PRELIMINARY CHARAKTERISTIC MINERALIZATION IN RIVER ALLUVIUM MAMUJA (HALMAHERA ISLAND, INDONESIA)

The study area is located in a north part of Halmahera Island. The drainage basin of Mamuja River is built by igneous rocks of calc–alkaline character, mainly tuffs, andesites, basalts, rhyolites and dacites. Fragments of mentioned rocks and heavy minerals are principal components of Mamuja’s alluvium. Two samples collected from Mamuja River terrace and seaside sands near estuary, were the object of detailed studies. The topic of these studies was determining the mineral composition of river’s alluvium. The fraction of samples is below 1mm. Sand in the samples is black in colour and also shows magnetic properties. Magnetic fraction was separated using hand size magnet. Both magnetic and non–magnetic fractions were objects of microscopic studies in transmitted and reflected light. Energy Dispersive Spectrometry (EDS) and XRD analysis were used for detailed studies minerals and their chemical composition. Black sands are composed at 75% - 80% magnetic fraction each. It is composed of fine magnetite crystals dispersed within at groundmass and glassy groundmass. Feldspars (anorthite, albite), pyroxene, olivine, magnetite, hematite, pyrrhotite, ilmenite, rutile and chalcopyrite have been identified. Crystals of magnetite and hematite occur as separate grains of sands or form intergrowtha with the waste rocks or minor sulphides. These crystals form the intercalation between cleavage planes of pyroxenes. Occasionally magnetite crystal is affected by martitisation. The presence of mineral phase of chemical composition between magnetite and ilmenite was confirmed by Energy Dispersive Spectrometry. The some admixtures of magnesium, aluminum, manganese and vanadium have been detected in this mineral phase. Apatite, pyrite and diopside were confirmed in samples using EDS method. Non – magnetic fractions comprise a 20 -25% of the samples. It is composed by feldspars, pyroxene, hematite and groundmass originating from igneous rocks. Hematite crystals replace waste rock. 1 µm magnetite inclusions were observed in rock fragments. The presence of sulphides was documented. There were mainly chalcopyrite, bornite and pyrite. Sulphides have rounded shape and its have less than 20 µm in diameter. Sulphides are occurred in the form of inclusions in feldspar and pyroxenes. The presence of Na – Ca feldspar and Mg – Ca pyroxenes were found out by XRD analyses.