Age and Relationship between Magmatites of a Pluton and Small Intrusions (Sorskoe Porphyry Cu–Mo Deposit, Khakassia)

Abstract

The Sorskoe (Sorsk, Sora) porphyry Cu–Mo deposit is located in the Kuznetsk Alatau, in the northwest of the Altai-Sayan folded region. Commercial Cu–Mo mineralization is closely associated with small porphyry intrusions (stocks, dikes) localized in the Uibat pluton. The magmatites of the pluton and small intrusions are composed of rocks of the gabbroid, monzonitoid, and granite/leucogranite associations. The rock associations of the pluton and small porphyry intrusions are similar in mineral composition, type of metallogenic specialization, petrogeochemical and isotopiс characteristics. The gabbroic and monzonitic rocks were probably generated by fractionation of the mafic magma and assimilation of lower crustal material. The geochemical characteristics indicate the absence of a genetic relationship between basic/intermediate rocks and the granite/leucogranite association. The rocks of the granite/leucogranite association were probably derived from the silicic magma generated by partial melting of the juvenile lower mafic crust due to the heat released from the mafic magma. U-Pb geochronological data indicate a time gap between the completion of pluton formation (∼478 Ma) and emplacement of small porphyry intrusions (from ∼467 to ∼457 Ma). Age differences between pluton and small intrusions suggest they are not coeval and were probably sourced from independent upper-middle crust magma chambers, which were formed by ascending melts derived from a deep large long-lived magma reservoir. The rocks of the Uibat pluton represent the remnant of the earlier solidified and eroded magma chamber. Periodic episodes of magma supply from a late shallow magma chamber resulted in the formation of small porphyry intrusions. Together these observations suggest that the magmatites of the pluton and small intrusions are not comagmatic. Their magmas may have been derived from a common deep-seated source, but possibly evolved independently, that is, they do not represent a single magma lineage, as often noted for porphyry Cu–Mo deposits. Changing tectonic environment before the emplacement of small intrusions triggered porphyry magma ascent, fluid saturation and exsolution and provided favorable conditions for large-scale mineralization.