ARTICLE WilliamA.Turner á JeremyP.Richards BruceE.Nesbitt á KarlisMuehlenbachs á JohnL.Biczok Proterozoic low-sul®dation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada Received: 25 April 2000 /Accepted: 18 March 2001 / Published online: 27 June 2001 Ó Springer-Verlag 2001 Abstract The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sul®- dation epithermal deposits. The deposits are hosted by rhyolitic ¯ows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades up to 24 g/t over 30 cm) occur in the Chalcedonic Stock- work Zone. Quartz veining occurs in two main types: barren A veins, characterized by ®ne- to coarse-grained comb quartz, with ¯uorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalce- donic silica and ®ne-grained quartz, locally intergrown with ®ne-grained gold or electrum. A third type of quartz vein C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with ®ne-grained hematite and rare electrum. Fluid in- clusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temper- ature Th=150 to 220 °C), low salinity 1 eq. wt% NaCl, with trace CO 2 ), locally boiling ¯uid; this ¯uid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposi- tion. Assemblage 2 inclusions represent a lower tem- perature Th=90 to 150 °C), high salinity calcic brine 23 to 31 wt% CaCl 2 -NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alter- nating ¯uxes of dilute ¯uid and local saline ground- water. No workable primary ¯uid inclusions were observed in the C veins. The A-vein quartz yields d 18 O values from 8.3 to 14.5& average=10.91.7& [1r], n=30), whereas d 18 O values for B-vein quartz range from 11.2 to 14.0& average=13.00.9&, n=12). Calculated d 18 O H2O values for the dilute mineralizing ¯uid from B veins range from ±2.6 to 0.2& average= ±0.80.9&, n=12) and are consistent with a domi- nantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz sam- ples contain mixed ¯uid inclusion populations. Howev- er, the fact that A-vein quartz samples extend to lower d 18 O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute ¯uid. Hydrogen isotopic ratios of ¯uid inclusion waters ex- tracted from eleven quartz samples of both vein types range from dD FI =±56 to ±134&, but show no partic- ular correlation with vein type. In most respects, the mineralogical and ¯uid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sul®dation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sul®dation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal ¯uid assemblage. The signi®cance of these sa- line brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition. Introduction Epithermal deposits are typically formed by the shallow circulation of low to moderate temperature hydrothermal ¯uids in terrestrial settings e.g., volca- Mineralium Deposita 2001) 36: 442±457 DOI 10.1007/s001260100181 Editorial handling: A.C. Brown Bruce Nesbitt is now deceased. W.A. Turner &) á J.P. Richards &) á B.E. Nesbitt K. Muehlenbachs &) Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Canada E-mail: Allan_Turner@gov.nt.ca E-mail: Jeremy.Richards@UAlberta.ca E-mail: Karlis.Muehlenbachs@UAlberta.ca J.L. Biczok &) Phelps Dodge Corporation of Canada, 223-530 Century St., Winnipeg, MB, R3H OY4; Canada E-mail: jbiczok@phelpsdodge.com