The genesis of giant lithium pegmatite veins in Jiajika, Sichuan, China: Insights from geophysical, geochemical as well as structural geology approach

Many studies focus on mineralogy and geochemistry of pegmatites and their parental source rocks to decipher the genesis of pegmatites as well as classify them. Recently, structures were regarded as an additional dimension on the classification of pegmatites. However, categorizing pegmatites by structure has not received much attention. The recent discovery of the Jiajika granite-pegmatite deposit in Sichuan, China, makes it the largest Li ore body in Asia. This study combines gravity survey, audio magnetotelluric sounding survey, structural geology, soil geochemistry as well as major element analysis together, and aims to decipher the genetic connection between the major pegmatite veins and their parental granites. The three-dimensional structure of the Jiajika granite body is exhibited by the density model obtained through inverting the Bouguer anomaly data. The diverse structures of pegmatites are analyzed through their geological occurrences. The hot spots are recognized through soil geochemistry exploration. Major elements from many samples in the X03 and the No. 134 lithium-rich pegmatites are used to investigate geochemical variations in pegmatites. The shallower structures of the granite top and the structures of pegmatites have been refined by the resistivity cross-sections as well as a geological profile with deep drilling. The geochronology data are integrated to present the cooling history of granites and pegmatites. The pegmatites in the Jiajika lithium ore field can be categorized into two types, one with small dips that close to the granite outcrops and another with large dips that far away from the granite outcrops. The majorities of the pegmatites exhibit similar geological occurrences under the regional stress field. Moreover, the delicate structures of the top of the granite dome impose a local stress field that controls the developments of pegmatites, which cause the pegmatites close to and far away from the granite outcrops have distinct geological occurrences as well as geochemical differentiation. The uplifts on the top of the granite dome correspond to the high concentration areas of boron, cesium, and lithium in soils. They also influence the regional zonation of pegmatites in Jiajika. Furthermore, the structures of the top of the granite dome affect the local heat field that regulates the metamorphism in Jiajika. The 3-D gravity model, the resistivity cross-sections, and the deep borehole have proven that most of the pegmatites in the Jiajika ore field are typically close to granites, sometimes even imbedded with granite layers. The gradient of the cooling curve for the Majingzi granite is large. This rapid cooling rate can explain why most pegmatites are distributed in the vicinity of the granites. The rapid cooling rate also provides crucial evidence that why fine-grained spodumene and microcrystalline spodumene account for the majority of all spodumene in Jiajika. Structures of pegmatites were treated as one criterion on the classification of pegmatites, and it also provides new perspectives on the relationships between pegmatites and their parental granites. Many explorations for pegmatite as a source of economic commodities rely on near-surface surveys. However, this study suggests that a deep earth probe may provide more useful information.