Why does the narrator compare Chillingworth to a miner? Nathaniel Hawthorne's The Scarlet Letter

In his
interview withwithin the prison,declares that he will discover the identity of who is the father
of Hester's child, and this man will be his and he will own the man's soul.  Then, in Chapter X
of , Hawthorne writes that Chillingworth has become the physician of the
Reverendinitially to merely to attain the truth; however, as he has proceeded with his
investigation, "a terrible fascination" has seized upon the old man, and he has become
obsessed.

[He] dug into the poor clergyman's
heart, like a miner searching for gold; or, rather, like a sexton
delving into a grave, possibly in quest of a jewel that had been buried on the dead man's bosom,
but likely to find nothing save mortality and corruption. Alas for his own soul, if these were
what he sought!

And, like the miner who seeks the mother
lode, Chillingsworth continually probes, questioning Dimmesdale on all sorts of topics,
constantly watching the minister, delving deeper and deeper into the soul of the minister.  As
he questions Dimmesdale, Chillingworth tells himself, "Let us dig a little further in the
direction of this vein!" as he seeks the secrets of Dimmesdale's heart.  Finally, the
physician strikes the vein that contains gold.  For, while the minister sleeps, the
"miner" pulls aside the vestment of Dimmesdale views that which makes him feels
ecstasy: he has discovered the mother-lode.