Geochemical Signature And Multifractal Analysis Of Hydrothermal Mineralization At Dexing Porphyry Copper Deposit, SE China

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 element concentration enrichment or depletion, caused by hydrothermal mineralization, is of great importance for mineral exploration and resource assessment. Researchers in recent decades have demonstrated that self-similarity exists at different scales, or non-scale distribution, which can be investigated by fractal/multifractal theory. Many authors have studied ore deposits at regional, deposit, or ore body scales; however, little has been done on the concentration micro-distribution of elements on the crystal surfaces of major ore minerals. This study was carried out at the Dexing porphyry copper deposit, located in a NE-trending magmatic belt along the southeastern margin of the Yangtze Craton; consisting of three porphyry copper ore bodies of Zhushahong, Tongchang, and Fujiawu, from northwest to southeast. The main objectives of this research were:(1) to constrain the geochemistry of the granodiorite porphyry, which is the main host for Cu mineralization in the area; and (2) to evaluate the potential of multifractal parameters, constructed by 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.On a deposit scale, the geology and geochemistry of the study area were constrained based on field investigation, petrographic study, major and trace element analysis. In addition, the existing geological, geochemical, and isotopic data were incorporated into our work. The origin of the main host rock, granodiorite porphyry, has remained a topic of debate for several decades. Most of the recent studies indicated that the Dexing porphyry copper deposit was emplaced in the Middle Jurassic (～171 Ma), and molybdenite Re-Os dating indicates that the associated Cu-Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The results of this study revealed that the Tongchang porphyry share similar compositional characteristics to adakites, possessing properties such as:(1) a calc-alkaline compositional trend; (2) medium to high-K calc-alkaline; (3) similar geochemical characteristics to Archean tonalite-trondhjemite-granodiorites (TTG), with relatively low Yb contents and high La/Yb ratios; and (4) a high Sr/Y ratio. The porphyry has higher Th content and Th/Ce ratios than those of oceanic slab-derived adakites, but showing high Th characteristics of a lower-crustal source. In SiO2 vs MgO and Mg# diagrams, most of the samples of the Tongchang porphyry are similar to lower crust-derived adakites. Additionally, the SiO2 vs Th/Ce and Th diagrams show Th contents and Th/Ce ratios different from those of subducted oceanic crust-derived adakites; however, some of the samples overlap with the field of the delaminated lower crust-derived adakitic rocks.Undertanding and describing the spatial and temporal distribution of elements can provide insight into the ore-forming mechanism. The non-linear theory indicate that singular geological processes, such as mineralization, are scale invariant and the resultant phenomena often satisfy the distribution of fractal or multifractal. In this study, two methods were proposed to quantify the distribution patterns of elements on pyrite crystal surfaces using multifractal analysis. Firstly, a new method for separating highly from weakly mineralized zone is presented. For this purpose, a total of 23 representative samples were collected from the inner porphyry zone (IPZ) and the outer and contact zone (OCZ) of the Tongchang ore body. Based on our new field observations and combined with petrological and mineralogical studies as well as some previous researches, the spatial distributions of alteration and mineralization zones were outlined, and then multifractal analysis were applied to two groups of samples, representing both the IPZ and OCZ; using elemental dot maps, generated by imaging capabilities of the Environmental Scanning Electron Microscope (ESEM), on pyrite crystal surfaces. The results revealed a symmetric zoning pattern from the center outward, with less common potassic alteration in the center through phyllic and propylitic alteration zones. The elements considered for multifractal analysis include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Te, Sb, Si, P, S, Ag, Au, Hf, Ta, and Hg. Significant multifractal parameters, such as the widths (△α) and the left-branch △αL of the multifractal spectra were computed using the so- called moment method. The values of these parameters are significantly higher in OCZ than in IPZ, especially for ore forming elements; such as Cu, Zn, Au, and Ag, suggesting that the spatial variability of studied element concentrations on the surface of pyrite crystals can be interpreted as representing two zones;(1) highly dispersed and mineralized OCZ with Cu grade >0.4 wt. %, and (2) weakly mineralized IPZ, with <0.4 wt.% Cu. These results strongly support the results obtained by traditional methods, which show that economic mineralization at Dexing porphyry Cu deposit are concentrated in outer flanks and contacts of the porphyry stock; whereas the inner parts are weakly mineralized. Therefore, the method used in this study may provide a potential tool to separate highly mineralized zones from weakly mineralized zones in the Tongchang ore body of the Dexing PCD and probably similar deposits elsewhere.Secondly, a new method for vein type identification and characterization was presented. This study has two objectives of identifying different vein types; and evaluating the potential of multifractal parameters by using the so-called method of moments, to characterize the spatial and temporal distribution patterns of concentration of elements on mineral surfaces. 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 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 different veins and veinlets were represented by multifractal spectra. Similar to the first type study, the multifractal parameters were calculated, however, the most favourable 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 also 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.