Characteristic Of Ore-controlling Structure And Distribution Of Mineralization Intensity In The Xinli Gold Deposit,Jiaodong Peninsula, China

The Jiaodong Peninsula, the most productive gold province in China, is dominated by Jiaojia-type gold deposits that formed at ca. 120 Ma. The deposits are characterized by widespread alteration and mineralization resulted from fluid–rock reactions in the footwall of regional faults. It is known that the interrupted appearance of continuous mineralization zones and barren zones along regional faults are controlled by changes in the dip direction and dip angle of these faults, which can cause local dilational sense. In the continuous mineralization zone, intensive fluid–rock reactions occurred. Yet how the structure and fluid–rock reaction impacted on the distribution of mineralization intensity is not well constrained. In this study, we selected the Xinli Jiaojia-type gold deposit associated Sanshandao–Cangshang fault(F1), located on the northwest margin of the Jiaodong Peninsula, to examine in detail the structural ore-controlling characteristics and the distribution of mineralization intensity.Structural observation indicate the F1 controls the orebody distribution in large-scale. Mineralogical analyses via XRD reveal that the kaolinite and quartz are the major components in the F1 fault gouge with small amounts of illite, pyrite and gypsum. Fault gouge of secondary fault(F2) dividing the sericite-quartz alteration and K-feldspar alteration consists of contain large amounts of quartz and sericite with subordinate kaolinite and mixed-layer illite-smectite. Constraints from the mineral occurrence and genetic conditions, sericite was considered to transform into kaolinite, then into the illite-smectite mixed-layer acted as the intermediate product, and finally into illite. The mineral transformations reflect that the aK+, or aK+/aH+(acid) is continuously decreased. The particle size distribution for the F1 fault gouge displays unimodal, composite bimodal and trimodal peaks. In contrast, the F2 fault gouge shows merely bimodal peaks. The fractal dimension of particle size distribution in F1 and F2 are 2.61-2.82 and 2.46-2.52, respectively. It can interpret that the F1 fault gouge has undergone the process of rolling abrasion, while the F2 fault gouge was in the status of constrained comminution. The dominant clay compositions, highly differentiated particle sizes and oriented microstructures in F1 fault gouge induce a low lateral permeability, making the fault act as a “barrier layer” for the ore fluid and resulting in the mineraliztion in the footwall of F1. F2 is the structrure of post-mineralization.We applied the number–size fractal model and the lacunarity method to quantify the distribution of gold concentration and mineralization intensity along drifts. All drifts share several common features.(1) Statistical results show that changes in the dip direction and dip angle of this fault bear almost no relationship to the thickness or grade–thickness of the orebody.(2) Mineralization along drifts is most intense at some distance from the fault plane rather than at locations immediately adjacent to the fault plane where fracturing is most extensive and, according to fluid infiltration theory, would be the most likely regions of intense mineralization.(3) Gold-bearing sulfide pods and veinlets in the alteration zone commonly possess irregular and corroded boundaries that are considered to be an indicator of fluid–rock reactions.(4) The threshold(the gold concentration that divides a segment with a lower fractal dimension in a lower concentration range from one with a higher fractal dimension in a higher concentration range) and the lacunarity(the parameter that evaluates the evenness of high gold concentration distribution) show positive and negative relationships, respectively, to the thickness and grade–thickness of a drift.We thereby argue on the basis of these features that fluid–rock reactions were an important factor responsible for the mineralization upgrade in continuous mineralization zones. An increase in the threshold and a decrease in the lacunarity indicate that fluid–rock reactions causing gold precipitation are more intensive and more evenly distributed throughout the footwall for better mineralization. These phenomena suggest that fluid–rock reactions responsible for the gold precipitation possible own the characteristics of spatial self-organization mechanism, which is widely developed in various geological fluid–rock reaction processes.