Ore Controls for the Salau Scheelite Deposit (Ariege, France): Evolution of Ideas and Present State of Knowledge

Abstract

In the Salau skarn-type scheelite deposit, ore controls are of three kinds: lithologic, structural and mineralogical. Lithologic controls include the proximity of a granodioritic narrow apex, and a special banded formation called ‘barrégiennes’, composed of thin alternating beds of limestone and shales or sandstone. Structural controls include the folding pattern of barrégiennes, early blastomylonite and two types of faults: f1, the drainage channel of fluids responsible for the main stage of mineralization; f2, a series of reverse faults with individual displacements of 100-m magnitude which delimit the main orebodies. The general pattern of the deposit as a whole is an aureole of orebodies in barrégiennes in or near an f 1 fault, around an apophysis of the granite. In the conclusion of this chapter, the predicted (already partly confirmed by drilling) location of new orebodies is deduced.

From a mineralogical standpoint, the Salau skarns can be described as having been developed in a two-stage process: the first stage produced clearly zoned skarn with grossularite and hedenbergite (always on the marble side) and lowgrade scheelite; the second stage was cross-cutting in relation to f 1 fractures, with almandine-spessartite rich garnet often in direct contact with the marble, and (somewhat later) an abundance of pyrrhotite and high-grade scheelite. The fluids of the first stage come from granite, whereas almandine-spessartite rich garnet is an indicator of the action of a fluid in equilibrium with country rocks (including limestone), and is the best mineralogical guide to high-grade scheelite ore in the Salau mine. The mechanism of precipitation of rich ore is inferred to be an interreaction in the later stage of two fluids of contrasting origins (one originating in the granite, the other one circulating in the country rocks). It is shown that the various types of chemical changes observed in rocks (through exchange or precipitation) can be explained by this model.

In the Appendix an outline of the case history is given, and explained by the successive development of new ideas induced both by the development of ore extraction and by the progress of the scientific analysis of structural setting, mineralogy and petrology.