关键词: 泥鳅河组, 锆石U-Pb定年, Hf同位素, 沉积时代, 物源区, 二道坎银矿床, 大兴安岭北段

Abstract: The Erdaokan silver deposit in Heilongjiang Province, located in the northern segment of the Great Khingan Range, is an independent hydrothermal vein-type silver deposit. Orebodies are mainly hosted in two members of the Devonian Niqiuhe Formation: The sedimentary member and the pyroclastic member. The wall rocks have undergone sericite-quartz alteration and silicification alteration via hydrothermal processes, forming two corresponding types of altered rocks. Taking the sericite-quartz altered rock and pyritized silicified altered rock representing the two members as research objects, this paper conducts systematic analyses of detrital zircon morphology, U-Pb geochronology, trace elements and Hf isotopes to investigate their provenance properties. The results indicate that zircons in the sericite-quartz altered rock from the lower sedimentary member are dominated by euhedral magmatic zircons. Their U-Pb age peaks are concentrated in the Early Devonian (397-395 Ma), with a small amount of Mesoproterozoic zircons (1 276 Ma). Zircons in the pyritized silicified altered rock from the upper pyroclastic member yield concordant U-Pb ages in the Late Devonian (369-358 Ma), with a few Paleoproterozoic and Mesoproterozoic zircons (1 924-1 593 Ma). With the exception of individual detrital zircons in the sericite-quartz altered rock, most detrital zircons exhibit trace element patterns characterized by HREE enrichment and LREE depletion, accompanied by pronounced positive Ce anomalies and distinct negative Eu anomalies, indicative of typical magmatic zircons. The Early Devonian zircons from the sericite-quartz altered rock have ε_Hf(t) values of 10.5-14.5, with two-stage Hf model ages ranging from 713 Ma to 469 Ma. The Late Devonian zircons from the pyritized silicified altered rock have ε_Hf(t) values of -23.3-7.4, with two-stage Hf model ages ranging from 2 811 Ma to 889 Ma. These characteristics suggest that the provenance of the lower sedimentary member is primarily intermediate-mafic volcanic rocks formed by the eruption of weakly enriched mantle-derived magmas in the Early Devonian, mixed with minor Mesoproterozoic strata. The provenance of the upper pyroclastic member is mainly derived from the reworking of ancient continental crust, as well as deep-seated intermediate-felsic intrusive rocks formed by the mixing of ancient crust and mantle-derived materials, with the addition of minor Paleoproterozoic to Mesoproterozoic strata. Integrating previous studies and trace element characteristics of detrital zircons, this study concludes that the Early Devonian sedimentary area was in a post-collisional extensional tectonic setting, while the Late Devonian sedimentary area was in a subduction continental arc tectonic setting.

Key words: Niqiuhe Formation, zircon U-Pb dating, Hf isotopes, sedimentary age, provenance, Erdaokan silver deposit, Northern Great Khingan Range