Strata-Bound Fe-Co-Cu-Au-Bi-Y-REE Deposits of the Idaho Cobalt Belt: Multistage Hydrothermal Mineralization in a Magmatic-Related Iron Oxide Copper-Gold System

Abstract

Mineralogical and geochemical studies of strata-bound Fe-Co-Cu-Au-Bi-Y-rare-earth element (REE) deposits of the Idaho cobalt belt in east-central Idaho provide evidence of multistage epigenetic mineralization by magmatic-hydrothermal processes in an iron oxide copper-gold (IOCG) system. Deposits of the Idaho cobalt belt comprise three types: (1) strata-bound sulfide lenses in the Blackbird district, which are cobaltite and, less commonly, chalcopyrite rich with locally abundant gold, native bismuth, bismuthinite, xenotime, allanite, monazite, and the Be-rich silicate gadolinite-(Y), with sparse uraninite, stannite, and Bi tellurides, in a gangue of quartz, chlorite, biotite, muscovite, garnet, tourmaline, chloritoid, and/or siderite, with locally abundant fluorapatite or magnetite; (2) discordant tourmalinized breccias in the Blackbird district that in places have concentrations of cobaltite, chalcopyrite, gold, and xenotime; and (3) strata-bound magnetite-rich lenses in the Iron Creek area, which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime. Most sulfide-rich deposits in the Blackbird district are enclosed by strata-bound lenses composed mainly of Cl-rich Fe biotite; some deposits have quartz-rich envelopes.Whole-rock analyses of 48 Co-and/or Cu-rich samples show high concentrations of Au (up to 26.8 ppm), Bi (up to 9.16 wt %), Y (up to 0.83 wt %), ΣREEs (up to 2.56 wt %), Ni (up to 6,780 ppm), and Be (up to 1,180 ppm), with locally elevated U (up to 124 ppm) and Sn (up to 133 ppm); Zn and Pb contents are uniformly low (≤821 and ≤61 ppm, respectively). Varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite, allanite, and xenotime; biotite and gold; detrital minerals; chalcopyrite and sparse stannite; quartz; and cobaltite with sparse selenides and tellurides. Significantly, Cu is statistically separate from Co and As, consistent with the general lack of abundant chalcopyrite in cobaltite-rich samples.Paragenetic relations determined by scanning electron microscopy indicate that the earliest Y-REE-Be mineralization preceded deposition of Co, Cu, Au, and Bi. Allanite, xenotime, and gadolinite-(Y) commonly occur as intergrowths with and inclusions in cobaltite; the opposite texture is rare. Monazite, in contrast, forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime, reflecting a later metamorphic paragenesis. Allanite and xenotime also show evidence of late dissolution and reprecipitation, forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral. Textural data suggest extensive deformation of the deposits by folding and shearing, and by pervasive recrystallization, all during Cretaceous metamorphism. Sensitive high resolution ion microprobe U-Pb geochronology by Aleinikoff et al. (2012) supports these paragenetic interpretations, documenting contemporaneous Mesoproterozoic growth of early xenotime and ...

Sulphide Re-Os geochronology links orogenesis, salt and Cu-Co ores in the Central African Copperbelt

“…570–520 Ma A-type magmatism that produced the Hook Batholith 45 , including its northern, magnetically interpreted subsurface extension (presently located several tens of kilometres to the south of the DRC-Zambia border) 4 , 46 . Magmatic-hydrothermal systems in connection with granite plutons have been identified as being at the root of multi-stage REE-Y-Co-Cu-Au sediment-hosted mineralisation within large Mesoproterozoic sedimentary basins characterised by large-scale fluxes of evaporitic brines, e.g., the Idaho Cobalt Belt within the Belt-Purcell Basin, USA 47 , 48 . Therefore, a contribution by the Hook Batholith and its northern subsurface extension as heat engine and source of Os (and possibly other metals such as Cu) for the ca.…”

Fluid sources and metallogenesis in the Blackbird Co-Cu-Au-Bi-Y-REE district, Idaho, U.S.A.: insights from major-element and boron isotopic compositions of tourmaline

“…Other ore minerals include pyrite, pyrrhotite, arsenopyrite, glaucodot, safflorite, bismuthinite, native bismuth, Bi-tellurides, and gold. Recent geochemical and mineralogical studies have documented previously unknown high Bi contents (up to 9.2 wt %) and occurrences of sparse uraninite and stannite (Slack 2011). Some of the sulfide deposits, such as Sunshine, Merle, Brown Bear, and Ram ( Fig.…”

“…Some of the sulfide deposits, such as Sunshine, Merle, Brown Bear, and Ram ( Fig. 1), locally contain minor to abundant allanite, apatite, xenotime, or monazite, which in whole-rock analyses produce total REE+Y oxide contents up to 3.66 wt % (Slack 2006(Slack , 2011. SHRIMP U-Pb dating of paragenetically early xenotime from the Merle deposit yields an age of 1370 ± 4 Ma, which is within analytical uncertainty of the 1377 ± 4 Ma age of the megacrystic granite (Aleinkoff et al 2011).…”

“…SHRIMP U-Pb dating of paragenetically early xenotime from the Merle deposit yields an age of 1370 ± 4 Ma, which is within analytical uncertainty of the 1377 ± 4 Ma age of the megacrystic granite (Aleinkoff et al 2011). The Northfield deposit has local concentrations of gadolinite-(Y) (Slack 2011), a Be-rich silicate containing essential Fe, light rare-earth elements (LREE), and Y [(LREE,Y) 2 Fe 2+ Be 2 O(SiO 4 ) 2 ]. Most of the stratabound sulfide deposits are interlayered with distinctive rocks composed mainly of Fe-and Cl-rich biotite, termed biotitite (Nash & Hahn 1989, Nash & Connor 1993, which likely formed by hydrothermal metasomatism of argillaceous sediments.…”

“…Stratabound sulfide deposits of this district, occurring within the central part of the ~60-km-long Idaho cobalt belt (ICB), have been interpreted for over two decades as products of syngenetic to diagenetic mineralization (e.g., Modreski 1985, Eiseman 1988, Nash & Hahn 1989, Nold 1990, Evans et al 1995, Bending & Scales 2001. Recent work has identified several diagnostic mineralogical and geochemical features in the deposits, including locally high contents of Bi, Y, rare earth elements (REE), and Be, which together suggest mineralization in a magmatic-related, iron oxide-copper-gold (IOCG) system (Slack 2011). Our study seeks additional constraints on the origin of these enigmatic deposits by using tourmaline as a geochemical tracer of boron sources and its evolution in the district.…”