Characteristics and genesis of the Fanshan lithocap, Zhejiang Province: Exploration implications from the largest alunite deposit of China

Le résumé fourni par la source

The Fanshan alunite deposit, located in Cangnan County, Zhejiang Province, is the largest alunite deposit in China, which developed a lithocap with an extensive advanced argillic alteration. Surface alteration mapping, through field geology and shortwave infrared (SWIR) spectroscopy analyses, has defined three alteration zones including Na-alunite-dickite-pyrophyllite-topaz alteration, K-alunite-pyrophyllite-phengitic muscovite alteration, and chlorite-illite alteration from center to margin. Alunite varies from K-rich to Na-rich and has different paragenesis and texture. Na-alunite, with quartz, pyrophyllite, dickite, and topaz, mainly developed in Pingpengling area of Fanshan, forming a typical advanced argillic alteration. The wavelength position of the alunite –OH spectral absorption feature at ∼ 1480 nm (Pos1480) varies from 1478 nm to 1492 nm with increasing content of Na in alunite. There exists a positive correlation between Na/K ratio value and Pos1480 of alunite. The TESCAN Integrated Mineral Analyzer (TIMA), coupled with LA-ICP-MS analyses of alunite has been conducted, aiming to systematically characterize the texture and mineral chemistry of alunite. Fanshan alunite is rich in Ga, V, rare earth elements (REEs), and large ion lithophile elements (LILEs). Na-alunite has higher LILEs and light rare earth elements (LREEs) than K-alunite. Thorium, U, Li, and B, vary greatly in the Na-alunite and K-alunite. The ratio of Na/K in alunite, together with Pos1480 has been used to vector the magmatic-hydrothermal center. Furthermore, the content of Pb and ratios of Rb/Sr*1000, Sr/Pb, La/Pb, have illustrated the consistent results as Na/K and Pos1480. Based on the spatial distribution of these vectors in conjunction with the occurrence of Na-alunite, we concluded that Pingpengling area is the most proximal area to the magmatic-hydrothermal center, which may imply a possible causative intrusion underneath. Further exploration is suggested to be conducted under Pingpengling to verify if there is porphyry Cu(-Au-Mo) mineralization.

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