Mineralogical Origins of Apparent Fe and Na Enrichment in Fangshankou Turquoise, Northwest China

Fangshankou turquoise, hosted by the Lower Cambrian phosphorus–vanadium–uranium black-shale ore system of Dunhuang, NW China, has been reported to contain anomalously high Na, Fe and Si, but the mineralogical origin of these anomalies is unknown. Micro-Raman screening combined with SEM-EDS, powder and single-crystal XRD, EPMA and LA-ICP-MS on 27 specimens shows that turquoise is intergrown at the 10–50 μm scale with cyrilovite (17.7–22.4 wt% FeO, 2.6–4.7 wt% Na2O), a poorly crystalline unnamed NaAl phosphate phase (4.9–11.1 wt% Na2O; diagnostic Raman band at 992 cm−1), fluorapatite, collinsite, alunite, jarosite, goethite, phengite and chabazite-Na. Raman-screened turquoise contains only 2.2–6.5 wt% FeO and ≤0.8 wt% Na2O; the higher values obtained on unscreened material arise from analytical volumes that intersect these Na- and Fe-rich phosphates, not from unusual substitution in turquoise, whose structure offers no site for Na. Screened turquoise inherits the Li–Cr–V–U signature of its black-shale host (615–1357 ppm Cr, up to 65 ppm U), whereas the origin of its moderate Si remains unresolved. Chabazite-Na in post-turquoise fractures records a late Si–Na–K-rich fluid stage. The Na-rich phosphate assemblage, not reported from other Chinese turquoise deposits, is itself a potential provenance indicator, and the results show that provenance criteria for microcrystalline turquoise must rest on phase-resolved rather than bulk analyses.