A Unique association of scheelite and magnetite in the Tiemuli W-Fe skarn deposit: Implications for Early Cretaceous metallogenesis in the Nanling Region, South China

摘要

The Tiemuli scheelite-magnetite deposit in the central part of the Nanling region, South China, is genetically related to a porphyritic biotite granite pluton. The W-Fe orebodies are hosted within upper Cambrian calcareous sandstone or impure limestone in the external zones. LA-ICP-MS U-Pb dating of zircons from the granite yielded a weighted mean age of 136.6 +/- 1.1 Ma. The main ore minerals are magnetite, scheelite and arsenopyrite, and the gangue minerals are clinopyroxene, grossularite, vesuvianite, amphibole and mica. Three paragenetic stages of skarn formation have been recognized: I. a skarn stage; II. a retrograde stage, and III. a quartz-sulphide-calcite stage. Massive magnetite and scheelite orebodies associated with hydrous minerals formed during Stage II. Molybdenite grains separated from Tiemuli skams yielded Re-Os model ages of 136.3-133.8 Ma, with a weighted mean age of 135.1 +/- 0.8 Ma. Biotite associated with fine-grained magnetite from the ores has a Ar-40/Ar-39 plateau age of 132.67 +/- Ma. These three independent dating techniques show that the granitic magmatism and tungsten mineralization were coeval (within error), whereas the late-stage alteration recorded by biotite could be ca. 2 m.y. younger than the early stage. The granite is classified as an A-type body on the basis of its geochemistry and mineralogy. Zircon grains from the granite show negative epsilon Hf (t) values of -7.7 to -2.3, and molybdenite grains from the skarn ores contain low Re contents of 0.93 to 2.97 ppm. These data indicate that the magma and ore-forming components at Tiemuli were derived mainly from crustal materials, although mafic xenoliths in the granites are consistent with some mantle contribution. The Tiemuli deposit was produced by crust-mantle interaction in an extensional regime, possibly caused by rollback of the subducting Paleo-Pacific plate during Early Cretaceous. The skarn scheelite and magnetite association formed by interaction between magmatic fluids and calcite-bearing sandstone or impure limestone of the host strata.