PETROGRAPHY OF MAIN ROCK UNITS AND PETROGRAPHIC ALTERATION OF THE HUGO NORTH DEPOSIT, OYU-TOLGOI, SOUTHERN MONGOLIA

Abstract

The Hugo deposits extend NNE over a stride length of 2.6kms, and are divided into two ore-bodies, Hugo South and Hugo North. At the northern end, a N70E trending high-angle reverse fault may truncate the Hugo deposit, whilst at the southern end grade gradually diminishes. The Hugo deposits are classified as Cu-Au porphyry and high sulphidation-related styles. Mineralization is hosted by quartz monzodiorite (qmd) and adjacent host rocks. The mineralized sequence comprises basaltic volcanics overlain by 80 to 600m of dacitic ash flow. These volcanics in turn are overlain by the Lower sedimentary-volcanic sequence. The volcanic and sedimentary cover rocks have been folded into a NNE-trending monocline which has been intruded by several stages of qmd along the steeply-dipping easterly limb and with final intrusions along the monoclinal axis. The final intrusion is bell-shaped in cross section and may be an elongate body, about 0.6 x 2km in plan at a depth 1km below the surface. The western boundary of the monocline is cut by a steeply dipping fault referred to as the West BAT Fault, and the eastern limb is faulted by a similar trending fault, referred to as the East BAT Fault.

Since 2002 year we have worked in cooperation with Ivanhoe Mines company at Oyu-Tolgoi project and have performed several ordered works as petrography, mineragraphy and XRD from Oyu-Tolgoi Au-Cu porphyry deposit. During this period we have analyzed over 500 samples and selected photomicrographs to emphasize perhaps the most critical aspects of rock type, alteration mineralogy and mineralization. But from Hugo deposit were logged several drill holes and from these OTD 514I drill hole was logged and sampled in detail. From these samples were prepared polished thin sections, thin sections and polished sections for petrography (514I-section figure). First module of my study is petrography of main rock units, i.e., intrusives, volcanic wall rocks, etc. These samples have been studied using petrographic and mineragraphic methods, supplemented by confirmation of the principal mineralogy by X-ray diffraction methods. During this study (petrography, mineragraphy and XRD analyses) have tried to identify all rock types, mineral component and origin of advanced argillic alteration relating to the copper-gold mineralization of the deposit.