THE NIEDERBOBRITZSCH GRANITE, ERZGEBIRGE, GERMANY: IMPLICATIONS FOR THE DIFFERENTIAL MOBILITY OF THE LREE AND Th DURING ALTERATION

A detailed survey of accessory minerals in the poorly to moderately differentiated F-poor biotite granites from Niederbobritzsch, Erzgebirge, Germany, reveals the presence of various, late magmatic to postmagmatic secondary rare-earth (REE) minerals, including cerite-(Ce), thorian synchysite-(Ce), synchysite-(Ce), and an unidentified Th-rich REE fluorocarbonate(?). Cerite-(Ce) is a REE silicate that, to date, has been found in only half a dozen occurrences worldwide. It had not previously been described from a granite. The composition of cerite-(Ce) from Niederbobritzsch is characterized by lower bulk REE contents but higher abundances of Si, Al, Ca, and F than that from other occurrences. Dissolution of thorian monazite-(Ce) during interaction with a F-, CO2- and Ca-bearing fluid gave rise to the formation of thorian synchysite-(Ce) containing up to 18.1 wt % ThO2. Previously reported contents of Th in synchysite-(Ce) did not exceed 1.6 wt % ThO2. The spatial relations between the secondary REE minerals and their precursor attest to a differential mobility of the LREE and Th during fluid–rock interaction. Under the prevailing PTX-conditions, the LREE were more soluble and, thus, mobilized further away from their site of removal relative to Th, which tended to be reprecipitated next to its precursor. Virtually unchanged whole-rock REE budgets and continuous, unfractionated chondrite-normalized LREE patterns of the secondary REE minerals, however, imply that the lanthanides were transported over distances of millimeters or centimeters only