Precambrian metamorphic crustal basement cannot provide much gold to form giant gold deposits in the Jiaodong Peninsula, China

Highlights

• Crustal xenoliths trapped by Guojialing granodiorites display rather low Au contents.

• The Precambrian metamorphic outcrop rocks also show low Au contents.

• The metal contents are consistent with the metamorphic devolatilization model.

• The Precambrian basement was not the main Au source for Jiaodong giant gold deposits.

Abstract

The Jiaodong gold deposits, eastern China define a giant gold province in the Precambrian high-grade metamorphic crustal basement. Given the early metamorphism, the crustal basement would have been so depleted in gold that cannot be the main source for the later Mesozoic gold deposits. However, previously reported gold contents from crustal rocks were variably high (e.g., >5–10 ppb) and often interpreted to reflect the contribution of crustal basement. In this study, after careful field sampling, we obtained high-precision Au and S contents (with Cu and Ag for many samples) to solve the controversial issue. The Au contents were determined by internal standardization to platinum which was obtained from isotope dilution via high-sensitivity Element XR, and/or another independent method by GFAAS after polyurethane foam pretreatment. These methods achieved low procedural blanks (<0.02 ppb) and reliable analyses of low-Au samples. The analyzed metamorphic crustal rocks are from surface outcrops (n = 30) and crustal xenoliths (n = 10) in the Jiaobei terrane, where hosts the predominant Jiaodong gold resources (>85% of 5000 t). The Archean and Paleoproterozoic metamorphic outcrop rocks were sampled far from gold deposits and faults with the minimum effect of ore-forming fluids, and they display overall low Au contents (0.20 ± 0.18 ppb and 0.47 ± 0.29 ppb, respectively). More importantly, the crustal xenoliths trapped in pre-mineralization Guojialing granodiorites were not affected by hydrothermal overprint and better reflect the pristine features of the metamorphic basement. Although the crustal xenoliths contain variable sulfides with a wide range of Cu, Ag and S contents, they show rather low Au contents (0.18 ± 0.15 ppb). These results indicate the strong depletion of Au contents in the Precambrian crustal basement, consistent with the metamorphic devolatilization model. The Precambrian metamorphic crustal basement thus was rather difficult as the main gold source for the Jiaodong gold deposits, suggesting alternative sources.

Introduction

The gold province in the Jiaodong Peninsula, eastern North China Craton (NCC) is renowned worldwide for its large resource of >5000 t gold. Much effort has been made to understand the sources of such large-scale gold endowment (e.g., Deng et al., 2020b, Feng et al., 2020, Goldfarb and Santosh, 2014, Groves and Santosh, 2016, Li et al., 2015). The Jiaodong gold deposits formed in a short peak period of around 120 ± 5 Ma and occur in the Precambrian crustal basement which underwent high-grade metamorphism of amphibolite-facies to granulite-facies at 1.9–1.8 Ga (e.g., Deng et al., 2020a, Feng et al., 2020, Liu et al., 2013, Yang and Zhou, 2001, Zhai and Santosh, 2013, Zhang et al., 2020, Zhu et al., 2015). The early-stage metamorphism would have removed much gold and other mobile elements (e.g., Goldfarb and Santosh, 2014, Patten et al., 2020, Pitcairn et al., 2015a, Pitcairn et al., 2015b, Zhong et al., 2015). Theoretically, the Jiaodong Precambrian crustal basement thus should have been so depleted in gold and other ore-forming components that cannot be the main source for the later Mesozoic giant gold deposits (Goldfarb and Groves, 2015, Goldfarb and Santosh, 2014, Zhai et al., 2004). Therefore, recent models increasingly emphasize the key contribution from other reservoirs such as the metasomatized lithosphere mantle (Deng et al., 2020c, Goldfarb and Santosh, 2014, Groves et al., 2020, Li and Santosh, 2014, Wang et al., 2020).

However, for rocks from the Precambrian crustal basement, the gold contents reported in 1980s–1990s overall showed high values of a few ppb and even up to tens of ppb (Fig. 1). Besides, radiogenic Pb isotopes of gold ores generally overlap those of the Precambrian metamorphic rocks (Yang et al., 2014a). The Precambrian crustal basement thus was often inferred as the main gold source, that is, the remobilization of gold from the Jiaodong Precambrian crustal basement for the Mesozoic gold deposits (e.g., Yang et al., 2014a, Zhai and Santosh, 2013). Recently, Chen et al (2019) reported much lower gold contents with a mean of 0.8 ppb for drill cores of the Precambrian crustal rocks, but still followed the model of scavenging gold from Precambrian crustal basement for gold mineralization. Such low gold contents were interpreted to be the residual results after strong extraction from the presumably gold-rich crustal basement to form the gold deposits (Chen et al., 2019). This model is difficult to reconcile with the well-constrained mobile behavior of gold during high-grade metamorphism (e.g., Goldfarb and Santosh, 2014, Patten et al., 2020, Pitcairn et al., 2014, Pitcairn et al., 2006, Wang et al., 2019, Zhong et al., 2015).

The previously reported gold contents of Precambrian crustal basement show a large range which could be ascribed to several reasons: 1) the variable uncertainty of employed analytical methods at different times; 2) the overprint by ore-forming hydrothermal fluids that led to gold enrichment for analyzed rocks (Chen et al., 2019, Taylor et al., 1994); and 3) the heterogenous distribution of gold in crust rocks. Therefore, it is very necessary to apply high-precision analytical methods for well-characterized samples, particularly those without hydrothermal overprint, to better constrain the exact role of Precambrian metamorphic crustal basement for the giant gold deposits. If the high-grade Precambrian metamorphic crustal basement was confirmed to show high gold contents for giant gold mineralization, it would be crucial to revisit the classic orogenic gold deposit model (Goldfarb et al., 2001, Groves et al., 1998). If not, alternative sources would be the main contributor.

In this study, we use high-precision methods to systematically analyze the contents of Au, S and other relevant elements on the typical rock types of the Precambrian metamorphic crustal basement in the Jiaobei terrane. The studied samples include both surface outcrops and crustal xenoliths. In particular, the crustal xenoliths were not overprinted by ore-forming hydrothermal fluids and thus better reflect the pre-gold mineralization, pristine features of the metamorphic crustal rocks. Based on careful field sampling and petrological observation, we will display the strong depletion of gold contents in the Precambrian metamorphic crustal rocks and discuss its limited capability to provide gold for the Jiaodong gold deposits.