Hydrothermal, Biogenic, and Seawater Components in Metalliferous Black Shales of the Brooks Range, Alaska: Synsedimentary Metal Enrichment in a Carbonate Ramp Setting

摘要: Trace element and Os isotope data for Lisburne Group metalliferous black shales of Middle Mississippian (early Chesterian) age in the Brooks Range northern Alaska suggest that metals were sourced chiefly from local seawater (including biogenic detritus) but also externally derived hydrothermal fluids. These are interbedded with phosphorites limestones sequences 3 to 35 m thick; deposition occurred mainly on a carbonate ramp during intermittent upwelling under varying redox conditions, suboxic anoxic sulfidic. Deposition at ~335 Ma was broadly contemporaneous sulfide mineralization Red Dog Drenchwater Zn-Pb-Ag deposits, which formed distal marginal basin. Relative composition average shale, ( n = 29) display large enrichment factors (>10) Zn (10.1), Cd (11.0), Ag (20.1). Small enrichments (>2–<10) shown by V, Cr, Ni, Cu, Mo, Pd, Pt, U, Se, Y, all rare earth elements except Ce, Nd, Sm. A detailed stratigraphic profile over 23 Skimo Creek area (central Range) indicates samples near top section, accumulated period major is correlative levels have highest Zn/TOC (total organic carbon), Cu/TOC, Tl/TOC ratios calculated marine fractions (no detrital component) these three metals. Average authigenic (detrital-free) contents Cd, Pb, Ge, Re, As, Sb, Tl, Au show 4.3 × 103 1.2 106 relative modern seawater. Such moderate enrichments, common other shales, wholly sources (seawater material) metals, given similar trends organic-rich sediments zones Namibian, Peruvian, Chilean shelves. The largest much higher (1.4 107 2.9 107, respectively), consistent an appreciable component. Other such as Tl concentrated several shale samples, locally abundant may partly origin this cannot be fully established available data. Enrichments Cr (up 7.8 106) attributed not processes. presence some Eu 6.1 107) small positive anomalies (Eu/Eu* up 1.12) considered unrelated activity, instead being linked early diagenetic processes within sulfidic pore fluids. Initial (187Os/188Os) paleontologically based depositional 335 reveal moderately unradiogenic values 0.24 0.88 four shale. proxy ratio coeval Chesterian provided initial (187Os/188Os)335 unaltered Kuna Formation 1.08, nearly identical 1.06 Evaluation possible suggests most likely source mafic igneous rocks leached This interpreted been deeply circulating fluids basin, followed venting into overlying seawater. We propose metal-bearing deposits or vented carried currents onto margins shallow-water platform, then deposited muds, together seawater- biogenically components, syngenetic sedimentary Metal concentration promoted high biologic productivity, sorption matter, diffusion across boundaries, low sedimentation rate, availability H2S bottom waters