Quartz-sericite and argillic alterations at the Peschanka Cu-Mo-Au deposit, Chukchi Peninsula, Russia

The porphyry Peschanka copper-molybdenum-gold deposit and the Nakhodka ore field located in the Baimka ore trend on the western Chukchi Peninsula are spatially related to monzonitic rocks of the Early Cretaceous Egdykgych Complex. Two types of quartz-sericite metasomatic rocks (QSR) have been identified at both the deposits and the ore field: (I) chlorite-quartz-muscovite rock with bornite and chalcopyrite (porphyry type) and (II) tourmaline-quartz-carbonate-muscovite ± phengite rock accompanied by veins with base-metal mineralization (subepithermal or transitional type), as well as carbonate-quartz-illite rock (argillic alteration) accompanied by veins with precious metal mineralization (epithermal type). The QSR I chlorite evolves from chamosite to clinochlore, which is caused by increasing H2S activity in mineralizing fluid and precipitation of sulfide minerals. The QSR I clinochlore is significantly depleted in silica as compared with that from the rocks affected by argillic alteration. The chemical composition of muscovite from both quartz-sericite alterations is similar. The QSR II carbonates evolve from calcite through dolomite to siderite, which results from the increasing activity of CO2 followed by the decreasing activity of H2S in mineralizing fluid. The Mn content in dolomite is similar to that in beresite (quartz-muscovite-carbonate-pyrite metasomatic rock) of the intrusion-related gold deposits. Illite from argillic alteration is depleted in Al as compared with that of postvolcanic epithermal Au-Ag deposits. However, carbonates from the discussed argillic alteration rhodochrosite and Mn-rich dolomite are similar to those from quartz-illite rock at postvolcanic epithermal Au-Ag deposits.