Geological Issues

Бурхан Дэл алтны ордын геологийн тогтоц, бодисын бүрэлдэхүүний судалгаа

М.Төмөрхуяг, Л.Оюунжаргал, Б.Батзориг, Л.Жаргал, Q.-M. Pei — Fri, 19 Jun 2026 00:00:00 +0800

The Burkhan Del deposit locates at an elevation of about 1250 m in the north part of the Gobi, south Mongolia. The deposit is 500 km south of Ulaanbaatar and about 120 km northwest of Dalanzadgad. Tectonically, the Burkhan Del deposit locates in center of Mandal Ovoo Terrane.
We sampled rock and ore specimens from drilling cores of study area. Based on the field investigation, petrography results, intrusive rocks are mainly biotite diorite, leucocratic diorite, altered schistosed biotite diorite, subalkaline diorite and diorite porphyry. Fine-medium grained biotite diorite composed of plagioclase (andesine, 80-85%), altered biotite (10-15%), quartz (~5%), and minor amounts of magnetite, pyrite, chalcopyrite, sphene and apatite. Altered biotite pseudomorphs are mostly flaky in shape, completely metamorphosed to chlorite. Altered biotite grains form veins up to 200 μm wide. Carbonate veins with a width of 20 μm are observed in the rock. The ore types can be classified into three main categories: (1) fine- to medium-grained titanite, chalcopyrite, and pyrite with rectangular and hexagonal crystal habits hosted in biotite diorite; (2) bluish-gray quartz enriched with iron hydroxides and containing pyrite, arsenopyrite, ±bornite, ±chalcocite, ±covellite, ±electrum, and ±altaite, typically forming vein-type mineralization; and (3) quartz-carbonate and carbonate-quartz vein types hosting gold, chalcopyrite, sphalerite, pyrite, ±bornite, ±chalcocite, ±covellite, ±electrum, and ±altaite.

Агуйт төмрийн илрэлийн эрдэслэг бүрэлдэхүүний судалгаа

М.Ренчин, С.Оюунгэрэл — Fri, 19 Jun 2026 00:00:00 +0800

This study was conducted to determine the structure, mineral composition, and origin of magnetite mineralization hosted within deep-seated ultramafic to mafic intrusive rocks of Cambrian age, located in the Khukh-Undur Mountain area, a subrange of the Khantaishir Range, near Biger soum, Gobi-Altai Province. These igneous bodies are intruded into the Khantaishir Metacomplex, which predominantly consists of hornblende-bearing granitic gneiss, dioritic gneiss, medium-grained amphibolite, and migmatites. Geological mapping, sampling, petrography, ore microscopy, and electron probe microanalysis (EPMA) were employed as primary investigation methods.

Petrographic analysis revealed that the studied samples are mainly composed of ultramafic plutonic rocks such as olivinite (harzburgite) and pyroxenite.

Ore microscopic observations identified magnetite, chromite, hematite, pentlandite, and chalcopyrite as primary ore minerals, occurring as disseminated, vein-type, and massive textures within the host rocks. Magnetite occurs in two distinct stages, indicating both early and late magmatic origins.

EPMA results show that magnetite contains tFeO = 81.06–88.22%, V2O3 = 0.58–0.96%, and TiO2 = 0.76–3.34%, supporting its magmatic origin. In some zones, tFeO content decreases (36.67–41.32%) and TiO2 increases (52.31– 55.75%), suggesting the presence of ilmenite phases. Chalcopyrite shows compositions of Cu = 30.35–34.12%, Fe = 29.94–30.77%, and S = 27.73–35.63%.

These findings suggest that the magnetite and ilmenite mineralization in the study area formed from magmatic processes in two stages and is associated with sulfide minerals such as pentlandite and chalcopyrite. This indicates the presence of massive iron– titan-nickel–copper mineralized bodies with economic potential. The study provides fundamental data for further research on ore genesis and for planning exploration activities in the region.

Баруунбүрэн орчмын Сэлэнгэ бүрдлийн геохими, геохронологийн судалгаа

Г. Оргил, Н. Тунгалаг, С. Болорцэцэг, Д.Энхцацрал, Г.Дашлхагва — Fri, 19 Jun 2026 00:00:00 +0800

The Selenge Complex, situated in the Orkhon–Selenge region of Northern Mongolia within the Central Asian Orogenic Belt (CAOB), constitutes a significant archive of Late Paleozoic to Early Mesozoic tectono-magmatic evolution. This study integrates petrographic obsevations, whole-rock major and trace element geochemistry, rare earth element (REE) characteristics, and zircon U–Pb geochronology to constrain the petrogenesis and geodynamic setting of the granitoid assemblages.

The investigated granitoid are characterized as calc-alkaline, metaluminous, and belong to the I-type intrusive suite. They exhibit enrichment in large-ion lithophile elements (LILE; e.g., Rb, Ba, Sr) and light rare earth elements (LREE), coupled with depletion in high field strength elements (HFSE; e.g., Nb, Ta, Ti), and display pronounced negative Nb–Ta–Ti anomalies. These geochemical features are indicative of magmatism in a subduction-related continental arc environment, generated from a mantle wedge source metasomatized by slab-derived fluids. Systematic geochemical variations further suggest that the magmatic evolution was predominantly controlled by fractional crystallization, progressing from a basaltic parental magma toward more evolved granodioritic to granitic compositions.

