Gold Mineralization Triggered by Contact Metamorphism at Mano Deposit in the South Kitakami Belt, Japan

ABSTRACT The South Kitakami Belt of Japan yields a number of Mesozoic vein‐type gold deposits, which have been classified as a ‘mesothermal’ type, related to the intrusions of Cretaceous granitic rocks. We investigated the Mano deposit, one of the gold deposits in the belt, to reveal the type and mineralization process of the deposit. The Mano deposit consists of thin auriferous quartz veins hosted by pyroxene hornfels that was metamorphosed from sandstone and mudstone of the Triassic Isatomae Formation by the intrusion of the Cretaceous Numazu gabbroic pluton. The pyroxene hornfels in the Mano deposit contains relatively less metamorphosed dark‐gray pockets, which consist mainly of quartz, plagioclase, orthoclase, prehnite, calcite, chlorite, clinopyroxene, clinozoisite, carbonaceous matters, and sulfides in the white‐colored completely metamorphosed hornfels. The mineral assemblage of the completely metamorphosed hornfels is similar to that of the less metamorphosed hornfels; although it contains titanite in addition to the minerals above, it is characterized by less amounts of sulfides and hydrous minerals. The bulk chemical compositions of the less metamorphosed and completely metamorphosed hornfels are both characterized by high concentrations of CaO (8.56–11.23 wt%); although the completely metamorphosed hornfels is more depleted in loss on ignition (including C, H and S) Co, Cu, Mo, and As, and has higher concentrations of Ca, K, Ba, Sr, and Rb, compared with the less metamorphosed hornfels. The sulfide minerals disseminated in the less metamorphosed hornfels are pyrrhotite, arsenopyrite, and chalcopyrite, with trace amounts of pyrite and alloclasite. Pyrrhotite contains trace amounts of Co, Ni, and Cu, while arsenopyrite contains trace amounts of Pb, Te, Co, Ni, Sb, Bi, and Au. The arsenopyrite geothermometer indicates an equilibrium temperature range of 460°C–485°C, which is slightly lower than the temperatures estimated from the crystallinity of carbonaceous matter (513°C to 539°C). The ore minerals in the quartz veins of the deposit are gold, bismuth, joséite‐A, joséite‐B, hedleyite, and Bi‐Au ± Te alloys, with trace galena and arsenopyrite. These minerals are oxidized from the grain margins to form various oxides. A large smirnite grain is associated with 1–20 μm size droplets of gold and Bi minerals that form trails aligning on planes together with a few micron‐size fluid inclusions. The lithology, mineralogy, and chemistry of the host rocks show that the hornfels metamorphism attained temperatures of 460°C to > 500°C. The chemistry of the sulfide minerals in the hornfels indicates that during this metamorphism, Bi and Te, the major low‐melting chalcophile elements included in sulfides, were scavenged into the metamorphic fluid as melts, together with other elements such as Au, As, Cu, Pb, Co, and Mo. These metal melts in the fluid precipitated gold, various Bi‐Te‐(Se + S) minerals, and Bi‐Au‐Te alloys in the quartz veins close to and/or within the source rock at the temperature range of 240°C–300°C. The Au mineralization at Mano could be categorized into an “orogenic type”, although it is related to contact metamorphism, which is uncommon in the orogenic gold deposits.