Carbon and oxygen isotope ratios in diagenetic siderite and magnetite from Upper Devonian ironstone, Wadi Shatti District, Libya

Abstract

Geology and stable isotope geochemistry of the Devonian ironstones of Wadi Shatti, Libya, suggests that they are shallow water, nonmarine deposits possibly formed in a deltaic environment. The Wadi Shatti occurrence is notable for the presence of magnetite, rarely reported in unmetamorphosed Phanerozoic ironstones, and for the presence of thin coal seams, uncommon in older ironstones.Oxygen isotope fractionation between magnetite and siderite, if produced by an equilibrium reaction, indicates that magnetite formed diagenetically at a temperature not exceeding 65 degrees C. The average delta 18 O of siderite is 18.6 per mil and the maximum delta 18 O is 21.2 per mil vs. SMOW. The minimum delta 18 O of magnetite is --9.7 per mil. These values are best explained in terms of an equilibrium reaction with fresh or brackish water to produce goethite which was later diagenetically altered to produce magnetite and siderite.Variable and extremely low delta 13 C values for Wadi Shatti siderite (-- 12 to -- 30 per mil, vs. PDB) indicate that complex organic reactions were important in the formation of this deposit.