Textural Signatures of Hardrock Ore Deposits, Madagascar: Insights from Pattern Recognition

J. Evensen
University of Oklahoma, School of Geology and Geophysics, Norman, OK

As the spatial components of any chemical or mechanical (equilibrium, textural signatures of minerals carry the potential to record key geologic processes relevant to the gamete of energy applications. The use of shape descriptors (crystal solidity, roundness, diameter aspect ratio) and multifractal measures on natural and experimental mineral-fluid systems (using image processing) provides statistical characterizations as a function of scale. Combined with experimental calibrations, such measurements allow for constraint of geologic systems in terms of P, T, composition, and time, thereby providing limits for ore-forming events and supplying new tools for future pays.

Pegmatite fields in central Madagascar likely contain the most fractionated silicic rocks on earth in terms of overwhelming abundances of Cs, Li, Be, B, and Ta at the magmatic stage. But ore deposit models for rare lithophile metals found (only) in granitic pegmatites are among the least constrained due to (1) complex crystallization scenarios induced by rapid heat loss and (2) the interpretation of highly complex textural patterns. Spatial signatures, however, appear to highlight critical processes that operated during their histories.

Dike size, and hence thermal regime upon emplacement, exerts the greatest control on crystallization mechanism and style of mineralization in Madagascar. Composite textures of dikes with widths <1 m suggest cooling below the glass transition temperature (~400 degrees C) prior to solidification. Dikes of greater thickness exhibit rhythmic zonation and morphological signatures that indicate initial (subliquidus) coolings of ~150 degrees C. This difference leads to advanced mineralization over broad regions in narrow dikes, whereas such ore is strictly concentrated in central regions (residual liquid pools) of thicker dikes.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid