Key processes determining secondary alterations in kimberlites

Abstract

A variety of hypotheses have been put forward as to the nature of secondary alteration. They were tested by using the amount of insoluble SiO₂ accumulated in the rocks (Q) as the index of secondary alteration degree. This parameter allows any sample to be characterized with the degree of its secondary alteration. By putting Q values together with contents of CaO and rare earth elements, features of diamond crystal size distributions within samples in most altered kimberlites, and the distribution of these parameters over kimberlite bodies, we show that secondary processes occurred when kimberlites were exposed to postmagmatic solutions: fluids containing water and carbon dioxide. Endogenous fluids rose from the bottom of pipes along narrow local zones and spread beyond them at the top, involving larger rock masses. There, they mingled with vadose and phreatic water. The basicity decreased abruptly, inducing profound changes of rock-forming minerals and alteration of diamond crystal weight distribution by recrystallization. The study invokes a great body of factual information, including hundreds and thousands of analyzes of kimberlites from Yakutian diamond pipes: Botuobinskaya, Nyur’inskaya, Maiskaya, Internatsional’naya, Mir, Aikhal, Yubileinaya, Sytykanskaya, Udachnaya-West, and Udachnaya-East.