ABSTRACT

REE and other trace elements in altered and unaltered phyllites, orebodies and quartz veins in the Yinshan deposit were determined in order to examine behaviors of trace elements during hydrothermal alteration. REE, especially LREE, were selectively leached from the phyllites that had been undergone recrystallization dominantly of the major minerals during hydrothermal alteration. The greater the degree of alteration was, the stronger the leaching of REE would be. LREE were leached from the rocks that are close to the intrusions and re-deposited in the part far away from the intrusions. Nevertheless, Eu was systematically leached from the rocks, resulting in strong negative Eu anomalies in the altered phyllites. The lower total REE contents in the altered rocks close to the intrusions, compared to those from the further part is probably attributed to the dilution of REE-barren minerals (quartz and sulfides), and the removement of REE from the rocks by hydrothermal alteration. REE in the hydrothermal solutions are characterized by enrichment in LREE, strong positive Eu anomalies and (La/Yb)_N ratios much smaller than those of the phyllites. The addition of REE from the hydrothermal solutions into the altered rocks resulted in the observed smaller (La/Yb)_N ratios in the altered phyllites than that of the unaltered equivalent. For other trace elements, Y behaved similarly to HREE. The LIL elements such as Rb, Sr and Ba behaved differently, which may be attributed to the different fates of their main hosting minerals. The ore-forming elements such as Cu, Pb, Zn, Ag and Sn were remarkably added to the altered phyllites, while Hf, Th, U, Nb, and Zr remained rather constantly during hydrothermal alteration. It is suggested by the REE features that ore-forming fluids probably found their ways upwards from the depth through faults to the locus of ore deposition. The degree of REE mobility increases with the sizes of the orebodies. Thus, REE is probably an effective geochemical indicator for distinguishing between small and large orebodies at the later stage of exploration.