Genesis of Fanshan lithocap, Luzong volcanic basin, Anhui Province, China - Indications from alunite and pyrite isotopes

Previous research in the Luzong volcanic basin in the middle-lower Yangtze River area, east China, mostly focused on iron oxide-apatite (IOA) deposits, but the area also hosts an extensive lithocap of previously unknown affinities. This study proposes a genetic model for the Fanshan lithocap and clarifies the relationship between the lithocap and the IOA deposits, based on stable isotope analyses.

There are four types of alunite in the Fanshan lithocap: granular alunite with APS cores, bladed alunite in vugs, needle alunite co-existing with pyrite, and very fine-grained powdery alunite co-existing with kaolinite and jarosite. We determined the S isotopic compositions of the different types of alunite: bladed and needle alunite produced results (20.30–22.29‰ and 18.30–22.98‰, respectively) indicating a magmatic-hydrothermal origin, whereas powdery alunite produced lower isotopic compositions (−4.38 to −4.48‰), consistent with the associated pyrite (−8.06‰), and indicating a supergene origin.

The δ³⁴S values of the needle alunite and of co-existing pyrite in the quartz–alunite–pyrite alteration zone reflect isotopic equilibrium between sulfate and sulfide at 180–220 °C. The pH increased from <2 to about 5, and the log ƒ_O2 decreased from −33 to −46 from proximal to distal alteration zones (quartz–alunite–pyrite to quartz–kaolinite–dickite to quartz–illite+/−smectite).

The O and H isotopes of the magmatic-hydrothermal alunite (δ¹⁸O 14.2–19.9‰ and δD −74.4 to −67.0‰) indicate that the hydrothermal fluid during the wall rock alteration was dominantly magmatic in origin (δ¹⁸O 3.34–9.04‰ and δD −73.28 to −80.68‰). The H₂S/SO₄ of the bulk hydrothermal fluid during acid alteration was ~3 due to the deposition of alunite, similar to that of high-sulfidation epithermal deposits worldwide.

The δ34S_ΣS of the hydrothermal fluid that produced the Fanshan lithocap was 0.12‰, suggesting the sulfur was probably derived from the mantle; that is significantly different from results obtained from previous work on the IOA deposits in the Luzong basin, which indicated that the sulfur was produced by mixing of a magmatic-hydrothermal fluid with sulfur from Triassic marine sedimentary rocks.

Based on our isotopic data we conclude that the Fanshan lithocap is not related to the IOA deposits and its fluids were not derived from the diorite porphyrites that are related to IOA deposits in the Luzong basin. We infer that the intrusion that supplied magmatic-hydrothermal fluids to the lithocap is probably at depth below the lithocap, and may have been similar to intrusions that in many parts of the world are responsible for developing copper–gold porphyry deposits below lithocaps similar to the Luzong lithocap.