Element behaviors due to rock weathering and its implication to geochemical anomaly recognition: A case study on Linglong biotite granite in Jiaodong peninsula, China

Abstract

Trace element contents of regolith materials are often orders of magnitude higher than those of their parent rocks. This weathering enrichment is often confused with mineralization. In order to quantitatively describe trace element behaviors due to weathering, we selected a regolith profile developed over the Linglong biotite granite in the western part of the Jiaodong peninsula, China. Eleven samples were collected from weathered soil to fresh biotite granite sequentially from top to bottom of the profile and 39 elements were analyzed for each sample according to the analytical specification of regional geochemical surveys in China. Based on the behaviors of major elements in samples from the studied regolith profile we present a new Weathering Index of Granite (WIG), which is more sensitive than the Chemical Index of Alteration (CIA), Weathering Index of Parker (WIP), and Weathering Index of Colman (WIC). The weathering behaviors of 12 trace elements are quantitatively described using an exponential relationship between element contents and WIG values in the studied regolith profile. Trace element contents calculated according to this exponential relation can be viewed as background values resulting only from differences of weathering degree of the same parent rock. For the case study in the Linglong–Jiaojia Au deposit region, an enrichment factor was defined as the ratio of trace element content to its background value calculated from WIG. The results of the case study indicate that the enrichment factor is a useful index to determine anomalies in regional geochemical survey data in a region where stream sediments and regolith materials are derived from the same parent material.