An experimental study of the mechanism of the replacement of magnetite by pyrite up to 300 degrees C

作者：Qian, Gujie; Brugger, Joel; Skinner, William M.; Chen, Guorong; Pring, Allan^*

来源：Geochimica et Cosmochimica Acta, 2010, 74(19): 5610-5630.

DOI：10.1016/j.gca.2010.06.035

• HOSTED GOLD MINERALIZATION
• REACTIONS FORMING PYRITE
• BANDED-IRON-FORMATION
• AQUEOUS-SOLUTIONS
• REDUCTIVE DISSOLUTION
• EPIGENETIC NATURE
• FLUID INCLUSION
• ORE PETROGRAPHY
• SOUTHERN INDIA
• H2S OXIDATION

We present the results of an experimental study into the sulfidation of magnetite to form pyrite/marcasite under hydrothermal conditions (90-300 degrees C, vapor saturated pressures), a process associated with gold deposition in a number of ore deposits. The formation of pyrite/marcasite was studied as a function of reaction time, temperature, pH, sulfide concentration, solid-weight-to-fluid-volume ratio, and geometric surface area of magnetite in polytetrafluoroethylene-lined autoclaves (PTFE) and a titanium and stainless steel flow-through cell. Marcasite was formed only at pH(21 degrees c) <4 and was the dominant Fe disulfide at pH(21 degrees c) 1.11, while pyrite predominated at pH(21 degrees c) >2 and formed even under basic conditions (up to pH(21 degrees c) 12-13). Marcasite formation was favored at higher temperatures. Fine-grained pyrrhotite formed in the initial stage of the reaction together with pyrite in some experiments with large surface area of magnetite (grain size <125 mu m). This pyrrhotite eventually gave way to pyrite. The transformation rate of magnetite to Fe disulfide increased with decreasing pH (at 120 degrees C; pH(120 degrees c) 0.96-4.42), and that rate of the transformation increased from 120 to 190 C.
Scanning electron microscope (SEM) imaging revealed that micro-pores (0.1-5 mu m scale) existed at the reaction front between the parent magnetite and the product pyrite, and that the pyrite and/or marcasite were euhedral at pH(21 degrees c) <4 and anhedral at higher pH. The newly formed pyrite was micro-porous (0.1-5 mu m); this micro-porosity facilitates fluid transport to the reaction interface between magnetite and pyrite, thus promoting the replacement reaction. The pyrite precipitated onto the parent magnetite was polycrystalline and did not preserve the crystallographic orientation of the magnetite. The pyrite precipitation was also observed on the PTFE liner, which is consistent with pyrite crystallizing from solution. The mechanism of the reaction is that of a dissolution-reprecipitation reaction with the precipitation of pyrite being the rate-limiting step relative to magnetite dissolution under mildly acidic conditions (e.g., pH(155 degrees c) 4.42).
The experimental results are in good agreement with sulfide phase assemblage and textures reported from sulfidized Banded Iron Formations: pyrite, marcasite and pyrrhotite have been found to exist or co-exist in different sulfidized Banded Iron Formations, and the microtextures show no evidence of sub-gm-scale pseudomorphism of magnetite by pyrite.

• 出版日期2010-10-1
• 单位华东理工大学