An evolving MVT hydrothermal system: Insights from the Niujiaotang Cd-Zn ore field, SW China

Abstract

• The carbonate rocks acted as a key role during sulfide mineralization . • The change of alkaline-oxidized to acid-reduced resulted in sulfide precipitation. • The fluids evolved from the SW to the NE ore deposits . • The MVT deposits are the coupled mineralization product of fluid–structure-lithology. The metallogenic characteristics of Mississippi Valley-type (MVT) deposits have been widely concerned, but the evolution of MVT hydrothermal systems still needs to be further explored. The Niujiaotang Cd-Zn ore field (5,300 t Cd @ 0.04–1.43 wt% and 0.35 Mt of 5.85–24.48 wt% Zn) is located along the southeastern margin of the Yangtze Block, SW China, and consists of seven ore deposits, i.e. Dongchong, Mapo, Daliang, 768, Caiyuanhe, Shuanglongquan and Duniu. The ore bodies occur as stratiform or lenticular and are hosted by dolostone of the early Cambrian Qingxudong Formation, and are structurally controlled by the NE-trending Mandong fault. The hydrothermal dolomite has δ 13 C values of −3.03 ‰ to + 2.94 ‰, with lower contents of ΣREE (6.76–53.5 ppm), negative Eu and weakly positive Ce anomalies, all of which are similar to those of wall rocks. This suggests that the early Cambrian carbonate rocks involved in sulfide mineralization through water/rock (W/R) interaction and provided part of ore-forming materials. The δ 18 O values (+17.14 ‰-+22.79 ‰) are within the range of metamorphic fluids (δ 18 O=+16 ‰-+25 ‰), implying the hydrothermal fluids were derived from the Precambrian basement metamorphic rocks. The sulfides are characterized by scattered sulfur isotopes (δ 34 S=+10.03 ‰-+32.82 ‰), implicating that the reduced sulfur was derived from a mixed source of metamorphic fluids and thermo-chemical sulfate reduction (TSR). In situ Pb isotopes indicate that both Precambrian metamorphic rocks and early Cambrian sedimentary rocks provided metals for the hydrothermal fluids. In addition, the increased δ 34 S Σ values and Pb isotopes from Dongchong (SW part of the Niujiaotang ore field) to 768, and then to Duniu (NE part of the Niujiaotang ore field), indicating the hydrothermal fluids likely evolved from the SW to NE ore deposition sites along the Mandong fault, which is also confirmed by other evidence. Hence, we propose an evolving MVT hydrothermal system and a fluid-tectonic-lithology combination coupled metallogenic model for the Niujiaotang ore field. The determination of such metallogenic model of this ore field is conducive to the research and exploration of the similar MVT deposits nearby and worldwide.