Investigating the Irregular Localization of Skarn Orebodies by Computational Modeling in the Fenghuangshan Ore Field, Tongling District, Anhui Province, China

Abstract

The localization patterns and mechanisms of orebody formation are critical in understanding and exploring for mineral deposits. Skarn ore deposits are especially difficult to study because of their complex features. The Fenghuangshan ore field is a maturely explored skarn copper ore field with many underground workings that reveal the skarn orebodies and related geological context in detail. This situation enables us to use computational modeling to investigate the irregular localization pattern of the skarn orebodies and their origin. Our 3D geometric modeling results show that the skarn orebodies are irregularly localized along the intrusion contact zone and that the irregular localization pattern is closely related to the topography and orientation of the contact zone. The largest orebodies and dominant reserves occur in the western contact zone (WCZ), mainly in the southern segment, where the contact zone is most concave (toward the intrusion). The thicknesses of the major orebodies in the WCZ are correlated with the Gaussian curvature of the contact surface. To investigate the localization mechanism of such irregular orebodies, we use numerical dynamic modeling to study the intrusion cooling process, which is critical for skarn orebody formation. The model consisted of a granodiorite intrusion and carbonate wall rocks, and it was ran on the FLAC3D platform to test three different scenarios: (1) syn-extensional cooling, which is the likely regional tectonic condition during the mineralization period; (2) syn-compressional cooling; and (3) cooling with no tectonic load. The dynamic modeling results for these three scenarios are distinctly different. Only the first scenario (NE–SW extension) can produce large-scale maximum dilation zones with a distribution pattern similar to that of the orebodies. As the maximum dilation zones are favorable for the formation of skarn ores, we conclude that the tectonic phase is also a critical constraint condition for the irregular localization of skarn orebodies.