Skarn Zonation of the Giant Jiama Cu-Mo-Au Deposit in Southern Tibet, SW China

Abstract

Abstract Jiama is a giant skarn-porphyry deposit in southern Tibet, southwestern China. It is the largest Cu (~7.4 Mt), second largest Au (~208 t), and fourth largest Mo (~0.62 Mt) skarn deposit in China. In addition, Jiama also contains considerable amounts of Zn, Pb, and Ag. The mineralization is related to a Miocene (~15 Ma) monzogranite porphyry that intruded the Upper Jurassic Duodigou Formation limestone (now mainly marble) and the overlying Lower Cretaceous Linbuzong Formation sandstone, siltstone, and shale (now mainly hornfels with minor slate). Porphyry-type orebodies occur in the intrusion and in the surrounding hornfels, whereas ~50- to 100-m-wide skarn-type orebodies wrap around the intrusion along the intrusion-marble contact, and as a ~50- to 150-m-thick stratabound blanket along the hornfels-marble boundary away from the contact. The stratabound skarns extend continuously for up to ~2 km on one side of the intrusion until the hornfels-marble boundary is exposed at the surface. Jiama is well zoned with respect to skarn minerals, metals, and stable isotope compositions, both laterally and vertically. Laterally, the proximal skarn is dominated by garnet, whereas the stratabound skarn contains progressively more wollastonite away from the intrusion center. The garnet composition changes from mixed Al and Fe rich (Ad21-79Gr20-77) in the endoskarn, to Fe rich in the proximal exoskarn (Ad70 to pure andradite), and Al rich (Gr69-88) in the stratabound skarn distal to the intrusion. Pyroxene is Mg rich (mostly Di66-97) in the deposit and is slightly more Fe rich in the distal zones. The garnet/pyroxene ratio remains high throughout the skarn. Vertically in the stratabound skarn, garnet is the dominant skarn mineral closer to the hornfels at shallower positions, whereas closer to the marble at deeper positions, there is more wollastonite, up to a wollastonite-dominant zone at the skarn-marble contact. The color of the garnet near the upper hornfels is dark red-brown, and gradually turns yellowish green downward. In terms of metals, the Mo-rich domains are dominantly within the intrusion, the Cu-Au-rich domains are in exoskarns near the intrusion, and the Zn-Pb-Ag mineralization occurs mostly in the distal zones. The Cu ore mineralogy changes vertically in the stratabound skarns, from chalcopyrite rich in the upper garnet zone, to bornite rich in the lower wollastonite zone. Values of δ34SV-CDT for chalcopyrite generally decrease from >–1‰ within the intrusion to –6‰ ~2 km away from the intrusion. The C and O isotope compositions of the marble samples show generally increasing trends downward in the stratabound part of the skarn, indicating that the fluid channel was closer to the hornfels. The lowest marble δ18OV-SMOW values (mostly <14%) occur close to the intrusion, consistent with the fluids being derived from the magmas. The organic carbon content decreases at shallower parts of the stratabound skarns and closer to the intrusion (from 0.412 to 0.003 wt %), indicating loss of organic carbon nearer to the fluid channel and source. These spatial zonation patterns are due to a combination of several factors, among which the oxidized intrusion-oxidized carbonate wall-rock environment likely played the most critical role. Other factors include the temperature gradient from the intrusion center outward, the compositions of protoliths (intrusion, hornfels and carbonate), and the changing fluid compositions caused by fluid-rock reaction and mineral precipitation during fluid flow. The development of the skarn zonation patterns at Jiama and the understanding of their controlling factors are helpful to mineral exploration in the Jiama area and elsewhere.