Diagenesis of ferriferous phases in the Northampton ironstone in the Cowthick quarry near Corby (England)

Abstract

-Berthierine, siderite and pyrite are the major ferriferous phases in the Northampton ironstone (NIS). Mineralogical and chemical data suggest a formation of these phases in a diagenetic marine environment changing from post-oxic to sulphidic conditions. Berthierine was formed first when the Fe 2+ activity in the diagenetic system increased. Later, this phase was partially replaced by siderite and/or pyrite. A second stage of the diagenetic development in the NIS with increasing CO 2 partial pressure (P C0 J is documented by siderite. The isotopic composition (5 18 O mean value: -1.7%oPDB; <$ l3 C mean value: -8.6%,,PDB) points to siderite precipitation from a marine porewater environment with a microbial CO 2 source. The shift from post-oxic to sulphidic conditions is indicated by the occurrence of pyrite and can be considered as a final stage. The diagenetic processes in the marine environment and the formation of the ferriferous phases were stopped by the influx of brackish or fresh water when the Midland Shelf turned estuarine.

An Fe-Berthierine From A Cretaceous Laterite: Part I. Characterization

“…Stable isotope values of ~3C from siderite associated with berthierine and non-berthierine-bearing pisolitic horizons are within the range of values reported by Weber et al (1964) in their study of freshwater siderites, and slightly enriched relative to Gehring's (1990) study of siderites from the ferriferous phases of the Northampton ironstones ( Figure 12). As a further check to confirm a freshwater origin of the siderites, values from modern tufa deposits collected 20 km west of Purgatory Creek fall extremely close to Keith and Weber's (1964) average for nonmarine limestones.…”

“…As a further check to confirm a freshwater origin of the siderites, values from modern tufa deposits collected 20 km west of Purgatory Creek fall extremely close to Keith and Weber's (1964) average for nonmarine limestones. The ~80 values of siderite are more depleted than Gehring's (1990) results, but are reasonably close to those of the freshwater limestone. Thus, in addition to the abundant plant remains, stable isotopic results from siderites also indicate that the depositional environment was nonmarine.…”

“…Examples include reworked ooids with an outer tangential berthierine fabric (Bhattacharyya 1983), and alternating layers of either berthierine and hematite (Bhattacharyya 1989 Table 3 for sample descriptions). Gehring's (1990) values are from freshwater siderites from the Northampton ironstones. Range bars at the bottom are from siderites collected from fresh-, brackish-, and marine rocks based on fossil evidence .…”

Constraints on the Preservation of Ferriferous Microfossils

“…The conditions for preservation can also be generalized, as shown by studies reporting the preservation of biological structures by Fe oxides (e.g., Collin et al 2005;Fernandez-Remolar and Knoll 2008;Gehring et al 1994;Hofmann et al 2008;Preat et al 2000). In contrast, there is no evidence that microbial casts develop in Fe-reducing, nonsulfidic environments, although stable ferrous phases can form in high amounts under such conditions, e.g., siderite, magnetite or berthierine (Gehring 1990).…”

“…Based on XRD, the Si-Al-Fe component consists of berthierine [Fe II ,Fe III ,Al,Mg) 2-3 (Si,Al) 2 O 5 (OH) 4 ]. Berthierine forms during diagenesis largely through the replacement of Al 3+ by Fe 2+ in kaolinite and is a major Fe constitutent in many Jurassic oolitic ironstones in NW Europe (Bhattacharyya 1983;Gehring 1990). Its presence indicates non-sulfidic, post-oxic conditions (Berner 1981).…”