Volcanic red-bed copper mineralisation related to submarine basalt alteration,Mont Alexandre,Quebec Appalachians,Canada |
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Authors: | Alexandre Raphael Cabral Georges Beaudoin |
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Institution: | (1) Département de géologie et de génie géologique, Université Laval, Pavillon Adrien-Pouliot, Québec, QC, G1K 7P4, Canada;(2) Present address: Department of Geology: Exploration Geology, Rhodes University, P.O. Box 94, Grahamstown, 6140, South Africa |
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Abstract: | Two types of native copper occur in Upper Silurian basaltic rocks in the Mont Alexandre area, Quebec Appalachians: (1) type
1 forms micrometric inclusions in plagioclase and is possibly magmatic in origin, whereas (2) type 2 occurs as coarse-grained
patches rimmed by cuprite in altered porphyritic basalt. Type 1 has higher contents of sulphur (2,000–20,263 ppm) and arsenic
(146–6,017 ppm), and a broader range of silver abundances (<65–2,186 ppm Ag) than type 2 (149–1,288 ppm S, <90–146 As, <65–928 ppm
Ag). No mineral inclusions of sulphide or arsenide in native copper were observed at the electron-microprobe scale. Primary
igneous fabrics are preserved, but the basaltic flows are pervasively oxidised and plagioclase is albitised. Chlorite replaces
plagioclase and forms interstitial aggregates in the groundmass and has Fe/(Fe+Mg) ratios ranging from 0.29 to 0.36 with calculated
temperatures between 155°C and 182°C. Copper sulphides in vacuoles and veinlets are associated with malachite, fibro-radiating
albite and yarrowite (Cu9S8 with up to 0.3 wt% Ag). Bulk-rock concentrations of thallium and lithium range from 70 to 310 ppb and 10 to 22 ppm, respectively,
and thallium is positively correlated with Fe2O3. Such concentrations of thallium and lithium are typical of spilitisation during heated seawater–basalt interaction. Spilitisation
is consistent with the regional geological setting of deepwater-facies sedimentation, but is different from current models
for volcanic red-bed copper, which indicate subaerial oxidation of volcanic flows. The volcanic red-bed copper model should
be re-examined to account for native copper mineralisation in basalts altered by warm seawater. |
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Keywords: | Native copper Volcanic red beds Spilitisation Mont Alexandre Quebec Appalachians Canada |
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