CONDITIONS OF METAMORPHISM OF THE HOST ROCKS IN THE BORZOVA RIVER GOLD ORE CLUSTER ( <i>northeastern Taimyr</i> )

The metamorphic rocks in the northeastern Taimyr region are a consequential result of its complex geologic history. Series of metamorphic processes were associated with the formation of lode copper and noble-metal mineralization. The aim of this study is to investigate the type and degree of metamorphism of the host basic volcanic rocks in the ore cluster. We have identified three stages of metamorphic process: The first stage led to the formation of actinolite and sodium plagioclase paragenesis (with its chemical compositions corresponding to temperature of about 400 °C); the second stage resulted in the crystallization of hornblende, plagioclase, spessartine–almandine garnet, biotite, feldspars (oligoclase–andesine and K–Ba types), and various Fe–Ti phases (mineral geothermometers show the highest temperature range of 450–600 °C), and the final stage produced clinochlore, pyrite, and fluorapatite (330–290 °C according to the AlIV contents in chlorite). We interpret the studied rocks as hornfels based on the highly discordant contours of the metamorphic aureole, the compositions of garnet (spessartine is a predominant component), and the K–Ar ages of biotite (253 ± 5 Ma) and plagioclase (239 ± 8 Ma), which coincide with the time of late Paleozoic–Mesozoic postcollisional granitoid magmatism. The absence of schistosity, the disequilibrium assemblages, and the abundance of volatile-rich minerals (containing F–, OH–, and SO42-) also indicate their contact origin. The discovered aureole includes the known Cu–Au–Ag veins and metasomatic occurrences, the age of which is therefore late- or postmetamorphic. We confirm the earlier hypothesis of our colleagues about the impact of a concealed granite massif on the host rocks and consider the high metamorphic grade to be a natural boundary for the ore cluster.