Origin of phosphorite of the late Precambrian Gangolihat dolomite of Pithoragarh, Kumaun Himalaya, India

Abstract

The Precambrian Gangolihat Dolomites of the Pithoragarh area in the Kumaun Himalaya, characterized by rich and widespread development of the stromatolites belonging to the Baicalica group, are poorly to moderately phosphatic. The intimate and constant association of collophane with the branching stromatolites suggests that algae played a significant role in the localization of phosphate in the dolomites formed in very shallow and warm waters of the sea. The phosphatization of the stromatolites occurred during the later, almost penultimate period of the Gangolihat times, when the Baicalica group of algal stromatolites was in its heyday of development. The replacement of carbonates by collophane took place a little before and during the sedimentation of the clastic calcarenites that fill the space between the columns of algal structures. What is uniquely distinctive and significant in the Pithoragarh area is the association with algal bioherms of the lentiform deposits of very coarsely crystalline magnesite. The magnesite that has replaced the phosphatized stromatolite-bearing dolomites not only preserves excellently the original stromatolitic structure, but its phosphatized parts as well. This shows that the phosphatization pre-dates the origin of magnesite.