Unravelling the origin of macroscopic helicoidal structures of the Red Dog District, northwestern Alaska

English Abstract:

The geological record contains rare instances of macroscopic helicoidal structures. Such structures are often attributed to biological processes, but may be actually be concretions – the products of localized mineral growth. The recent discovery and investigation of enigmatic helicoidal structures in northwest Alaska has provided insight into controls on their formation and has implications for the geological history of the region.

The helicoidal structures are found near the Red Dog Zn-Pb-Ag mine in the Brooks Range, within black shale of the Mississippian Ikalukrok unit that hosts massive sulfide mineralization. The helicoidal structures are 12-15 cm in diameter, up to 80 cm in length, and show both left and right-handed forms. Spherical structures are also sometimes associated with the helicoidal structures. The regular helicoidal form is inconsistent with known helicoidal trace fossils, which are characteristically smaller diameter and comprise open, longer wavelength coils.

Powder XRD shows the structures to be much richer in calcite (83 wt%) than the surrounding black shale (10% calcite and 78% quartz). Arrays of thin sections reveal the interior of the concretions to be dominated by fibrous calcite radiating from a central axis (helicoidal forms) or point (spherical forms). Electron microprobe analysis shows that the material contains abundant microfossils that are partially obscured by fibrous calcite overgrowths.

Our observations indicate that these concretions formed via an unusual crystallization phenomenon rather than by biological processes. The growth of radial-fibrous calcite, possibly in the presence of organic molecules and silica (as observed by others in analogous lab-grown synthetic calcite), is a probable origin for these unique structures.