Mountains graphic with white diamonds on itGeoscience and Exploration

Reconstruction of the Volcanic and Ore-forming Environment of the Neoarchean Sunrise VMS Deposit, Beaulieu Belt: Implications for VMS Formation in the Slave Craton

Tuesday, November 19, 2019 - 11:20am to 11:40am Theatre One


Y.M. DeWolfe (Presenting)
Mount Royal University
B. Knox
Northwest Territories Geological Survey

The Sunrise VMS deposit is located within the Neoarchean Beaulieu volcanic belt of the Slave craton, ~110 km northeast of Yellowknife. The belt is comprised of mafic to intermediate volcanic flows with lesser felsic volcanic rocks. The deposit (historic indicated resources of 1.52 Mt at 5.99% Zn, 2.39%Pb, 0.08% Cu, 262 g/t Ag, and 0.67 g/t Au) is a banded polymetallic Zn-Pb-Cu-Ag-Au sulfide lens hosted by rhyolitic rocks, but the age, stratigraphy, and volcanic setting of the deposit remain enigmatic.

Detailed mapping and core-logging of rocks in the Sunrise area indicate a complex stratigraphy comprising numerous lithofacies. The footwall to the deposit consists—from oldest to youngest—of pillow basalt (>200 m), formerly glassy rhyolite lobes surrounded by hyaloclastite (~100 m), felsic volcaniclastic rocks ranging from tuff breccia to tuff with depositional units between 2–10 m totaling ~100 m, and a massive (~100 m), weakly quartz and plagioclase porphyritic rhyolite dome with brecciated margins. Stratigraphically above the porphyritic rhyolite is a sequence of strongly sericitized and silicified, interbedded felsic tuff to tuff breccia units. These units contain semi-massive, massive, and stringer-style mineralization. The mineralization is dominated by pyrite and sphalerite with lesser amounts of pyrrhotite, galena, arsenopyrite, and chalcopyrite. The felsic tuff to tuff breccia units that host the mineralization are locally overlain by argillite. Where present, the argillite is up to 14 m thick, dark grey to black, finely laminated, and commonly contains pyrite (5–15%), sphalerite (1–2%), and trace amounts of galena, chalcopyrite, and arsenopyrite. The argillite, or the felsic volcaniclastic unit where the argillite is not present, is overlain by pervasively carbonate altered pillowed basaltic-andesite of the hanging wall. Whole rock trace element and Sm–Nd and Lu–Hf isotopic data of the basaltic-andesite units in the footwall exhibit calc-alkaline, arc-like signatures. The mantle source is interpreted to be enriched mantle (E-MORB-type) with a minor degree of subduction zone input (slab metasomatism or sediment subduction). 

The rhyolitic rocks of the footwall, totaling over 500 m of stratigraphy, terminate abruptly along strike to the south where the stratigraphy is dominantly intermediate to mafic pillow lavas with subordinate amounts of volcaniclastic rocks. The rhyolite flows and domes indicate a vent proximal environment and the thick sequences of rhyolitic volcaniclastic rocks define a synvolcanic subsidence structure in which they accumulated. This suggests that the deposit, and host volcaniclastic rocks correspond with an area of subsidence, fracturing and the accumulation of permeable debris on the margin of a rhyolite dome. Synvolcanic faults associated with subsidence would have acted as both magma and hydrothermal fluid pathways adding to the favorable conditions for the formation of a VMS deposit.