A Geochemical And Isotopic Study Of Fe-Mn Carbonates From The Lengshuikeng Ag-Pb-Zn Deposit, Jiangxi Province

The Lengshuikeng deposit is a giant Ag-Pb-Zn deposit in the northern Wuyi Mountain volcanic and metallogenic belt in South China. There are two major mineralization types in the Lengshuikeng deposit, which the upper is the porphyry orebody and the deep is the stratiform orebody.The porphyry orebody:belonging to the sub-volcanic hydrothermal mineralization Ag-Pb-Zn ore body, large-scale but low grade, mostly lenticular orebodies, the shape is relatively regular, ore fabric shaped granular structure fine-grained semi-euhedral, veinlet disseminated structure-based. Ore-forming temperature is 120-490℃. Ore forming materials mainly come from containing mine granite porphyry which originated from the melting of the crust.The stratiform orebody:high grade, thickness and with many kinds of useful substances. The ore body and surrounding rock was deposited conformable contact relationship, was bedded rules lenticular bedding output. Most of ore fabrics are fine-grained semi-euhedral, accountable structures mainly are brecciated and disseminated structure. Ore-forming temperature is 170-400℃. The stage of ore forming has experienced a volcanic eruption-sedimentary mineralization and hydrothermal mine period.In this study, we performed a systematic study on trace and rare earth element analysis and Sr-Nd isotope analysis for the bedded Fe-Mn carbonate rocks related to the stratiform Ag-Pb-Zn mineralization in the Lengshuikeng ore district. Three types of Fe-Mn carbonates are mainly distinguished, namely, the massive, breccia, and vein types. Both carbonate and silicate fractions in the samples are analyzed for their trace and rare earth element concentrations using a step acid-leaching technique.Our results show that the carbonate fractions in the massive type samples show the lowest REE concentrations but pronounced positive Eu and Y anomalies with Eu/Eu* value from 1.3 to 6.19 and Y/Ho value from 40.12 to 59.53, and similar characteristics are also shown for the silicate fractions in the massive type samples (Eu/Eu*=1.01-6.7, Y/Ho=20.68-55.14). These REE characteristics are similar to those of Sedex type massive sulfide deposits worldwide, and we suggest the massive type Fe-Mn carbonate rocks were formed from an exhalative volcanic-hydrothermal fluid feedering the depression basin of a volcanic lake. The high concentrations of redox-sensitive elements and ratios such as U/Th, V/Cr and V/V+Ni indicate a dysoxic environment for the Fe-Mn carbonate deposition.In contrast, the breccia type and vein type Fe-Mn carbonate samples show different trace and rare earth element features from those of massive type samples, and they are more similar to the volcanic rocks and magmatic-hydrothermal fluids in the Lengshuikeng ore district and may reflect strong overprinting from volcanic and sub-volcanic magmatism related to the porphyry type mineralization in the district.The characteristics of Sr-Nd isotope are similar to the volcanic rocks in the Lengshuikeng ore district.In short, the genesis of the Lengshuikeng ore deposit may have complex multi-stage mineralization characteristics. In terms of the stratiform orebody in the ore deposit which has huge economic value, it was formed from a volcanic-hydrothermal exhalative mineralization in the depression basin of a volcanic lake, and experienced latter overprinting from volcanic and sub-volcanic magmatic-hydrothermal mineralization.