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ALTERATION OF ILMENITE IN THE CRETACEOUS SANDSTONES OF NOVA SCOTIA, SOUTHEASTERN CANADA

¹ Department of Geology, Saint Mary’s University, Halifax, N.S., Canada B3H 3C3
² Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, PO Box 1006, Dartmouth, N.S., Canada B2Y 4A2

^* E-mail address of corresponding author: gpiper{at}smu.ca

Most detrital ilmenite grains in sandstones of the Chaswood Formation are completely altered to pseudorutile, leucoxene and rutile. The textural, chemical and mineralogical changes involved in alteration were tracked using electron microprobe analyses, backscattered electron images, and elemental maps. Ilmenite grains (Ti/(Ti+Fe)0.48) alter patchily to pseudorutile (Ti/(Ti+Fe) 0.5–0.7) with volume loss, forming a porous structure and this process continues with the development of leucoxene (Ti/(Ti+Fe) 0.7–0.9). Within the pseudorutile and leucoxene, stubby prismatic rutile crystals have been precipitated. Si and Al occur in the altered ilmenite, either (1) inherited from original quartz and muscovite inclusions in the parent crystal or (2) as kaolinite altered from muscovite inclusions or precipitated in the pore space, under pedogenic or early diagenetic conditions. Distribution of alteration phases has been related to facies and diagenetic variations. With increasing amounts of leaching in different types of paleosols, there was increasing alteration of pseudorutile to leucoxene. In light gray mudstones and interbedded sandstones with diagenetic kaolinite that formed beneath the water table from percolating meteoric water, most leucoxene was converted to rutile. Burial diagenesis (to vitrinite reflectance values >0.4%) also promoted the change from leucoxene to rutile. The alteration of ilmenite is an important source of Fe for diagenetic minerals in the Chaswood Formation and correlative offshore deltaic and marine facies of the Scotian basin.

Key Words: Alteration • Diagenesis • Ilmenite • Pseudorutile • Rutile

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