Mapping of district-scale potential targets using fractal models

Abstract

The Pulang porphyry-Cu deposit is one of the most important copper deposits discovered in China in the last five years. In this deposit, the irregular distributions of high concentrations of metals are associated with four intrusive complexes and fault structures. In the present study, fractal models including box-counting dimension (Bd), power-law frequency and Hurst exponent are applied to characterize the vertical distributions of Cu values along boreholes in the deposit, and to delineate target areas in the Pulang copper district. The resulting box-counting model shows that the vertical distributions of Cu in both mineralized and non-mineralized boreholes exhibit self-similarity, with values of Bd ranging from 1.01 to 1.43, and mineralized boreholes have values of Bd higher than those of non-mineralized boreholes. The resulting power-law frequency model shows that the vertical distributions of Cu in mineralized boreholes are bifractal whereas they are monofractal in non-mineralized boreholes. The bifractal vertical distributions of Cu values are generally associated with multiple ore-forming stages/periods or complicated ore-forming functions of geological background. The Hurst exponents of Cu data from all continuously mineralized boreholes are >0.5, indicating that the Pulang porphyry-Cu deposit has good vertical continuity of mineralization, or that the development of orebodies was relatively stable in this geological setting. High Hurst exponents (>0.85) can be utilized to identify subsurface mineralized targets, whereas lower Hurst exponents (<0.85) represent discontinuous mineralized rocks along the margins of the porphyry-Cu deposit near country rocks. Based on calculated values of average Cu grade, coefficient of variation, Bd and Hurst exponent for the Cu data along boreholes, interpolated values of these variables are excellent for mapping of potential targets. The results show that the Complex II (intrusive body) is a potential copper target in the north of the Pulang district, because it has a similar Bd as the Complex I that hosts the Pulang porphyry-Cu orebody in the south of the Pulang district. In addition, the Complex I has a potential porphyry-Cu target at depth between explorations lines 7 and 15 based on high Hurst exponents and favorable geological setting. The application of the fractal models discussed in this study is convenient, simple, rapid and direct for outlining potential exploration targets without processing multiple geological, geophysical, and geochemical datasets from disparate sources.