Are the Fe-rich-clay veins in the igneous rock of the Kansas (USA) Precambrian crust of magmatic origin?

Abstract

The North American Mid-Continent rift (MCR) is a 1.1 Ga aborted rift, which has recently become an area of intense focus for energy resource exploration following the report of H2 emissions. To document the nature of the producing rocks, we conducted a multi-scale study on preserved drill-core samples from the DR1-A well located in the same area as the H2-producing wells in Kansas. We showed that this well reaches an unmapped part of the MRS composed of fayalite-bearing monzo-diorites in which we identified atypical veins of iddingsite, a complex mixing of Fe-rich phyllosilicates. Combining scanning and transmission electron microscopy (SEM and TEM) with scanning transmission X-ray microscopy (STXM) allowed us to differentiate at least two types of sub-micro-meter Fe-rich veins. A central reduced vein cutting the fayalite, other mafic minerals, the plagioclase, and to a lesser extent the alkali feldspar, contains a complex mix of serpentine, chlorite, and mica. Furthermore, a border vein, with a higher degree of Fe-oxidation, is found to be contained only within the fayalite. This external vein mainly contains iron and silicon, together with a few percent of potassium and calcium, and can be divided into two sub-veins composed of Fe3+-rich interstratified chlorite-smectite and Fe3+-rich serpentine in direct contact with the fayalite. Textures and microstructures of these phyllosilicates suggest that they have crystallized from a late magmatic and differentiated fluid, which precipitation produced the central vein together with the exsolution of an H2O-enriched fluid phase. This exsolved fluid, chemically far from equilibrium with the fayalite, appears to have induced a deuteric alteration of the fayalite, leading to the crystallization of the external veins enriched in ferric iron. These observations bring new perspectives on the history of formation of iron-rich clay minerals, which may, somehow, be related to H2 production.