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

Abstract

Tourmaline is a widespread mineral in the Mesoproterozoic Blackbird Co-Cu-Au-Bi-Y-REE district, Idaho, where it occurs in both mineralized zones and wallrocks. We report here major-element and B-isotope compositions of tourmaline from stratabound sulfide deposits and their metasedimentary wallrocks, from mineralized and barren tourmaline breccia pipes, from late barren quartz veins, and from Mesoproterozoic granite. Tourmalines are aluminous, intermediate schorldravite with Fe/(Fe+Mg) ratios of 0.30 to 0.85 and 10 % to 50 % X-site vacancies. Compositional zoning is prominent only in tourmaline from breccias and quartz-veins; crystal rims are enriched in Mg, Ca and Ti, and depleted in Fe and Al relative to cores. The chemical composition of tourmaline does not correlate with the presence or absence of mineralization. The δ 11 B values fall into two groups. Isotopically light tourmaline (-21.7 to-7.6 ‰) occurs in unmineralized samples from wallrocks, late quartz veins and Mesoproterozoic granite, whereas heavy tourmaline (-6.9 to +3.2 ‰) is spatially associated with mineralization (stratabound and brecciahosted), and is also found in barren breccia. At an inferred temperature of 300°C, boron in the hydrothermal fluid associated with mineralization had δ 11 B values of-3 to +7 ‰. The high end of this range indicates a marine source of the boron. A likely scenario involves leaching of boron principally from marine carbonate beds or B-bearing evaporites in Mesoproterozoic strata of the region.The δ 11 B values of the isotopically light tourmaline in the sulfide deposits are attributed to recrystallization during Cretaceous metamorphism, superimposed on a light boron component derived from footwall siliciclastic sediments (e.g., marine clays) during Mesoproterozoic mineralization, and possibly a minor component of light boron from a magmatic-hydrothermal fluid. The metal association of Bi-Be-Y-REE in the Blackbird ores suggests some magmatic input, but involvement of granite-derived fluids cannot be conclusively established from the present database.

Geologic history of the Blackbird Co-Cu district in the Lemhi subbasin of the Belt-Purcell Basin

“…Some of these quartz-tourmaline breccias are enveloped by biotitized host rocks. According to Trumbull et al (2011), tourmaline of the Blackbird district is aluminous schorl-dravite with B-isotopic ratios that indicate an isotopically heavy, saline marine-fluid source for the contained boron.…”

Tourmaline occurrence and gold mineralization at a granitoid-metasediment contact in the Upper Lom Basin, east Cameroon

“…Low Fe content can also be attributed to the precipitation of hematite [70]. The absence of Al-Fe 3+ elemental substitution in tourmaline may suggest tourmaline crystallization from a reducing hydrothermal fluid [11]. Our data show high Mg/(Fe + Mg) ratio 0.47-0.7, low Ca content 0.18-0.47apfu, low Fe content 1.0-1.9 apfu, and low Na content 0.2-0.9 apfu, (Fig.…”

“…Tourmaline chemical composition can provide exploitable information on the source and evolution of ore fluid [2,[7][8][9], including possible fingerprints on the origin of associated gold mineralization [10]. The elemental composition of tourmaline, including boron isotopic data, have been widely employed in studies aimed at unraveling: (1) The source of the boron, (2) Fluid-rock interaction processes and transport, (3) The evolution of the magmatic fluid, (4) Temperature of the fluids related to B-metasomatism, and (5) The influence of magmatic-hydrothermal process on formation of orogenic gold deposits and the timing of magmatic, metamorphic and hydrothermal processes [1,5,6,[11][12][13][14]. The relative abundance of δ 11 B in nature [~ 60%; 5] makes the boron isotope system a valuable tracer of boron source in magmatic-hydrothermal and metamorphic systems [14].…”

“…The elemental composition of tourmaline, including boron isotopic data, have been widely employed in studies aimed at unraveling: (1) The source of the boron, (2) Fluid-rock interaction processes and transport, (3) The evolution of the magmatic fluid, (4) Temperature of the fluids related to B-metasomatism, and (5) The influence of magmatic-hydrothermal process on formation of orogenic gold deposits and the timing of magmatic, metamorphic and hydrothermal processes [1,5,6,[11][12][13][14]. The relative abundance of δ 11 B in nature [~ 60%; 5] makes the boron isotope system a valuable tracer of boron source in magmatic-hydrothermal and metamorphic systems [14]. There is a plexus of views on the source of fluid and metals in hydrothermal and orogenic gold deposits based on the interpretation of δ 11 B isotopic data and the geologic setting of such gold deposits in the world [15].…”

“…The relative abundance of δ 11 B in nature [~ 60%; 5] makes the boron isotope system a valuable tracer of boron source in magmatic-hydrothermal and metamorphic systems [14]. There is a plexus of views on the source of fluid and metals in hydrothermal and orogenic gold deposits based on the interpretation of δ 11 B isotopic data and the geologic setting of such gold deposits in the world [15]. A metamorphic source of fluid associated with boron metasomatism is now widely considered to be the major source of mineralizing fluids in orogenic gold deposit systems, derived from prograde to peak metamorphic devolatilization of metasedimentary basement units [16,17].…”