Genesis of Mesozoic alkaline granites and biotite granites in Nigeria and its significance for tin-polymetallic prospecting

There are tin(niobium and tantalum)polymetallic granites in the Mesozoic ring complexes in Nigeria. The two main series are alkaline granite and biotite granite. The former is mainly characterized by alkaline mafic minerals and the latter is mainly peraluminous series. The possible restriction mechanism of the two main series on tin(niobium and tantalum)mineralization has not been deeply discussed by predecessors. The comprehensive analysis of previous research data in this paper provides a new perspective for understanding the tin(niobium-tantalum)mineralization of non-orogenic A-type granite and further defines the prospecting direction. Two series both emplaced during the Jurassic. In one complex, the alkaline granite crystallized slightly earlier than the biotite granite and the former is slightly weaker in the degree of differentiation than the latter. Alkaline granites were formed through fractional crystallization of enriched mantle-derived magma and contaminated with a small amount of crustal material. Peraluminous biotite granites unlikely came from the melting of crustal material like orogenic peraluminous granites. Peraluminous biotite granites were more likely to come from the same primitive magma as the alkaline granite but belong to another branch contaminated with more crustal material. However, like the ore-forming granites in the subduction background, the tin-polymetallic enrichment is mainly related to the weakly peraluminous biotite granite in Nigeria, which is significantly affected by the exsolution of fluids during the late-magmatic stage. The enrichment of tin(niobium and tantalum)elements in some alkaline granites roughly represent the content of tin(niobium and tantalum)in the paragenetic biotite granites that had been not affected by fluids. The ore-forming materials were mainly from Pan-African basement rocks, and the enrichment of elements is mainly controlled by the fractional crystallization and fluid exsolution. The content of tin(niobium and tantalum)in the unmineralized ring complexes in Nigeria is relatively low in biotite granite and alkaline granite, whose Sn content range from 4×10^-6 to 13×10^-6. As for the mineralized ring complexes, the content of tin(niobium and tantalum)is higher than that of the unmineralized complexes. However, the content of tin(niobium and tantalum)in the biotite granite closely related to mineralization is lower than that of the alkaline granite. Sn content range of these alkaline granites and biotite granites are from 21×10^-6 to 205×10^-6 and 10×10^-6 to 62×10^-6, respectively. The above evidence shows that the content of ore-forming elements in the melt and the later exsolution of hydrothermal fluids are the key factors to determine the mineralization. Therefore, the biotite granite with low tin(-niobium and tantalum)content in the ring complexes with corresponding high ore-forming elements is the first choice for searching for tin polymetallic deposits.