[H-RE13] 資源地球科学

[HRE13-P02] Petrographic characterization of alkaline igneous rock related REEs in the Semarule Igneous Complex, Southeast Botswana

*Rita Gorata Ngwanaotsile¹、Akira Imai¹、Kotaro Yonezu¹、Akane Ito¹ (1.Kyushu University)

キーワード：Rare earth elements, Alkaline igneous rocks, Syenite , Semarule Igneous Complex

Rare earth element (REE) bearing deposits are found primarily in four geologic environments: carbonatites, alkaline igneous systems, ion-absorption deposits, and monazite-bearing placer deposits. This study focuses on REE deposits associated with alkaline igneous rocks. The Semarule Igneous Complex is a small 3 km wide intrusion that occurs within the Neoarchean Gaborone Granite of the Kaapvaal craton. The inner part of the intrusion is more complex and compositionally varied, consisting of fine grained syenite, coarse to pegmatitic syenite and zoned syenite veins. A thick 50 m, sub horizontal, dolerite sheet intrudes the eastern part of the intrusion. The Botswana Geological Institute (BGI) investigated textural, mineralogical composition and the emplacement sequence of the syenites, dolerite intrusion and zoned dykes within the complex. According to the report by BGI the complex is a result of continuous emplacement of magmatic intrusions with contrasting composition intruding the Gaborone granite.In this study we present geological, petrography and mineralogy data aiming to investigate the nature and origin of REEs in the Semarule Igneous Complex. Based on sample observations the syenite samples are medium to coarse texture and primarily composed of orthoclase feldspar as its dominant mineral, along with significant amounts of albite and amphibole. Similarly petrographic observations of the Semarule syenite samples depict a composition rich in orthoclase (70 modal %), with a significant portion of microcline. Notably, phenocrysts of amphibole, interstitial sphene and minor clinopyroxene are present. There are also accessory amounts of apatite and zircon. The primary REE-bearing minerals identified in the syenite samples are amphibole and apatite, exhibiting elevated peaks of La, Ce, and Nd.