Journal of Geochemical Exploration, 36 (1990) 73-102 73 Elsevier Science Publishers B.V., Amsterdam m Printed in The Netherlands The Laforma deposit, an atypical epithermal-Au system at Freegold Mountain, Yukon Territory, Canada BRENT I.A. McINNES', JAMES H. CROCKET 2 and WAYNE D. GOODFELLOW3 1Department of Geology, University of Ottawa, Ottawa-Carleton Geoscience Centre, Ottawa, Ont. KIN 6N5, Canada 2Department of Geology, McMaster University, Hamilton, Ont. L8S 4M1, Canada aMineral Resources Division, GeologicalSurvey of Canada, Ottawa, Ont. KIS 0,48, Canada (Received March 12, 1989; revised and accepted June 20, 1989) ABSTRACT McInnes, B.I.A., Crocket, J.H. and Goodfellow, W.D., 1990. The Laforma deposit, an atypical epithermal-Au system at Freegold Mountain, Yukon Territory, Canada. In: J.W. Hedenquist, N.C. White and G. Siddeley (Editors), Epithermal Gold Mineralization of the Circum-Pacific: Geology, Geochemistry, Origin and Exploration, II. J. Geochem. Explor., 36: 73-102. Gold mineralization at Freegold Mountain is temporally and spatially related to Late Creta- ceous rhyolitic volcanism. Gold-bearing quartz veins such as the Laforma deposit are high-grade, low-tonnage deposits with grades ranging from 0.4 to 1.0 oz/t. Fluid inclusions from the lower portions of the Laforma mine homogenize at 185 °C and have salinities between 2 and 4.5 eq. wt.% NaCl. Above the deeper ore, fluid inclusions homogenizingto the liquid phase show extreme ranges in homogenization temperatures ( 165-430 ° C) due to two phase entrapment of a boiling fluid, and a wide variation in salinity (4-43 wt.% NaC1). Although evidence for boiling is commonly reported in epithermal systems, fluids of such high salinity have not been recorded in past studies. These highly saline fluids are interpreted to be residual brines, left from a period of extensive boiling of a hydrothermal fluid with an initial concentration of around 4 wt.% NaC1. To achieve such high salinities, a high-enthalpy, steam-dominated system closed to dilution by surface water is required. Fluid-inclusionwaters from the lower levels of the deposit, below the boiling zone, have stable isotopic signatures indicative of meteoric water with 5D = - 138%o and J180 = - 18.4%o. Boiling of the Laforma fluid has caused a marked shift towards heavier hydrogen and oxygen isotopic values and has resulted in the precipitation of carbonate and silicate gangue minerals and gold. Fluid-inclusion evidence indicates that boiling zones at Laforma formed between 1 and 1.5 km below the Late Cretaceous paleosurface and stable isotopic evidence suggests that the paleolati- tude of Freegold Mountain during the Late Cretaceous time was south of the present-day Yukon- B.C. border. 0375-6742/90/$03.50 © 1990 Elsevier Science Publishers B.V.