Sulphur-isotope and elemental geochemistry studies of the Hemlo gold mineralization, Ontario: sources of sulphur and implications for the mineralization process: Discussion

Abstract

Undoubtedly, geochemical studies can provide key information on the genesis of an ore deposit but it is important to constrain these studies by reliable ore paragenesis. This is particularly important for the Hemlo gold deposit, which is characterized by a long history of deformation and metamorphism. In their paper the authors have used sulfur-isotope studies to determine the temperature of isotopic equilibrium and to speculate on the possible source of sulfur of barite and sulfides. In our opinion the methodology of their study is very questionable because they seem to give no consideration to the ore paragenesis and the relative timing of deformation, metamorphism, and gold mineralization. In the Golden Giant mine Michibayashi (1991) has recorded the presence of two distinct types of barite: highly folded and sheared-banded barite and massive barite filling fractures within the feldspathic-altered ore bodies. The massive barite appears to have formed after the potassic alteration as a result of syntectonic remobilization of the banded barite. The massive barite is characterized by microstructures (polygonal to dynamically recrystallized) indicating that barite was deformed plastically. Based on the mesoscopicand microscopic-scale structures the pyrite grains can be divided into three types: sub to euhedral pyrite, polygonal pyrite, and elongate pyrite with their long axes parallel to the schistosity. Onetching, the individual pyrite grains commonly show zoned structure with a core overgrown by a later generation of possibly metamorphic pyrite. In the Golden Giant mine, microstructural studies reveal a difference in the timing of formation of molybdenite and stibnite. molybdenite has formed synchronously with the development of S, and S, cleavages and was associated with the formation of potash-feldspar and muscovite. Stibnite occurs along rnicrofractures that cut across both S, and S, cleavages and is coeval with the development of S, (Michibayashi 1991). Gold is associated with both molybdenite and stibnite mineralization and appears to be unrelated to barite. The authors have used the equilibrium fractionation between barite and pyrite to determine the temperature. According to them, the temperatures determined by sulfur-isotope fractionation (varying between 492 and 557°C) are close to those determined by the oxygen-isotope fractionation between quartz and muscovite, although the oxygen-isotope geothermometer