Why would a species of subterreanean worms consume metal?

In my latest draft, a worm-like species buries itself deep below the crust of their world and consumes/gathers tiny amounts of heavy metals.

Can you think of any plausible reason why a wormlike creature would do so? Why would they spend time and energy digging deep and gathering small pieces of metal?

Some Details:

• Location: Earth-analog
• The species itself is not really fleshed out right now.
• They do return to the surface. Its either part of their lifecycle or daily/weekly/monthly routine.
• the intake of metal takes places many hundreds of metres below the surface, at least 1000 metres below. I should make the heat-resistant, or should I?
• the metals take a significant amount of their worms bodyweight.

Bonus Question: Is that deep below the surface a suitable place for heavy metals or do I need to mess with the geology of my earth analog for this?

Edit: As Suggested by John Dallman the plot reason for this:

The worms are harvested by humans, who need the heavy metals. It kind of harvesting spider silk in terms of quantity, maybe a order of magnitude higher in output. This work should be dirty, dangerous and really unpleasent for the poor peasants.

posted over 8 years ago

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