Trace element zoning and temperature variation in sphalerite from the giant Nannihu porphyry-skarn-epithermal polymetallic system, central China

The Nannihu ore field at the southern margin of the North China Craton is a typical magmatic-hydrothermal metallogenic system consisting of the large Nannihu porphyry Mo-W deposit, Luotuoshan skarn Zn-Cu-W deposit, and Lengshuibeigou epithermal Pb-Zn-Ag deposit. Sphalerite is present as a minor to major phase in all these deposits, providing an ideal object to investigate its trace element zoning and temperature variation. The field data and associated mineral assemblages indicate that only one type of sphalerite is present at the Nannihu porphyry deposit, whereas two types of sphalerite are present at both the Luotuoshan skarn deposit and Lengshuibeigou epithermal deposit. Sphalerite from the Nannihu porphyry deposit has Ga, Cd and In concentrations of 0.64–7.26 ppm, 1426–1687 ppm and 291–460 ppm, respectively. In the Luotuoshan skarn deposit, sphalerite contains relatively high In (147–616 ppm) and moderate Cd (1234–1558 ppm), with low Ga (below 43 ppm). Sphalerite from the Lengshuibeigou epithermal deposit has Ga, Cd, and In contents of 3.70–589 ppm, 1453–2673 ppm, and 26.7–308 ppm, respectively. Germanium concentrations are low in sphalerite (< 5 ppm) from all three deposits. Infrared microthermometry of sphalerite-hosted fluid inclusions reveals that the sphalerite in the Luotuoshan deposit precipitated from intermediate- to low-salinity fluids (4.0 ~ 13.7 wt% NaCl equiv) at 398 to 308 ℃, whereas the Lengshuibeigou deposit formed from low-salinity fluids (2.5 ~ 7.4 wt% NaCl equiv) at 305 to 238 ℃. Indium tends to be enriched in sphalerite of the proximal high-temperature Luotuoshan skarn deposit, whereas Ga is more concentrated in sphalerite from the distal lower-temperature Lengshuibeigou deposit. Gallium and In distributions, combined with data collected from other magmatic-hydrothermal ore fields and deposits, reveal that In-enriched sphalerite occurs in proximal skarn deposits of magmatic-hydrothermal systems, and Ga-enriched sphalerite usually occurs in distal and shallow epithermal deposits. Ga/In values of sphalerite are related to the distance between mineralization and the causative intrusion of the magmatic-hydrothermal systems, and can be an indicator for exploration targeting.

Mineralium Deposita

Mineralium Deposita

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1432-1866

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