Discovery of microdiamonds in Taojiang, Hunan, and its implications for primary diamond prospecting in the southeastern margin of the Yangtze Craton

Objective Located on the southeastern margin of the Yangtze Craton, the Taojiang area in Hunan Province is one of the favorable metallogenic segments for primary diamond deposits in China. This study aims to clarify the lithogeochemical characteristics of microdiamond−bearing diabase and the mineralogical characteristics of microdiamonds in the study area, and to discuss the source properties of microdiamond−bearing diabase.

Methods Lithogeochemical analysis was carried out on diabase samples collected from Jiangshiqiao, Taojiang area.Mineralogical observation, Fourier−transform infrared (FTIR) spectroscopy and Raman spectroscopy were adopted to test the microdiamonds.

Results The diabase is enriched in large ion lithophile elements (LILEs) such as Ba and Th, but depleted in high field strength elements (HFSEs) including Ta, Nb and Ti. The Nb/U, Nb/Ta and Zr/Hf ratios range from 10.68 to 29.09, 5.90 to 13.06, and 40.77 to 62.79, respectively. The microdiamonds have grain sizes of 0.2–0.6 mm, pale yellow to yellow hues, and are predominantly cubic−octahedral composite crystals with characteristic Raman peaks of natural diamonds. The diamonds are classified as natural IaA and IaAB types containing isolated nitrogen centers.

Conclusions Evidence indicates that the diabases in Taojiang were generated via partial melting, and their mantle source records mixed crust−mantle genetic signatures. Nitrogen impurity centers of the local diamonds show an evolutionary trend transitioning from single C centers to aggregated A and B centers, which is highly consistent with the features of Ib−type diamonds from northern Anhui−Jiangsu. This similarity implies an extremely short mantle residence time, confirming that these diamonds are juvenile diamonds. This discovery verifies that diabase represents a new significant host rock type for diamonds along the southeastern margin of the Yangtze Craton. The findings advance the regional metallogenic theory of primary diamonds and provide critical guidance for subsequent studies on diamond metallogenic mechanisms and prospecting targets.