Distribution of orebodies and gold grades and their structural controls at the Chaihulanzi gold deposit, NE China: constraints from 3D modeling and fractal analysis

Abstract

The multi-scale controls on the spatial distribution of orebodies and gold grades at the Chaihulanzi gold deposit remain enigmatic. The orogenic gold mineralization is hosted by graphite-bearing schists, gneisses, and diorites. Our study employed Surpac 3D modeling technology to define the spatial morphology of orebodies and applied the number–size fractal model to analyze the gold grade distribution, quantitatively characterizing the influence of ore-controlling fault systems. Our 3D model is based on geological profiles, outcrop observations and drill hole data. It reveals that the NW–SE-trending and nearly E–W-trending ore-controlling faults are compatible with a Riedel shear system. The NW–SE-trending transpressional faults controlled continuous, tabular orebodies that extend into deep en-echelon veins, whereas the nearly E–W-trending transtensional faults formed discontinuous, lens-shaped orebodies. Fractal analysis indicates that gold grades in most cross-cuts display a bifractal pattern. The orebodies controlled by transpressional faults with more intense mineralization have higher fractal dimensions (D2: 0.59–5.75; median 1.84) and thresholds (median 3.1 g/t) compared to those controlled by transtensional faults (D2: 0.75–5.25; median 1.4; threshold median 1.03 g/t). This can be explained by the dense and interconnected ore-controlling microfractures in transpressional faults associated with variable gold grade distribution, relative to sparse and dispersed microfractures in transtensional faults associated with uniform gold grade distribution. The gold grades of orebodies are decreasing in wall rocks from graphite-bearing schist to gneiss to diorite porphyry, which, together with a decreasing intensity of quartz-sericite alteration, can be attributed to the influence of fluid–rock reactions. Our study contributes to the better understanding of deposit-scale and micro-scale controls on mineralization and gold grades at the Chaihulanzi deposit with significant implications for optimizing deep and regional exploration strategies.