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Trace elemental and sulfur-lead isotopic variations in metamorphosed volcanogenic massive sulfide (VMS) mineralization systems: an example from the Keketale Pb-Zn(-Ag) deposit, NW China
Ore Geology Reviews ( IF 3.3 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.oregeorev.2020.103685 Peng-Peng Yu , Yi Zheng , Cheng-Ming Wang
Ore Geology Reviews ( IF 3.3 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.oregeorev.2020.103685 Peng-Peng Yu , Yi Zheng , Cheng-Ming Wang
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Abstract The effects of post-VMS (volcanogenic massive sulfide) deformation/metamorphism on sulfide trace element and sulfur-lead isotopic compositions remain unclear. The greenschist to lower amphibolite facies metamorphosed Keketale VMS Pb-Zn(-Ag) deposit, located in the Devonian Maizi volcanic-sedimentary basin in the Chinese Altay, NW China, provides an opportunity to resolve the abovementioned issue. Two mineralization stages are recognized at Keketale, including (1) primary banded, massive and disseminated ores derived from sea-floor hydrothermal mineralization and (2) ore remobilization, as represented by intensively-deformed ores and quartz-polymetallic sulfide ore veins that crosscut the primary ores. Sulfide trace elements data suggest that Stage II pyrite and pyrrhotite have higher contents of Cu, Zn, Ag, Sb and Pb, whereas contents of these elements are higher in Stage I sphalerite and galena than their Stage II counterparts. Such elemental remobilization was probably caused by the rapid intragrain diffusion and subsequent fluid-mediated liberation with trace elements re-precipitated as particles within the same grain and nearby minerals, respectively. The δ34SCDT values in both stages show a trimodal distribution (−25.9 to −22.7‰, −17.0 to −10.6‰ and 0.6 to 2.1‰). Sulfide mineral pairs formed during primary VMS mineralization are not in sulfur isotopic equilibrium, and only localized re-equilibration occurs during deformation and metamorphism. In addition, the lead isotopic values of galena from remobilized sulfide veins overlap with those of the Stage I galena and pyrite. These data indicate that regional deformation and metamorphism of the Keketale deposit can be approximated as a closed system in terms of sulfur-lead isotopes. Whilst the presence of the anomalously low δ34S values (minimum −25.9‰) and the distinct lead isotopic ratios of sulfides from the calcite-quartz veins argue for that in the late part of the metamorphic event, some exotic lead and light sulfur were sourced from outside the ore field. In situ δ34S values of Stage I sulfides vary widely (−17.0 to +2.1‰), suggesting that the sulfur was originated from bacterial sulfate reduction (BSR) of marine sulfates with probably some magmatic input. Lead isotopes of the sulfides and mineralized meta-sedimentary-volcanic rocks display a linear distribution between the mantle and upper crust line in the 206Pb/204Pb vs. 207Pb/204Pb diagram. The lead source may be derived from both felsic volcanic rocks and mafic igneous rocks (and/or magmatic volatiles), and the sulfur-lead isotopes may result from a shallow-level seawater circulating process. An additional group of lead isotopic compositions of the meta-volcanic rocks show elevated present-day 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values, probably due to decay of Th and U carried by deeply-penetrative circulation (~13 km) of CO2-F-Cl-rich hydrothermal (metamorphic) fluids. Both syn-VMS shallow (seawater) and post-VMS deep (metamorphic fluids) circulation processes are critical for the metal concentration at Keketale.
更新日期：2020-10-01
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