Trace elements and C-O isotopes of calcite from Carlin-type gold deposits in the Youjiang Basin, SW China: Constraints on ore-forming fluid compositions and sources

摘要

The Carlin-type gold deposits in the Youjiang Basin, SW China can be further classified into three sub-types; strata-bound, fault-controlled, and diabase-associated. The compositions and sources of the ore-forming fluids for each deposit type are evaluated using the texture of calcite by CL (Cathode luminescence), its chemistry by EMPA (Electron probe microanalysis) and LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry), and its C-O isotopic compositions by SIMS (Secondary ion mass spectroscopy). The studied deposits are the stratabound Zimudang deposit, the fault-controlled Lannigou deposit, and the diabase-associated Badu deposit. Calcite in the Zimudang and Lannigou deposits formed at three different stages: pre-ore stage porefilling and massive calcite (Cal 1), late-ore stage vein calcite (Cal 2) associated with realgar and pyrite, and postore stage vein calcite (Cal 3). Calcite in the Badu deposit formed in two stages: ore stage vein and replacement calcite (Cal 2) associated with pyrite and post-ore stage calcites (Cal 3). Calcite with brighter red CL color has higher Mn and lower Fe content than calcite with darker red CL color. Furthermore, the higher the total REE content, the brighter the CL color. The host rocks are characterized by light REE (Rare earth element) enrichments. In contrast, the ore stage calcite shows middle REE enrichments. The ore stage calcite has higher concentrations of Fe, Mn, Mg, Si and Sr, and lower concentrations of Cu, Sb and Zn than both the pre- and post-ore calcite. The ranges of carbon (PDB) and oxygen (SMOW) isotope compositions of calcite are from -11.29 to 6.89 parts per thousand and from 16.49 to 22.60 parts per thousand for the Zimudang deposit, from -13.31 to 2.80 parts per thousand and from 9.12 to 29.71 for the Lanningou deposit, and from -11.94 to 0.499 parts per thousand and from 10.51 to 18.97 parts per thousand for the Badu deposit. Generally, the ore stage calcite has a wider range of delta C-13 and delta O-18 values than the other stage calcite. The isotope data indicate that a deep crustal fluid was involved in the formation of all of these three deposits. Meteoric water containing abundant organic matter was also involved in the formation of the Badu deposit as well as the Zimudang deposit, whereas a basin brine was also involved in the formation of the Lanningou deposit.