The Mechanisms Of The Five-floor Vertical Morphological Zonation At The Dajishan Vein-type Tungsten Deposit, Jiangxi

Vertical morphological zonation is a common phenomenon at exo-contact vein-typetungsten deposits in South China. Field geologists have identified five zones based on thevein morphology since1960s, and proposed a five-floor vertical morphological zonation. Thefive-floor zonation has played an important role in prospecting this type of tungsten depositsin the last few decades. However, the reasons for such a zonation are still unclear. In thispaper, we combine fractal methods, structural analysis, and finite element based numericalmodeling to investigate the mechanisms of the five-floor vertical morphologicalcharacteristics at the Dajishan tungsten deposit in Jiangxi Province.The vertical morphological zonation at Dajishan indicated by vein thickness and spacingdistributions is that veins become increasingly thick and sparse downwards. Fractaldimensions of vein thickness and spacing suggest that vein growth at the deeper parts ofDajishan is dominated by vein growth rate, and vein nucleation is inhibited. These veingrowth mechanisms favor formation of thick veins at the deeper part of vein systems. Thevein growth mechanisms at Dajishan are controlled by the coupling of stress field andmagmatic fluids. Therefore, fractal distributions of veins thickness and spacing are powerfultools to illustrate the vertical morphological zonation of the veins at Dajishan and offerinsight into the vein growth mechanims.The NWW trending subvertical joints are important conduits for mineralized fluids andthe main sites of mineralization. The permeability and aspect ratios of these joints controlledby stress state affect the fluid focusing in these joints and the other potential conduitssignificantly. The higher permeability or aspect ratios at the deeper parts of the joint systemslead to more fluid focusing in the lower part of ore bodies, which provides necessaryconditions for forming thick veins and mineralization. Formation of these subvertical jointsis related to the regional tectonic evolution and multiple granite emplacements.Transient fluid flow in joints is very fast and follows non-Darcy law. The hydrostaticcompression exerted by high-pressure fluids on the low-porosity wallrock decreases thewallrock permeability significantly in a short geological time. The deeper part of the depositis more likely to create low wallrock permeability and form infill textures than the shallowerpart, therfore favoring formation of thick veins at deeper parts. The hydrostatic compressionalso prohibits the fluid flow from the fracture zone to the wall rock and limits water-wallrockinteractions. Therefore, the hydrostatic compression facilitates both the fluid focusing and the high ore-formation efficiency in fracture zones. The hydrostatic compression mechanism maybe one of reasons why the Dajishan deposit has high-efficiency mineralization and thealteration at the deeper part of deposit is weaker than that at the shallower parts.The emplacment mechanisms of the Dajishan deposit are that the ore-bearing fracturesinitate from existing fractures and then propagate driven by by high-pressure magmatic fluids,and the orientation of these hydraulic fractures is also controlled by regional stresses.Generally, formation of the five-floor zonation at Dajishan was controlled by the complicatedcoupling of high-pressure fluids released at magmatic-hydrothermal transition, stress field,and the lithology of wallrock. Their interaction determines the unique vetical morphologicalzonation at the Dajishan tungsten deposit.