ALBERT

Description: Magnetite (Fe 3 O 4 ) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe analysis (EMPA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis, and oxygen isotope analysis to test whether magnetite from the five following geologic settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist facies burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform Cu-Ag deposits (Spar Lake and Rock Creek) in Belt Supergroup metasedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d’Alene district, (4) extensively deformed and partially altered Belt Supergroup host rocks from the Coeur d’Alene district, and (5) two Cretaceous postmetamorphic igneous intrusions. EMPA results show that magnetite from each of these five settings is essentially pure Fe 3 O 4 , but LA-ICP-MS analyses results show that magnetite from these five settings has trace element concentrations that generally vary over less than one order of magnitude. These magnetite occurrences show subtle compositional differences that generally correlate with temperatures, as determined by oxygen isotope geothermometry. Burial metamorphic magnetite from the Coeur d’Alene host rocks has the smallest overall trace element contents. Chromium, Co, and Zn are depleted in both hydrothermal and host-rock magnetite from the Coeur d’Alene district. In contrast, magnetite from postmetamorphic igneous rocks in the Belt terrane has relatively large Mg, V, Co, and Mn values, consistent with its formation at relatively high temperatures and subsequent subsolidus reequilibration. Factor analysis was used to trace any underlying or latent relationships among elements that are likely to be incorporated into the magnetite structure. Factor analysis provides geochemical discrimination of at least three types of magnetite in the Belt terrane: (1) Mg-Mn, (2) Ga-Zn-Cr, and (3) Co-Ni-V magnetite. Hydrothermal magnetite from the Gold Hunter siderite vein shows characteristically high values for factor 1. Factor 2 is most pronounced in magnetite from the burial metamorphic host rocks and the sediment-hosted Cu-Ag deposits. Furthermore, factor 2 indicates that Ga, Zn, and Cr concentrations are lower on average in hydrothermal and host-rock magnetite from the Coeur d’Alene district. Factor 3 divides igneous magnetite from other magnetite occurrences. This factor also subdivides magnetite of an alkalic-ultramafic intrusive complex from that of the granitic stock. Hydrothermal magnetite from siderite and calcite veins in the Coeur d’Alene district has consistently low scores for factor 3. The geochemistry of magnetite can be a useful discriminator and pathfinder for hydrothermal deposits. The relatively low formation temperature and the metamorphic history of the Belt terrane led to low trace element concentrations and subtle differences between magnetite from different geologic settings. Nevertheless, by combining LA-ICP-MS analysis and factor analysis, compositional variations between groups of magnetite samples from different geologic settings can be recognized.

Description: Volcanogenic massive sulfide (VMS) Zn-Pb-Cu-Ag-Au deposits of the Bonnifield mining district formed during Late Devonian-Early Mississippian magmatism along the western edge of Laurentia. The largest deposits, Dry Creek and WTF, have a combined resource of 5.7 million tonnes at 10% Zn, 4% Pb, 0.3% Cu, 300 grams per tonne (g/t) Ag, and 1.6 g/t Au. These polymetallic deposits are hosted in high field strength element (HFSE)- and rare-earth element (REE)-rich peralkaline (pantelleritic) metarhyolite, and interlayered pyritic argillite and mudstone of the Mystic Creek Member of the Totatlanika Schist Formation. Mystic Creek metarhyolite and alkali basalt (Chute Creek Member) constitute a bimodal pair that formed in an extensional environment. A synvolcanic peralkaline quartz porphyry containing veins of fluorite, sphalerite, pyrite, and quartz intrudes the central footwall at Dry Creek. The Anderson Mountain deposit, located ~32 km to the southwest, occurs within calc-alkaline felsic to intermediate-composition metavolcanic rocks and associated graphitic argillite of the Wood River assemblage. Felsic metavolcanic rocks there have only slightly elevated HFSEs and REEs. The association of abundant graphitic and siliceous argillite with the felsic volcanic rocks together with low Cu contents in the Bonnifield deposits suggests classification as a siliciclastic-felsic type of VMS deposit. Bonnifield massive sulfides and host rocks were metamorphosed and deformed under greenschist-facies conditions in the Mesozoic. Primary depositional textures, generally uncommon, consist of framboids, framboidal aggregates, and spongy masses of pyrite. Sphalerite, the predominant base metal sulfide, encloses early pyrite framboids. Galena and chalcopyrite accompanied early pyrite formation but primarily formed late in the paragenetic sequence. Silver-rich tetrahedrite is a minor late phase at the Dry Creek deposit. Gold and Ag are present in low to moderate amounts in pyrite from all of the deposits; electrum inclusions occur in Dry Creek sphalerite. Contents and ratios of trace elements in graphitic argillite that serve as proxies for the redox state of the bottom waters in the basin indicate that Dry Creek mineralization took place in suboxic to periodically anoxic bottom waters. Trace element data show higher contents of Tl-Mn-As in pyrite from the Anderson Mountain deposit compared to the Dry Creek or WTF deposits and thus suggest that Anderson Mountain may have formed at lower temperatures or under slightly more oxidizing conditions. No exact modern analogue for the tectonic setting of the Bonnifield VMS deposits is known, although the back-arc regions of the Okinawa Trough and Woodlark Basin satisfy the requirement for a submarine, extensional setting adjacent to a continental margin. Limited occurrences of peralkaline volcanic rocks occur in these two potential analogues, but the peralkalinity of those rocks is much less than that of the Mystic Creek Member metarhyolites in the Bonnifield district. The highly elevated trace element (e.g., Zr, Nb) contents of Mystic Creek metarhyolites suggest that a better analogue may be a submarine rifted continental margin. The calc-alkaline composition of the host rocks to the Anderson Mountain deposit suggests that mineralization there formed in a continental margin arc, outboard of the extended continental margin setting of the peralkaline-hosted Dry Creek and WTF deposits.