The granitic rocks and related molybdenite mineralisation op the Emeksan area, Espiye, N.E.Turkey

Abstract

The intrusive rocks of the Emeksan area mainly, consist of granoaiorite, variousadamellites and granites. These rocks were intruded into the pre-Mesozoic basement and the Mesozoic to Tertiary volcanic rocks. The batholith was cut by a series of dolerite, andesite porphyry and microgranite dykes, termed the late magmatic series, which show closely similar chemistry to the Tertiary hypabyssal and volcanic rocks of the Pontids. The chemical trends of the late magmatic series are typical of calc-alkaline suites. The calc-alkaline fractionation is also thought to be responsible for the generation of the intermediate Plutonic rocks in the area although variation diagrams indicate that other processes were involved before the solidification of the granodiorite and Esenli porphyritic adamellite. Field occurrences and factor analyses of these intermediate rocks imply that their chemical compositions were affected by xenoliths of quartz-feldspar-mica schists derived from basement rocks which resulted in higher levels of Nb, Y and the lithophile elements K and Rb. An earlier leucogranite intrusion shows a distinctly different character relative to the other intrusive rocks. It has gradational, contacts to the basement rocks, is highly enriched in quartz and alkali feldspars of lower temperature form, and shows, coarser grain-size and abnormal perthite abundance. The leucogranite contains anomalous values of Na(_2)O, K(_2)O, Rb, Kb and Y relative to the normal calo-alkaline fractionation trend. The leucogranite plots in the low temperature-high-pressure fields of quartz-alkali feldspar phrase diagrams. The evidence suggests that the leucogranitic magna probably derived from the basement rocks as a result of partial fusion. N-S compressive forces were the primary causes of the structural features in the area. The E-W extension of phenocrysts and the N-S, NE-SW and NW-SE trends of dykes, veins and faults are related to these forces which were probably produced by subduction processes during the Tertiary. The variation of K(_2)O, and other indices, in the magmatic rocks across the Pontids support a northward inclined Benioff zone. The molybdenum mineralisation is related to the intermediate to acidic calo-alkaline intrusive rocks showing porphyritic or fine-grained texture. Fractures with a narrow dispersion about the N-S direction are the important structural features controlling the sulphide mineralisation. The molybdenum is introduced in K-feldspar-quartz-biotite and quartz veins and sometimes occurs as disseminations. Chalcopyrite is very limited in the molybdenum zone but relatively enriched in the surrounding areas in which pyrite is also widespread. Pb-Zn-Cu sulphide veins are occasionally found in the outermost sulphide zone. Hydrothermal alteration associated with the ore mineralisation also shows a zonal distribution. In the potassic alteration zone the host rocks were replaced by fine-grained material, including K-feldspar-quartz-biotite. Quartz, sericite and clay minerals are the main minerals of rocks in the phyllic to argillic alteration zones, which include most of the Mo-mineralisation. The development of chlorite, after mafic minerals, and day minerals after feldspars represents the outermost alteration zone, which contains pyrite, chalcopyrite and some carbonate veins. The primary distribution of elements during bydrothermal alteration was used to examine the types of host rock alteration, and to locate there mineralisation more precisely. Higher Mo, Kb, Si0(_2) and K(_2)O and lower Fe(_2)0(_3), MgO, CaO, Na(_2)0, Ti0(_2), Sr and Zn values are recorded in rocks from the pervasive hydrothermal alteration centre. Cu, Zn, S and Rb are enriched in the halo zone and may be used in exploration for the similar ore deposits in other areas. The Mo-mineralisation in the area shows a close similarity to porphyry ore deposits which are economically important. Exploration for this type of deposit should therefore be continued in the Pontids.