Zircon U–Pb geochronological data obtained from granodiorite and biotite granite samples in the Baruunburen area yield crystallization ages ranging from 258 to 252 Ma. These ages correspond to Late Permian–Early Triassic tectono-magmatic activity, consistent with regionally documented events throughout the CAOB.

Collectively, the results support a model in which the Selenge granitoids formed within a subduction-related magmatic arc during the terminal stages of subduction. This magmatic activity records a transitional geodynamic regime, marking the shift from active convergence to post-collisional evolution.

Lower Cretaceous Ostracods from the Undurbogd Region (Southern Mongolia): Implications for Biostratigraphy and Paleoenvironments

Belgutei Bold, Undariya Jalbaa — Fri, 19 Jun 2026 00:00:00 +0800

This study presents new findings on ostracod assemblages from the Lower Cretaceous deposits in the area surrounding Undurbogd mountain, southern Mongolia. Three species of the genus Cypridea — C. copulenta, C. trita Lubimova, 1956, and C.
unicostata Galeeva, 1955— were identified from lacustrine sediments in the study area. In Biostratigraphy, the Cypridea assemblage indicates a Lower Cretaceous age for the sediments and correlates well with ostracod-based stratigraphic frameworks of other lacustrine successions in East Asia.
Morphological analysis reveals characteristic features common to Cypridea species, such as a rostrum, ventral rib, and distinct surface ornamentation including reticulation and spines. These features suggest adaptive responses to variations in salinity and
hydrological conditions during deposition. Our study was indefined of C. copulenta, C. trita, and C. unicostata three species in Undurbogd area.

Баянголын баруун VI төмрийн ордын интрузив чулуулгийн геохими ба геохронологи

Г.Дашлхагвa, Н.Тунгалаг, Л.Оюунжаргал, Г.Оргил — Fri, 19 Jun 2026 00:00:00 +0800

The Bayangol West VI iron deposit in northern Mongolia represents a multi-stage mineralization system genetically associated with dioritic magmatism developed in a subduction-related geodynamic setting. Petrographic observations indicate that the host rocks are predominantly composed of amphibole–plagioclase diorite with porphyritic textures, locally affected by metasomatic and hydrothermal alterations.

Geochemically, the rocks are characterized by low SiO₂ (42.45–46.99 wt%) and relatively high MgO (7.12–8.11 wt%) and CaO (10.43–14.01 wt%), with Na₂O/K₂O ratios of 1.22–2.09, indicating a calc-alkaline affinity typical of arc-related magmatism. Chondrite-normalized REE patterns show enrichment in light rare earth elements (LREE) with weak negative Eu anomalies, suggesting fractional crystallization. Primitive mantle-normalized trace element patterns display enrichment in large-ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), consistent with subduction-related magma signatures. However, similarities to OIB-type patterns imply contributions from a relatively deep mantle source.

Zircon U–Pb geochronology constrains the timing of magmatism. Cathodoluminescence images reveal euhedral to subhedral zircons (50–300 μm) with well-developed oscillatory zoning, indicative of magmatic origin, supported by Th/U ratios of 0.17– 1.30. Weighted mean U–Pb ages from concordant analyses (n = 29) yield 439 ± 2 Ma (MSWD = 1.3), corresponding to crystallization during the Middle Ordovician to Early Silurian.

Integrated petrographic, geochemical, and geochronological results indicate that the mineralization formed in an island arc to continental arc transitional setting related to Paleozoic subduction. The iron mineralization is interpreted as a skarn-type system formed through multi-stage processes involving magmatic intrusion, contact metasomatism, structural control and subsequent hydrothermal overprinting.

Гүнбаян формацын вулканоген болон мандах бүрдэлийн Эрүү Хар Толгой массивын цацраг идэвхит изотопийн (Rb-Sr) изохрон болон үнэмлэхүй (U-Pb) нас

Б.Тамир — Fri, 19 Jun 2026 00:00:00 +0800

Мандах-Манлайн металлогений бүсийн зүүн захын хэсэг Сайхандулааны хүдрийн дүүрэгт зүүн хойноос баруун урагш чиглэсэн хагарлаар хянагдсан Оюут Улаан Овоогийн алт-зэсийн порфир, Өндөрнаран, Улаан, Цацын булаг, Өндөр /Өндөр талбайгаас зүүн хойш 15 км/ алтны (судал ба эрдэсжсэн бүс) орд, илрэлүүд бий. Өндөрнаран ордын хүдрийн талбай дахь алтны хүдэржилт нь Гүнбаян формацын бялхмал чулуулгийн хэмжээнд түгээмэл тохиолдох ба хожуу карбоны Мандахын интрузив чулуулгийн хил заагийн метасоматоз хувирлаар бий болсон хүдрийн эрдэсжсэн бүсүүдээр илэрдэг. Эрдэсжсэн бүс нь зүүн урдаас баруун хойш чиглэлтэй хагас цагираг хэлбэртэй суналын дагуу 500-600 м үргэлжилсэн, 250-300 м өргөнтэй нэлээд хэдэн /16/ тооны алт, сульфид агуулсан судлын хүдрийн биет байна.

Үнэмлэхүй насны судалгаа болон Rb-Sr-ийн цацраг идэвхит изотопын судалгаагаар изохрон нас 303±18 сая жил, SHRIMP багажаар ордын талбайд тархсан диоритоос ялгасан цирконы дан эрдэс дээр хэмжсэн нас U-Pb аргаар 303±5.3 сая жил болохыг давхар тогтоов.

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Редакцийн зөвлөл — Fri, 19 Jun 2026 00:00:00 +0800

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