Objective This paper focus on conducting petrological and geochemical studies on the volcanic-sedimentary Ga-Nb-REE enrichment layer discovered at the bottom of Emeishan basalt in Dongfengcun area, Yanbian County, Sichuan Province, analyzes its ore-forming material sources and metallogenic conditions, discusses its metallogenic mechanism, and provides new enlightenment for prospecting and exploration.

Methods On the basis of field investigation, representative rock and ore samples were collected, and the research work was carried out by microscopic identification and analysis of main elements, trace elements and rare earth elements.

Results The Ga-Nb-REE enrichment layer is an aluminum-rich paleo-weathering crust formed by strong weathering of basic volcaniclastic rocks. It is mainly composed of altered tuffaceous volcanic breccia, bauxitic clay rock, tuffaceous clay rock and other rocks. It is controlled by the contact interface between the karst landform at the top of the underlying Yangxin Formation and the overlying basalt. The mineralized enrichment layer extends stably and has significant mineralization enrichment characteristics of niobium, tantalum, gallium, titanium and rare earth element, and the mineralization is uniform. The rare earth elements in the mineralized enrichment layer shows strong enrichment of light rare earth elements and loss of heavy rare earth elements. The enrichment degree of niobium, tantalum and titanium elements increases with the increase of rock aluminum content, and the content of gallium is more uniform. The ore-forming mineral source mainly comes from basalt and basic pyroclastic rocks formed by the activity of Emeishan mantle plume. The humid and hot paleogeographic environment, gentle and open landform and acidic oxygen-containing medium have created favorable conditions for the activation, migration and enrichment of gallium, niobium, rare earth and other elements. The ore-forming elements have undergone multiple dissolution, complexation, migration, hydrolysis, precipitation and adsorption, and finally formed a mineralized enrichment layer.

Conclusions The volcanic-sedimentary Ga-Nb-REE enrichment layer found at the bottom of Emeishan basalt is a new metallogenic type of rare and rare earth element.It not only has good prospecting prospects and great resource potential, but also has great significance for studying metallogenic theory and expanding prospecting ideas.