Overprinting mineralization in the Paleozoic Yandong porphyry copper deposit, Eastern Tianshan, NW China-Evidence from geology, fluid inclusions and geochronology

摘要

The Yandong porphyry copper deposit is located in the southern part of the Dananhu-Tousuquan Island Arc belt in Eastern Tianshan (Xinjiang, NW China). The Dananhu-Tousuquan belt contains a number of large and medium Cu deposits, among which the Yandong deposit (similar to 2.0 Mt Cu @ 0.67%) is the largest. The Yandong Cu deposit is hosted by plagiogranite porphyry and the Carboniferous Qi'eshan Group mafic to intermediate calc-alkaline volcanic and pyroclastic rocks. According to the crosscutting relationships and mineral assemblages, the alteration and mineralization processes at Yandong comprise three episodes: the porphyry mineralization episode (including potassic, propylitic, early phyllic and late phyllic stages), the overprinting mineralization episode (including main- and post-mineralization stages) and lastly supergene alteration episode. Fluid inclusions (FIs) in quartz from the potassic, early late phyllic and post-mineralization stages, and anhydrite from the main-/post-mineralization stages include four types: 1) vapor-liquid two phases (W-type), 2) daughter mineral-bearing (S-type), 3) pure liquid (PW-type), and 4) pure vapor (PV-type). The potassic stage contains W-, S- and PV-type FIs, which yielded homogenization temperatures of 304-445 degrees C (mainly 340-400 degrees C) and salinities of 3.1-16.4 (mainly 4.0-10.0) wt% NaCl eqv. At the early phyllic stage, three types of FIs are identified, i.e., the W-, S-, and PV-type. The S-type and W-type FIs homogenized at 213-441 degrees C (mainly 280-300 degrees C) and 241-352 degrees C (mainly 300-320 degrees C), respectively, with corresponding salinities of 33.1-52.0 (mainly 34.0-40.0) wt% NaCl eqv. and 0.9-15.9 (mainly 0.9-4.0) wt% NaCl eqv., respectively. The late phyllic stage contains W-, S- and PW-type FIs, which homogenized at 146-248 degrees C (mainly 160-180 degrees C), with salinities ranging from 3.9 to 18.2 (mainly 4.0-8.0) wt% NaCl eqv.