Abstract: A slightly altered volcanic rock sample and the bearing pyrite near NW Eifuku hydrothermal vent in the northern part of the Mariana Arc were selected. The major elements of the sample were analyzed, the olivine and pyrite were observed by electronic microprobe, and the S-isotope of pyrite was investigated. In addition, the geochemical characteristics, origination, and microbial mineralization of the pyrite were scrutinized. Results show that the sample is basaltic andesite in lithology, and contains four types of pyrite in occurrence, namely pyrite in altered porphyritic crystals, in porphyritic fissure, in matrix, and in air bubbles. The last three types of pyrite are idiomorphic or semi-idiomorphic, and are the products of early hydrothermal activity, while the pyrite in altered porphyritic crystals are allotriomorphic and colloidal and the S/Fe ratio is greater than those of the other three types, indicating their low temperature origin due to temperature drop caused by the mixing of hydrothermal fluids with seawater. The δ³⁴ S values of the four types of pyrite are negative(−6.58‰～−19.60‰), and some of the idiomorphic pyrite have iron oxide layer edges, indicating that those pyrites may had been affected by microbial action during its formation. In particular, the Co and Ni contents of pyrite in altered peridotite is significantly greater than those of other types of pyrite, indicating the influence by the change in hydrothermal fluid composition before and after alteration and the temperature drop due to hydrothermal fluid mixture with seawater, resulting in the easier entry of elements into the lattice of pyrite. The difference in the chemical element content between the core and the rim of pyrite might be caused by microbial action, while the effect of olivine alteration on the element content of pyrite was relatively small.