Prospectivity

Prospectivity is a working term for how likely an area is to contain a mineral deposit of a particular type. It is a judgment, not a measurement: it ranks ground by how well its geology fits the settings where known deposits of that type have formed.

How USGS approaches the question

USGS national mineral resource assessments work from mineral deposit models. A 1997 USGS summary of the national assessment describes the models as "sets of data that describe a group of deposits that have similar geologic characteristics, form in similar geologic settings, and have distinctive sizes and distinctive amounts of metal." Assessors outline regions whose geology broadly matches a model, then "outline much smaller 'tracts' of land having geologic features that more closely match those of the models." Those tracts "are considered to be permissive for the occurrence of a specific type of mineral deposit." A USGS open-file report on assessment methods puts it the same way: "The permissive tracts are defined by the geologic environments of formation described in the deposit model."

The same summary explains that assessors then estimate the number of undiscovered deposits "at several levels of certainty," for example "a 90-percent chance that the area has at least 1 or more undiscovered deposits of a specific type."

Undiscovered resources

The USGS and Bureau of Mines resource classification (USGS Circular 831) gives two categories for deposits that are thought to exist but have not been found:

• Hypothetical resources: "similar to known mineral bodies" and "reasonably expected to exist in the same producing district or region under analogous geologic conditions"
• Speculative resources: those that "may occur either in known types of deposits in favorable geologic settings where mineral discoveries have not been made, or in types of deposits as yet unrecognized for their economic potential"

Evidence used

• Geologic maps and the deposit model for the target commodity
• Known mines, prospects, and occurrences, such as those recorded in USGS MRDS
• Sampling, assays, and drilling from earlier work
• The pattern of past and present mining claims, which shows where others have staked, not where deposits are proven

Prospective is not the same as available

Circular 831 recognizes "restricted resources," those "restricted from extraction by laws or regulations." On federal land, ground can be highly prospective and still closed to mineral entry by a withdrawal. Prospective ground may also already be covered by mining claims.

Why it matters in mining claim research

A prospectivity judgment says where to look. Whether the ground can be claimed is a separate question, answered from land status records, master title plats, and BLM mining claim records. See also Mineral Deposit, MRDS, Claim Density, and Open to Entry.