Sulphur-isotope and elemental geochemistry studies of the Hemlo gold mineralization,. Ontario: sources of sulphur and implications for the mineralization ...
DISCUSSIONS
Sulphur-isotopeand elemental geochemistry studies of the Hemlo gold mineralization, Ontario: sources of sulphur and implications for the mineralization process:' Discussion KATSUMICHIBAYASHI AND SUBHASH JAIRETH Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Harbin Industrial University on 06/03/13 For personal use only.
Geology Department, James Cook University, Townsville, Queensland 481 1, Australia
Received August 19, 1991 Accepted September 25, 1991 Can. J. Earth Sci. 28, 2069 (1991)
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-bandedbarite and massive barite filling fractures within the feldspathic-alteredore 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 mesoscopic- and 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 associatedwith both molybdeniteand 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 'Paper by H. G. Thode, T. Ding, and J. H. Crocket. 1991. Canadian Journal of Earth Sciences, 28: 13-25. Rinted in Canada
gives temperatures of 489 + 77, 613, and 499 2 30°C. They rejected one determination, which gave a temperature of 727"C, on the basis that that sample might not reflect isotopic equilibrium.A close examinationof the sulfur-isotopedata of coexisting minerals (their Table 4) raises doubts about the attainment of equilibrium between different sulfides. It is true that at high temperatures the isotopic fractionation between sulfur species is significantly small, which can complicate any judgement about isotopic equilibrium. The equilibrium isotopic fractionation factors indicate (Ohmoto and Rye 1979) that at isotopic equilibrium, the pyrite sulfur should be lighter than the molybdenite sulfur and heavier than the stibnite and pyrrhotite sulfur. On this basis the molybdenite-pyrite pair does now show isotopic equilibrium (their Table 4). Similarly, some values for stibnite and pyrrhotite (their Table 4) do not indicate isotopic equilibrium with pyrite. The lack of isotopic equilibriumbetween coexisting sulfides throws doubt on the attainment of complete isotopic equilibrium between pyrite and barite, which is used by the authors as a geothermometer. We believe that the ore paragenesis at Hemlo is characterized by more than one generation of pyrite. Hence sulfur-isotope analysis of the bulk pyrite fraction from the sample is not going to give the true temperature of isotopic equilibrium. Even if the sulfur-isotope composition between pyrite and barite is close to the equilibrium fractionation and is able to give the true equilibration temperature, it is doubtful that these temperatures reflect those of the gold-forming event, because the ore paragenesis has clearly shown that most of the gold in Hemlo was formed synchronously with molybdenite and stibnite and is not related to barite.
Acknowledgments K.M. gratefully acknowledges Hemlo Gold Mines Inc. for its financial support and for access to the mine. K.M. was supported by an Overseas Postgraduate Research Scholarship and a James Cook Postgraduate Scholarship. K. 1991. The role of deformation on mineralization MICHIBAYASHI, within the Hemlo Gold deposit, Canada. Kalgoorlie '91, Structural Geology in Mining and Exploration, Extended Abstracts, Geology Department and University Extension, University of Western Australia Publication 25, pp. 90-92. OHMOTO,H., and RYE,R. 0. 1979. Isotopes of sulfur and carbon. In Geochemistry of hydrothermal ore deposits. Edited by H. L. Barnes. Wiley, New York, pp. 509-567.
