The poem turns the act of listening into a kind of drilling. A trepan is a tool for boring a hole in the skull, and the speaker imagines the ear as exactly that: something that cuts its way into the head. Once the skull is open, the mind becomes a mine, and the speaker becomes a prospector digging out whatever valuable material lies inside.
The "gold and silver" and "noon and night" pair up bright things with the cycle of the day, so the digging seems to go on around the clock, through light and dark alike. The last line is the uneasy one. Each bit of treasure taken out costs something: "losing a little more of my head." The speaker seems to feel both the pull of the work and the toll it takes.
The mood is strange, quiet, and a little unsettling. The poem is short and calm in tone, yet the image of an opened skull gives it a slow, creeping sense of loss, as if creating or discovering something means giving part of yourself away.