Stratigraphy, Physical Volcanology, and Hydrothermal Alteration of the Footwall Rocks to the Winston Lake Massive Sulfide Deposit, Northwestern Ontario

Abstract

The Winston Lake Zn-Cu-Ag massive sulfide deposit is situated above a sequence of metamorphosed Archean calc-alkaline volcanic and volcaniclastic rocks. A detailed mapping, petrographic, and chemical study was undertaken to evaluate the stratigraphic and hydrothermal development of the footwall rocks with regard to depositional environment and spatial controls on metasomatism and mineralization. The footwall rocks are dominated by interlayered successions of metamorphosed volcaniclastic and volcanic rocks that have been extensively intruded and block faulted. Volcaniclastic-sediments were deposited at the base of the stratigraphy where they were interlayered with felsic pyroclastic deposits and/or their turbiditic equivalents. Locally massive sulfide and cherty exhalative beds were deposited. A relatively thick section of interlayered felsic and mafic lava flows were erupted and deposited above the basal volcaniclastic rocks; minor interflow elastic and base metal-poor exhalative sediments accumulated during pauses in mafic volcanism. An upper elastic succession accumulated above the lava flows; basinal volcaniclastic-sediments were deposited and were overlain in part by felsic pyroclastic material that was erupted from a distant, extraneous source. Interlayered mafic lava flows and volcaniclastic rocks cap the footwall stratigraphy and host the Winston Lake deposit and stratigraphically equivalent mineralized occurrences. Facies analysis of lava flows, along with the basinal distribution of volcaniclastic-sediments indicates the Winston Lake footwall stratigraphy developed in a subsiding, subaqueous rift environment. Subsidence was focussed in the rift axis; associated stresses resulted in development of synvolcanic faults within and distal to the rift axis. The dominance of passive eruption products indicates volcanism occurred in relatively deep water beneath the volatile fragmentation depth. Approximately 50% of the footwall stratigraphy has been hydrothermally altered in subconcordant to cross-stratal zones. Interaction of the rocks with metasomatic fluids, followed by isochemical metamorphism has resulted in unusual modal abundances of tremolite/actinolite, biotite, sillimanite, staurolite, anthophyllite/gedrite, chlorite, and quartz relative to metamorphosed primary compositions. Microprobe analyses indicate extreme Fe/Mg enrichment offerromagnesian silicates near the base of the stratigraphy. Mass balance analysis indicates variable enrichment of MgO, Fe2O3T, and K2O, and depletion of CaO and Na2O in altered rocks; TiO2 and Al2O3 were relatively immobile. Overall mass losses, indicative of metasomatic leaching, dominate alteration towards the base of the stratigraphy, whereas both gains and losses occurred in the upper portions of the section. Mg enrichment occurred in stratiform zones through shallow circulation of seawater-based hydrothermal fluids during progressive stratigraphic growth. Minor associated base metal-poor exhalites developed during intermittent pauses in volcanism and sedimentation. Substratiform zones of iron-aluminous-potassic alteration developed as chemically evolved fluids, which originated at depth, interacted with permeable lithologic units through which they buoyantly migrated. The distribution of alteration indicates that chemically-evolved fluids rarely reached the sea floor environment but were generally confined beneath impermeable stratigraphic units. Metalliferous fluids periodically passed through the footwall rocks to the sea floor; no distinct chemical or mineralogical fingerprint of their passage is evident in the rocks, suggesting the metalliferous fluids were similar to chemically-evolved fluids except in metal content. The metalliferous fluids reached the sea floor during at least two stages of stratigraphic growth in which metals were deposited as massive sulfides. The first stage was at the Pick Lake deposit, near the base of the stratigraphy and the second stage was at the Winston Lake deposit at the top of the section. The distribution and composition of alteration and associated base metal sulfide and cherty exhalative occurrences indicates the Winston Lake hydrothermal system was multistaged and involved multiple hydrothermal fluids. Stratigraphic development in a subsiding rift environment spatially controlled the movement of buoyant hydrothermal fluids through permeable lithologic units. Periodic synvolcanic faulting released metalliferous fluids to the sea floor where base metal sulfides were deposited.