L. I. Marushchenko

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.

The Au-Mo-Cu porphyry Peschanka deposit is confined to the Egdygkych Pluton (K₁). The deposit comprises three major types of metasomatic rocks: quartz-K-feldspar-biotite, propylites and quartz-sericite. In their chemical compositions, amphiboles of propylites range from early magnesiohornblende to tremolite-actinolite. Compared to magmatic K-feldspar, the metasomatic K-feldspar is enriched in barium (up to 7.3 wt % BaO). Tourmaline belongs to the intermediate member of the oxydravite-povondraite series. The Al content in chlorite decreases to the outer zones of propylitization, which is caused by a decrease in the temperature of mineralization.