Geochemistry and geochronology of the Banxi Sb deposit: Implications for fluid origin and the evolution of Sb mineralization in central-western Hunan, South China

Abstract

Central-western Hunan in South China hosts the largest antimony belt in the world with two types of Sb deposits identified: Sb-Au Woxi-type and Sb only Xikuangshan-type. Banxi is the most representative deposit in the region with vein-type Sb mineralization hosted in Neoproterozoic clastic rocks, stibnite developing in ores, and arsenopyrite mainly occurring in altered country rocks. Trace element contents (including rare earth elements; REE) and isotopic ratios (S, Pb, Sr, Nd, He and Ar) in stibnite and/or arsenopyrite were analyzed to determine the timing of ore formation and to elucidate fluid origin and evolution processes. Most sulfides have high ΣREE contents (45–103 ppm), moderate fractionation between LREE and HREE (LREE/HREE = 6.38–11.56) and slightly negative Eu anomalies (Eu/Eu* = 0.53–0.72). The δ34S values are 3.88–5.81‰ for stibnite and 9.25–11.82‰ for arsenopyrite. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios range from 18.617 to 18.635, 15.695 to 15.739, and 38.965 to 38.981 for stibnite, and from 18.594 to 18.609, from 15.587 to 15.698, and from 38.893 to 38.926 for arsenopyrite. The 87Rb/86Sr and 87Sr/86Sr ratios are 0.3016–3.538 and 0.711463–0.717591, respectively for stibnite, and 0.2251–2.214 and 0.711244–0.711565, respectively for arsenopyrite. The 147Sm/144Nd and 143Nd/144Nd ratios are 0.1174–0.9816 and 0.511942–0.512768 for stibnite, with εNd(t) values (t = 130 Ma) ranging from − 12.4 to − 6.6, and two-stage model ages (T2DM) ranging from 1457 to 1932 Ma. The 40Ar/36Ar and R/Ra ratios (where Ra is the atmospheric 3He/4He ratio of 1.4 × 10− 6) are 409–545 and 0.0088–0.0348, respectively for stibnite. The Rb–Sr and Sm–Nd isotopic analyses of the sulfides yield isochron ages of 129.4 ± 2.4 Ma (2σ, MSWD = 1.3) and 130.4 ± 1.9 Ma (2σ, MSWD = 1.6), respectively. The combined trace element and isotopic data-set indicates that the ore-forming fluids were a mixture of dominantly deep basement-derived, solute-rich parent water and a small amount of dilute, heated meteoric water. The decreases in ΣREE, δ34S, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 87Rb/86Sr, 87Sr/86Sr, 147Sm/144Nd and 143Nd/144Nd and increases in 40Ar/36Ar and 4He contents in the stibnite from the earlier quartz-stibnite mineralization stage (Stage II) through to the later stibnite mineralization stage (Stage III) indicate that the influx of meteoric water increased during Sb mineralization. The deep-sourced, relatively high temperature, rock-reacted fluid mixed with increasing amounts of more oxidized, low temperature meteoric fluids, causing the precipitation of arsenopyrite and stibnite sequentially at the Banxi deposit. A two-period genetic model has been proposed for the world-class Sb mineralization in the central-western Hunan region. The Woxi-type Sb–Au deposits were derived from orogenic fluids generated from folding and shearing during the Caledonian and Indosinian Orogeny, whereas the Xikuangshan-type Sb deposits were formed by mixing heated basement-derived hydrothermal fluid circulating during the flare-up of magmatism in South China, with lower temperature meteoric water.