Integrating Ancient Submarine Arc - Back Arc Successions in Mining Provinces and Modern Eruptions: Evidence for the Nature of Submarine Felsic Pyroclastic Eruptions

Abstract:

Deep water (1-2 km) basins within ancient submarine volcanic arcs and back arcs are the main host for Volcanogenic Zn-Pb-Cu-Ag-Au Massive Sulphide (VMS) mining districts. Consequently, exploration, mining and research of VMS ore deposits can provide much information about the nature of deep submarine volcanism.

Studies of ancient successions have the advantage over studies of modern submarine eruptions in that the successions can be well exposed in outcrop and drill cores and the sequence of events both before and after the eruption in question can be demonstrated. However, the study of modern eruptions provides the understanding of eruption and depositional processes and pyroclast textures that are required to interpret the ancient successions.

Integrating data from several VMS mining districts and modern felsic eruptions suggests that:

• In relatively shallow water (<200m), felsic pyroclastic deposits and their resedimented and reworked equivalents are more abundant than lavas.
• In deep water (>1 km) environments, felsic lavas, cryptodomes and shallow intrusions are more abundant than locally erupted pyroclastic rocks. However, mass flow deposits of pyroclastic debris derived from shallower or extra-basinal vents may be abundant.
• Major (>1km3) deep-water felsic pyroclastic eruptions are pumiceous magmatic eruptions rather than phreatomagmatic or phreatic, suggesting that water did not interact significantly with erupting magma until after explosive fragmentation. However, textures and sorting characteristics of the deposits indicate transport and deposition from water-supported mass flows.
• Deep water felsic pyroclastic eruptions commonly vented from dome complexes, formed small craters or larger calderas, and were directly followed by intrusion of degassed magma (domes).
• Basaltic fissure eruptions commonly occurred directly after felsic eruptions in deep water, indicating active deep faulting and a likely role in triggering felsic eruption.

Rhyolite with 5 wt% H₂O and O.1 wt% CO₂ ascending to the sea floor at 2km water depth may have vesicularity of 67% in the hypothetical case that no degassing occurs during ascent; this suggests that deep water pumiceous pyroclastic eruptions associated with VMS ore deposits resulted from ascent of very volatile-rich (>> 5 wt% H₂O) felsic magma batches.