Petrographic relations and uranium distribution in the Skaloti granitoid complex, Paranestion District, Greek Macedonia

Abstract

The Skaloti granitoid complex consists, for the most part, of biotite granites. The granites are composed of quartz, K-feldspar (mainly microcline), plagioclase, and biotite, with less common muscovite and rare hornblende. Common accessories are allanite, apatite, sphene, epidote, monazite, and garnet. The complex is divided on the basis of texture into two groups--granular and foliated--which have distinct accessory mineralogy. Epidote and sphene coexisting with biotite are relatively abundant in the foliated types; garnet is found only in the granular types. The granular granitoid group includes a leucocratic facies that occurs along the central part of the granitoid terrain. The leucocratic rocks show alteration and mechanical deformation and consist of quartz, feldspar, minor amounts of mica, garnet, and disseminated sulfides (pyrite, chalcopyrite). Sulfide mineralization and hematitization are associated with the microfractures.The Skaloti whole-rock chemical compositions indicate a metaluminous to peraluminous character, a potassium affinity, moderate to high thorium contents, apparently high Th/U ratios, and a tendency toward alkali enrichment. A shallow to moderate depth of emplacement and fractional crystallization paths highly sensitive to variations in total pressure, fluid pressure, and temperature are inferred from a consideration of the chemical data and appropriate phase relations. On the basis of present evidence, the Skaloti granitoid can be tentatively regarded as an S-type granite.The alkali enrichment, potassium affinity, moderate to high thorium contents, and presence of uranium-bearing accessory minerals render the complex a possible uranium source rock. The two groups of granitoids (granular, foliated) identified in Skaloti differ in the crystal chemical character of the uranium that they contain. In the foliated granites, the accessory minerals contain most of the uranium and represent refractory uranium sites. Granular granites, on the other hand, contain relatively high soluble uranium; these uranium sites in which uranium is not firmly chemically bound, are related to fragmentation and hydrothermal alteration undergone by the leucocratic granites, and to deuteric and low-temperature alteration which, overall, has affected the rocks of this group.The total uranium contents (U T ) of the analyzed surface samples range between 1 and 39 ppm. High total uranium values related to refractory uranium sites are attained by rocks of the foliated group and also by some aplitic-pegmatitic rocks; the extractable to total uranium ratios (U Ex /U T ) of these rocks fall below 0.39. Anomalously high total uranium values not attributable to refractory uranium sites and accompanied by anomalously high (U Ex /U T ) ratios (>0.50) are found in rocks of the leucocratic facies and also in samples from areas where dikes invade country rocks, and define localized uranium anomalies.The field petrographic and chemical relations of the Skaloti rocks suggest that fractional crystallization of the silicate melt, undersaturated in its aqueous phase, proceeded in the isobaric quinary granitic system (albite-anorthite-orthoclase-quartz-H 2 O) producing sodic plagioclase followed by a single alkali feldspar. This process of crystallization under hypersolvus conditions was terminated by an event of vapor saturation, related to the formation of the aplitic-pegmatitic rocks in the area. Conceivably, myrmekite and associated deuteric alteration and resorption are the textural evidence of boiling. The expelled fluid phase caused deuteric alteration and created areas rich in soluble uranium. These areas correspond to those radio-active-geochemical anomalies defined by exploration in Skaloti, which are located within or peripheral to the granitoid terrain and are associated with aplitic-pegmatitic rocks. These uranium anomalies, which show no structural control but can be identified as alteration areas, may represent the surface expression of uranium-rich, late-stage magmatic alteration channels at depth.Within the Skaloti granitoid terrain, significant concentration of uranium may be anticipated to occur in two types of setting: in uranium anomalies that may signify the presence of late-stage magmatic alteration channels at depth and in anomalies affected by way of secondary remobilization and deposition processes that are controlled by Tertiary faulting and fracturing.