Research Article: A geochemical characterization of lead ores in China: An isotope database for provenancing archaeological materials

Date Published: April 24, 2019

Publisher: Public Library of Science

Author(s): Yiu-Kang Hsu, Benjamin J. Sabatini, Qun-Ke Xia.

http://doi.org/10.1371/journal.pone.0215973

Abstract

A well reasoned lead (Pb) isotope-driven provenance study lies in concert with a comprehensively evaluated database of geological ore sources and accompanying archaeological and contextual information. In this paper we have compiled and evaluated all currently available Pb isotope data for galena and K-feldspars in China, and provided geological interpretations for how their ore-forming substances evolved across relevant tectonic terrains. We pay particular attention to the geological settings of host ore deposits that were likely exploited in ancient and historic China, detailing the heterogeneity and homogeneity of their ore formation across different metallogenic provinces and belts. Using the isotope database, and supportive geological and archaeological background information, three case studies are presented that detail the provenancing of Chinese cultural materials. The isotope data themselves are presented in ternary diagrams that allow for their concise and accurate comparison.

Partial Text

Lead (Pb) isotope geochemistry has played a vital role in tracing the fluid pathways and sources of metal in ore deposits [1–4], revealing chronological information of ore body formation [5], evaluating the economic potential of ore exploration [6,7], and in the provenancing of archaeological materials. In archaeology, the method of comparing the Pb isotopic “fingerprints” of artifacts and other materials to that of ores has been widely applied despite initial and ongoing debates regarding their application and overall validity [8–11]. Indeed, the approach has had a long and controversial history of proof of concept, setback, and eventual acceptance [12–17]. Often cited in these debates, and the basis for the usefulness of Pb isotopes in provenance studies, lies in the relative abundances of four stable isotopes (204Pb, 206Pb, 207Pb, 208Pb), of which 204Pb is primordial and has no long-lived radioactive parent. Primordial Pb can therefore be leveraged to model the elapsed time since the initial formation of a given Pb-bearing mineral. The abundance of primordial Pb remains unchanged while the concentrations of the other three increase over time due to the radioactive decay of their U and Th parent isotopes of 238U, 235U, and 232Th, respectively. The relative abundances of these four isotopes impart age information and often a unique geochemical signature in ores [18–24], of which the latter is commonly the focal point in archaeological provenance studies.

In order to make informed interpretations of Pb isotopes in provenance studies, one must consider the geochemistry and dynamic geological history of crustal formations that host ore deposits. In China, these deposits can be broadly separated into several metallogenic provinces by their tecontic ages or the metamorphism of basement rocks. The older continental plates, such as the NCC, contain the least radiogenic Pb as opposed to the relatively high radiogenesis that occurred in the younger SCB. High-grade metamorphism in the Dabie lithosphere resulted in low Th/U ratios, which are characteristic of lower crustal materials. Unique Pb isotopes usually occur along the geochemical boundaries between provinces such as the metallogenic belts in the Yan, Taihang, and Dabie mountains. The distinctiveness of these ore deposits may allow one to accurately distinguish Pb sources, however it is important to understand that significant overlap occurred between deposits that formed during the Mesozoic period under similar geological conditions. Examples of these similarities can be found in the Middle-Lower Yangtze, Qinling, and Xiaoqinling belts. As a result, some artifacts cannot be assigned to a specific source unless backed by additional scientific, archaeological, and textual evidence.

 

Source:

http://doi.org/10.1371/journal.pone.0215973

 

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