Mineral Deposit

A mineral deposit is a natural concentration of a useful mineral or metal. The U.S. Bureau of Mines' Dictionary of Mining, Mineral, and Related Terms (1968) gives two senses: "any valuable mass of ore," and "a body of mineral matter in or on the earth's surface which may be utilized for its industrial mineral or metal content." Its entry for "deposit" adds that the term designates "a natural occurrence of a useful mineral, or an ore, in sufficient extent and degree of concentration to invite exploitation."

In the Mining Law

The Mining Law is written around deposits: "all valuable mineral deposits in lands belonging to the United States, both surveyed and unsurveyed, shall be free and open to exploration and purchase" (30 U.S.C. 22). A mining claim depends on finding one; see Discovery.

Lode and placer deposits

The kind of deposit decides the kind of mining claim. Under BLM's regulations (43 CFR 3832.21):

• A lode claim is for a mineral that "occurs as veins, lodes, ledges, or other rock in place"
• A placer claim is for minerals in "river sands or gravels," or "hosted in soils, alluvium ... colluvium ... talus, or other rock not in its original place," and for "bedded gypsum, limestone, cinders, pumice, and similar mineral deposits"

BLM's mining claims page notes that lode deposits "may be broad zones of mineralized rock," including "large volume, but low-grade disseminated gold deposits."

Occurrence, prospect, deposit

USGS's Mineral Resources Data System (MRDS) grades sites by development status. Its definitions draw a useful line:

• Occurrence: "Ore mineralization in outcrop, shallow pit or pits, or isolated drill hole. Grade, tonnage, and extent of mineralization essentially unknown."
• Prospect: "A deposit that has gone beyond the occurrence stage," where enough work has been done "to at least estimate grade and tonnage"
• Producer and Past Producer: mines operating, or formerly operating, at the time of data entry

MRDS also records each site's deposit type, the form and shape of the ore body, and the deposit models that characterize it.

Deposit, resource, reserve

Whether a deposit is economic is a separate question. USGS Circular 831 defines a resource as "a concentration of naturally occurring solid, liquid, or gaseous material in or on the Earth's crust in such form and amount that economic extraction of a commodity from the concentration is currently or potentially feasible." The SEC's mining rules call a concentration with "reasonable prospects for economic extraction" a mineral resource (17 CFR 229.1300). The profitable part is ore, or in SEC terms a mineral reserve. See Ore, Grade, Mineral Resource, and Mineral Reserve.

Why it matters in mining claim research

The type and shape of a deposit decide how mining claims over it should be laid out: lode claims along a vein, placer claims over gravels or bedded deposits. Comparing known deposits from MRDS with the mining claims that cover them shows whether the mining claim type and layout fit the deposit.