MAGMATIC FLUIDS ARE A PRIMARY SOURCE OF ARCHEAN GOLD AT MAGINO; FLUID INCLUSION AND STABLE ISOTOPE EVIDENCE

The Magino gold deposit is located ~45 km's northeast of Wawa, Ontario, Canada in the Michipicoten Greenstone Belt (MGB). Earlier gold is found encased in pyrite and in veins and is associated with Cu, Zn, Fe and As sulfides. Later gold is found hosted in quartz veins and is associated with Au, Ag, Pb and Bi telluride minerals.

Fluid inclusion microthermometry identified three main fluid types of carbonic (CO₂-CH₄, ±N₂), aqueous (H₂O-NaCl), and aqueo-carbonic (H₂O-CO₂-CH₄-NaCl) fluids. Carbonic inclusion Tm range from -69 to -56.6 °C and Th range from -69 to +28.5 °C. Aqueous fluid inclusions are often suspected of necking but when measurable were determined to have Tm(ice) ranging from -22 to 0 °C (23.7 to 0 equiv. wt. % NaCl) and Th ranging from +70 to >+525 °C. Rarely observed eutectic melting temperatures range from -55 to -21 °C (suggesting variable amounts of divalent cations in the fluids). Aqueo-carbonic Tm(car) range from -63.9 to -56.6 °C, Th(car) range from -43 to +26 °C and Tm(cla) range from -10 to +23.6 °C. At least four generations of fluids (Fluid I-IV) have been identified in at least two auriferous (Fluid II, IV?) and multiple barren fluid (Fluids I, III, more?) events at Magino. Fluid II and IV are variably saline magmatic derived aqueous fluids. Fluids I and III are carbonic and aqueo-carbonic fluids associated with the host intrusion and regional deformation respectively.

Mineral (ankerite, calcite) δ¹⁸O values range from 8.85 ‰ to 11.14 ‰ (VSMOW) and mineral (ankerite, calcite) δ¹³C values range from -6.74 ‰ to -1.48 ‰ (PDB). Calcite δ¹³C values fall in a narrow range between -3.27 ‰ to -1.48 ‰ (PDB).The mineral δ¹⁸O values show a relatively low δ¹⁸O value for deposits in the MGB area when compared to deposits in the Abitibi suggesting similarities in the hydrothermal system(s) of the MGB (Studemeister and Kilias, 1987; Samson et al., 1997). Fluid δ¹⁸O values range from 5.22 ‰ to 7.51 ‰ (VSMOW) calculated using equations from (Zheng, 1999) and an estimated temperature of mineralization (350 °C). Both the mineral δ¹³C values and the calculated fluid δ¹⁸O values suggest a dominantly magmatic fluid source.

Our interpretation is that a metamorphic fluid derived orogenic system was present, but relatively barren in the MGB area, and magmatic derived fluids played a more significant role in gold mineralization.