Diagnostic REE patterns of magmatic and hydrothermal apatite in the Zhuxi tungsten skarn deposit, China

Abstract

Apatite is increasingly used as a tracer for petrogenetic and hydrothermal processes. We studied the cathodoluminescence (CL) texture and composition of igneous and hydrothermal apatite from the Zhuxi biotite granite and its associated hydrothermal tungsten system in the Late Mesozoic Jiangnan porphyry-skarn tungsten belt in South China. Both magmatic and hydrothermal apatite is anhedral to subhedral, with up to >1 mm grain size. Igneous apatite has relatively constant ΣREE + Y contents in the 2700–6500 ppm range (3970 ± 1003 ppm; n = 52). Hydrothermal apatite in exoskarn has lower ΣREE + Y contents in the range from <1 ppm to ∼1500 ppm, i.e. a strongly heterogeneous REE distribution (101 ± 253 ppm; n = 68). The ΣREE + Y contents correlate positively with CL brightness in both magmatic and hydrothermal apatite.The REE patterns of magmatic apatite display patterns similar to the host rock (Zhuxi biotite granite): generally flat ((La/Yb)N = 1.19–8.06), with moderately negative Eu anomalies (EuN/EuN* = 0.11–0.51), slightly positive Ce anomalies (CeN/CeN* = 1.05–1.12), and chondritic Y/Ho ratios (25–36, average = 30). The hydrothermal apatite is much different: downward-convex (MREE depleted) patterns ((La/Yb)N = 2.48–211), slightly to strongly positive Eu anomalies (EuN/EuN* = 1.11–540), slightly positive to moderately negative Ce anomalies (CeN/CeN* = 0.40–1.16), and non-chondritic Y/Ho ratios (40–152, average = 84). Since the hydrothermal samples are located outside of the pluton, excluding the influence of feldspar on Eu content, the differences of the Ce and Eu anomalies between magmatic and hydrothermal apatite from the retrograde skarn suggest that the retrograde skarn formed in a more oxidized environment than the granite.The low and variable contents of REEs, low Th/U, and non-chondritic Y/Ho ratios can be diagnostically used to distinguish hydrothermal from magmatic apatite, and the MREE-depleted REE pattern may be a common feature of apatite from retrograde tungsten skarn deposits.