Abstract: The Lala Iron-Copper Deposit, once considered submarine volcanic sedimentary mineralization, is now regarded by most scholars as one of the largest IOCG deposits in the Kang Dian copper belt. However, existing case studies of typical IOCG deposits had been problematic, at the center is the controversy over the widely distributed albitites and their relationship to mineralization in Lala area. Here, through petrographical, geochemical and scanning electron microprobe analyses of spilite-keratophyre formation in the Lala Xiaolutian pit, we confirm that the mineralization period was later than the spilite-keratophyre formation, therefore volcanic sedimentary mineralization did not occur. Our petrography data showed that both primary albite phenocrysts and secondary albites were in the keratophyre matrix; secondary albites replaced primary albites with small anhedral granular albites; and ore-bearing capacity of keratophyre was low. Electron microprobe analysis showed that magnetite is distributed along the albite, and the late ore-forming fluids are rich in K and Al, suggesting spilite was affected by the late tectonic movement. Moreover, the ore-forming fluid intruded into spilite foliation in a vein pattern, producing large amount of biotite and muscovite and enriching metal oxides and sulfides in the schistositilization zone. Geochemical composition revealed that spilite-keratophyre formation was significantly enriched in Nb, Ta, Zr and Hf, and depleted greatly in Ba, Pb, Sr and Ti; ∑REE changed largely; and the ore-forming fluid had lower Na but higher K, Al, CO2 and H2O contents. The initial 87Sr/86Sr ratio lied between 0.701260.76241, indicating it has been affected by crustal contamination. Furthermore, five keratophyre samples yielded a εNd(t) value of -1.096.75, consistent with a mantel-crust process, such that the ore-forming fluid formed initially in the deep magma chambers, ascended then to the crust and intruded into the weak zone or foliated rock. This process is similar to the transmagmatic fluid model.

Key words: Lala Iron-Copper Deposit, spilite-keratophyre formation, Sr-Nd isotopic geochemistry, ore-forming relationship