeast. The dominant structure at the mine is an asymmetric doubly plunging anticline with the axis trending N 45 deg E, probably related to late Ordovician (456 + - 2 Ma) regional metamorphism and deformation. Two stages of pyrite growth are recognized. Stage 1, primary, spongy pyrite, is present in thin massive sulfide layers parallel to bedding and intergrown with pyrrhotite, chalcopyrite, sphalerite, and sparse arsenopy-rite. Stage 2, secondary pyrite, is present as euhedral, idiomorphic rims on spongy pyrite and as veinlets that parallel the regional foliation. Stage 1 pyrite was deposited syngenetically or diagenetically during or soon after sedimentation. Stage 2 pyrite overgrowths and veins resulted from recrystallization or remobilization of stage 1 pyrite, probably during deformation related to Ordovician regional metamorphism. It is proposed that the spongy texture of the stage 1 pyrite represents nucleation and growth of pyrite on organic matter, possibly of bacterial origin. Other textures, such as geometric voids in spongy pyrite, As-rich zones surrounding central voids, 60-mu m ring structures composed of spongy pyrite, and layers of spongy pyrite interlayered with bedding laminations, also appear to indicate replacement of organic matter by pyrite. Stage 1 pyrite contains up to 0.06 wt percent gold. The delta~(34)S values of pyrite in and near pyritic ore (3.5-4.5 per thousand), in the rhyolite dome (5.1-5.4 per thousand) and in the Tillery Formation (5.9-6.2 per thousand) are interpreted to reflect mixing of sulfur derived from igneous and seawater sources. Whole-rock delta~(18)O values of nearby unaltered mudstone are about 11 per mil whereas those from altered mudstone at the deposit are 7.4 to 10.6 per mil. The lower delta~(18)O values are interpreted to indicate possible high-temperature exchange between relatively low delta~(18)O hydrothermal fluids and the wall rocks. Gold, As, K, and Mo are enriched relative to the regional background in both the ore zones and in pyrite veins and disseminations in the nearby rhyolite dome, suggesting a possible genetic link between the rhyolitic volcanism and the gold mineralization.