The Crescent Lake copper deposit, central Newfoundland: deep levels of a volcanogenic hydrothermal system?

Abstract

The Crescent Lake Mine, near the village of Robert's Arm in central Newfoundland, produced small tonnages of high-grade copper ore from quartz veins in the late 1800's and the 1920's. Although a Crescent Lake-type deposit is not a viable exploration target in itself, its presence has been used to suggest a potential for either volcanogenic massive sulphides (VMS) such as the nearby Pilley's Island deposit, or for mesothermal or epithermal gold. Field relationships, petrography, geochemistry, mineral chemistry, and fluid inclusion and stable isotopic studies are used to constrain the genesis of the deposit. Mineralization occurs in four sulphide stages and a late oxide stage, distinguished by ore microscopy. Sphalerite, galena, chalcopyrite, and newly recognized carrollite were deposited with quartz between two pyrite stages. Covellite and iron oxides indicate later oxidation. Fluid inclusions in vein quartz are aqueous fluid-vapour type with no daughter crystals, no C 0 2 or CH4, no boiling, and a narrow range of salinities (-3-7% NaCle<)ttiv). The sulphides were likely deposited from homogeneous fluids at >430 m depth. The wide range of homogenization temperatures ( -1 12-250°C) can be interpreted as the result of either changing temperature or fluctuating fluid pressures during the episodic crack-seal vein formation. Ore-related chlorite has a distinctive blue birefringence and higher Fe/Fe+Mg values (~ 0.51 vs. 0.42) than chlorite in barren host rock, similar to other VMS deposits. The average calculated crystallization temperature of hydrothermal chlorite (240 ± 2°C) is close to the maximum fluid inclusion homogenization temperatures and likely closer to the true fluid trapping temperature. Sulphur isotope determinations (334Spy = 5.4; 534Scp = 2.8; 934Sfp = 2.2) are similar to the Gull Pond VMS deposit; they indicate isotopic disequilibrium between coeval sulphides but a homogeneous sulphur source during the ore-forming event. This study concludes that the Crescent Lake copper deposit is most likely part of a volcanogenic hydrothermal stockwork, possibly genetically related to the Pilley's Island deposit.La mine de Crescent Lake, situee dans la region centrale de Terre-Neuve a proximite du village de Robert's Arm, produisit de faibles tonnages de minerai a forte teneur en cuivre, issu de veines de quartz, vers la fin des armies 1800 et dans les annees 1920. Bien qu'un gite de type Crescent Lake tout seul ne constitue pas une cible d 'exploration viable, sa presence a ete utilisee pour suggerer l'dventualite soit d 'amas sulfures volcanogenes(AS V), tel que le gite avoisinant de Pilley's Island, soit d'or mesoou epithermal. La confrontation des donnees concemant les rapports sur le terrain, la petrographie, la geochimie, la chimie des mineraux, ainsi que 1'etude des inclusions fluides et des isotopes stables, est employee pour definir la genese du gite. La mineralisation se presente sous forme de quatre stades a sulfures et d 'un stade tardif a oxydes, que l'on reconnait a 1'aide de la microscopie mdtallograp...

Low-Temperature Chlorite Geothermometry and Related Recent Analytical Advances: A Review

“…This empiricism could not account for the varying influences of TK and DT substitutions on the variation of Al IV content-some have tried to correct their proposed equation with a corrective factor based on the Fe/Mg ratio, i.e., on FM substitution. Thus, there are no less than five other empirical thermometric equations, in addition to that of Cathelineau (Table 1), proposed in the literature [24,38,[45][46][47][48], in some cases being used as a thermometer when they had not been suggested by their authors for this [24].…”