Rare-earth element mobility in alteration pipes below massive CuZn-sulfide deposits

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A convincing demonstration of trace-element mobility should satisfy two criteria: (1) the range of trace-element concentrations in the rock before and after alteration must be known with confidence; and (2) the absolute abundances and the ratios of the mobile trace elements should vary between altered and unaltered rocks. These criteria have been applied to the host felsic volcanics and alteration pipes associated with several Canadian massive CuZn-sulfide deposits. Although there is no evidence of rare-earth element (REE) mobility in the pervasive low-grade alteration envelopes adjacent to the deposits, the light and middle REE (LREE and MREE) plus Y have been systematically mobilized relative to the heavy REE (HREE) and other trace elements in the alteration pipes beneath the ore zones. There is evidence of minor LREE and MREE depletion in the most altered rocks at the South Bay Mine (Uchi Lake) and of significant LREE depletion in the alteration pipe beneath the Mattabi Mine (Sturgeon Lake). However, the strongest mobility is found at the Kidd Creek Mine where the LREE were leached from the rocks in the footwall stringer zone and redeposited in a metasomatic chlorite zone directly beneath the massive ore horizon, and the MREE and Y were preferentially lost from the system producing distinctive chondrite-normalized REE patterns with ( Tb Yb ) n ratios less than unity. Eu 2+ was found to be more mobile than the trivalent REE, especially at Kidd Creek where there is a marked increase in the magnitude of the Eu anomaly in the most altered samples. High-field-strength elements such as U, Th, Hf and Zr have not decoupled and apparently remained relatively immobile during all but the most intense alteration (i.e. Kidd Creek). Preliminary data suggest that the degree of REE mobility increases with the size of the deposit. Thus, REE mobility is a potentially useful method of distinguishing between small and large massive sulfide deposits at an early stage in exploration, however, this hypothesis requires further testing.