ABSTRACT
Titanite is associated with scheelite in altered biotite granite, clinopyroxene skarn, actinolite skarn, clinozoisite skarn, and quartz veins in the Jiepai W-Cu deposit, South China. Textural and geochemical characteristics indicate a hydrothermal origin for the titanite. There is compositional variability with respect to the rare earth element (REE) and high field strength element (HFSE) components in titanite from the different rock types. Interstitial titanite from the clinopyroxene (-garnet-vesuvianite) skarn displays low to moderate concentrations of REE (1212–1693 ppm), Nb (1337–1911 ppm), Ta (16–24 ppm), and W (26–42 ppm); low LREE/HREE (0.36–0.47) yet high Nb/Ta (47–85) ratios; along with weak negative Eu (0.71–0.90) and positive Ce (1.1–1.2) anomalies. By contrast, titanite from the actinolite and clinozoisite skarns shows generally higher concentrations of REE (2721–11,550 ppm), Nb (4350–24,228 ppm), Ta (1346–11,781 ppm), and W (32–337 ppm); highly variable LREE/HREE (0.14–0.70) but lower Nb/Ta (0.61–5.6) ratios; along with stronger yet variable negative Eu (0.02–0.14) and positive Ce (1.2–1.6) anomalies. Furthermore, the quartz vein-hosted titanite differs from those occurring in retrograde skarns in its significantly higher LREE/HREE ratios (0.78–6.3) and distinct Eu anomalies, which vary from negative to positive (0.15–1.2). Accordingly, the shift from relatively oxidizing to reducing conditions, as recorded by δEu and δCe in titanite, together with the different LREE/HREE and Nb/Ta ratios of the mineralizing fluids, as constrained by fluid composition and fractional precipitation, took place during emplacement of the hydrothermal W mineralization. In situ LA-ICP-MS U-Pb dating of hydrothermal titanite from the mineralized clinozoisite skarn, quartz vein, and actinolite skarn, respectively, yielded weighted mean 207Pb-corrected 206Pb/238U ages of 427 ± 5 Ma, 427 ± 4 Ma, and 426 ± 7 Ma (1σ), indicative of the dominant Silurian W skarn mineralization at Jiepai. Our new U-Pb data are consistent with published ages for igneous and ore-forming activities in other major W (-polymetallic) deposits in the Miao'ershan-Yuechengling pluton, highlighting the capability of Early Paleozoic granites to develop W (-polymetallic) deposits in South China. Additionally, hydrothermal titanite carrying considerable concentrations of Nb, Ta, and W along with variable Nb/Ta ratios holds potential for deciphering the fluid chemistry and sources for W-skarn deposits elsewhere.
In situ LA-ICP-MS U-Pb geochronology and geochemical characteristics of garnet from the Zhuxi skarn W-Cu deposit, South China