Formation of hydrothermal sulphide deposits on the Niua volcano, Tofuaarc

Peterkin, Ben Robert (2021) Formation of hydrothermal sulphide deposits on the Niua volcano, Tofuaarc. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Niua South hydrothermal vent field is an intraoceanic arc volcano-hosted hydrothermal system at the northern terminus of the Tofua-Kermadec arc. The vent field is hosted within a 500-m wide volcanic crater on the eastern flank of the southern summit of Niua volcano and contains several hydrothermal mounds that host clusters of active, high-temperature (up to 325°C) chimneys and abundant inactive spires. This thesis presents results from the first detailed exploration and sampling of this vent field, including descriptions of the mineralogy and geochemistry of the hydrothermal deposits. Two distinct mineralogical suites were identified from 33 sulphide-sulphate chimney and talus samples: a high-temperature (≥300°C), Cu-rich suite and a lower-temperature (<300°C), Ba-Zn-rich suite. The high-temperature, chalcopyrite-dominated mineralization contains elevated Bi, Te, Se, and Sn relative to the low-temperature suite, which is dominated by barite, sphalerite, galena and sulphosalts, and contains relatively higher As, Sb, Ag, Cd, and Sr concentrations. The hydrothermal mounds and associated chimneys in the central and eastern regions of the crater are associated with relatively lower temperature fluids and a higher degree of subsurface fluid mixing. In contrast, an isolated hydrothermal mound in the northern end of the crater has more focused, high-temperature venting with visible boiling at the orifice on one high-temperature vent. The enrichment of mantle incompatible elements (Ba, Pb, As, and Sb) in the hydrothermal deposits is consistent with a more felsic substrate at Niua relative to typical mid-ocean ridge settings. The composition and morphology of the hydrothermal deposits at Niua South are similar to the seawater-dominated hydrothermal system at the northwest caldera vent field at Brothers volcano. The presence of primary bornite and euhedral hematite associated with chalcopyrite suggests that oxidizing magmatic fluids may be contributing to the hydrothermal output. High-resolution bathymetry and visual surveys obtained by autonomous underwater vehicle and remotely-operated vehicle, respectively, provide enough detail of the seafloor to accurately identify the extent of hydrothermal deposits on the seafloor. This thesis also summarizes results of radioisotope age determinations and volumes of hydrothermal deposits within the Niua South crater. The deposit consists of three main groupings of active chimneys and mounds, with the bulk of the deposited material located in the central part of the crater. The oldest sample age indicates that venting within the crater has occurred over at least the last ~3,000 years. Volume calculations for hydrothermal material, based on the high-resolution bathymetry indicate 50,600 ± 5,060 m³ of sulphide-sulphate material has accumulated on the seafloor within the crater. Using a bulk density estimate of the average composition of material of 2.65 ± 0.24 t/m³, the crater contains an estimated 134,000 ± 21,700 t of deposit material. Based on the maximum age of the deposit, sulphide-rich material for the Niua South has a maximum mass accumulation rate of 47 ± 8 t/yr, comparable to other sites along mid-ocean ridges where data are available.

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