Qihai Shu, Jun Deng, Zhaoshan Chang, Qingfei Wang, Xudong Niu, Kai Xing, Xiang Sun, Zhongkun Zhang, Qingwen Zeng, Hesen Zhao, Fan Yu
{"title":"Skarn Zonation of the Giant Jiama Cu-Mo-Au Deposit in Southern Tibet, SW China","authors":"Qihai Shu, Jun Deng, Zhaoshan Chang, Qingfei Wang, Xudong Niu, Kai Xing, Xiang Sun, Zhongkun Zhang, Qingwen Zeng, Hesen Zhao, Fan Yu","doi":"10.5382/econgeo.5038","DOIUrl":null,"url":null,"abstract":"\n 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.\n 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.\n 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.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":null,"pages":null},"PeriodicalIF":5.5000,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Economic Geology","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.5382/econgeo.5038","RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
引用次数: 0
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.
期刊介绍:
The journal, now published semi-quarterly, was first published in 1905 by the Economic Geology Publishing Company (PUBCO), a not-for-profit company established for the purpose of publishing a periodical devoted to economic geology. On the founding of SEG in 1920, a cooperative arrangement between PUBCO and SEG made the journal the official organ of the Society, and PUBCO agreed to carry the Society''s name on the front cover under the heading "Bulletin of the Society of Economic Geologists". PUBCO and SEG continued to operate as cooperating but separate entities until 2001, when the Board of Directors of PUBCO and the Council of SEG, by unanimous consent, approved a formal agreement of merger. The former activities of the PUBCO Board of Directors are now carried out by a Publications Board, a new self-governing unit within SEG.