A HYDROTHERMAL SOURCE FOR METALS IN METALLIFEROUS BLACK SHALES OF THE BROOKS RANGE, NORTHERN ALASKA: GEOCHEMICAL AND OSMIUM ISOTOPE EVIDENCE

The origin of metals in metalliferous black shales (MBS) has been debated for over three decades. Two principal sources have been proposed: hydrothermal fluids and seawater. We evaluate these sources in the Brooks Range, where Mississippian strata of the Lisburne Group contain MBS, deposited in outer to middle ramp settings, and coeval sedimentary-exhalative (sedex) Zn-Pb-Ag ores of the Red Dog district, deposited in a deeper basinal setting. Massive sulfide ores in this district are noteworthy for high contents of Tl (up to 313 ppm) and Sb (up to 919 ppm). Fossil ages for the host black shales and the MBS, mainly from conodonts, are Osagean to early Chesterian. Vein pyrite from the Red Dog deposit yields a Re-Os isochron age of 338.3 ± 5.8 Ma and unradiogenic initial ¹⁸⁷Os/¹⁸⁸Os (Os_i) of 0.20 ± 0.21 (Morelli et al., 2004, Econ. Geol. 99: 1569-1576; this study). The latter Os_i value suggests a significant component of mantle-derived Os, which we propose was derived by leaching of mafic igneous rocks by the Red Dog hydrothermal system.

The MBS, typically interbedded with phosphorite, have up to 2831 ppm V, 1690 ppm Cr, 4820 ppm Zn, 312 ppm Cu, 58.0 ppm Ag, 12.3 ppm Tl, and 19.0 ppm Sb. Whereas the principal metal source is likely seawater, two samples of MBS have an additional hydrothermal Zn component based on Zn/Cu ratios of 40 and 35 (data exclude terrigenous fractions), which are much higher than those of modern seawater (~3) and modern plankton (~6). Correlative unmineralized black shale host rocks (Kuna Formation) in the Red Dog district lack metal enrichments and have radiogenic Os_i values (calculated at 338 Ma) of 0.98-1.09 (avg 1.06; n = 4), recording a predominantly seawater Os source and little if any mantle-like Os component. In contrast, calculated Os_i values for the coeval MBS are uniformly less radiogenic, ranging from 0.16 to 0.85 (avg 0.64; n = 5). The least radiogenic value, determined on the most metal-rich black shale, is consistent with a large mantle-like Os component from a hydrothermal source. We suggest that this component in the MBS, and large portions of other metals such as Zn, Cu, Ag, Tl, and Sb originated in a basinal sedex-type Pb-Zn-Ag system such as Red Dog and were transported in a hydrothermal plume >100 km to the basin margin where upwelling, nutrient-rich seawater deposited metals in the MBS and related phosphorites.