Abstract
The genesis of the Bayan Obo REEs deposit has been debated for a century. The most outstanding issue concerns the genesis of the ore-hosting dolomite and the contribution of the Paleozoic overprint events in the rare earth budget. Pyrite offers a unique window into those debates in that it is the only common fingerprinting mineral shared by both events. In this contribution, LA-ICP-MS trace element mapping, in-situ elemental and sulfur isotopic analyses on pyrite are integrated in order to constrain the genesis of ore-hosting dolomite and evaluate the contribution of the Paleozoic thermal events to enormous REEs resources. Five types of pyrite were distinguished in ore-hosting dolomites, various types of ore and skarns based on mineral paragenesis and geochemical signatures. The dolomites contain early euhedral pyrite (Py-1) overgrown by magnetite and euhedral pyrite (Py-2w in the West Open Pit and Py-2d in the deep boreholes) associated with pyrrhotite, monazite, dolomite and apatite. By contrast, pyrite in the banded ores (Py-3b) and the massive ores (Py-3 m) was co-precipitated with magnetite, aegirine, fluorite, riebeckite, REEs minerals and apatite. Pyrite in the vein ores (Py-4) usually coexists with barite, quartz, aegirine and calcite, and those (Py-5) in skarns associated with biotite and amphibole. Py-1 is enriched in Ti, Ni, Nb, Hf and REEs, with high Co/Ni and Y/Ho ratios (ca. 66), and high δ34S values from +8.6‰ to +13.8‰. In comparison, Py-2w and Py-2d have higher Co/Ni yet lower Y/Ho (ca. 11) ratios, but exhibit magmatic δ34S values (-1.0‰ to +3.0‰; −1.0‰ to +3.7‰). Py-3b and Py-3 m are relatively enriched in Cr, Mn, Zn and LREEs, and possess intermediate δ34S values (+5.7‰ to +7.7‰, and +0.2‰ to +2.4‰, respectively). Py-4 is generally depleted in trace elements with magmatic δ34S values (-2.0‰ to +2.7‰). Compared with Py-3 m and Py-3b, Py-5 is characterized by high Co yet low Ti, Cr, Mn, Cu and Zn with variable REEs, and is enriched in 34S (δ34S: +9.1‰ to +10.8‰). The high δ34S value and Y/Ho ratio in Py-1 indicate that the ore-hosting dolomite is an igneous carbonatite originated from partial melting of upper mantle contaminated by altered and subducted oceanic crust. The marked difference in trace elements and sulfur isotopes between Py-3, Py-4 and Py-5 implies that the Paleozoic thermal events contributed little to the primary REEs mineralization except remobilizing preexisting ore metals.
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