Abstract
The Laochang Ag-Pb-Zn-Cu deposit, located in the southern margin of the Sanjiang Tethys Metallogenic Belt (STMB), is the typical Ag-Pb-Zn-Cu deposit in this region. Its orebodies are hosted in the Carboniferous Yiliu Formation volcanic-sedimentary cycle and occur as stratiform, stratoid and lenticular. Whether or not the stratabound ore belong to the volcanogenic massive sulfide (VMS) deposit remains unclear and controversial. In this paper, the whole rock geochemistry, trace elements in sphalerite, U-Pb zircon chronology and Pb isotopes were investigated, aiming to provide significant insights into the genesis and geodynamic setting of the Laochang deposit. Lead isotope ratios of pyrite and sphalerite from the stratabound ore are 18.341 to 18.915 for 206Pb/204Pb; 15.376 to 15.770 for 207Pb/204Pb; and 38.159 to 39.200 for 208Pb/204Pb—which display a steep linear trend on Pb-Pb diagrams. This indicates a binary mixing of lead components derived from leaching between the host volcanic rock and mantle reservoir. Sphalerite from stratabound ores is relatively enriched in Fe, Mn, In, Sn, and Ga—similar to typical VMS deposits. Moreover, the Carboniferous volcanic rock hosting the stratabound Ag-Pb-Zn-Cu ores has a zircon U-Pb age of 312 ± 4 Ma; together with previous geochronological and geological evidences, thus, we consider that the stratabound mineralization occur in the Late Paleozoic (~323–308 Ma). Collectively, these geologic, geochemical, and isotopic data confirm that the stratabound ores should be assigned to Carboniferous VMS mineralization. In addition, volcanic rocks hosting the stratabound ore exhibit elevated high field strength elements (HFSEs, Nb, Ta, Zr and Hf) abundance, slight enrichment of light rare earth element (LREE), and depletion of Ba and Sr with obvious Nb-Ta anomalies. Such characteristics suggest that their magma is similar to typical oceanic island basalt. In addition, the oceanic island basalt (OIB)-like volcanic rocks were formed at Late Paleozoic, which could be approximately synchronous with the VMS mineralization at Laochang. Thus, it is suggested that the Laochang VMS mineralization was generated in the oceanic island setting prior to the initial subduction of the Changning-Menglian Paleo-Tethys Ocean.
Highlights
The Sanjiang Tethys Metallogenic Belt (STMB), one of the premier polymetallic belts in China (Figure 1A), is located within the eastern Himalayan-Tibetan Orogen
We present a comparative geochronology study (U-Pb dating using zricon) for stratabound ores hosting volcanic rocks and previous studies obtained the timing of stratabound mineralization to assess their temporal relationship
This study mainly focuses on high field strength elements (HFSEs), rare earth elements (REEs), and transitional elements (e.g., V and Sc) for rock classification and petrogenetic discussion of the Laochang samples
Summary
The Sanjiang Tethys Metallogenic Belt (STMB), one of the premier polymetallic belts in China (Figure 1A), is located within the eastern Himalayan-Tibetan Orogen. Cu-Mo, Cu, and Cu-Au, orogenic Au, and Mississippi Valley-type (MVT) Pb-Zn deposits, were formed from Paleozoic to Cenozoic These different genetic types of ore deposit formation are closely associated with the tectonic evolution in the Sanjing region [1,2,3,4,5]. The Laochang Ag-Pb-Zn-Cu deposit is located at the southern part of the STMB, and has a proven reserve of 866,000 t Pb at 4.5%, 336,000 t Zn at 3.3%, 1700 t Ag at 155 g/t, 116,000 t Cu at 0.5–0.9%, 2.84 Mt Pyrite and accompanying 0.8 t Au [6,7,8]. Over the past four decades, several studies have been carried out on the Laochang
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