Geochronology and geochemistry of the Aobaoshan crystalline graphite ore cluster in the Dunhuang Block, Gansu, NW China

IF 3.6 2区地球科学 Q1 GEOLOGY

Ore Geology Reviews Pub Date : 2025-08-19 DOI:10.1016/j.oregeorev.2025.106840

Qingyan Tang , Zhuoming Li , Hong Song , Chi Zhao , Pengfei Di , Tianyu Qin , Yan Zhang , Tengda Yang , Min Qiao , Wei Liu

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Abstract

The Dunhuang block is located in the southwest of the Central Asian Orogenic Belt (CAOB). Within this region, the Aobaoshan crystalline graphite ore cluster (ACGOC) occurs in the southern part of the Dunhuang block. These graphite deposits are hosted by two-mica quartz schist of Dunhuang group. We present the petrography, major and trace elements, zircon U-Pb ages, and carbon isotopes for the mineralized two-mica quartz schist from the ACGOC. Zircon U-Pb age dating reveals that the ages of 2442 ± 15 Ma and 2462 ± 20 Ma are interpreted as the maximum depositional ages of the sedimentary protoliths, while those of 1756 ± 10 Ma and 1821 ± 20 Ma reflect high-grade metamorphism in the Aobaoshan (ABS) and Hongliuxia (HLX) crystalline graphite deposits. A Silurian-Devonian-magmatic tectonic-thermal event at 407 ± 11 Ma is recorded in the Daaobaogou (DABG) deposit. Collectively, these results suggest at least two major tectonic-thermal events in the Dunhuang block. The whole-rock samples are enriched in silicon, and exhibit low total rare-earth element abundances (∑REE = 50.9–164.9 ppm). Their (La/Yb)_N ratios vary widely (0.48–7.35), accompanied by consistently negative Eu anomalies (δEu = 0.55–1.19). Primitive mantle-normalized trace element patterns reveal obvious depletions in Nb, Ta, Zr, and Hf, coupled with enrichments in Rb, Ba, and Sr, indicating a marine sedimentary environment for the ACGOC rocks. The restoration of metamorphic protoliths indicates that the protoliths of metamorphic rocks in the study area are mainly shale, clay rock, and greywacke, which is resulted from the shallow water environment with weak hydrodynamic transport conditions, likely associated with island arc and active continental margin. The identified paleosalinity type of paleo-water body was fresh-brackish water. The crystalline graphite yields δ¹³C_V-PDB in the range of –21.8 ‰ to –25.6 ‰, consistent with a predominantly biogenic organic matter. Our findings indicate that the formation of crystalline graphite deposits in the ACGOC was controlled by at least two tectonic-thermal events, including Paleoproterozoic high-grade metamorphism (1821 ± 20 Ma and 1756 ± 10 Ma) and a Silurian-Devonian magmatic-tectonic-thermal event (407 ± 11 Ma).

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甘肃敦煌地块敖宝山结晶石墨矿团的年代学及地球化学特征

敦煌地块位于中亚造山带西南部。在该区域内，敖宝山结晶石墨矿团（ACGOC）赋存于敦煌地块南部。这些石墨矿床赋存于敦煌群的二云母石英片岩中。本文介绍了ACGOC矿化双云母石英片岩的岩石学、主微量元素、锆石U-Pb年龄和碳同位素。锆石U-Pb年龄测定结果表明，沉积原岩的最大沉积年龄为2442±15 Ma和2462±20 Ma，而1756±10 Ma和1821±20 Ma反映了鳌宝山（ABS）和红柳峡（HLX）结晶石墨矿床的高变质作用。道包沟（DABG）矿床记录了407±11 Ma的志留系—泥盆系—岩浆构造—热事件。总的来说，这些结果表明敦煌地块至少有两个主要的构造-热事件。整体岩石样品富含硅，稀土元素总丰度较低（∑REE = 50.9 ~ 164.9 ppm）。它们的（La/Yb）N比值变化很大（0.48 ~ 7.35），且Eu呈负异常（δEu = 0.55 ~ 1.19）。原始地幔归一化微量元素模式显示Nb、Ta、Zr和Hf明显缺失，Rb、Ba和Sr富集，表明ACGOC岩石为海相沉积环境。变质原岩恢复表明，研究区变质岩原岩主要为页岩、粘土岩和灰岩，形成于水动力输运条件弱的浅水环境，可能与岛弧和活动大陆边缘有关。确定的古水体盐度类型为淡淡咸水。结晶石墨δ13CV-PDB值在-21.8‰~ -25.6‰之间，主要为生物成因有机质。研究结果表明，ACGOC中结晶石墨矿床的形成受古元古代高变质作用（1821±20 Ma和1756±10 Ma）和志留纪-泥盆纪岩浆-构造-热事件（407±11 Ma）至少两次构造-热事件控制。

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来源期刊

Ore Geology Reviews 地学-地质学

CiteScore

6.50

自引率

27.30%

发文量

546

审稿时长

22.9 weeks

期刊介绍： Ore Geology Reviews aims to familiarize all earth scientists with recent advances in a number of interconnected disciplines related to the study of, and search for, ore deposits. The reviews range from brief to longer contributions, but the journal preferentially publishes manuscripts that fill the niche between the commonly shorter journal articles and the comprehensive book coverages, and thus has a special appeal to many authors and readers.