Evidence for Cu-(Ag) mineralization by magmatic-meteoric fluid mixing in Keweenawan fissure veins, Mamainse Point, Ontario

Abstract

Locally rich (up to 40% Cu) Cu-(Fe) sulfide mineralization with minor Ag occurs in fault-related fissure veins cutting, but probably penecontemporaneous with, Keweenawan plateau basalt lava flows at Mamainse Point, Ontario. Paragenetic studies of samples from the Coppercorp mine define four stages of mineralization: (1) early pyrite-(chalcopyrite) with strong silicification and sericitic alteration; (2) a transitional stage of chalcopyrite-hematite and bornite-hematite assemblages, with propylitic alteration; (3) main-stage chalcocite-hematite, often replacing earlier sulfides, with propylitic alteration; and (4) late-stage, low-temperature calcite deposition, with local Cu degrees , Ag degrees , Cu arsenides, and cuprite. This paragenetic sequence suggests an increase in f (sub O 2 ) , perhaps accompanied by an increase in pH and a decrease in total sulfur activity.The mineralized fissures crosscut but are coeval with regional-scale low-grade (zeolitic to local epidotic) alteration in the plateau basalt sequence; their mineralogy, particularly the early sericitic wall-rock alteration, is at variance with the regional alteration mineralogy. This contrast is supported by fluid inclusion and light stable isotope data. Temperatures of the regional alteration reached little higher than 300 degrees C at the base of the lava pile, and fluid salinities were generally below approximately 2 equiv wt percent NaCl, suggesting the dominant presence of meteoric water. However, in the fissure veins, fluids varied from or = 450 degrees C and approximately 22 equiv wt percent NaCl. Fluid inclusions representing the hot saline fluids were only found in the presence of copper mineralization, and it is therefore concluded that these fluids were responsible for metal transport. Analysis of decrepitation residues from fluid inclusions in chalcocite indicate that the fluids were dominantly NaCl-CaCl 2 -H 2 O solutions; suppression of the critical curve by the presence of Ca (super +2) permits the interpretation that these saline, high-temperature fluids were near their critical point at approximately 450 degrees C. Their high temperatures compared to the ambient fluids suggest that they were derived from below the Keweenawan lava pile and that they may have been associated with intrusive magmatic activity. Approximately positively correlated arrays of homogenization temperature vs. salinity data from fluid inclusions in ore samples suggest that mixing between the hot saline fluid and the cooler meteoric ground waters was an important mechanism for deposition of Cu-(Fe) sulfides. The combined effects of rapid cooling, dilution, neutralization, and oxidation resulting from this process would cause a significant drop in copper solubility with consequent deposition of ore; this process would also explain the observed paragenetic sequence of reduced to oxidized assemblages.The delta 13 C values of calcite from the fissure veins average -3.9 + or - 0.9 per mil (n = 12), and fluid delta 18 O ratios calculated from seven samples of quartz and epidote intergrown with sulfides range from 10.7 to 2.5 per mil (+ or -2ppm due to temperature uncertainties). These isotopic ratios overlap the ranges for magmatic carbon and water, respectively. Eight samples of late calcite from mineralized veins indicate fluid delta 18 O values ranging from 4.8 to -3.1 per mil (+ or -0.5-4.5ppm). Calcite from barren veins (n = 3) and from veinlets associated with the regional alteration (n = 4) extend this range down to -6.9 per mil; delta 13 C ratios of between -2.7 and +0.7 per mil were obtained from these last four samples. Negative fluid delta 18 O values and delta 13 C values of carbonates near zero per mil suggest the dominant presence of meteoric waters and atmospheric carbon, respectively, in the regional system and the dominance of meteoric water in barren veins in the fissures; this interpretation is in agreement with the fluid inclusion data. The range of delta 18 O from magmatic to lighter values in the mineralized veins suggests that meteoric water was also present during ore deposition and was mixed in varying proportions with a magmatic fluid. The isotopic data, therefore, support a model of meteoric ground water and magmatic (or highly exchanged) fluid interaction in the fissure veins.Comparisons are made to the nearby Keweenawan Jogran Cu-(Mo) porphyry, and the Tribag Cu-(Mo) breccia pipes. Similarities between these deposits and the Mamainse Point fissure veins in mineralogy, fluid properties, and light stable isotope compositions suggest a genetic link. It is proposed that the Coppercorp, Tribag, and Jogran deposits represent successively deeper levels of exposure in Keweenawan porphyry-related hydrothermal systems. Richest ore deposition occurred in the high-level fissure veins at Coppercorp, due to focused mixing of hot metalliferous fluids with meteoric ground waters.