Petrological and geochemical characteristic of the rocks of the Voznesensky intrusive massif (Southern Urals): Оn the question of the composition and sources of magma producing gold and copper porphyry mineralization

Research subject. The petrological and geochemical features of the rocks of the Voznesensky intrusive massif and its dyke series were studied in order to clarify the composition, possible sources and geodynamic settings of magma generation that produced Au- and Cu-porphyry mineralization.Methods. The content of petrogenic oxides was determined by the chemical method, trace elements – by ICP-MS analysis.Results. Among the rocks of the Voznesensky massif, which have the geochemical characteristics of suprasubduction formations, varieties with calc-alkaline and adakite-like properties were established. The main phase of the massif is represented by gabbro-diorites and diorites belonging to the calc-alkaline series. Ore-bearing dykes of gabbro-diorites, diorites and granodiorites of the Au-porphyry Bolshekaransky deposit are of calc-alkaline composition, while the post-ore dykes of granodiorites and plagiogranites of this deposit exhibit adakite-like characteristics.Conclusions. The ore-bearing dyke series of the Voznesensky deposit is represented by calc-alkaline diorites and adakite-like granodiorites and plagiogranites. The metallogenic specialization of the dykes was influenced by the silicic acidity and the redox state of the ore-generating melts. Granitoids with Cu-porphyry mineralization, compared to their gold-bearing varieties, crystallized from more acidic melts with a higher degree of oxidation. It is assumed that the main mantle component of magmas for the Voznesensky rocks were relatively weakly depleted spinel peridotites of the suprasubduction lithospheric mantle. Calc-alkaline magmas were melted from a mantle substrate previously metasomatized by aqueous fluids, and magmas with adakite-like properties – metamorphosed by melts of basalts and sedimentary rocks of slab. Melting of slab rocks may have been associated with additional heating due to friction caused by changes in direction and/or velocity of oblique subduction.

GEOCHEMICAL CHARACTERISTIC OF THE SILURIAN ROCKS FROM THE EASTERN PART OF MOLDAVIAN PLATFORM (ROMANIA)

The scope of this paper is to assess the hydrocarbon potential of Silurian, respectively, the maturity of this stratigraphical interval, based, especially, of the geological and geophysical data derived from eight boreholes located in the north-eastern part of Moldavian Platform - Romania. The main conclusion is that the organic matter contents measured in the core and cutting samples of the different wells are low with the Organic Carbon (TOC) (residual) mainly clearly lower than 1%. Due to the poorness of the sample and the high maturity the petroleum potential and organic matter cannot be assessed. Estimation of the initial TOC allows to differentiate a richer interval at 24 – 27m thick below the top of Silurian. It shows higher TOC with estimated initial TOC reaching 3.6 % weight at the most. The maturity is high and shows a rapid increase with depth between around 2% at 200m and 4% eq. VRr at 1100m. An estimation of the eroded cap-rock is of around 3000m. Mineral carbon content obtained from Rock Eval permits to separate two lithological intervals: carbonated in the upper part (thickness of 250-300m), argillaceous in the lower part.

Fahlbands of the Keret archipelago, White Sea: the composition of rocks and minerals, ore mineralization

This paper presents a complex mineralogical and geochemical characteristic (based on SEM-EDS, ICP-MS analysis) of the fahlband rocks of the Kiv-Guba-Kartesh occurrence within the White Sea mobile belt (WSMB). The term “fahlband” first appeared in the silver mines of Kongsberg in the 17th century. Now fahlbands are interlayers or lenses with sulfide impregnation, located in the host, usually metamorphic rock. The level of sulfide content in the rock exceed the typical accessory values, but at the same time be insufficient for massive ores. Fahlbands are weathered in a different way than the host rocks, so they are easily distinguished in outcrops due to their rusty-brown color. The studied rocks are amphibolites, differing from each other in garnet content and silicification degree. Ore mineralization is represented mainly by pyrrhotite and pyrite, and pyrrhotite grains are often replaced along the periphery by iron oxides and hydroxides, followed by pyrite overgrowth. At the same time, the rock contains practically unaltered pyrrhotite grains of irregular shape with fine exsolution structures composed of pentlandite, and individual pyrite grains with an increased Ni content (up to 5.4 wt.%). A relatively common mineral is chalcopyrite, which forms small grains, often trapped by pyrrhotite. We have also found single submicron sobolevskite and hedleyite grains. The REE composition of the fahlband rocks suggests that they are related to Archean metabasalts of the Seryakskaya and Loukhsko-Pisemskaya structures of the WSMB, rather than with metagabbroids and metaultrabasites common in the study area.

