Role of melt–mush interactions in the ore mineralization of Burpala alkaline syenite massif: Experimental results

   Research subject. Alkaline and nepheline ore-bearing syenites of the Burpala massif (Western Baikal).

   Aim. To experimentally model crystallizing much–melt interactions during the early magmatic stages of syenite body formation in the Burpala massif to identify ore concentration mechanisms.

   Materials and methods. Experimental modeling of interactions between a crystallizing mush (alkaline syenite of the Burpala massif) and an interstitial, volatile-rich, alkali-saturated melt (in the form of glass obtained by preliminary melting of ore-bearing nepheline syenite from the Burpala massif) was carried out in hydrothermal autoclaves equipped with a rapid-quench system at 750 °C and 200 MPa. The starting materials and experimental products were analyzed using SEM and LA–ICP–MS methods.

   Results. Experimental runs at 750 °C and 200 MPa revealed intensive recrystallization of clinopyroxenes and localized reworking of K-feldspar at crystal–melt interfaces in alkaline syenites. The melt produced as a result of glass melting interacts with the syenite and changes its composition, retaining an alkaline (agpaitic) character although with a lower agpaitic index. From this melt, a discrete assemblage of HFSE–REE phases crystallizes, including zircon, Zr-titanite, and sorosilicates of the chevkinite–perrierite group. A sharp enrichment of clinopyroxene rim zones in Mn, Sr, LREE, Y, and Nb is observed, while the feldspar composition shows relatively minor changes, highlighting the role of clinopyroxene as the main temporary “buffer” for rare elements at the early reaction stage.

   Conclusions. The results support the model of local redistribution of Zr–Ti-Nb–REE in the intercrystalline melt at the crystal–melt interface, which allows the formation of ore mineralization before the participation of later hydrothermal processes and can be directly applied to the interpretation of ore-bearing syenites of the Burpala massif.

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