Lode gold deposit characterization using evidence from stream sediments: an example from Brush Creek, Montgomery County, Virginia

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Virginia Polytechnic Institute and State University


Placer ore minerals are commonly intergrown with "relict" phases that coexisted with the ore mineral in the original lode deposit. Studying these relict phases can yield important information about the nature, and formation of the lode deposit. This type of study can be useful in areas with poor exposure, areas that are remote, or areas where discretion is important.

Analysis of the heavy mineral suite of stream sediments from the Brush Creek area shows no correlation between the heavy minerals and the gold. However, analysis of the relict phases intergrown with the gold grains yields important results.

Placer gold grains recovered from streams draining the Brush Creek deposit, in southwestern Virginia, contain relict quartz, orthoclase, ilmenite and mica. Textures, and fluid inclusion composition and character in the relict quartz, indicate that the gold mineralization post dated the mylonitization associated with the Fries ductile deformation zone, which hosts the gold mineralization. The relict orthoclase is interpreted to be adularia, which is common in low-temperature, hydrothermal environments. The intergrowth textures of the gold and ilmenite show that the ilmenite was present in the country rocks prior to gold mineralization, and was not, therefore, cogenetic with the gold. The relict mica was not positively identified, but is believed to be chlorite, which is consistent with the proposed low temperature mineralization. The textures of the relict phases indicate that gold mineralization occurs in late, brittle fractures, with little or no significant alteration.

The study of the relict phases intergrown with the alluvial gold grains has yielded information that otherwise could only have been obtained by more advanced, but also much more expensive, exploration techniques.