Metal remobilization from country rocks into the Jiaodong-type orogenic gold systems, Eastern China: New constraints from scheelite and galena isotope results at the Xiadian and Majiayao gold deposits

Abstract

Orogenic gold deposits in the Jiaodong Peninsula contain >5000 t of known gold resources. Precambrian metamorphic rocks and Mesozoic granite represent the main host rocks for these deposits, but gold deposition (~120 Ma) is significantly younger and it remains unclear whether some of the metals are sourced from the local country rock to these mineral systems. This study uses in situ geochemistry and isotope signatures of scheelite (Sr isotopes) and galena (Pb isotopes) at the Xiadian and Majiayao gold deposits to determine the role, if any, of country rock compositions on the formation of gold ore systems in the Jiaodong Peninsula. Scheelite from the two gold deposits occurred as part of a syn-mineralization stage with quartz, siderite, ankerite, and pyrite in the main orebody and/or its footwall. New results show that scheelite yields high Sr (>200 ppm) and low Mo (<40 ppm) and As (<5 ppm) concentrations, bell to flat REE profiles, and variable Eu anomalies. These scheelite compositions are very similar to previously published results from orogenic-style gold deposits in greenstone belts, indicating fluids with similar geochemical properties. The Xiadian scheelite with negative Eu anomalies may be controlled by REE substitution mechanisms and the composition of parent fluids that may have interacted with graphitic formations of the Jingshan Group, while the Majiayao scheelite with positive Eu anomaly is most likely the result of crystallographic controls on REE partition. New scheelite Sr isotope results for the Xiadian (87Sr/86Sr: 0.71258–0.71540) and Majiayao (87Sr/86Sr: 0.71568–0.71930) deposits further support interaction between hydrothermal fluids and local to distal country rocks. The high radiogenic Sr isotope signature of scheelite from the Majiayao deposit is primarily derived from the regional Jingshan and/or Penglai Group meta-sedimentary rocks. Galena in the Xiadian deposit, which occurs with pyrite in the syn-mineralization stage, has Pb isotope signatures of 206Pb/204Pb: 17.185 ± 0.004 that may reflect mixing between the Linglong granite and/or metamorphic rocks. The Majiayao galena yields the least radiogenic Pb isotope results (206Pb/204Pb: 16.554 ± 0.040), which are consistent with sulfide in the Jiaodong Group meta-volcanic rocks (206Pb/204Pb: 16.270–16.718) and suggest remobilization of Pb from sulfide within the previously metamorphosed formation. New geochemistry and isotope results document a mixed metal contribution that likely occurred as a consequence of fluid-rock interaction between ore-forming fluids and wall rocks at shallow crustal depths (<10 km). Such processes may have played a critical role in generating similar metallogenic features of the Jiaodong gold deposits as well as syn-metamorphic orogenic gold deposits in other greenstone belts. Our study highlights that the remobilization and precipitation of metals (Pb, Sr, W, and possible Au) scavenged from country rocks are perhaps ubiquitous processes, whether in syn- and/or post-metamorphism orogenic gold systems.