Cousens, B.L., Falck, H., van Hees, E.H., Farrell, S., and Ootes, L. 2005: Pb Isotopic Compositions of Sulphide Minerals from the Yellowknife Gold Camp: Metal Sources and Timing of Mineralization; Chapter 24 in Gold in the Yellowknife Greenstone Belt, Northwest Territories: Results of the EXTECH III Multidisciplinary Research Project, (ed.) C.D. Anglin, H. Falck, D.F. Wright and E.J. Ambrose; Geological Association of Canada, Mineral Deposits Division, Special Paper No. p. 24. PB ISOTOPIC COMPOSITIONS OF SULPHIDE MINERALS FROM THE YELLOWKNIFE GOLD CAMP: METAL SOURCES AND TIMING OF MINERALIZATION Brian L. Cousens 1 , Hendrik Falck 2 , Edmond H. van Hees 3 , Sean Farrell 4 , and Luke Ootes 2 1. Ottawa-Carleton Geoscience Centre, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6 2. C.S. Lord Northern Geoscience Centre, PO Box 1500, Yellowknife, NT X1A 2R3 3. Geology Department, Wayne State University, Detroit, MI 48202 USA 4. Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5 INTRODUCTION The Yellowknife Volcanic Belt and surrounding metatur- bidites, commonly referred to as the Yellowknife Supergroup, host two producing gold mines, several past- producing deposits, and many showings and potential pro- ducers (Franklin and Thorpe, 1982; Falck, 1992; Thorpe et al., 1992). The host rocks are of late Archean age, but the gold is commonly found in quartz veins and shear zones that postdate the deposition of the host rocks (Padgham, 1992). These veins and shear systems do not include minerals that provide high-precision geochronological data, such as zircon or monazite (titanite is present but is poor for dating purpos- es). Thus the absolute age of the mineralizing event (or events) is not well constrained. The goals of this study are to utilize isotopes of lead (Pb), a common trace element in many sulphide minerals and a major element in galena and sphalerite, to attempt to date gold deposition and to deter- mine what rocks may have been sources of the Pb (and gold?) in these shear zone or quartz-vein systems. Pb Isotope Systematics Lead has four isotopes with masses 204, 206, 207, and 208. 208 Pb is produced by the decay of 232 Th, whereas 207 Pb and 206 Pb are produced by the decay of 235 U and 238 U, respec- tively (Faure, 1986). 204 Pb is a stable isotope whose abun- dance does not change over time, and for this reason it is common to refer to the abundance of any other isotope of Pb relative to 204 Pb, e.g., 206 Pb/ 204 Pb. Lead is a common ele- ment in sulphide minerals such as galena, sphalerite, arsenopyrite, chalcopyrite, pyrrhotite, and pyrite. The Pb is inherited from the fluids that precipitate the sulphide miner- als, and at the time of deposition the minerals will therefore record the isotopic composition of the ore-bearing fluid. However, most of these minerals exclude U and Th, such that U/Pb and Th/Pb are approximately zero. Thus, the Pb isotopic composition of a sulphide mineral will not change appreciably over geological time. Surface weathering has no effect on Pb isotope ratios, and post-crystallization meta- morphic or intrusive events may modify isotopic ratios if new Pb is introduced into the sulphide mineral during recrys- tallization (Thorpe, 1982). Mineralizing fluids generally pass through a large volume of crust, and as a result the Pb isotopic compositions of the fluids will be averages of the crust traversed by the fluids (including any initial hydrothermal magmatic component). This average allows for reconstruction of the Pb isotopic evolution of the crust with time, providing that the age of Pb- bearing deposits can be firmly established by independent methods (usually conformable massive sulphide deposits, summarized in Stacey and Kramers, 1975; Faure, 1986; Zartman and Haines, 1988). Pb isotope ratios in a sulphide mineral from a deposit of unknown age can then be plotted against the isotopic evolution curve, which gives a “model age” for that mineral. Several models for the isotopic evolu- tion of the crust have been proposed, some more complex than others, and thus “model ages” differ depending on the model used ( Stacey and Kramers, 1975; Zartman and Haines, 1988; Kramers and Tolstikhin, 1997). The most common mineral used to determine model ages is galena, because of its extremely high Pb content, and its common occurrence in conformable massive sulphide deposits. Some sulphide minerals such as pyrite can include minor amounts of U, and thus U/Pb will be greater than zero and allow radioactive decay to form new 207 Pb and 206 Pb, thus increasing 207 Pb/ 204 Pb and 206 Pb/ 204 Pb as time progresses after crystallization (termed radiogenic ingrowth). Provided that pyrite occurs with other minerals with different U/Pb (and preferably one mineral with U/Pb = 0), then after any time, the minerals will lie on a straight line (termed an isochron) in a plot of 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb, and the slope of that isochron will be a function of the time that has passed since the minerals crystallized (Faure, 1986). Critical to this analysis is knowing that all minerals in the system are cogenetic and that they have remained closed to U or Pb dif- fusion since crystallization, either of which can be difficult to ascertain. This analysis also assumes that the isotopic composition of the fluid was constant during the deposition of the minerals being analyzed. Linear arrays of data points in a Pb-Pb plot could also be the result of mixing of Pb with distinct isotopic composi- tions, either as a result of mixing of Pb from different crustal sources as mineralizing fluids traverse the crust, or as a result of a post-crystallization “disturbance event”. In some