Formation, remobilisation and alteration processes at inactive hydrothermal vents: insights from elemental analysis of Cu-(Fe-)S sulfides from TAG, Mid-Atlantic Ridge

Chalcopyrite is the main Cu mineral in mafic-hosted marine hydrothermal systems. Its trace element budget and that of its alteration products may hold valuable information on formation, remobilisation and alteration processes of the hydrothermal system. In this study, we analysed chalcopyrite from five inactive seafloor massive sulfide (SMS) sites from the TAG hydrothermal field on the Mid-Atlantic Ridge by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for 24 elements. Twelve of them are discussed in detail. In general, trace element concentrations range between sub-parts per million (ppm) to several hundreds of ppm. The elements Se and Co are incorporated into the lattice at high temperatures of > 300 °C, whereas As, Ge, Ga substitute into the structure at intermediate to low temperatures. Other elements, e.g. Zn, are either accommodated into the mineral lattice or form inclusions, whereas V and Mn, which originate from seawater, get adsorbed onto the mineral surface. Idaite, chalcocite, and covellite exhibit similar trace element patterns to those of the precursor chalcopyrite. However, the secondary copper minerals show enrichment of Ag and Mo. Factors controlling the incorporation are predominantly related to changes in physicochemical conditions with the host rock composition playing only a minor role.