43 C.F.R. § 3832.21
(a) Lode claims.
(2) Locating a lode claim. You may locate a lode claim for a mineral that:
(3) Establishing extralateral rights. If the minerals are contained within a vein, lode, or ledge and the vein, lode, or ledge extends through the endlines of your lode claim, you have extra-lateral rights to pursue the down-dip extension of the vein, lode, or ledge to the point where the vein, lode, or ledge intersects a vertical plain projected parallel to the end lines and outside the sideline boundaries of your lode claim if—
(4) Preserving extralateral rights. In order to preserve your extralateral rights, you should determine, if possible, the general course of the vein in either direction from the point of discovery in order to mark the correct boundaries of the claim. You should expose the vein, lode, or ledge by—
(b) Placer claims.
(3) You may locate a placer claim for minerals that are—