Outline of Mineralization in Zoned Granitic Pegmatites

Abstract

Abstract An introduction to the general problems and procedures of the paper is given, and the background and approach are indicated. Zoned granitic pegmatites are divided into four groups with respect to major constituents. Simple pegmatites, composed mainly of biotite, plagioclase, microcline, and quartz, are distinguished from more complex ones with muscovite, late albite, and lithium minerals respectively. Two types of internal zoning are recognized. They differ in the nature of outer zones, which are made up of granite and graphic granite in one pattern, and of quartz and quartz-muscovite assemblages in the other. The first type is preferentially developed in simple pegmatites, and the other one in more complex deposits. Outer zones of the second type pattern mineralogically resemble inner zones, especially the quartz core and its margin, and therefore this pattern is considered to be reversed, and the pattern of simple pegmatites is chosen as a basis for the later discussion. It is shown that pegmatites crystallized from the walls inward and that internal zoning developed rapidly, and not in response to secular changes in geologic conditions. Internal spatial zones are not accepted as strictly synchronous or paragenetic formations. The following main paragenetical groups are recognized instead: the granitoid, the graphic granite, and the pegmatoid zones, the core margin, alkali replacement, and core margin mica groups, and the quartz core and late alterations. A schematic two-dimensional synopsis of mineralization in pegmatites is given, showing the correlation of minerals with development of pegmatites and paragenetical groups within these. Some features of wall rock alteration and of primary pegmatitic cavities are presented. In the discussion specific genetical problems of pegmatites are indicated, especially those involved in the strong geochemical fractionation, and the formation of the pegmatitic texture, the quartz core, and alkali replacements. For geological reasons a magmatic approach is chosen as a starting point, since it is valid for at least some pegmatites. Claims of a general harmony between mineral association in pegmatites and degree of metamorphism in the surrounding region are rejected and it is shown that they are based on insufficient data and mistakes in reasoning. Interpretation is started with deductions of temperatures, pressures, and gross composition of pegmatite systems. Pegmatitic crystallization is explained as a consequence of low magmatic temperatures, which imply low energy-content and poor nucleation in pegmatite systems. Fractional crystallization is thus the result of heat — and concentration gradients towards the walls and scarcity of growth centres. It is essentially non-eutectic. Scarcity of growth-centres is also the cause of the formation of giant crystals, crystal size being expressed as the mass of crystallized matter divided by the number of available growth centres. Crystallization of quartz is shown to be abnormal by the stop in the formation of graphic granite and the subsequent late formation of the core. After exclusion of alternatives it is explained as being caused by immobilisation of silica due to increased activity of CO2. Muscovite marks increased activity of H2O and CO2, i. e. hydration and extraction of potassium and silica. Alkali replacements are caused by supersaturation (sugary albite) and by changes in stability relations upon the appearance of an aqueous, carbonated phase (cleavelandite, spodumene, petalite), i. e. increased activity of H2O and CO2 and an excess of alkalies over the amount necessary to balance Al2O3. Interpretation is extended to explain reversed (American) zoning and the occurrence of minor minerals. Outer zones rich in quartz and muscovite denote an originally high content of H2O and CO2 whereas the distribution of minor minerals reveals the influence of the same fundamental factors, which ruled the occurrence of major constituents. A special factor is the inactivation and precipitation of core-silica, which raises the concentration of rare constituents in the residual system and favours precipitation of compounds with other anions in the core margin. For a general background brief statements on the mechanisms of the chemical differentiation of pegmatites, and their magmatic and metamorphic genesis are given, along with some comments on the structural behaviour of pegmatites, its relation to metamorphism, and some indications of analogies with other types of rocks and mineral deposits.