Multifractal distribution of geochemical elements in Dexing porphyry copper deposit, Jiangxi Province, China

Abstract

Hydrothermal mineralization can cause enrichment or depletion of elements in ore bodies and surrounding country rocks. Local metal-enriched rocks can become an ore body that can be mined at an economic profit. Therefore, understanding the spatial distribution of elements caused by mineralization is crucial for mineral exploration and resource assessment. In this study, a total of 24 representative samples were collected from different veins within the Tongchang ore body of the Dexing porphyry Cu–Mo Deposit. The objectives of this work were firstly to identify different vein types and secondly to evaluate the potential of multifractal parameters, constructed using the so-called method of moments, to characterize the spatial and temporal distribution patterns of concentration of elements on mineral surfaces, which may provide insight into hydrothermal mineralization and its effect on spatial and temporal distribution of element concentration. Based on the analytical results and previous investigations, the vein systems within the deposit can be divided into four main types as follows: (1) early quartz veins (A-type veins) related to K-feldspar alteration; (2) quartz-pyrite-molybdenite-chalcopyrite veins (B-type veins) related to chlorite+illite alteration; (3) quartz-pyrite-chalcopyrite veins (D-type veins) related to late phyllic (muscovite+quartz) alteration; and (4) the latest carbonate-sulphate-oxide veins (H-type veins). The multifractal distributions of concentration of chemical elements on the surface of pyrite crystals selected from veins of different types were investigated using digital images generated by means of ESEM. To achieve our goal, the distribution patterns of element concentration values of eight elements (Cu, Fe, Mn, Ni, P, S, Si and Ti) on the pyrite surface selected from B-type, D-type and H-type veins and veinlets were represented by multifractal spectra. Several multifractal parameters were calculated; however, the most significant ones are ∆α and ∆αL. These parameters are found to be higher in the D-type veins related to late phyllic alteration and mineralization in comparison with other veins and vein systems, indicating that the ore minerals containing Cu and Fe sulphides are highly enriched in D-type veins. This result is consistent with previous research on the area as well as new field observations, chemical and petrographic analysis carried out for this study. Therefore, the multifractal parameters, particularly ∆α and ∆αL, appear to be promising to discriminate between mineralized veins developed during different stages of hydrothermal activity which led to development of the porphyry system at Tongchang ore body.