The Baiyinnuoer deposit (32.74 Mt ore with grades of 5.44% Zn, 2.02% Pb and 31.36 g t -1 Ag), the largest Zn-Pb-Ag deposit in northern China, is hosted by crystalline limestone and slate of the Early Permian Huanggangliang Formation. Detailed cross-section mapping indicates stratigraphic and fold structural controls on the mineralization. The Zn-Pb-Ag mineralization is hosted predominantly by skarn, which occurs as bedding-parallel lens that pinch out at the margins of the main economic zone. Three skarn stages are identified at the deposit: (i) garnet-clinopyroxene; (ii) sulfides; and (iii) carbonate-epidote. Lead isotopic compositions were determined for galena and sphalerite of the ores, whole rock samples of the Yanshanian granite and granodiorite, Permian marble and tuff, and Jurassic volcanic and subvolcanic rocks in and around the Baiyinnuoer area in order to discuss the sources of ore-forming materials and the relationship between the ore formation and these whole rocks. Galena Pb,). These results indicate that the mineralization was not related to the Jurassic volcanism, but to the Yanshanian magmatism. The Permian strata may have a slight contribution to the mineralization. All features show that the Baiyinnuoer deposit is related to the Yanshanian granitic magmatism, and can be classified as a zinc-lead-silver skarn deposit.
A B S T R A C TAn integrated geological and geophysical investigation was carried out in the near-mine areas of the Chaihulanzi gold-field, Inner Mongolia, northern China, to determine the south-eastern strike extension of the principal mineralized shear system, in the search for new resources. In this case study, surface geophysical surveys played an important role in defining the mineralized structures beneath cover. A re-investigation of the mine geology by observing the underground exposures of mineralization and its hosting structures, coupled with re-evaluation and re-examination of previous exploration data, revealed that the mineralization style at the Chaihulanzi gold-field is not the skarn-type as previously assumed, but is structurally controlled by an oblique-sinistral shear structure. Moreover, a southeastward shallow-plunging mineralized corridor within the principal mineralized shear system was also identified, implying that the ore-forming fluid-flow might move through the mineralizing system, from the south-east at depth to the north-west. These new geological findings imply that the mineralized shear system should extend much farther south-eastwards along strike beneath cover, and thus a conceptual target area was proposed. A detailed geophysical survey program, involving the application of VLF-EM, Stratagem EH4, CSAMT and gradient IP measurements, was carried out to test the validity of the conceptual target. Orientation surveys on the geologically known Line-18 traverse revealed that VLF-EM and Stratagem EH4 were the most effective methods of detecting the unseen mineralized system, whereas CSAMT could only provide low-resolution data, and IP proved to be unsuitable in this environment due to the ubiquitous presence of graphite-bearing schists in and around the mineralized system. A follow-up systematic VLF-EM survey highlighted the principal mineralized shear system as a linear conductive belt, tracing the principal mineralized system for a further 750 m of strike length beneath cover from the previous exploration limit (Line 18). Stratagem EH4 soundings over six parallel traverses perpendicular to the mineralized trend revealed that the principal mineralized shear structure extends for more than 500 m in the dip direction, a strong indication of the presence of potential deep mineralization under the surveyed area. Detailed modelling of the Stratagem EH4 sounding images *
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