Geology and geochemistry of magnesite occurrences, Akamas Area, Northwest Cyprus

Brydie, James Ronald (1995) Geology and geochemistry of magnesite occurrences, Akamas Area, Northwest Cyprus. Masters thesis, Memorial University of Newfoundland.

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Abstract

A complex Late Cretaceous suture zone between two distinct geological terranes is preserved in southern Cyprus in a series of erosional windows through Tertiary-Recent cover sediments. The two terranes are the Mamonia and Troodos Complexes, and the suture zone contains evidence of two major contractional tectonic events - primary north-northeast- and secondary west-southwest-directed thrusting. Delamination of the Mamonia Complex during juxtaposition against the Troodos Complex resulted in Mamonia rocks being thrust beneath and over the Troodos Complex Later west-southwest-directed back-thrusting resulted in imbrication of the Troodos Complex and also Mamonia rocks being thrust over Troodos rocks (Malpas et al., 1993). The serpentinite sheet containing magnesite mineralisation was tectonically emplaced from east to west over sheeted diabase dykes and lower- and upper pillow lavas during this latter contractional tectonic episode. -- Magnesite deposits are found in a large sheet of serpentinised harzburgite in the Akamas Peninsula and trend north-south along a 5km line adjacent to Tertiary limestone cover. The largest magnesite occurrence, the Magnisia mine, exhibits many clues to magnesite vein genesis prior to, and during, the tectonic emplacement of the serpentinite. Several temporally and spatially related generations of magnesite are identified. The magnesite deposits contain two distinct styles of mineralisation within the serpentinite, and have been divided into Zones i and II. Zone I encompasses veins and stockwork magnesite and Zone II contains nodular magnesite. Antitaxial magnesite veins have mineralised serpentinite along planes of weakness, encapsulating serpentinite fragments and exhibiting classic features associated with vein minerals formed in active shear zones, i.e. well developed median lines in veins, comb textures, brecciated veins recemented by further mineralisation and spalled wallrock lying parallel to the edges of vein walls. These veins are interpreted as having formed during emplacement of the serpentinite, and define the main phase of magnesite mineralisation. The magnesite veins and stockworks define Zone I style mineralisation. Voids within these magnesite veins have subsequently been filled by epigenetic calcite and dolomite, probably derived from solutions passing through the limestone cover above the deposits. A later generation of magnesite is present within sheared serpentinite above the veins and stockworks. This carbonate is present as a replacement style of mineralisation with magnesite nodules prominent in a sheared, carbonatised serpentinite matrix. This magnesite defines mineralisation of the Zone II type. -- Petrographic examination and chemical analyses of magnesite and associated lithologies provides further clues to magnesite genesis in the Akamas area. Stable isotope analyses of magnesite, calcite and dolomite show two distinct populations of magnesite with a singular source of carbon, one precipitated around 165°C (veins and stockworks) and the other around 40°C (nodular magnesite). Calcite and dolomite from overlying karst sediments lie in an isotopic field comparable with typical marine carbonates. Trace element analyses of magnesite, calcite, dolomite, serpentinite and organic-rich sandstone samples indicate calcite, dolomite and serpentinite to be unlikely sources of carbon present within the magnesite. From isotope and trace element data, the source of carbon within the magnesite is likely decarboxylation of organic material. -- A suggested model for magnesite formation would be as follows. Magnesite veins were formed during the initial disruption of the serpentinite, probably initiated by the production of CO₂ upon decarboxylation of organic-rich sediments underthrust beneath the serpentinite. The earliest magnesite veins were then brecciated by continued compressional tectonism and cemented by further cryptocrystalline magnesite precipitation. The serpentinite (including magnesite veins and stockworks) was eventuallv emplaced westward over gabbro and sheeted diabase dykes resulting in the formation of a prominent c-s shear fabric present at the base of the serpentinite. Thrusting also resulted in the formation of an intra-serpentinite shear zone which was mineralised by nodular magnesite, defining a later generation of magnesite formation. No magnesite veins are present in this shear zone. Isotope data suggest a similar source of carbon for both veins and nodules but veins were precipitated at elevated temperatures, while nodules were formed at lower temperatures. The magnesite nodules are most likely a result of remobilisation of vein material followed by precipitation as nodules in the overlying shear zone. -- Recent precipitates of magnesite and dolomite around the study area may be the continuance of magnesite mineralisation.

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