Evolution of chemical composition of rock-forming carbonates in Sallanlatvi carbonatites, Kola Peninsula, Russia

Abstract

The problem of whether calcio, magnesioand ferrocarbonatites are related to each other by fractionation, produced by metasomatic alteration, or derived separately remains central to the understanding of carbonatite petrogenesis. The Sallanlatvi Complex is one of the least well studied alkalinecarbonatite intrusions in the Kola Alkaline Province, but it consists of a simple concentric structure with, in order of intrusion, melteigite, ijolite and then a central carbonatite stockwork containing all of the main types of carbonatite. Calcite, dolomite, magnesite-dolomite and siderite types have all been distinguished (Subbotina and Subbotin, 1990). The authors have studied the composition of the rockforming carbonates in order to determine their relationships. Calcite, major in calcite carbonatite, is also an accessory in the other types of carbonatites. It contains minor Mn, Sr, Fe, Mg and Ba, of which Sr and Mn are the most abundant. The Sr contents range from 0.0-I .O wt% SrO, and MnO contents range from 0.0-1.4 wt%. In fresh carbonatite, zoned crystals of calcite are common and have centres with high SrO (1 .O wt%) and low MnO (<0.3 wt%). From the centre to the rim, SrO content decreases (to 0.2 wt%), whilst MnO increases slightly (up to 0.4 wt%). In altered carbonatites, secondary calcite is developed along cleavages in primary calcite and is characterised by high MnO (up to 1 .I wt%) and low SrO (up to 0.2 wt%). Calcite, which recrystallised accompanying the formation of chlorite and carbonate-apatite, also has the same composition. Calcite from cavities, where it is associated with barite, Ba zeolites, strontianite and ancylite, is very poor in SrO (0.1-0.2 wt%) and rich in MnO (up to 0.9 wt%). In dolomite carbonatites, calcite is observed as inclusions in pyrite and has high MgO (2.1-2.3 wt%), whereas in siderite carbonatites the mineral forms thin veins cutting siderite and has high Fe0 (0.7-I .8 wt%).