粤北大宝山铜多金属矿区赋矿地层时代及对矿床成因的指示

Depositional age and provenance of ore-hosting strata in the Dabaoshan copper polymetallic deposit,northern Guangdong Province:Implication on ore genesis

广东大宝山铜多金属矿床是中国著名的大型矿床之一,主要由斑岩型-矽卡岩型钼–钨矿和层状—似层状铜–铅–锌多金属矿组成。矿区内层状—似层状多金属矿的成因一直存在古生代喷流沉积成因或属于侏罗纪斑岩-矽卡岩成矿系统之争,关键问题之一是赋矿地层的时代及与层状火山岩的关系未得到很好的约束。通过对赋矿地层碎屑锆石进行LA-ICP-MS U-Pb测年和Hf同位素研究表明,2件样品的碎屑锆石最小年龄分别为430±8 Ma和431±4 Ma,2件层状火山岩底板碎屑岩中最年轻且谐和的锆石年龄分别为239±3 Ma和189±3 Ma,将矿区层状—似层状矿体的赋矿地层、层状火山岩底板砂岩的沉积时代分别限定在中泥盆世和早侏罗世。碎屑源区主要为中—新元古代(1200~750 Ma)和早古生代(440~420 Ma)的物质,并有少量古元古代(2500~1600 Ma)甚至泛非期(650~520 Ma)物质来源,后者还有少量晚古生代—早中生代(340~210 Ma)物质记录,它们是华夏地块早期微陆块聚合演化、扬子和华夏地块拼合及其显生宙演化过程的响应。Hf同位素特征暗示这些物质主要源自再循环的古老地壳物质及元古宙新生地壳。综合分析认为,矿区赋矿的泥盆纪地层不整合覆盖在志留纪层状火山岩之上,后被推覆在侏罗纪碎屑岩之上,层状—似层状矿体先经历了泥盆纪海底喷流沉积成矿作用,后受到燕山期岩浆热液叠加成矿。

The Dabaoshan deposit is a large-sized polymetallic deposit in northern Guangdong province,South China,and consists mainly of porphyry-and skarn-type Mo–W mineralization and adjacent stratiform Cu–Pb–Zn mineralization.It has been debated for a long time whether the stratiform Cu–Pb–Zn mineralization is related to a Jurassic porphyry Cu–Mo system,or to a Devonian exhalative event with Jurassic overprinting.One of the key issues is that the depositional age of the ore-hosting strata and petrogenetic relationship to the layered volcanic rocks were not well constrained.In this paper,we conducted LA-ICP-MS zircon U-Pb dating and Hf isotope study for the ore-hosting strata in the Dabaoshan deposit.The results show that the youngest ages of detrital zircon from two ore-hosting strata samples yield 430±8 Ma and 431±4 Ma,whereas those from two footwall rocks samples of the layered volcanic rocks yield 239±3 Ma and 189±3 Ma,respectively.It suggests that the depositional age of the ore-hosting strata and footwall rocks of the layered volcanic rocks belongs to the Middle Devonian and the Early Jurassic,respectively.They were mainly derived from Neoto Meso-proterozoic(1200~750 Ma) and Paleozoic(440~420 Ma) crust with minor input of Paleo-proterozoic(2500~1600 Ma) and Pan-African(650~520 Ma) material.In addition,the footwall rocks include the Late Paleozoic to the Early Mesozoic detrital zircons(340~210 Ma).The Hf isotopic signature suggests that these materials were mainly originated from recycled ancient crust and the Proterozoic juvenile crust.All above mentioned were the response to the early micro-continental evolution of the Cathaysia Block,amalgamation between the Yangtze Craton and the Cathaysia Block and their Phanerozoic evolution.The layered volcanic rocks were unconformable overlaid by the ore-hosting Devonian rocks,and they were both thrusting on the Jurassic clastic rocks.The stratiform Cu-Pb-Zn orebodies were suffering from the Devonian sedimentary exhalative,then overprinted by the Yanshanian magmatic-hydrothermal activity.