Geochemical indicators of major, trace, and REE elements for the alteration lithofacies in the Oyu Tolgoi copper-gold deposit in Mongolia

Abstract

The Oyu Tolgoi world-class porphyry copper-gold deposits in Mongolian are that supergene and epithermal hydrothermal system superimposed in the early stage of porphyry system. Lithofacies of alterations, mineral and geochemical comprehensive methods were employed. Objective of this article are to investigate their formation environment of alteration rocks and mineralization in order to establish REEs and comprehensive prospecting indicators of alteration lithofacies, especially to find ore-forming center for this superimposed mineralization systems. Based on this study, REEs and geochemical prospecting indicators of alteration veins are of evident positive Eu anomaly, and are higher in concentrations of Fe₂O₃, CaO, S and TiO₂, with obvious positive U–K, Pb, P and Ti anomalies. It is very useful to catch the advanced argillic alteration lithofacies to delineate coverage of the later stage epithermal system, in which they are characterized by “U”-shape pattern normalized by chondrite as well as obviously anomalous concentrations of Cu, As, Se, Ba, Pb and Bi, with higher concentrations of K₂O, Al₂O₃ and S. Moreover, it is very valuable to discovery the intermediate argillization lithofacies to outline the mineralized center in which are typified by veinlets and brecciated hematite-magnetite silicification rock, and hematite-ferrocarbonatization rock. The mineralized center is typified by silicification altered rock in structure of veinlets-shape or the lithocap. The hydrothermal breccia zone may be an indicator of structure-lithology for the excrete channel and feeder of metallogenic fluid transportation in the epithermal system. Therefore, it is very more important that they are characterized by noticeable negative Eu anomaly (Eu*＝－0.57) and smooth-shape pattern normalized by chondrite. Copper mineralized center is implied by transitional environment of oxidation-reduction characterized by the ratios of Fe³⁺/Fe²⁺ from 0.72 to 2.13, plus Cu、As、Se、Ba、Pb and Bi anomalies, occurred lithofacies of the intermediate argillic alteration are considered to be localization markers whenever associations of Cu、V、Zn、Se and Mo anomalies may be signs for orebody finder.