Geochemical domains and their characteristics in the middle-southern part of the Sanjiang orogenic belt in southwestern China

Highlights

• Seven element associations reflecting comprehensive information were determined.

• Six geochemical domains were studied and considered reasonable.

• More accurate spatial distribution patterns of geochemical anomalies were delineated.

• Some favorable mineralization areas of element associations are identified.

Abstract

Principal component analysis (PCA) was used to extract geological information about regional strata and mineralization from many high-quality stream sediment geochemical data in the middle-southern part of the Sanjiang orogenic belt. According to the geological reality, the 32 main ore-forming elements in the study area were grouped into seven element associations that reflected the stratum features and mineralization. The study area was further divided into six geochemical domains that reflected the geological background. The strata and structure were combined with mineral data and revealed that the geological domains proposed in this paper represented the geological background of the study area. The corresponding elements within these geochemical domains have higher concentrations and enrichment factors, showing that the geological domains in the study area are reasonable. In the study area, the observed anomalies in the geochemical domains highlighted effective anomalies and reduced false anomalies. These anomalies can not only be used to identify potential metallogenic areas, but also provide information on tectonics; for instance, the middle axial fault of the Lanping-Simao basin was reflected in the linear anomalies of the LiB element association. The results of this study can help understand the combination and distribution characteristics of ore-forming elements and the regional geological structure of the middle-southern part of the Sanjiang orogenic belt.

Introduction

Since the 1930s, a large amount of stream sediment geochemical data has been obtained from geochemical mapping projects around the world (Wang et al., 2007; Birke et al., 2009; Grunsky et al., 2009; Andersson et al., 2014; Xie and Cheng, 2014). The multifaceted information contained in massive amounts of geochemical data has played a significant role in areas such as mineral exploration, environmental zoology protection, and land use management (Matschullat et al., 2000a, Matschullat et al., 2000b; Plant et al., 2001; Lima et al., 2003; Albanese et al., 2007; Cheng, 2007; Wilford et al., 2016). In mineral exploration, the focus has always been on recognizing and interpreting geochemical anomalies, particularly how to reduce false anomalies in high-background areas and highlight effective anomalies in low-background areas (Cheng and Agterberg, 2009; Zuo et al., 2016). However, there are few extended applications. Previous studies have shown that a new theory or an improved method can greatly enhance anomaly recognition, such as fractal/multifractal models, singularity theory, and combinations of methods (Cheng, 2007, Cheng, 2012; Agterberg, 2012； Lancianese and Dinelli, 2015; Zuo et al., 2016; Tian et al., 2018). It is also feasible to divide several background areas or geological tectonic units to investigate their geochemical characteristics. Most previous studies have divided units based on lithologic associations or structural features, while fewer have considered unit division from the perspective of microgeochemistry (Shi et al., 2004; Tang et al., 2016). However, structural systems can be divided from their macro-tectonic environment, and geological unit divisions can also be distinguished by micro-element geochemistry because the same geological tectonic units undergo similar geological development processes, with similar material sources and geochemical environments (Shen et al., 2003).

Currently, regions are divided into several subareas to perform regional geochemical surveys because of the inherent information contained in geochemical samples. The purpose of such a division is to identify the spatial distributions of elements in the crust and exhibit their geochemical characteristics in different subareas. This method provides useful information for studying the global characteristics of different units, helping to deepen people's understanding of regional geological structures and mineral resources distribution (Wang et al., 2017). Another key step of geochemical division is to determine element associations to explain the corresponding geochemical anomalies. Factor analysis, especially PCA (using R-factor analysis with varimax rotation), has been wildly applied (Steenfelt, 1990; Ziaii et al., 2011). As a multivariate statistical method, PCA is an effective method for studying symbiotic combinations of elements because it summarizes the original complicated variables into several new variables according to inherent relationships. Each new variable or component indicates an element association that represents a combined relationship between elements with similar genetic connection. This information can be used to conduct other analyses, and to explain the genesis of geological or geochemical phenomena by analyzing the geochemical behavior of elements or element associations (Guo et al., 2008).

The Sanjiang orogenic belt in southwestern China is one of the most active Phanerozoic orogenic belts in the world. It is geographically located in the western Yunnan Province, China, and tectonically located in the eastern segment of the Tethys orogenic belt. The Sanjiang orogenic belt contains many large ore-concentrated areas currently under production, and it seems that the region has a large metallogenic potential and good exploration prospects (Gao et al., 2016). Following previous studies, major concerns in the region include not only the regional geological surveys and basin-mountain coupling processes for revealing the basic characteristics of the regional stratigraphic framework and tectonic evolution (Deng et al., 2014a, Deng et al., 2014b), but also the geochemical investigation and exploration of critical mineral resources for serving the needs of economic development and national strategy (Wang et al., 2016; Fu et al., 2016). Multiscale regional geochemical exploration has been completed in this area, but analysis has primarily occurred in local areas, while macroscopic research is rare. Although Xie et al. (2017) attempted to create geochemical domains for the entire Yunnan province – which covers nearly half of the Sanjiang orogenic belt – there were several shortcomings due to the large spatial scope. Some effort has been made to identify different background areas in parts of the Sanjiang orogenic belt in terms of the abnormal descriptions according to geological structure unit boundaries (Shen et al., 2003), which could also be controversial owing to the various division schemes of tectonic units.

Therefore, in this study, the middle-southern part of the Sanjiang orogenic belt in the western Yunnan province was selected because it has a single natural landscape. The data from 1:200,000 regional geochemical data were used as the research object. Principal component analysis was utilized to divide geochemical domains, and the relationships between the geochemical domains, and geological background of mineralization and lithologic-structural units were also discussed. Then, geochemical anomalies of element associations were delineated to identify possible targets. An attempt was also made to establish corresponding relationships between geochemical domains and ore-forming elements to identify connections between the characteristics of ore-forming elements, geological background, and mineralization in different geochemical domains.