干旱区生态环境知识资源中心

The South Urn Mongul prospect is occupied by Tonian porphyritic dacite (773 +/- 6.9 Ma) intruded by Ediacaran post-collisional hornblende gabbro (603 +/- 3.5 Ma) and monzogranite (558 +/- 4.6 Ma). The dacite porphyry was formed in a continental arc and hosts a weakly mineralized porphyry system. The earliest veinlet stockwork contains mainly chalcopyrite and molybdenite, which is associated with biotite-rich potassic alteration. The sulfide -bearing system evolved to intermediate sulfidation veins and hosts chalcopyrite and tetrahedrite with minor sphalerite and cobaltite, while the alteration evolved to sericitic-argillic assemblages.

A younger iron oxide -rich hydrothermal system is associated with the monzogranite overprinting the previous sulfide -bearing one. The iron oxide -rich system evolved from quartz -magnetite veins with potassic selvages to quartz-specularite and barite-specularite veins associated with more pervasive sericite-chlorite alteration. The sulfur isotopic signature of the barite indicates mainly a magmatic-hydrothermal origin with a limited involvement of meteoric water. The sulfur source may have been mafic melts accompanying the monzogranite, while the Ba was most likely leached from the host rocks, especially the varieties that were affected by potassic alteration. The magnetite hosts chalcopyrite inclusions. The monzogranite was probably derived from melting of an amphibolitic lower crust related to the formation of the earlier Tonian porphyry copper -hosted dacite porphyry. The earlier Tonian melting might have caused the low sulfide contents in the hydrous lower crust, which became fertile for Au -rich magmas during later remelting process, which caused the formation of monzogranite. This model is substantiated by the close association between the native gold and the supergene oxidation products of the chalcopyrite inclusions in the magnetite rather than the oxidation assemblages of the sulfides of the earlier Tonian porphyry system in the prospect.

Supergene oxidation occurred under near -neutral to mildly alkaline conditions due to the scarcity of pyrite in the prospect. It is manifested by the formation of atacamite and chrysocolla in association with goethite and widespread incipient argillic alteration, which resulted in plagioclase alteration to clay minerals. Only the sericitic alteration, which is related to the early porphyry copper systems, contains jarosite, alunite and svanbergite, which indicate more acidic conditions during the formation of such argillic alteration. This reflects pyrite oxidation which is related to either supergene processes or hypogene processes accompanying the younger iron oxide -rich hydrothermal system.