Minerały rudne w serpentynitach Szklar (Dolny Śląsk)

Jerzy Niśkiewicz


Ore minerals in the Szklary Serpentinites (Lower Silesia)

The Szklary serpentinites occur among metamorphic — mylonitic rooks of the Niemcza zone (Oberc, 1972) and form a massif situated 7 km north of Ząbkowice Śląskie (Fig. 1). As regards its geological structure, the massif includes, besides the rooks mentioned above, amphibolites, cataelasites, mylonites, leueocratic and melanocratic veins and magnesite veins. All these rocks are strongly weathered in the surface zone (Niśkiewicz, 1967). The Szklary serpentinites are massive Tocks with an aphanitic texture, ranging in colour from olive-green to greenish-black. The most remarkable rocks among them are the olivine serpentinite, composed of olivine and minerals of the serpentine group, and the true serpentinite, containing minerals of the serpentine group among which olivine relics are found. The microscopic picture of the Szklary serpentinites reveals their alveolar texture and massive disorderly structure (in places, directional — flaser). Genetically, they represent a peridotitic rock serpentinized to various degrees (Juskowiak, 1957). Microscopic studies in the reflected light permitted the detection of magnetite, hematite and chromite in olivine serpentinite and true serpentinite. Magnetite reveals typical optical characteristics (Ramdohr, 1960) and is represented by two varieties: primary (Pl. I, Fig. 3, 4, 5) and secondary (Pl. II, Fig. 1—5). Primary magnetite is more frequently encountered in the olivine serpentinite, while secondary magnetite occurs mainly in the true serpentinite and prevails quantitatively over the former. Hematite forms inclusions as a rule, and less frequently, individual grains. The form of its occurrence indicates that it resulted martitization. If this process takes place along divisional planes, occasionally found in magnetite (Betechtin, 1964; Ramdohr, 1960), then hematite lamellae form a relatively regular network in it (Pl. IV; Fig. 1, 2). The occurrence of hematite only in the true serpentinite points out that the martitization process took place at the stage of advanced serpentinization of peridotitic rocks. Chromite sporadically occurs in the true serpentinite, forming inconsiderable aggregations which are often accompanied with magnetite grains (blasts). Bigger grains of chromite are usually cracked, and around some of them magnetite rims are observed (Pl. IV; Fig. 3, 4). The analysis of interrelation between nontransparent ore minerals and transparent ones, mainly silicates, corroborates the occurrence of the two genetic types of ore minerals — primary and secondary — in the Szklary serpentinites. The primary minerals are represented by the primary magnetite and chromite. They were formed as a result of magmatic processes, together with peridotitic rocks which were later transformed into serpentinites. The secondary minerals include the secondary magnetite and hematite whose origin is related to the process of serpentinization of peridotitic rocks. Hematite was formed mainly as a result of martitization which took place at the stage of advanced serpentinization of peridotitic rocks.

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