Wall-rock alteration at the porphyry copper-molybdenum deposits of the Baimka ore cluster, Chukchi Peninsula

Nagornaya E. V.

Faculty of Geology, Moscow State University, Moscow, Russia

chp312@gmail.com

 

Large porphyry copper-molybdenum deposits (Peshanka, Nakhodka, Malysh) in the Baimka ore cluster, Chukchi Peninsula are related to the Early Cretaceous quartz monzonite intrusions. Monzonite is composed of potassium feldspar, plagioclase, biotite, clinopyroxene, and amphibole. During three subsequent stages of potassium metasomatic alteration, this rock was altered to quartz-biotite-potassium feldspar rock, potassium propylite, and quartz-sericite rock (phyllic alteration). Hosted volcano-sedimentary and volcanic rocks were propylitized. Phyllic alteration is more abundant in Nakhodka, whereas K propylite and quartz-biotite-potassium feldspar alteration, at Peschanka. This indicate shallower erosion level at the Nakhodka deposit.

Potassium propylite is composed of chlorite, epidote, tremolite-actinolite, potassium feldspar, albite, and occasional calcite, tourmaline, and prehnite. Chlorite is classified as intermediate member of the clinochlore-chamosite series [Fe/(Fe+Mg) = 0.25-0.61). Elevated content of Mn (up to 0.43 apfu) and F (up to 0.10 apfu) is a feature of the chlorite studied here. It has been established that the Al content in chlorite drops toward flanks of the Peschanka deposit probably indicating decreases of temperature of the mineral formation. Epidote occurred as spotty zoned crystals replaces magmatic clinopyroxene and plagioclase. Tremolite-actinolite is simultaneous with epidote replacing magmatic clinopyroxene and amphibole and in turn, is replaced by chlorite. The Mg/(Fe²⁺ + Mg) ratio in the mineral ranges from 0.70 to 0.91. Tremolite-actinolite contains insignificant fluorine (up to 0.10 apfu). Like other porphyry-copper deposits, tourmaline classified as intermediate member of the “oxy-dravite”-povondraite series is characterized by the major isomorphic substitution Fe → Al.

Quartz-sericite metasomatic rock is composed of quartz, muscovite-phengite (sericite), albite, occasional potassium feldspar, tourmaline and chlorite. Fluorine content in white mica of phyllic alteration at Peschanka, Nakhodka and Malysh is different. At the first two deposits it ranges from 0.05 to 0.22 wt % and close to that at large Mo-poor porphyry copper deposits Erdenet (Mongolia) and Aksug (Russia). At the Malysh Cu-Mo deposit, F concentration in white mica (0.25-0.43 wt %) similar to that in Mo-rich porphyry deposits Zhiriken and Sora (Sotnikov et al., 2002). Therefore F content in white mica of phyllic alteration may indirectly indicate a grade of Mo in porphyry deposits.

 

References

Sotnikov, V.I., Berzina, A.T., and Berzina, A.P. (2002): Fluorine in light micas of Siberian and Mongolian copper-molybdenum deposits and its probable origin. Geochem. Iternat. V. 40 (10), 1004-1012.