In Act 4, scene three, of Shakespeare's Julius Ceasar, what does Cassius mean when he refers to "Pluto's mine?" (line 111)

In Act Four, scene three, of Shakespeare's
Julius Caesar, Cassius and Brutus are arguing. Cassius calls aloud
that Antony and Octavius could come and kill him.

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Come, Antony, and young Octavius, come,

Revenge yourselves alone on Cassius,
For Cassius is aweary of the
world:
Hated by one he loves...
 (IV.iii.102-105)

Then
Cassius offers Brutus the same opportunity, to kill him if he wishes it by stabbing him
in the heart.

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There is my dagger,
And here my naked
breast; within, a heart (110)
Dearer than Pluto's mine, richer than gold.
(IV.iii.109-111)

Depending
upon what text you refer to, Pluto is sometimes "translated" to mean "Plutus." Pluto is
the Roman god of the underworld and "judge of the
dead."

Plutus
is the Greek god of abundance or wealth, a personification of
ploutos (Greek:
“riches”).

It is difficult to
say which one is being referred to as Cassius is offering his dagger and his naked
breast for Brutus to kill him if he so desires. If this were the case, Cassius would be
conveyed to the underworld at that point, and Pluto would be an accurate
reference.

However, Cassius has also mentioned the value of
his heart ("...a heart / Dearer than Pluto's mine, richer than gold"), so if I had to
choose one, I would select Plutus—Plutus' mine would be overflowing
with abundant wealth; Cassius is saying that his heart is more
valuable than such a cache of
riches.