Garnierites from the ultramafic rocks in Swat, northwestern Pakistan
Abstract
The serpentinized and/or carbonated ultramafic rocks of the Swat valley ophiolite locally contain accessory to trace amounts of microscopic to megascopically visible grains of garnierite. Such grains mostly occur in samples where the serpentinisation and/or carbonation are rather pervasive and accompanied by supergene alteration. Their optical properties and chemical compositions are extremely variable and differ greatly from spot to spot within individual grains which appear to be intimate mixtures of different compositions. The highly variable and, in most cases, high to very high concentration of FeO (up to ~54 wt. %), and abnormally high CaO contents (ranging up to ~3 wt. %) in the chemical analyses make them distinct from the serpentine varieties of garnierites reported from other localities.
Most of the garnierite grains seem to be pseudomorphic after, and contain relic patches of, pentlandite, and might have developed as a result of reaction between the 'normal' matrix serpentine or talc and the original magmatic sulfide. As their occurrence along fractures in the rocks is clear at least in some cases, their formation probably took place after the main episodes of serpentinisation and carbonation and was probably brought about by the processes of wreathing and supergene alteration. The association of calcite with garnierites in some of the rocks indicates that the fluid, which caused the alteration, was charged with calcium and CO2 and this factor probably also accounted for the anomalously high values of CaO in the garnierites.
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