Early carbonate veining and gold mineralization in the Timmins camp: Depositional context of the Dome mine ankerite veins

Abstract

The earliest auriferous veining event in the development of the over 16 Moz gold endowment at the Dome mine was an extensive set of ankerite veins (over 5400 m strike length). The depositional context of the Dome mine ankerite veins has been historically contentious and their role in the mineralization history of the deposit not well understood. We present a deposit wide geochemical study of the ankerite veins and their host rocks, with observations from new underground mapping. Ankerite veining across the mine is almost exclusively restricted to the Vipond Formation, which is Fe rich and tholeiitic in nature. Metamorphic fluids and ankerite vein formation generally focused along flow boundaries in the volcanic sequence, however, carbonatization is ubiquitous. The flows of the Vipond Formation maintain their protolith geochemical signatures as REE, Zr, Al2O3, TiO2, and Y are largely immobile during mineralization and alteration, which mobilized Na2O, K2O, and locally Fe2O3. The ankerite vein structures are reactivated and overprinted by quartz and quartz-tourmaline veining, and gold grade is variable, ranging from <1 to 98 g/t with the highest grades related to the overprinting veining. Gold in the ankerite veins occurs primarily at the vein margins associated with pyrite mineralization as inclusions, in fractures, or along pyrite grain boundaries. There are no correlations with trace metals and gold, making vectoring towards mineralization difficult. The δ13C and δ18O values for dolomite from the ankerite veins range from −2.2 to 1.0‰ δ13CPDB and 10.8–14.9‰ δ18OVSMOW with averages of −0.7 ± 0.7‰ δ13CPDB and 13.2 ± 1.1‰ δ18OVSMOW (n = 36). There is a trend of increasing δ18OVSMOW values from 10.8 to 14.9‰ down dip of the ankerite veins and to the Northwest, a likely result of decreasing fluid temperatures away from the Dome fault deformation zone. The Dome Fault deformation zone is a region of intensely deformed and carbonatized ultramafic and felsic rock that likely acted as a fluid conduit for the ankerite-forming metamorphic fluids. The intense carbonatizatin that accompanied this fluid event played an important role in the mineralization history of the deposit by facilitating a rheological and geochemical regime favoring D3-4 auriferous vein formation in several regions of the mine by reducing permeability and creating new competency contrasts.