Dehui Zhang

Influences of hydraulic fracturing on fluid flow and mineralization at the vein-type tungsten deposits in Southern China

Wolframite is the main ore mineral at the vein-type tungsten deposits in the Nanling Range, which is a world-class tungsten province. It is disputed how wolframite is precipitated at these deposits and no one has yet studied the links of the mechanical processes to fluid flow and mineralization. Finite element-based numerical experiments are used to investigate the influences of a hydraulic fracturing process on fluid flow and solubility of CO2 and quartz. The fluids are aqueous NaCl solutions and fluid pressure is the only variable controlling solubility of CO2 and quartz in the numerical experiments. Significant fluctuations of fluid pressure and high-velocity hydrothermal pulse are found once rock is fractured by high-pressure fluids. The fluid pressure drop induced by hydraulic fracturing could cause a 9% decrease of quartz solubility. This amount of quartz deposition may not cause a significant decrease in rock permeability. The fluid pressure decrease after hydraulic fracturing also reduces solubility of CO2 by 36% and increases pH. Because an increase in pH would cause a major decrease in solubility of tungsten, the fluid pressure drop accompanying a hydraulic fracturing process facilitates wolframite precipitation. Our numerical experiments provide insight into the mechanisms precipitating wolframite at the tungsten deposits in the Nanling Range as well as other metals whose solubility is strongly dependent on pH.

Metallogenic efficiencies of ore-forming elements in the Shizhuyuan ore-field, Hunan Province, SE China: implications for ore-generating potential and mineral prospecting

Metallogenic efficiency (ME), is the ratio of reserves of a metal to its total amount in both the deposits and alteration zones. We present a new method for calculating ME on a regional scale, based on stream sediment geochemical anomalies. Given a study area, the MEs of ore-forming elements should be weighted by their concentration factors, in order to quantitatively assess the potential of these elements for generating economic deposits. Taking Shizhuyuan ore-field as a case study, ore-forming elements with greater MEs exhibit greater ore-generating potentials. Accordingly, regions with geochemical anomalies of several elements with high-ranking WMEs (weighted metallogenic efficiencies) are favorable sites for exploration of polymetallic deposits. Moreover, in terms of elements such as W or Sn, the WME in the Shizhuyuan ore-field was greater than in the Wangxianling ore-field, consistent with the greater tonnage of mineral reserves in the former. Given a certain ore-forming element, therefore, its ore-generating potentials in different regions are quantitatively comparable to its WMEs. In summary, WME is an important parameter for quantitative evaluation of resource potentiality and resource prospection.

Hydraulic Fracturing Leads to Wolframite Deposition at Magmatic-hydrothermal transition

The Nanling Range in southern China is a world-class tungsten province. It is disputed how wolframite as the main ore mineral in the tungsten deposits in this area is precipitated. Wolframite solubility is only weakly dependent on pressure (Wood and Samson, 2000). Therefore, a pressure change cannot alter wolframite solubility. However, fluid inclusions record high-pressure fluids that trigger the initiation and propagation of veinbearing fractures in these deposits. It is enigmatic whether a decrease in fluid pressure after hydraulic fracturing causes wolframite precipitation from hydrothermal solutions. We first demonstrate that fluid pressure drop after hydraulic fracturing could cause a significant decrease in CO2 solubility, increase pH, and facilitate wolframite precipitation at magmatic-hydrothermal transition using finite element-based numerical experiments.