Subaqueous strombolian eruptions, and eruption-fed pyroclastic deposits in a Paleoproterozoic rifted-arc: Hidden formation, Flin Flon, Canada

Abstract

The Hidden formation of the Paleoproterozoic Flin Flon arc assemblage comprises the stratigraphic hanging wall to the volcanogenic massive sulfide (VMS) deposits at Flin Flon, Manitoba, Canada. It represents a basaltic volcanic edifice erupted within a large subsidence structure developed in a rifted-arc environment. It is predominately pillow lavas with subordinated amounts volcaniclastic rocks.Rocks that stratigraphically overly the Schist and Mandy VMS deposits ∼5 km southeast of the main Flin Flon deposits have been correlated through litho- and chemo-stratigraphy with the Hidden formation. They display evidence of pyroclastic volcanism and concomitant subsidence. Pillow lavas, and hyaloclastite are unequivocal evidence of subaqueous eruption.The Hidden formation in the Schist – Mandy area comprises mafic tuff, mafic megabreccia, basalt lava, fluidal-clast breccia, and plagioclase crystal-rich volcaniclastic lithofacies, intruded by synvolcanic dikes and sills. The megabreccia lithofacies is characterized by angular basalt clasts (1–20 m) in a matrix of finer basalt clasts and is interpreted to have been deposited by mass flows due to slumping of the caldera wall. A within flow vertical transition in the lavas from massive to pillowed flows suggests a slowing of emplacement velocity, decrease in temperature, and an increase in the viscosity during eruption. Based on the monolithic nature of the fluidal-clast breccias, and the fluidal shapes of clasts, with delicate tails and intact glassy margins, they are interpreted to be the result of strombolian eruptions. An increase in the ingestion of water in the vent(s) lead to a change from magmatic to phreatomagmatic fragmentation. Planar and cross laminations, graded beds and the abundance of broken plagioclase crystals, in combination with scoria and lithic clasts suggest the mafic tuff and plagioclase crystal-rich volcaniclastic lithofacies were deposited by subaqueous, eruption-fed density currents derived from a phreatomagmatic eruption.Detailed mapping of the volcanic lithofacies in the Schist – Mandy area has facilitated the identification of synvolcanic faults defining inner caldera margins and volcanic vents. These structures when traced downwards through stratigraphy define structural corridors corresponding with the Schist and Mandy VMS deposits, indicating they represent long-lived structures that also acted as magmas and fluid pathways.