1990 Volume 40 Issue 222 Pages 223-244
The Tengumori copper deposit of the Kamaishi mine exists along an unconformity between a Permian limestone and a Cretaceous sandstone. The post-mineralization Tengumori Fault has displaced this deposit into two major ore bodies. A vertical zonation of calc-silicate minerals is recognized as follows, from the hanging-wall sandstone to the footwall limestone: the weakly skarnized sandstone, the upper garnet-clinopyroxene zone, the clinopyroxene zone, the lower garnet-clinopyroxene zone and the unmineralized limestone. Copper ores are developed mainly in the clinopyroxene zone.
The mineral paragenesis at the Tengumori deposit suggests that the hydrothermal processes may be divided into three stages: 1) calc-silicate, 2) ore mineralization and 3) calcite stages. Clinopyroxene and two types of garnet were formed during the talc-silicate stage. Garnet-1 (earlier type) dominates in the hanging-wall side of the deposit, especially in the upper garnet-clinopyroxene zone, and possesses an intermediate chemical composition (And 40-60). Garnet-2 (later type) occurs mostly in the lower garnet-clinopyroxene zone and its chemical composition is almost pure andradite. The ore mineralization stage is characterized by sulfide and minor quartz mineralization replacing clinopyroxene. The major sulfide minerals are chalcopyrite, hexagonal and monoclinic pyrrhotite, cubanite, and pyrite. There exists a lateral zoning of the Fe and Cu sulfide minerals concentric from the Tengumori Fault: the later sulfur-rich sulfide minerals tend to occur near the fault, while the earlier sulfides at the margin. The Fe content of hexagonal pyrrhotite appears to increase with distance away from the fault. The paragenesis, the vertical zoning of the calc-silicate minerals, and the lateral zoning of the sufide minerals strongly suggest that the hydrothermal solutions ascended along the Tengumori Fault and spread laterally to the marginal parts of the deposit.
Fluid inclusion data show that the formation of the calc-silicate stage involved hydrothermal solutions with higher temperatures and salinities than those of the ore mineralization stage. Fugacities of H 2 and H2S in the hydrothermal fluids for the calc-silicate and the ore mineralization stages were calculated. The results show that the sulfide minerals were formed under higher H2S conditions than the calc-silicate minerals. When sulfide minerals crystallized, considerable proportions of the ferrous iron came from clinopyroxene, and H2S from ore-forming fluids. This is supported by the fact that the chemical compositions of clinopyroxene relicts in sulfides are Fe-poor relative to those of the unaltered clinopyroxene.
