Lead and noble gas isotopic constraints on the origin of Te-bearing adularia-sericite epithermal Au-Ag deposits in a calc-alkaline magmatic arc, NE China

Abstract

Abstract Tellurium (Te)-bearing adularia-sericite epithermal Au-Ag deposits are widely distributed in calc-alkaline magmatic arcs and are an important current and future source of precious and critical metals. The source of ore-forming fluids in these deposits remains unclear due to the lack of in situ isotopic evidence on Au-, Ag-, and Te-bearing minerals. To advance the understanding of the source and evolution of Te and precious metals, herein, we combine in situ Pb isotope analysis with He, Ne, and Ar isotope and microthermometric analysis of fluid inclusions in ore and gangue minerals from two Te-rich and two Te-poor epithermal Au-Ag deposits that occur in an Early Cretaceous magmatic arc in the North Heilongjiang Belt, northeastern China. Ore minerals (hessite, petzite, calaverite, altaite, pyrite, chalcopyrite, and galena) from Te-rich Au-Ag deposits, including Sandaowanzi and Yongxin, have the least radiogenic Pb isotope compositions (206Pb/204Pb from 18.1 to 18.3) and the lowest μ1 values (the 238U/204Pb ratio of the lead source down to 9.14) of the deposits studied. For these Te-rich deposits, noble gas isotope data show that fluid inclusions in ore minerals contain a large proportion of mantle He (up to 25%), whereas barren early-stage minerals do not (<1%). The Pb, noble gas isotope, and fluid inclusion microthermometric results suggest that Te-rich ore-forming fluids were likely discharged from mafic magmas into convecting meteoric flow systems at shallow levels (<2 km). In contrast to the Te-rich deposits, ore minerals from the Te-poor Dong’an Au-Ag deposit have radiogenic Pb isotope compositions (206Pb/204Pb from 18.8 to 18.9) and the highest μ1 values (up to 10.54). Fluid inclusions in ore minerals contain a small proportion of mantle He (1% to 5%). The results suggest that metals and ore-forming fluids in these deposits were discharged from either more crustally contaminated intermediate-felsic magmas or leached from upper crustal rocks by convecting meteoric flow systems. Although the Te-poor Tuanjiegou Au-Ag deposit has a non-radiogenic Pb isotope composition consistent with a mafic magma source, Te is much less abundant (electrum [>95%] is the major gold- and silver-bearing mineral) than Au. The main exploration implication of these results is that unexplored volcano-plutonic centers in the northeast Xing’an Block with less radiogenic Pb isotope compositions (206Pb/204Pb < 18.3) and containing fluids with a high proportion of mantle He are more likely to generate Te-rich epithermal Au-Ag deposits than other volcano-plutonic centers in NE China.