Stratigraphy and Alteration of the Footwall Volcanic Rocks beneath the Archean Mattabi Massive Sulfide Deposit, Sturgeon Lake, Ontario

Abstract

Subaqueous mafic lava flows and breccias, mafic debris-flow and felsic pyroclastic-flow deposits, and felsic lavas form a 2 km thick succession beneath the Archean Mattabi massive sulfide deposit in northwestern Ontario. The lowermost 500 m is composed of massive amygdaloidal mafic flows, flow breccias, and heterolithic debris flows. Thin (<50m) amygdaloidal felsic lava flows and felsic block and ash deposits overlie the basal mafic flow sequence. This felsic horizon thickens both eastward and westward away from the Mattabi deposit and suggests the former existence of localized felsic vents on a broad shield volcano. Rocks interpreted to be mafic debris-flow and felsic pyroclastic-flow deposits lie above the felsic horizon and represent a change in eruptive style from lava extrusion to phreatomagmatic volcanism. The change is believed to be a result of a shallowing upward sequence and/or an increasing water/magma ratio. The mafic debris-flow deposits are massive to thick-bedded, poorly graded and composed of scoriaceous to amygdaloidal mafic clasts. Felsic pyroclastic-flow deposits include a) massive basal beds and overlying bedded ash tuff, b) well-bedded., graded lapilli tuff and c) massive pumice-rich beds. Felsic pyroclastic deposits intercalate and intermix with mafic debris flow deposits west of Mattabi and together these constitute the upper 500-600 m of the footwall succession. Massive pyroclastic beds truncate mafic debris-flow deposits and mark the culmination of explosive felsic volcanism prior to the ore-forming event. Massive pumiceous pyroclastic beds and quartz-porphyritic ash-flow tuff form the immediate mine footwall strata. Alteration within the footwall strata has been divided into four major mineralogical assemblages: 1) least altered (typical greenschist facies assemblages with moderate carbonatization), 2) iron carbonate-chlorite, 3) sericite and 4) chloritoid. Least-altered assemblage rocks are largely amygdaloidal mafic lava flows and mafic debris-flow deposits which are situated 4 to 5 km west of the Mattabi deposit. Iron carbonate-chlorite alteration is confined largely to felsic pyroclastic rocks and lavas within the upper 600 m of the footwall strata; sericitization is also widespread within these rocks. Chloritoid is developed in both sericite and iron carbonate-chlorite assemblage rocks. Mass balance computations indicate that constant volume has been maintained within all altered lithologies except iron carbonate-chlorite assemblages within felsic lavas; these rocks have undergone a 10 to 20% volume reduction. Iron carbonate-chlorite assemblage rocks display elemental gains of Fe, Mn and CO2, and losses of Si. Sericitization produces marked K and Rb gains at the expense of Na. Comparisons of iron carbonate-chlorite and sericite-assemblage rocks to similar chloritoid-bearing equivalents reveal no consistent elemental trends. It is envisioned that heated connate seawater/rock interactions within mafic lava flows and breccias produced a large reservoir of metal-rich hydrothermal solutions. Synvolcanic faulting allowed the rapid discharge of fluids from the reservoir. Diffuse, semiconformable alteration zones were developed in overlying felsic pyroclastic rocks and lavas as the solutions migrated upward to the seafloor surface. Focused discharge of fluids at several locations resulted in large-scale precipitation of iron sulfides on the seafloor.