Question

Which one of the following mineral associations is diagnostic of the highly-oxidized zone of a weathered primary base metal vein?

• A. Smithsonite, Malachite, Anglesite
• B. Calcite, Fluorite, Pyrite
• C. Limonite, Realgar, Chalcocite
• D. Goethite, Orpiment, Boehmite

Hint:

In the oxidized zone of base metal veins, minerals like Smithsonite (zinc), Malachite (copper), and Anglesite (lead) are commonly found as secondary products of weathering and oxidation.

Answer 1

The Correct Option is A

Step 1: What the oxidized zone holds.
The top, highly oxidized part of a weathered metal vein fills with new secondary minerals. These are mostly carbonates, sulfates and oxides formed in air and water.

Step 2: Read the right group.
Smithsonite is a zinc carbonate, malachite is a copper carbonate, and anglesite is a lead sulfate. All three are classic oxidation products of base metal ores.

Step 3: Check the chemistry fit.
This trio covers zinc, copper and lead in their oxidized forms, exactly what we expect near the surface of such a vein.

Step 4: Reject the others.
The other sets mix in pyrite, realgar, chalcocite or orpiment, which belong to fresh or less oxidized ore, not the highly oxidized cap.

Step 5: Final choice.
So the diagnostic group is smithsonite, malachite and anglesite.
\[ \boxed{\text{Smithsonite, Malachite, Anglesite}} \]