Journal Article

Disturbance of the Sm-Nd isotopic system by metasomatic alteration; a case study of fluorapatite from the Sin Quyen Cu-LREE-Au deposit, Vietnam

Li Xiaochun, Li Xiaochun, Yang Yueheng, Yang Yueheng, Zhao Xinfu, Zhao Xinfu, Zhou Meifu, Zhou Meifu, Gao Jianfeng, Gao Jianfeng

2018

Overview

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 147Sm/144Nd ratios ranging from 0.0359 to 0.0549, and 143Nd/144Nd ratios from 0.51147 to 0.51172. Their initial 143Nd/144Nd 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 147Sm/144Nd vs. 143Nd/144Nd 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 147Sm/144Nd ratios varying from 0.0667 to 0.1348, and 143Nd/144Nd ratios from 0.51160 to 0.51199. The calculated initial 143Nd/144Nd ratios range widely from 0.51114 to 0.51141, with most values lower than those of the allanite-(Ce) and monazite-(Ce). In the 147Sm/144Nd vs. 143Nd/144Nd 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 143Nd/144Nd 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.

• Publisher: Mineralogical Society of America,Walter de Gruyter GmbH
• Subject: absolute age / accessory minerals / allanite / Apatite / Asia / Case studies / Cerium / chevkinite / chevkinite group / Composition / Copper / copper ores / Crystals / dates / Domains / Economic geology / epidote group / Far East / Fluids / Fluorapatite / Geochemistry / Geological time / Gold / gold ores / Grains / hydrothermal alteration / igneous and metamorphic rocks / isochrons / isotope ratios / Isotopes / Mathematical analysis / metal ores / metals / metasomatic alteration / metasomatism / Mineralization / Minerals / Monazite / Nd-144/Nd-143 / Neodymium / Neodymium isotopes / Neoproterozoic / orthosilicates / Petrology / phosphates / Precambrian / Proterozoic / radioactive isotopes / rare earth deposits / rare earths / Ratios / Rock / Rocks / Samarium / Samarium isotopes / silicates / Sin Quyen Deposit / Sm-147/Nd-144 / Sm-Nd isotopes / Sm/Nd / sorosilicates / stable isotopes / upper Precambrian / Vietnam