Sulfur isotope investigation of the gold-quartz deposits of the Yellowknife District [Northwest Territories]

Sulfides in the ore bodies and their associated alteration haloes, the metamorphic facies of the greenstone belt, and the neighboring granodiorite have been examined in detail. Most of the samples investigated are slightly enriched in S ³⁴ with respect to the meteoritic standard. Although the over-all range is small (about 1.5%), significant variations were found between sulfides in the granodiorite and in the metamorphic facies of the greenstone belt. Direct correlation exists between the isotopic ratio and the distance from the granodiorite-greenstone contact; sulfides in the granodiorite are the most enriched in S ³⁴ whereas sulfides in the metamorphic facies contain progressively less S ³⁴ outward from the granodiorite contact. Sulfides in early tension fractures are consistently lighter (i.e. they contain more S ³² ) than the sulfides in the surrounding greenstones, whereas sulfides in the later Au-quartz lenses are heavier than those in the associated alteration zones. In addition, profiles across the country rock, alteration zones, and Au-quartz lenses indicate that the heavy isotope has been preferentially retained in the alteration zones with the maximum enrichment in the ore. However, in the late cross-cutting quartz-carbonate lenses the lighter S isotope has been preferentially concentrated. The isotopic studies indicate that the primary S isotopic distribution of the district was modified by granitization and metamorphism during which the S ³² was mobilized to a greater extent, leaving the sulfides in the regions with the higher thermal histories enriched in S ³⁴ . The subsequent formation of the Au-quartz lenses was associated with intense chemical activity, and the S ³² was again more mobile, resulting in an enrichment of S ³⁴ in the alteration zones and in the ore.

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