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1 year ago in EconomicGeology By Martinamame

What is the typical mineral alteration assemblage around the core mineralized zone within an Archaean lode gold deposit?

I'm a field geologist mapping a greenstone belt, and recognizing subtle alteration is critical for vectoring. The textbooks list many possible minerals. From a practical standpoint, what is the typical, diagnostic paragenesis I should see in the mudlog that indicates the fluid pathway and proximity to gold-bearing quartz veins?

 

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By Farheen Ahmed Answered 2 years ago

From my experience in the Abitibi and Dharwar cratons, the zoning is quite consistent. As you approach the ore shoot, you'll typically see a distinct inner halo dominated by fine-grained sericite (giving the rock a bleached, sugary look) and an increase in fine pyrite. This often sits within a broader envelope of chlorite and carbonate alteration. I have seen the presence of fuchsite (green chromium mica) or scheelite as a very positive local sign. Log the intensity of these minerals the bleaching and sulphidation spike right at the vein contact. This pattern is your best field guide.

 

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