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

• Graphite deposits was formed by biological organic matter transformation. • ACGOC formed in island arc/active margin settings with fresh-brackish paleo-water. • Graphite formation linked to at least two significant tectono-thermal events. 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 M a 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 δ 13 C 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).