CONSTRAINTS FROM FLUID-INCLUSION DATA ON THE ORIGIN OF THE JUBILEE CARBONATE-HOSTED Zn-Pb DEPOSIT, CAPE BRETON, NOVA SCOTIA*

GUOXIANG CHI, MARTINE M. SAVARD and YVON HÉROUX
Geological Survey of Canada, Quebec Geoscience Centre, P.O. Box 7500, Sainte-Foy, Quebec G1V 4C7

* Geological Survey of Canada contribution number 31794.


Abstract

Aqueous (Aq) and hydrocarbon (HC) fluid inclusions were studied in fibrous calcite (pre-ore), sphalerite, and coarse, anhedral calcite (syn- to post-ore) of the Jubilee Zn-Pb deposit, Cape Breton, Nova Scotia. The deposit is hosted by a limestone breccia at the top of the Macumber Formation (limestones) and under the Carrolls Corner Formation (evaporites). Aq inclusions from different minerals have similar temperatures of first melting (- 62.1 to -46.7 C), indicating a H2O-NaCl-CaCl2 compositional system. Salinities (wt%), NaCl/(NaCl + CaCl2) weight ratios, and temperatures of homogenization (Th) of Aq inclusions range from 21 to 27%, 0.16 to 0.73, and 53 to 79 C for fibrous calcite, 15 to 24%, 0.26 to 0.64, and 59 to 228 C for sphalerite, and 12 to 24%, 0.17 to 0.72, and 66 to 224 C for anhedral calcite. The Th -salinity correlation suggests that a high-temperature, high-salinity fluid first mixed with a low-temperature, high-salinity fluid, then mixed with a fluid of low temperature and salinity. HC inclusions coexist with Aq inclusions. The Th of HC inclusions ranges from 34 to 89 C for fibrous calcite, 46 to 87 C for sphalerite, and 31 to 238 C for anhedral calcite. Fluid pressures estimated from the intersection of isochores of coexisting Aq and HC inclusions ranges from 175 to 363 bars for the fibrous calcite, and from 160 to 398 bars for the anhedral calcite. The microthermometric data suggest that the Jubilee deposit formed from a hot, metal-carrying brine in a shallow environment (probably less than 1500 m) characterized by a low background temperature. Hydrocarbons migrated into the limestone breccia before the mineralizing event, reacted with oxidized sulfur derived from the evaporites, and produced a reservoir of reduced sulfur available for mineralization when the metal-carrying brine migrated through.


Keywords: Zn-Pb deposit, carbonates, aqueous fluid inclusions, hydrocarbon fluid inclusions, basinal brines, Jubilee, Cape Breton, Nova Scotia.