Abstract
The Ulaan Del deposit is located in the Lake Zone, Western Mongolia. In the area, middle-late Devonian alkali dykes of the Khalzan Complex are hosted in the middle-late Cambrian granodiorite-tonalite of the Togthohiinshil Complex. The alkali dykes of the Khalzan complex comprise medium- to fine-grained syenite, microsyenite, syenite-porphyry and trachyte, trachyrhyolite, and trachyandesite. The dykes are replaced to silica, sericite, albite, fluorite and are brecciated. They crosscut by quartz and quartz-carbonate veinlets. The dykes contain zircon (>0.19% Zr) with a total of rare earth elements oxides >0.1%. The host rocks of the Togtokhiinshil complex are mid-K, metaluminous, I- type granite, depleted in HFSE. Based on geochemical and mineralogical data, economic REE mineralization is concentrated in syenite and syenite porphyry of calc-alkaline high K to shoshonite series of A- type granite, emplaced at within a plate setting. Syenite dykes are enriched in REE. Ore minerals are zircon, apatite, sphene, monazite, xenotime, synchysite, parisite, fluorite and REE complex minerals, pyrite, rutile and limonite. Magmatic, metasomatic and hydrothermal processes significantly contributed to the formation of Zr, Nb, REE and Y mineralization at the Ulaan Del deposit.
Highlights
The Ulaan Del deposit is located southwest of Khyargas Lake within the Lake Island arc terrane (Lake Zone) of the Northern Mongolian domain (Fig. 1)
Petrography The Khalzan complex alkali dykes mostly represented by quartz syenite and microsyenite are fine to medium-grained pinkish, ranging from pale to reddish in color
Syenite-porphyry composed of K-feldspar or albite (70-75%), goethite, hydrogoethite (10-15%), few quartz grains, carbonate (5-10%), pseudomorphs of chlorite with rutile (3-5%) and sericite-muscovite (5%), a minor amount of fluorite (3-5%) and apatite
Summary
The Ulaan Del deposit is located southwest of Khyargas Lake within the Lake Island arc terrane (Lake Zone) of the Northern Mongolian domain (Fig. 1). Geological investigation of the region, including the Ulaan Del deposit area started in the early 1980’s by 1:200000 scale geological mapping (Samozvantsev et al, 1982) and an airborne geophysical survey on the scale of 1:50000 (Gavrilov et al, 1987, 1989 and 1990). The geological probable reserves were assessed at REE bearing ore of 19,900 tonnes of Ta2O5, 19,000 tonnes of Nb2O5 and 1.4 Mt of ZrO2 (Minin et al, 1991). Since 2009, geological prospecting and exploration works in the Ulaan Del deposit area (Fig. 2) were carried out by Geo-Info LLC, Mongolia, As a result, inferred and probable (C+P) reserves estimated 6.1 Mt of ore at 0.16%
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