The Idaho cobalt belt is a 60-km-long alignment of deposits composed of cobaltite, Co pyrite, chalcopyrite, and gold with anomalous Nb, Y, Be, and rare-earth elements (REEs) in a quartz-biotite-tourmaline gangue hosted in Mesoproterozoic metasedimentary rocks of the Lemhi Group. It is the largest cobalt resource in the United States with historic production from the Blackbird Mine. All of the deposits were deformed and metamorphosed to upper greenschist-lower amphibolite grade in the Cretaceous. They occur near a 1377 Ma anorogenic bimodal plutonic complex. The enhanced solubility of Fe, Co, Cu, and Au as chloride complexes together with gangue biotite rich in Fe and Cl and gangue quartz containing hypersaline inclusions allows that hot saline fluids were involved. The isotopes of B in gangue tourmaline are suggestive of a marine source, whereas those of Pb in ore suggest a U ± Th-enriched source.

The ore and gangue minerals in this belt may have trapped components in fluid inclusions that are distinct from those in post-ore minerals and metamorphic minerals. Such components can potentially be identified and distinguished by their relative abundances in contrasting samples. Therefore, we obtained samples of Co and Cu sulfides, gangue quartz, biotite, and tourmaline and post-ore quartz veins as well as Cretaceous metamorphic garnet and determined the gas, noble gas isotope, and ion ratios of fluid inclusion extracts by mass spectrometry and ion chromatography.

The most abundant gases present in extracts from each sample type are biased toward the gas-rich population of inclusions trapped during maximum burial and metamorphism. All have CO₂/CH₄ and N₂/Ar ratios of evolved crustal fluids, and many yield a range of H₂-CH₄-CO₂-H₂S equilibration temperatures consistent with the metamorphic grade. Cretaceous garnet and post-ore minerals have high R_H and R_S values suggestive of reduced sulfidic conditions. Most extracts have anomalous ⁴He produced by decay of U and Th and ³⁸Ar produced by nucleogenic production from ⁴¹K. In contrast, some ore and gangue minerals yield significant SO₂ and have low R_H and R_S values of a more oxidized fluid. Three extracts from gangue quartz have high helium R/R_A values indicative of a mantle source and neon isotope compositions that require nucleogenic production of ²²Ne in fluorite from U ± Th decay. Two extracts from gangue quartz have estimated ⁴⁰K/⁴⁰Ar that permit a Precambrian age.

Extracts from gangue quartz in three different ore zones are biased toward the hypersaline population of inclusions and have a tight range of ion ratios (Na, K, NH₄, Cl, Br, F) suggestive of a single fluid. Their Na, Cl, Br ratios suggest this fluid was a mixture of magmatic and basinal brine. Na-K-Ca temperatures (279°–347°C) are similar to homogenization temperatures of hypersaline inclusions. The high K/Na of the brine may be due to albitization of K silicate minerals in country rocks. Influx of K-rich brines is consistent with the K metasomatism necessary to form gangue biotite with high Cl. An extract from a post-ore quartz vein is distinct and has Na, Cl, Br ratios that resemble metamorphic fluids in Cretaceous silver veins of the Coeur d’Alene district in the Belt Basin.

The results show that in some samples, for certain components, it is possible to “see through” the Cretaceous metamorphic overprint. Of great import for genetic models, the volatiles trapped in gangue quartz have ³He derived from a mantle source and ²²Ne derived from fluorite, both of which may be attributed to nearby ~1377 Ma basalt-rhyolite magmatism. The brine trapped in gangue quartz is a mixture of magmatic fluid and evaporated seawater. The former requires a granitic intrusion that is present in the bimodal intrusive complex, and the latter equatorial paleolatitudes that existed in the Mesoproterozoic. The results permit genetic models involving heat and fluids from the neighboring bimodal plutonic complex and convection of basinal brine in the Lemhi Group. While the inferred fluid sources in the Idaho cobalt belt are similar in many respects to those in iron oxide copper-gold deposits, the fluids were more reduced such that iron was fixed in biotite and tourmaline instead of iron oxides.