Influences of fluid properties on the hydrothermal fluid flow and alteration halos at the Dajishan tungsten deposit, China

Abstract

The Nanling Range located in South China is a world-class tungsten province. Those tungsten deposits are spatially and temporally associated with the granitic magmatism during the Late Mesozoic (J2-K2). It is common that alteration halos decrease with increasing depth at the vein-type tungsten deposits. However, the mechanisms forming the alteration characteristics are still enigmatic. In this contribution, we investigate the influences of variable fluid density and viscosity on the fluid flow and species diffusion from fractures to adjacent wallrock at the Dajishan tungsten deposit using finite element based numerical experiments. Fluid density has a negative influence on the transient pressure-driven fluid flow at Dajishan. Large-density fluids slow fluid flow and heat transfer. High-viscosity fluids flow slowly. Variations of fluid viscosity at Dajishan are large and influence hydrothermal fluid flow and species diffusion from fractures to surrounding wallrock. Depth-dependent porosity and permeability of wallrock are effective mechanisms forming the alteration halos at Dajishan. Hydraulic and geometric characteristics of joint zones influence diffusion of chemical species at the initial stages of hydrothermal fluids injected into fractures. High-permeability narrow joint zones favor formation of thick alteration halos at shallower levels. Depth-dependent wallrock porosity also impedes diffusion of metals from fractures to adjacent wallrock increasing ore formation efficiency at Dajishan.