Crustal Mercury Addition Into the Giant Jinchuan Ni-Cu Sulfide Deposit, China, and Its Geological Implications

Mercury (Hg) isotopes have shown their power of tracing Hg pollution sources in ecosystems, but their potentials for petrogenetic tracing are yet to be explored. Here we conducted Hg isotope analysis for samples collected from major orebodies of the world-class Jinchuan Ni-Cu sulfide deposit, China. These samples show large variations of delta Hg-202 (-2.65 to +0.19 parts per thousand) and Delta Hg-199 (-0.16 to +0.19 parts per thousand). Some of the Delta Hg-199 values significantly deviate from current estimates on the primitive mantle (Delta Hg-199: 0.0 +/- 0.1 parts per thousand, 2SD). The pronounced Hg mass-independent fractionation (Hg-MIF) signals, with significant positive (>0.1 parts per thousand) and negative (<0.1 parts per thousand) Delta Hg-199 values similar to marine sediments and terrestrial soils, respectively, suggests the addition of crustal materials into the Jinchuan deposit, via crustal assimilation during mantle-derived magma ascending to the crust. These samples show delta S-34 values (-1.09 to +1.38 parts per thousand) identical to that of the primitive mantle (0.0 +/- 2.0 parts per thousand), which may indicate a major sulfur source from the mantle. However, delta S-34 provides poor constraints on the sulfur source, and the early reported anomalous Delta S-33 values (+0.12 to +2.67 parts per thousand) in the Jinchuan deposit support the involvement of external sulfur from Archean and Proterozoic sedimentary rocks during the formation of this deposit, similar to the case of Hg. This study shows the powerful use of Hg isotopes as a petrogenic tracer and highlights the importance of interaction between mantle-derived magmas and crustal materials on the formation of the Jinchuan Cu-Ni sulfide deposit.