On the Composition of Garnets in the Host Rocks and Ores of the Eldorado Deposit

A. M. Sazonov

(Presented by the seminar of the Department of Petrography)

Minerals of the garnet group are widely distributed in the host and vein rocks of the deposit. The development of garnet minerals both in the host rocks and in the ores is not accidental. This is one of the facts proving the broad participation of metamorphic processes in the formation of the deposit.

The deposit belongs to a multistage type. Many minerals composing the rocks are transitional. A change in mineral associations is reflected in their composition and properties. Minerals of the garnet group may be considered a vivid example of this.

In the rocks and ores of the deposit, three principal garnet generations of different ages are distinguished; they occur in various mineral associations that characterize both the facies features of their formation and the features of the chemistry of the processes of their formation.

At the initial stage of the process, characterized by the emplacement of northwestern-trending zones of crumpling and by metamorphic alteration of the host rocks, schists of the epidote-amphibolite facies and quartz-vein zones are formed with broad participation of the mineral association garnet + biotite + quartz. This mineral association occurs both in the rocks and in the ores. Its manifestation in vein formations is quite distinctive. Biotite flakes and garnet crystals larger than those found in the host rocks are localized along the selvages and in places where veins and lenses pinch out. Study of the physical properties (refractive indices, specific gravities, color, crystal forms) and chemical compositions of the minerals of this association gives similar results. The color of the mineral is pink with a violet tint. The refractive index is \(N = 1.793—1.800 \pm 0.004\), and the specific gravity is 4.10—4.15. The chemical composition of the mineral, recalculated into end members, is as follows: almandine — 76—78; grossular — 3—2; spessartine — 6—5; andradite — 5—7; pyrope — 10—8.

In connection with the renewal of tectonic activity and a change in the direction of stresses, rather large disruptions of NNW trend appeared. Along these disruptions infiltration metasomatism developed intensively, leading to the formation of garnet-bearing quartz-actinolite rocks. The bulk composition of the rocks changes from essentially aluminosilicate to calcareous-aluminosilicate. The previously existing parageneses were completely destroyed, and new ones formed in their place. It is noteworthy that at this stage vein-

host rocks in the form of thin and short lenses and veinlets of quartz–actinolite–garnet composition. The boundaries of these lenses, veinlets, and nests are often indistinct. The garnet–actinolite aggregate is localized mainly in the salband parts of the lenses. The position, morphology, and textural and structural features of these bodies indicate their metasomatic genesis. The garnet from rocks of this stage, as in the first case, has similar physical and chemical properties, which, together with all the other features, indicates a single process of formation of the host and vein rocks. The color of the mineral is light brown. The chemical composition of the mineral, recalculated into end-members, is as follows: almandine — 63–60; grossular — 2–6; spessartine — 6–7; pyrope — 6.5–7; andradite — 22.5–20.

The third generation of garnet is associated with the formation of calcite veinlets. Garnet of this generation is encountered rather rarely. Its appearance is associated with recrystallization of the first generation under the influence of high-temperature carbonate solutions. Here the garnet is associated with calcite, clinozoisite, epidote, and quartz. This garnet is characterized by a honey-yellow color. The refractive index $N$ reaches 1.816. Chemical silicate analysis shows the following composition of the mineral: almandine — 41; grossular — 35.5; spessartine — 16.5; andradite — 5; pyrope — 2.

Summarizing the foregoing, it may be noted that the physical properties and chemical composition of minerals of the garnet group are consistent with the processes that took place during formation of this deposit. Interpretation of the research results helps in part to reconstruct the complex process of genesis. As is evident from the chemical compositions of the mineral, toward the end of the process the amount of the almandine and pyrope components in the garnets decreases, and the content of the isomorphic admixture of the spessartine and grossular end-members increases. A significant content of the andradite end-member is characteristic of the second generation of garnet.

In the change in the chemical composition of garnets, along with the specificity of the chemistry of the processes of each stage, pressure and temperature also play a substantial role. On the basis of the existing changes in the composition of garnets, it may be assumed that the deposit formed against a background of decreasing temperatures and pressures.