Mineral chemistry and apatite Sr isotope signatures record overprinting mineralization in the Duobaoshan porphyry Cu deposit, Heilongjiang Province, NE China

Abstract

Porphyry copper deposits can be deformed by subsequent magmatic-tectonic-metamorphic evolution, resulting in the remobilization of pre-existing ores and the consequent overprinting mineralization. This process might potentially increase the ore grade and/or resource. We focus on the Duobaoshan porphyry Cu (Mo) deposit, two periods were identified in this study, i.e., porphyry period (including Stage 1: potassic alteration; 2: propylitic alteration, and 3: chlorite-sericite alteration and porphyry mineralization) and overprinting period (including Stage 4–1: ductile–brittle deformation; Stage 4–2 and Stage 4–3: overprinting mineralization). Metal sulfide veins occur along the schist in Stage 4–2 and Stage 4–3, features that differ from the typical porphyry mineralization of Stage 3 (veinlets, stockworks, and dissemination). Here, we present in situ Sr isotope data for apatite and compositions of chlorite, epidote, and apatite to reveal the source and nature of fluids and mechanism of precipitation during overprinting mineralization. The in situ 87Sr/86Sr ratios of apatite from Stages 3, which range from 0.7030 to 0.7046, are slightly higher than those observed in the Ordovician granodiorite. Meanwhile, the Sr isotope ratios for apatite from Stages 4–2 and 4–3 are 07033–0.7040 and 0.7013–0.7045 respectively. These ratios broadly consistent with those of the surrounding Ordovician granodiorite and basaltic andesite, suggesting that the ore-forming fluids for all these stages originated from magmatic sources. Chlorite in Stage 4–2 has a higher Mn, Co, Ni, Zn content compared to chlorite in Stage 3, 4–3, whereas epidote and apatite in Stage 4–2 have higher Ca, Sr, Pb content compared to those in Stage 3, 4–3, implying that the fluids in Stage 4–2 underwent strongly water–rock reactions and extracted metal elements from surrounding rocks. Copper was only detected in Stage 4–3 chlorite (15.13–500.20 ppm) and epidote (7.91–19.29 ppm), indicating a high concentration of copper in the fluid, and due to the pre-existing magnetite dissolution, chlorite, epidote, and apatite in Stage 4–3 exhibit high V and Fe relative to other stages. Fluid mixing likely occurred during Stage 3, leading to the precipitation of copper. Strong water-reaction in Stage 4–2 produce changes in fluid natures that facilitate sulfide deposition during this stage. While in Stage 4–3, copper precipitation is controlled by a combination of rising pH and changing sulfur content and valence. Our study highlights that in the Duobaoshan deposit, late magmatic-hydrothermal fluids remobilized pre-existing ore bodies, and the copper precipitation mechanisms differed during the main copper mineralization events.