The Neoproterozoic (840 Ma) Sin Quyen deposit in northwestern Vietnam contains replacement Cu-LREE-Au orebodies in Proterozoic metasedimentary rocks. In this deposit, LREE-bearing minerals include allanite-(Ce), monazite-(Ce), chevkinite-(Ce), and fluorapatite. Fluorapatite from orebodies has undergone variable degrees of metasomatic alteration. Samarium-neodymium isotopic analyses were conducted on altered fluorapatite, and also on allanite-(Ce) and monazite-(Ce), to investigate whether such metasomatism can affect the Sm-Nd isotope system.

Allanite-(Ce) and monazite-(Ce) have Sm-147/Nd-144 ratios ranging from 0.0359 to 0.0549, and Nd-143/Nd-144 ratios from 0.51147 to 0.51172. Their initial Nd-143/Nd-144 values at the time of mineralization range from 0.51126 to 0.51148, but mostly cluster between 0.51135 and 0.51145. Thus, the primary ore-forming fluids were relatively homogeneous in their Sm-Nd isotopic compositions. In the Sm-147/Nd-144 vs. Nd-143/Nd-144 diagram, the compositions of allanite-(Ce) and monazite-(Ce) generally plot along a Sm-Nd isochron of 840 Ma, implying that the Sm-Nd isotopic systems of these minerals were either closed or only slightly modified. In contrast, altered fluorapatite crystals have Sm-147/Nd-144 ratios varying from 0.0667 to 0.1348, and Nd-143/Nd-144 ratios from 0.51160 to 0.51199. The calculated initial Nd-143/Nd-144 ratios range widely from 0.51114 to 0.51141, with most values lower than those of the allanite-(Ce) and monazite-(Ce). In the Sm-147/Nd-144 vs. Nd-143/Nd-144 diagram, their compositions mostly plot below the 840-Ma Sm-Nd isochron. Petrographic observations and trace elemental analyses show that metasomatic modification of fluorapatite grains led to increases of their Sm/Nd ratios. The unaltered domains in the grains have Sm/Nd ratios varying from 0.114 to 0.200, with an average value of 0.161; whereas the altered domains have Sm/Nd ratios varying from 0.111 to 0.254, with an average value of 0.183. The increased Sm/Nd ratios can cause the calculated initial Nd-143/Nd-144 ratios to be lower than actual initial isotopic ratios, and can also result in compositional deviations from the reference Sm-Nd isochron.

This study demonstrates that the traditionally assumed inert Sm-Nd isotopic system can be metasomatically disturbed due to changes in the Sm/Nd ratio. Therefore, care must be taken when interpreting the Sm-Nd isotopic data from apatite/apatite-rich rocks that have undergone metasomatic alteration.