Texture, mineralogy and geochemistry of hydrothermally altered submarine volcanics recovered southeast of Cheshire Seamount, western Woodlark Basin

• Brecciated andesitic to rhyolitic volcanics have distinct hydrothermal overprint.
• Multiple fluid flow episodes in the system lead to five quartz formation stages.
• Chlorite geothermometry constraints alteration temperature at 240–300 °C.
• Trace element patterns in Au-bearing sample are distinct from barren counterparts.

The western Woodlark Basin lies within a gold-rich metallogenic province. This area is characterized by detachment faults that record ongoing extension and act as major pathways for the circulation of hydrothermal fluids. Dredging from the flanks of a submarine ridge southeast of Cheshire Seamount, western Woodlark Basin retrieved hydrothermally altered monomictic to polymictic crackle, mosaic and chaotic breccias with at least 30% clasts > 2 mm in diameter. The precursor rocks are andesitic to rhyolitic in composition, but have been intensely hydrothermally altered, with about 90% of the volcanic glass replaced by secondary minerals. The breccias show five generations of quartz growth, with the first being related to magmatic processes and the remaining four to alteration stages including silicification, chloritization, illitization, sericitization, albitization, and sulfidation. Needle-like crystals of mordenite (zeolite) with multiple growth centers grow on the fourth generation of quartz. Notable textural variants in the breccias are vesicles, perlitic cracks, and zoned alteration halos that mantle the rims of clasts. Electron microprobe analyses on chlorite from breccia samples have identified clinochlore as the main chlorite type and indicate a formation temperature in the range of 210–304 °C. This and the elevated Au–As–Ag–Hg–Zn–Pb–Sb contents of a mineralized sample indicate hydrothermal alteration temperatures > 200 °C suggesting that these breccias may represent the upflow zone of a hydrothermal system and highlight the potential for seafloor massive sulfides in the area. The breccias show elevated contents of immobile trace elements and LREE as well as a depletion in Ta and Nb suggesting that the precursor rocks were formed in a rift-related suprasubduction environment.