Summary

Weathering characteristics of Ultramafic rocks and the concentration of nickel
in Ha Tri massif (Cao Bang)

Pham Thanh Dang, Nguyen Van Pho, Pham Tich Xuan, Doan Thi Thu Tra,

Nguyen Thi Lien, Nguyen Xuan Qua

The Ha Tri ultramafic intrusive block are part of the Cao Bang complex occurred in Song Hien tectonic structure, Northeast Vietnam. Tropical weathering process of Ha Tri ultramafic rocks has produced lateritic profile consists of two generalized zones: saprolitic zone (weathered and decomposed zone) and the limonitic zone. The limonitic zone is characterized by strong leached of elements (Si, Mg, Ca, Na, K); concentration of (Al, Co, Mn), especially Fe with gain factor up to 224.52% at the top, where iron-rich minerals (goethite, hematite) dominate. Down to the saprolite zone, Ni showed significant concentration with a gain factor up to 784.30% in the presence of garnierite group minerals; other elements such as Co, Mn, Fe tend to get insignificant enrichment compared to the bedrock. Nickel is enriched in saprolite because they are leached from geothite in limonite, where they are first concentrated with cobalt and manganese and then moved to the lower part of the profile, where Ni²⁺ replaces Mg²⁺ in the structure of serpentine due to their similar ionic radius, finally precipitates as hydrous Mg-Ni silicates (garnierite). Using the index (S/SAF = 0.25-0.71, UMIA = 17-80) and the SAF (SiO₂-Al₂O₃-Fe₂O₃) chart indicate that the Ha Tri lateritic profile has experienced a not very strong lateritization process (to moderate level lateritization process).

Key words: weathering of ultramafic rocks; nickel laterite; garnierite, Hà Trị (Cao Bằng)