Fenite and syenite are present in the margins of my study area (ultramafic-alkaline-carbonatite complex). If fenitization takes place along fractures and are relatively thick, then how can I differentiate it from syenitic intrusions? Without geochemistry, can someone suggest the differences from field and petrography. Thank you.
Hi Damepaia, you should be able to distinguish them by their different textures. Syenite intrusions have porphyritic textures, while fenitization is a texture obliterating alteration product. The latter is a metasomatic (biotite and amphibole bearing) replacement product overprinting the original wall rocks, usually along the contact zones of carbonatite intrusions.
We can view fenitization as a type of contact metasomatism. The fenite may vary with host rock (syenite or ultramafics) and distribution of carbonatite within them. The fenitizing fluid composition will vary with the type and composition of the intruded carbonatite. Sodic fenite and potassic fenite are commonly associated with syenite. The sodic fenite develops 'albitite' along the contact zone and may be controlled by the fracture system present. Pre-existing potash feldspar is altered to massive albitite. Potassic fenite develops 'ultrapotassic' feldspar. This may be confirmed by variation in petrography from syenite to the suspected fenite zone (and carbonatite). If the fenitization is pervasive, massive and thick, it is possible to distinguish the original and altered minerals at the outcrop. Otherwise we may need geochemical confirmation. Accessory minerals such as pyroxene in the syenite, may be altered to blue riebeckite (asbestiform), following desilication reactions, the silica generated will possess riebeckite needles as inclusions and will be noticed as greyish blue to dark blue (quartz) pebbles (Samalpatti and Pakkanadu alkaline complex). Pyroxene may be altered to green epidote or dark to greenish black biotite or vermiculite (with or without apatite) , and may be distributed as streaks in the fenite zone. Carbonatite related minerals such as monazite, apatite, corundum may occur in the fenitized zone. You can get more information from the articles:
Carbonatites: related ore deposits, resources, footprint, and exploration methods by George J. Simandl & Suzanne Paradis. Article Carbonatites: related ore deposits, resources, footprint, an...
Article Fenites associated with carbonatite complexes: A review
Heinrich, E.W.M, 1985. Infinite variations on a fenite theme. Indian Mineralogist. Sukheswala Volume, pages 151–162.
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