Genesis of the Longtougou gold deposit, South Qinling Orogen, China: Constraints from ore geology, trace element, and S–Pb–H–O isotopes

Abstract

Orogenic gold deposits are a significant source of gold production, although the geological processes and metallogenic material sources remain somewhat controversial. The Longtougou deposit is located in the South Qinling Orogen, China, and is a newly discovered moderate tonnage gold deposit. The deposit can be split into four stages of mineralization and contains three types of pyrite (Py0, Py1, and Py2). The Py0 is fine-grained, present as subhedral–anhedral aggregates, and formed during diagenetic stage of sandstones of the Devonian Dafenggou Formation. Coarse- to medium-grained euhedral Py1 is hosted by early-ore stage quartz–pyrite veins. The last pyrite to form was fine-grained anhedral Py2, which coexists with barite within main-ore stage quartz–barite–polymetallic sulfide veins. The final stage is associated with the formation of quartz–barite–carbonate veins and stockworks. Early-ore stage samples yield δ18OH2O values from 9.8‰ to 13.9‰ and δDV-SMOW values from −75.4‰ to −65.3‰, indicative of fluids may be derived from a mixing of metamorphic and magmatic fluids. Main-ore stage samples yield δ18OH2O and δDV-SMOW values of 5.4‰ and −70.6‰, respectively, whereas late-ore stage samples yield δ18OH2O values from −9.0‰ to 3.3‰ and δDV-SMOW values from −74.5‰ to −56.7‰. These data indicate that the main-ore stage involved meteoric and organic water, whereas the late-ore stage was dominated by meteoric water. The Py0 has significantly negative δ34S values (−15.1‰ to −14.5‰), suggesting that the sulfur in this pyrite was derived from bacterial sulfate reduction. Early-ore stage sulfides have restricted ranges of δ34S values (−5.7‰ to 5.6‰) that are indicative of a magmatic–hydrothermal fluid origin. The Py2 yields lower positive δ34S values (2.4‰ to 5.3‰) and coexisting barite has higher positive δ34S values (15.3‰ to 17.5‰), indicative of a transition into an oxidizing environment. The Δ34SA–B−δ34SB diagram yields a δ34SΣS value of 8.2‰, indicative of sulfur derived from a mix of sedimentary and magmatic sources. All of the sulfides have Pb isotopic ratios that plot within the field of underlying Paleozoic strata and are close to the Indosinian and Yanshanian granites, suggesting that the metals were derived from a combination of underlying Paleozoic sedimentary and granitic sources. Most of the hydrothermal pyrites have Co/Ni and S/Se ratios that are indicative of a magmatic-hydrothermal origin, suggesting that the Au was derived predominantly from magma but with possible contributions from sedimentary and metamorphic fluid sources. In addition, hydrothermal pyrites have trace-element variations that differ from Py0, suggesting that the gold was not derived from the local host rocks. An underlying Paleozoic sedimentary sequence enriched Au that underwent greenschist facies metamorphism may be another source of the gold. The majority of invisible Au is present as structurally bound, with visible gold present as inclusions or as fracture fills within Py2. The gold was precipitated as a result of fluid mixing, a process that caused an increase in fO2 values and a change from reduced to oxidized conditions during the main-ore stage. This study suggests that the Longtougou deposit is an orogenic gold deposit that formed during the transition between compression and extension in an intracontinental orogeny.