Abstract:

The Late Carboniferous Huanglong Formation in Chaohu area in the lower Yangtze terrane consisted of red biomicrite
limestone, with a upper belt of mixed mudstone and limestone. The red-colored Huanglong limestone provides a good opportunity for
studying ferric minerals in the limestone and their relationships with the limestone bulk color, and for tracing the influence from
hydrothermal activities. Here we present a comprehensive study on the Huanglong Formation, and discuss the implications for
sedimentary environments and regional hydrothermal episodes. We collected red limestone samples, conducted thin section
microscopical observations and performed instrumental analyses including X-ray powder diffraction (XRD), Diffuse Reflectance
Spectroscopy (DRS), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Inductively Coupled
Plasma-Optical Emission Spectroscopy (ICP-OES). With DRS, we detected ferric oxide/hydroxide minerals (hematite and goethite) in
the red limestones. We employed “redness”, a widely used paleoclimate index, to quantitatively describe the rocks’red color. Our
data show that for the limestone samples, the hematite/goethite peak intensity on the DRS spectrum correlates well with the measured
redness of the samples, but the bulk iron content shows no such good correlation, which suggests that the ferric oxide/hydroxide
minerals likely control the abnormal red color of the Huanglong red limestones. In various diagrams characterized by distinct elemental
distribution, element data for the limestone samples fall into the zones representing hydrothermal deposits, revealing the potential
influence from hydrothermal activities. Though no direct evidence for hydrothermal systems was observed in the field (e.g.“, black
chimney”relics), findings of Late Carboniferous hydrothermal-associated depositions have been reported in the neighboring area.
Based on our results, the red-colored Huanglong limestones likely indicates a local response to the regional hydrothermal activities: the
hydrothermal systems released ferrous solutes-enriched fluids to surface environments and these ferrous solutes were oxidized to ferric

limestone. We conclude that the Huanglong Formation may record the imprint of the regional hydrothermal activities in the Late
Carboniferous. Our study contributes to the decadal debates on the occurrences of large scale hydrothermal events during the Late
Carboniferous in the lower Yangtze region, South China. Further investigations on sedimentary temperature reconstruction are needed
for unraveling the lower Yangtze hydrothermal episodes.

Key words: lower Yangtze region, red limestone, Huanglong Formation, hydrothermal sedimentary process, elemental geochemistry