Magma chamber recharge and sulfide mineralization in a Proterozoic mafic sill from the Franklin magmatic event, Victoria Island, NWT, Canada.

English Abstract:

Four drill cores provide continuous sampling through a 43 to 45m thick tholeiitic sill exposed over an area >90 km². The sill has a classic S-shaped profile when Mg# is plotted relative to height, and shows little change in thickness or internal stratigraphy along strike. The lower ~8m of the sill is a picritic gabbro in which % olivine increases upward from the lower contact to ~50% at the top of the picritic zone. The upper 35-37m of the sill, is a dolerite in which the Mg# decreases upward from ~62 at 9m to ~36 at 36m above the base, and then increases upward to ~64 at the upper contact. Geochemical modeling indicates that the basal picritic zone resulted from redistribution of initial phenocrysts by either crystal settling or flow segregation, and that in-situ crystal fractionation (~15% plagioclase, ~12% augite, ~3% olivine) and interstitial melt migration produced the Fe-enrichment trend in the dolerite. All four cores contain a small excursion marked by an abrupt shift to higher Mg# (~54 below to ~59 above) at the same stratigraphic level (~25m above the base). Magma chamber recharge would require ~15% new magma with the chilled margin composition in order to produce the observed change. The Mg# shift horizon is associated with an increase in Mg, Ca, Co, Ni, & S and a decrease in Fe, Ti, Mn, P & Cr. No change in Cu (~100ppm), Pt (~10ppb), or Pd (~10ppb) is observed. Core compositions of plagioclase (An71), augite (En47Fs13Wo40), & opx (Mg#75) are unchanged across the horizon, but plagioclase and augite grain-size decreases, and augite zoning is less Fe-rich above the horizon. Modal abundance of Fe-Ti oxides decrease (from ~3% to ~0.5%) and sulfides increase (from ~0.05% to ~0.5%) above the horizon. Oxides above the horizon are lower in Ti than those below. Sulfides above the horizon have pyrite/chalcopyrite >10, while below pyrite/chalcopyrite ~1. The dolerite above the horizon is more extensively altered than that below, and contains abundant titanite.