New constraints on regional brecciation in the Wernecke Mountains, Canada, from He, Ne, Ar, Kr, Xe, Cl, Br and I in fluid inclusions

Abstract

Regionally-distributed brecciation in the Proterozoic Wernecke Mountains of Canada is associated with hydrothermal alteration, that is similar to that seen in the Fe-oxide-Cu-Au class of ore deposit and characterized by variably saline fluid inclusions. Two phase liquid-vapour (LV) fluid inclusions (< 26-30 wt.% salt) in fluorite and barite, plus most quartz samples, are characterized by: greater than seawater Br/Cl values of up to 3.0 × 10- 3; I/Cl values of up to 65 × 10- 6; elevated36Ar concentrations of 4 to > 100 ppb and40Ar/36Ar of < 1000-2000. Similar fluid inclusions in sulphide have near atmospheric20Ne/22Ne and21Ne/22Ne values of ~ 9.8 and 0.029, respectively; and are enriched in4He* with4He*/40Ar* and4He*/21Ne* values of greater than the crustal production ratios. All these characteristics are typical of low temperature (< 250 °C) sedimentary formation waters. Halite saturated liquid-vapour-daughter (LVD) fluid inclusions, with salinities up to 44 wt wt.% NaCl-CaCl2 eq., are present in all of the samples but are predominant in only two quartz samples. One of these samples, from the Slab mega-breccia is situated close to a horizon of meta-evaporitic scapolite and it has the lowest measured Br/Cl and I/Cl values of 0.37 × 10- 3 and 0.32 × 10- 6, respectively. These data favour fluid interaction with meta-evaporitic scapolite, or halite, as an important mechanism for increasing fluid salinity. Fluid inclusions with variable salinities (LV and LVD) in a sample from the Hoover locality have the highest measured40Ar/36Ar values of ~ 40,000 and variable36Ar concentrations of 0.7 to 9 ppb. Similar fluid inclusions in sulphide have crustal20Ne/22Ne and21Ne/22Ne values of 6.5 and 0.35;3He/4He of ~ 0.002; and4He*/40Ar* and4He*/21Ne* values only slightly above the average crustal production ratios. Post-entrapment ingrowth of radiogenic40Ar* is minor and correctable, ingrowth of radiogenic4He* and nucleogenic21Ne* and22Ne* are also minor, suggesting that with care these isotopes can also be used to constrain fluid origins in Proterozoic samples. The noble gas data are most easily explained by mixing magmatic fluids derived from magmas generated in U-rich basement rocks and surface-derived sedimentary formation waters. Therefore, these data confirm similar fluid sources at Wernecke as in some other Fe-oxide-Cu-Au deposits, and further constrain a probable episode of cryptic magmatism in ancestral North America.