Indikasi Mineralisasi Tipe Porfiri di Daerah Sumbersari, Kompleks Pengunungan Kulon Progo, Purworejo, Indonesia

ABSTRAK Pegunungan Kulon Progo merupakan produk magmatisme Busur Sunda-Banda tersusun atas formasi andesit tua. Daerah Sumbersari merupakan bagian dari gunung api Gajah, batuan gunung api tertua Kulon Progo. Indikasi mineralisasi tipe porfiri ditemukan di daerah ini sehingga menarik untuk diteliti lebih lanjut. Penelitian ini bertujuan untuk mengetahui potensi keterdapatan mineral logam berharga (Cu-Au). Metode penelitian yang digunakan adalah pemetaan geologi, analisis petrografi dan mikroskopi bijih, serta analisis geokimia menggunakan XRF dan ICP-MS. Geologi daerah penelitian terletak pada fasies sentral-proksimal Khuluk Gajah, terususun atas intrusi mikrodiorit, mikrodiorit kuarsa, andesit, andesit basaltik-diorit, dan batugamping. Alterasi hidrotermal berkembang pada batuan beku diorit, mikrodiorit, dan sebagian pada andesit. Alterasi hidrotermal dibagi menjadi beberapa kelompok, yaitu ilit-serisit±biotit sekunder, epidot-aktinolit-kalsit±ilit, epidot-kalsit±ilit, dan ilit-serisit±kuarsa. Beberapa fase mineralisasi berkembang, antara lain fase epidot-aktinolit yang diikuti mineralisasi magnetit-kalkopirit, fase biotit-magnetit-kalkopirit-bornit, dan fase akhir serisit-mineral lempung-pirit menggantikan keseluruhan sistem. Analisis geokimia pada batuan teralterasi menunjukan indikasi mineralisasi Cu-Au dengan kadar 491–1447 ppm (0,14%) Cu dan 0,02–0,3 ppm Au dengan rasio elemen Cu:Au adalah 1,01. Karakter geokimia menunjukkan adanya korelasi kuat Cu terhadap Au.ABSTRACT Kulon Progo Mountain is Sunda-Banda Arc magmatism product composed of an old andesite formation. Sumbersari Area is part of the Gajah volcanic, which is the oldest rock of Kulon Progo volcanics. Indication of porphyry type mineralisation has been found in the area which makes the area interested for further research. The research methodologies are geological mapping, petrography and ore microscopy, and geochemical analysis using XRF and ICP-MS. Geology of the area located in central-proximal facies of Khuluk Gajah, consist of microdiorite, quartz-microdiorite, andesite, basaltic-dioritic andesite intrusions, and limestone. Hydrotermal alteration is developing into certain groups like illite-sericite ± secondary biotite, epidote-actinolite-calcite ± illite, epidot-calcite ± illite, and illite-sericite ± quartz. Some mineralisation phases are developed like epidote-actinolite followed by magnetite-chalcopyrite mineralisation, biotite-magnetite-chalcopyrite-bornite phase and the late phase of sericite-clay-pyrite replacing the entire system. Geochemical analysis on altered rocks show Cu-Au mineralisation indication ranging from 491-1,447 ppm (0.14%) and 0.02-0.3 ppm respectively, with Cu:Au ratio is 1.01. Geochemical characteristic shows strong correlation of Cu to Au.