Constraints of magmatic differentiation on epithermal mineralization at Dongan, NE China: Insights from zircon geochronology, elements and Sr-Hf-Nd isotope geochemistry

摘要

Low-sulfidation (LS) epithermal deposits formed under relatively low temperature and pressure conditions are generally associated with volcanic activity. To better understand magmatic processes prior to epithermal mineralization, field geology, petrology, U-Pb zircon geochronology, whole-rock major, and trace element geochemistry along with Hf-Nd isotopic geochemistry of the cogenetic volcanic rocks at Dongan were assessed. The Early Cretaceous (109.8 to 107.0 Ma) volcanic suite consists of high-K calc-alkaline andesitic-rhyolitic tuff and granite porphyry dikes or/and stocks. Arc magma affinity characterizes the suite with enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depletions in high field intensity elements (HFSEs) and heavy REEs (HREEs). The extrusive andesitic rocks showing enriched mantle features (epsilon Nd (t) = + 0.76 to +1.83, epsilon Hf = 3.4 to +3.7, T-DM2 = 933 to 1124 Ma) are the result of the hybridization between the mantle and lower crust melts with the contamination of upper crustal materials during eruption or ascending of melts. The intrusive granite porphyry (epsilon Nd(t) = +3.81 to +3.9, epsilon Hf =-0.3 to +3.7, T-DM2 = 901 to 1187 Ma) is geochemically comparable with andesitic rocks and was likely formed by the fractional crystallization of plagioclase and amphibole minerals from cooling andesitic melts at middle-upper crustal levels. Note that the rhyolitic magmas show distinct features of lower epsilon Nd(t) (-3.05 to - 2.78), epsilon Hf (-0.6 to +1.6) and total REE values (54.78 to 70.09 ppm), older T-DM2 (1464 to 1662 Ma) and obvious negative europium anomalies (0.34 to 0.37), suggesting an origin of upper crustal remelting (compositionally equivalent to water-poor felsic granulite or tonalite) that results from the heating of the underlying andesitic magmas. In summary, the sequential emplacement of the ore-related andesitic and granitic melts is attributed to the magmatic processes of the active magma chamber in a continental extensional setting, and the metal-rich bubbles focused at the tops of this chamber due to prolonged differentiation are released to produce epithermal veins at shallower levels (< 1.5 km).