The mechanisms and time scale of alteration halos in vein-type tungsten deposits in southern China

Abstract

Alteration halos are the fingerprints left by hydrothermal flow in rocks and indicative of ore forming processes. The Nanling Range in southern China is a world-class tungsten province. However, it is poorly understood why alteration halos normal to veins in many tungsten deposits in the area decrease with increasing depth, and the time it takes to form the halos. Finite element based numerical experiments were conducted to investigate hydrothermal flow and species diffusion processes from fractures to adjacent wallrock in the tungsten deposits. The vertical distribution of alteration halos is influenced by wallrock porosity, fracture permeability, and the duration of hydrothermal flow in fractures. When hydrothermal fluids flow upwards, wider alteration halos are formed at deeper levels, when the wallrock has a constant porosity for a depth of around one kilometre. This is inconsistent with the characteristics of alteration halos in the tungsten deposits in southern China. Alteration halos in the tungsten deposits could be formed when the wallrock porosity decreases with increasing depth. The silicification and greisenization of the tungsten deposits could form over a period from tens of years to a few thousand years. Such a period is much shorter than the uncertainties of existing isochronal ages and may represent the duration of mineralizing pulses rather than all the geological events associated with tungsten mineralization. Tungsten diffusion from fractures to adjacent wallrock cannot produce large-scale disseminated tungsten mineralization during such a short period of time.