A structural, petrographic, and geochemical study of the Jubilee Zn-Pb deposit, Nova Scotia, Canada, and a new metallogenic model

Abstract

The Jubilee Zn-Pb deposit (Cape Breton Island) is hosted by a thin brecciated unit at the top of the Macumber Formation (Carboniferous, Visean). This thin unit is capped by evaporites of the Carrolls Corner Formation, both formations being at the base of the Visean Windsor Group.Mapping in the vicinity of the Jubilee Zn-Pb deposit shows that the Mabou Group and part of the underlying Windsor Group lie unconformably over the Lower Windsor Group, implying a local truncation of 545 m of Windsor Group strata. Another omission of 500 m is recognized near the deposit, where the Upper Windsor lies directly on the Lower Windsor Group. These unconformities are considered to be associated with the regional Ainslie detachment, which is consistently found on Cape Breton Island at the top of the Macumber Formation, i.e., at the limestone and evaporite contact in the Lower Windsor Group.The fact that the limestone breccia which hosts mineralization is confined to a single stratigraphic level, as well as the angularity and the puzzlelike texture of thebreccia fragments, points to a link between the breccia and the Ainslie detachment. It is suggested that during extension, the high rheological contrast at the limestone-evaporite contact generated a tectonic brecciation in the relatively less ductile limestone, thus, a tectono-hydraulic origin is suggested for the mineralized breccia.Stratigraphic correlations using the contact between the Windsor Group and the Horton Group as a marker horizon indicate the presence of the Jubilee and Road subvertical faults in the vicinity of the deposit. They also show that the faults postdate lithification of the Macumber Formation.At the Jubilee deposit, the premineralization events comprise (1) stylolitization parallel to bedding, (2) brecciation of a finely laminated lithofacies, (3) migration of hydrocarbons, and (4) precipitation of brown fibrous calcite. The mineralization stage comprises (1) development of subvertical microfractures crosscutting breccia fragments, (2) precipitation of pyrite-marcasite-sphalerite-galena-chalcopyrite, and (3) precipitation of anhedral calcite.Low delta 13 C values are observed in the premineralization fibrous calcite (avg = -25ppm) and in the syn- to postmineralization anhedral calcite (avg = -15ppm). These signatures are interpreted to reflect the dominance of bicarbonate ions derived from organic carbon in the buried sequence. Low delta 18 O values (avg = -10ppm) in the anhedral calcite are interpreted to originate from precipitation in the presence of warm fluids. Radiogenic strontium (avg = 87 Sr/ 86 Sr of 0.70860) in the carbonate phases supports a clastic-dominated basin origin for the metal-rich brines.The proposed metallogenic model for the Jubilee Zn-Pb deposit comprises the following: (1) brecciation of a precursor limestone, creating a reservoir which is later filled by hydrocarbons, (2) sulfate reduction by hydrocarbons, increasing H 2 S concentrations, (3) ascent of metal-rich fluids toward the hydrocarbon and H 2 S reservoir along subvertical faults, (4) mixing of metal-rich fluids with H 2 S, leading to sulfide precipitation (pyrite, marcasite, sphalerite, and galena), and (5) filling of residual pores with anhedral calcite and sulfates (barite, gypsum, and anhydrite). During their ascent, the hot metal-rich brines leached the underlying rocks of the Horton Group. Sulfates most likely originated from the thick evaporite cap of the Carrolls Corner Formation.