Geochemical composition of apatite from the Zhuxi tungsten granite and the Zhenzhushan granite porphyry in the Jiangnan porphyry-skarn tungsten belt, China

Abstract

Apatite is increasingly used as a tracer for petrogenetic and metallogenic processes. We present the cathodoluminescence (CL) texture, and composition of igneous apatite from the tungsten-bearing Zhuxi biotite granite and the nearby Zhenzhushan granite porphyry in the Late Mesozoic Jiangnan porphyry-skarn tungsten belt in South China. Most apatite grains show homogeneous CL-dark texture, but some apatite grains have CL-dark cores and CL-bright rims, or CL dark-bright-dark-bright (from core to rim) growth zoning bands. The CL-bright bands are Mn depleted. Apatite from the Zhuxi biotite granite has elevated W contents (mean: 0.16 ppm; n = 75) compared to the Zhenzhushan granite porphyry (mean: ≤0.02 ppm; n = 32). The CL-bright rims of apatite from Zhuxi have more W than the CL-dark cores, i.e. average 0.58 ppm W versus average 0.07 ppm W, respectively, implying that the late-stage magmatic system was enriched in W. The chondritic Y/Ho ratios (22−33) of Zhuxi apatite, and the similar or even slightly more elevated REE contents of the CL-bright rims, compared to the cores, rule out hydrothermal effects. The Zhuxi apatite samples have distinctly negative δEu, F-rich and sulfur-depleted features, which are interpreted as signature of igneous apatite from magmatically evolved granitic intrusions associated with W deposits, while the inverse trend applies to apatite from Cu porphyry systems. In-situ microanalysis of apatite may be a useful exploration tool.