Abstract
In this paper we show that the pegmatite-forming processes responsible for the formation of the Malkhan pegmatites started at magmatic temperatures around 720 °C. The primary melts or supercritical fluids were very water- and boron-rich (maximum values of about 10% (g/g) B2O3) and over the temperature interval from 720 to 600 °C formed a pseudobinary solvus, indicated by the coexistence of two types of primary melt inclusions (type-A and type-B) representing a pair of conjugate melts. Due to the high water and boron concentration the pegmatite-forming melts are metastable and can be characterized either as genuine melts or silicate-rich fluids. This statement is underscored by Raman spectroscopic studies. This study suggested that the gel state proposed by some authors cannot represent the main stage of the pegmatite-forming processes in the Malkhan pegmatites, and probably in all others. However there are points in the evolution of the pegmatites where the gel- or gel-like state has left traces in form of real gel inclusions in some mineral in the Malkhan pegmatite, however only in a late, fluid dominated stage.
Highlights
In recent years a number of contradictory suggestions as to the genesis of gem-bearing pegmatites from the famous Malkhan deposit are found in the pegmatite literature [1,2,3,4]
The Malkhan complex was intruded by Mesozoic granites of the larger Bolsherechensky massif and the smaller (7 km × 5 km) Oreshnyi massif
Most of the Malkhan pegmatites are located between both massifs [4]
Summary
In recent years a number of contradictory suggestions as to the genesis of gem-bearing pegmatites from the famous Malkhan deposit are found in the pegmatite literature [1,2,3,4]. This paper gives a more detailed discussion of our results which were collected over a period of more than 10 years, since these results provide strong support for the melt-melt-fluid immiscibility concept (see to this [8]), which remains disputed by some others working on these pegmatites. A detailed description of the Malkhan deposit, their history, geology and mineralogy is given in [4]. Limited information on rare minerals and the high boric acid [H3BO3] (3 to 27% (g/g)) in fluid inclusions (FI) are given, data about melt inclusions (MI) are very sparse and liquid-liquid immiscibility processes were not considered
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