Ore fluid origin recorded by apatite chemistry: A case study on altered dolerite from the Badu Carlin-type gold deposit, Youjiang basin, SW China

摘要

The Paleozoic-Mesozoic Youjiang sedimentary basin in SW China is developed on a Lower Paleozoic low-grade metamorphic basement, and contains dozens of Carlin-type gold deposits with a total Au reserve of over 900 metric tonnes (t). Although most of these deposits are hosted in sedimentary rocks, similar to 70% of the gold (similar to 24.5 t) in the Badu deposit is hosted in altered OIB-type dolerite. To date, the ore-fluid source of Carlin-type gold deposits in the basin is highly debated, hampering the formulation of genetic models. Here, for the first time, we present ore-related apatite chemical and Sr isotope compositions from the altered dolerite at Badu to provide new insight into the ore-fluid source of this key gold province. @@@ Petrography studies distinguished three apatite generations (Ap1 to Ap3): Pre-ore Ap1 is needle-shaped or fine-grained (<20 mu m), and occurs mainly in the unmineralized dolerite; Main-ore Ap2 is euhedral-subhedral coarse-grained (>100 mu m) and spatially associated with auriferous pyrite and arsenopyrite in the mineralized dolerite; Late-ore Ap3 is also euhedral-subhedral coarse-grained (>100 mu m), and hosted in quartz +/- ankerite veins crosscutting the mineralized dolerite. Geochemically, Ap1 contains relatively high Cl (similar to 0.134 wt%), MnO (similar to 0.038 wt%), MgO (similar to 0.155 wt%), and SiO2 (similar to 0.285 wt%) contents, whereas Ap2 and Ap3 have relatively high SrO contents (similar to 0.448 and similar to 0.45 wt%, respectively). This geochemical difference can be used to distinguish magmatic from hydrothermal apatite at Badu. Ap2 has similar chemistry to Ap3 except for the higher SO3 contents (up to similar to 0.318 wt%), and they both have similarly higher Sr-87/Sr-86 ratios (0.709489-0.711787) than those of the unmineralized dolerite (0.706295-0.706323) and the sedimentary ore host in the basin, but similar to the Lower Cambrian basement rocks (0.70909-0.82733). This similarity indicates that the ore-forming fluids were dominantly sourced from the basinal basement or from deep magmatic fluids with radiogenic Sr addition from the basement rocks. The ore-forming fluids may also have leached MREEs from the sulfate-rich basement rocks, leading to the observed MREE enrichments in Ap2 and Ap3. Our study highlights that the basement material contribution is critical for the Carlin-type gold mineralization in the Youjiang basin.

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