On prospecting for porphyry copper mineralization in intracontinental mobile zones (Mongol-Okhotsk belt, Mongolian People's Republic)

Abstract

Abstract Porphyry copper mineralization within the Early Mesozoic magmatic province of Mongolia is found in a typical intracontinental Mongol-Okhotsk mobile belt. Porphyry Cu mineralization in intracontinental mobile zones of the Mongol-Okhotsk type associated with terrains of andesitic and basalt andesitic volcanism of relatively high alkalinity surrounds internal zones of calc-alkaline magmatism of normal alkalinity. A typical magmatic environment in the ore regions includes an earlier granodiorite-granite association as well as trachybasaltic andesites-trachyandesites and porphyry associations providing a single magmatic series. The Bajan-Ula ore-magmatic system (OMS) has been explored in Central Mongolia and is interpreted as a large, weakly eroded system of stratovolcano-intrusion type. The depth at which mineralization was formed was 1.5 or 2 km and its extent exceeds 500 m. The ensemble of hydrothermal-metasomatic rocks consists of diverse propylites, biotite-quartz-albite metasomatites, quarts sericitic and quartz-tourmaline metasomatites, quartz-tourmaline breccias, almost monomineralic quartz metasomatites, and argillic rocks. The ore-magmatic system is accompanied by regional (B, Bi, As) and local (Sn, Mo, Cu, Zn, Pb, Ag, Sb, etc.) haloes of indicator elements in bedrock, soils and stream sediments, a positive magnetic anomaly in the area of a blind intrusive cupola, and an apparent electric conductivity anomaly. The patterns of anomalies and the plot of multielement geochemical associations conform with the metasomatic zoning and structural blocks in the ore field. The zoning sequence of element indicators is: (Sn, Mo, Cu) — (Ag, Pb, Zn) — (As, Bi, Sb). The predictive assessment stage should involve a comprehensive analysis of the paleomagmatic environment with a special emphasis on structures of central type, aeromagnetic and airborne radiometric surveys, structural-geological interpretation, and evidence of mineralization. A stream-sediment survey is effective as well. Prospecting of promising areas is conducted along with a complete geological (1:50,000-scale) survey followed by a geochemical survey from secondary haloes, aeromagnetic and airborne radiometric surveys. Bi, As, Ag, Pb and B anomalies show the most contrast in the case of a weakly (at a scale of 1:50,000) as well as geochemical prospecting for secondary haloes, and aeromagnetic and airborne radiometric surveys. For prospecting weakly eroded ore-magmatic systems such as that at Bajan-Ula, Bi, As, Ag, B, and Pb anomalies are especially useful. In cases of more deeply eroded ore systems, the use of Ag, Pb, Bi, B, Mo and Cu anomalies are recommended. For deeply eroded systems of significant importance are Sn, W and Co anomalies. Follow-up geochemical surveys should be conducted at a scale of 1:10,000 and accompanied by ground electrical and magnetic surveys. In all cases, if the landscape conditions permit, it is desirable to perform the petrographic-geochemical survey, providing abundant geochemical and petrological information economically.