MAGMATISM AND THE ORIGIN OF VOLCANOGENIC MASSIVE SULFIDE (VMS) DEPOSITS

The role of magmatism in hydrothermal systems that produce Volcanogenic Massive Sulfide (VMS) deposits has been debated for decades. Modern seafloor deposits show abundant evidence that magmas provide (1) heat that drives hydrothermal circulation, (2) volatiles that contribute water, carbon, and sulfur, and in some cases HCl and HF. However, the role of direct contributions of metals from magmatic volatiles remains unresolved.

On the modern seafloor, known hydrothermal activity is located along mid-ocean ridges (MOR) (65%), in back-arc basins (22%), along volcanic arcs (12%), and on intraplate volcanoes (1%). In contrast, most VMS deposits preserved in the geologic record have formed in extensional volcanic arc and back-arc settings like those of the modern Okinawa Trough, Lau Basin, and Manus Basin.

High-temperature (350-400°C) black smoker vents are the most recognizable features of seafloor hydrothermal activity. Black smoker vents on mid-ocean ridges provide empirical geochemical evidence that copper solubility and transport increases exponentially at temperatures above 300°C. Subvolcanic heat from axial magma chambers drives sustained hydrothermal venting at temperatures above 250-300°C. However, MOR magmatic fluid components are limited to CO₂, He, and some H₂S with little evidence for magmatic water, other volatiles, or metals.

In back-arc settings like Manus Basin, SW Pacific, acidic hot springs hosted in intermediate to felsic rocks have significantly different hydrothermal fluids due mainly to addition of abundant magmatic volatiles (H₂O, CO₂, SO₂, HCl, HF), resulting in fluids with pH<1 and δD values as low as -11‰ (Reeves et al., 2011). Magmatic water end-members are inferred to have δD values of about -25‰. Additionally, δ³⁴S systematics indicate disproportionation of SO₂ in the vent fluids, giving δ³⁴S_H2S of about -4 to -7.5 and δ³⁴S_SO4 of 9.5 to 20‰. Sulfide deposits at Manus Basin vent sites are Cu, Zn, and Au rich. Studies of melt inclusions in associated volcanic rocks by Yang and Scott (2005) indicate vigorous degassing and contributions of Fe, Cu, Zn, and Pb to the hydrothermal systems. Thus, current evidence suggests that hydrothermal leaching of metals dominates in MOR settings and direct magmatic supply of metals may be more important in arc and back-arc settings.