Carbon-isotope systematics of Archean Au–Ag vein deposits in the Superior Province

Abstract

Abundant carbonate is a characteristic feature of most Archean mesothermal Au–Ag vein deposits, but the source of the C is controversial. For Superior Province deposits collectively, the maximum variation of average δ13C values is from −9.0 ± 0.7‰ (1σ, n = 19; Darius) to −0.6 ± 1.6‰ (1σ, n = 7; Cochenour–Willians), and limiting δ13C values are−13.6 and + 1.3‰. At the deposit scale, Fe dolomites in nongraphitic lithologies are for the most part isotopically uniform, where δ13C = −3.4 ± 0.4 (1σ) (Hollinger), −3.2 ± 0.3 (McIntyre), −4.7 ± 1.7 (Dome), −2.8 ± 0.6 (Buffalo Ankerite), −3.6 ± 0.5 (Macassa), −3.2 ± 0.3 (Bousquet), −5.4 ± 0.9 (Lamaque), and −5.3 ± 0.5‰ (Hasaga): the restricted individual ranges of δ13C values imply a corresponding uniformity to the ambient temperature and δ13CΣC of the ore-forming fluids.Within individual deposits, small systematic variations of δ13C carbonate arise from (i) interaction of hydrothermal fluids with carbonaceous rocks, (ii) immiscible separation of CO2 + CH4, or (iii) Rayleigh fractionation effects. Positive shifts in δ13C result from buffering of the fluid to lower Eh by reaction with reduced C, whereas negative shifts reflect partial isotopic equilibration between 13C-depleted C (δ13C ≈ −26‰) and aqueous hydrothermal C species. Transient immiscibility of CO2 + CH4 acts to precipitate carbonates enriched relative to the main population of Fe dolomites. The δ13C values of carbonates in unmineralized alteration halos (−2.2 ± 1.1‰, n = 42) at the McIntyre deposit are enriched in 13C relative to the main gold-bearing vein systems (δ13C = −3.2 ± 0.3‰): the enrichment is attributed to a Rayleigh fractionation accompanying progressive consumption of CO2 as hydrothermal fluids infiltrate laterally from veins into wall rocks. Fe dolomite and calcite are variably enriched in 18O with respect to equilibrium quartz-carbonate fractionations for ambient temperatures of 270–340 °C. Carbonate δ18O values diminish in an irregular manner with depth, converging on values of ~11‰ (Fe dolomite, 6800 ft (2073 m), McIntyre). Variable degrees of oxygen-isotope disequilibrium represent overprinting of carbonates by post-Archean brines in the Canadian Shield.Synvolcanic vesicle calcite in three groups of metabasalts (δ13C = −4.3 ± 2.1; −2.8 ± 1.5; −2.7 ± 1.3‰) and calcite in two groups of clastic sediments (−6.4 ± 1.8; −4.6 ± 2.5‰) remote from deposits are systematically depleted of 13C relative to average Precambrian limestones (~0 ± 1‰), owing to the involvement of CO2 derived from 13C-depleted organic matter. Consequently, calcite in greenstone belt supracrustal rocks is not restricted to approximately 0‰. The total spread of average δ13CFe dol values (−9.0 ± 0.7 to −0.6 ± 0.6‰) in the Au deposits, which goes in hand with a geographical provinciality in O-, Sr-, and Pb-isotope compositions of the ore-forming fluids, is too large to be accounted for by mantle CO2 (−6 ± 2‰) or magmatic CO2 (−6 ± 2‰) alone but rather is interpreted as reflecting generation of hydrothermal fluids in crustal or subcreted rocks heterogeneous in terms of the distribution of 13C-enriched (carbonate) and 13C-depleted (reduced C) lithologies.