The Tan (Nio, Third Creek) carbonatite occurrence is located on a west- flowing tributary of the Anstey River, referred to as ‘Third’ Creek, approximately 12 kilometres east of Hunakwa Lake.

The area lies within the Monashee Complex along the western margin of the Frenchman Cap Dome. The core of the dome is composed of a mixed, probably Aphebian, paragneiss and orthogneiss succession, and is mantled by an unconformably overlying succession of metasedimentary rocks, locally intruded by a suite of alkalic gneiss. The metasediments consists of a basal quartzite unit overlain by a succession of interbanded pelitic schists, pelitic gneiss, calc-silicate and marble.

Locally, a carbonatite sill(?) is hosted within the calc-silicate unit near a basic dike. The carbonatite is composed primarily of calcite with lesser disseminated apatite, tremolite, phlogopite, columbite, pyrrhotite and minor pyrite, garnet and scheelite. The carbonatite strikes 174 degrees and dips 34 degrees.

In 2001, a 1 metre chip sample (T-0+10S) assayed 0.105 per cent niobium and 0.002 per cent tantalum, while a 10 metre long by 0.35 metre wide chip sample (T-00A) yielded 0.37 per cent di-niobium pent-oxide with grab samples yielded up to 0.53 per cent di-niobium pent-oxide (Assessment Report 26791). In 2010, a rock sampling yielded up to 0.366 per cent niobium and 0.226 per cent tantalum (Assessment Report 31920).

During 1999 through 2001, the area was prospected and sampled as the Tan claims. In 2010, the area was prospected as the Nio claims.