Mineralogical and Geochemical Characteristics of Banded Agates from Placer Deposits: Implications for Agate Genesis

The phase composition and geochemical characteristics in banded agates with different structural sequences have been investigated in detail. The results reveal that the agate bands have a combination of a pseudo-granular silica → fibrous chalcedony → crystalline quartz (type I) sequence and a newly discovered pseudo-granular silica → crystalline quartz (type II) sequence. The banded agates mainly consist of α-quartz, moganite, and a minor amount of amorphous silica, goethite, hematite, kaolinite, illite, and carbonates. With the evolution of two structural sequences, the content of α-quartz and moganite increases and decreases, respectively. There is no moganite in crystalline quartz. The increased concentration of trace elements like Li, Na, Al, K, Ca, Ti, Mn, and Fe in different bands may correspond to the decrease in the water content in the mineral-forming fluid. The increased trace elements promote the structural transformation process of silica. With the evolution of the type I sequence, the thermal gradients between adjacent bands are 17 and 51 °C, respectively. In contrast, a significantly higher thermal gradient of 53-66 °C is exhibited when pseudo-granular silica transforms directly to crystalline quartz. It is inferred that a slightly increased thermal gradient between adjacent bands promotes the structural transformation process of the type I sequence. The sharply increasing thermal gradient between adjacent bands leads to the formation of the type II sequence from pseudo-granular silica to crystalline quartz. The formation process of different structural sequences in agate may be controlled together by trace element concentrations and thermal gradients.