广东大宝山铜矿英安斑岩的同位素组成与蚀变特征及其找矿意义

Isotopic Composition and Alteration Characteristics of Dacite Porphyry, and Their Prospecting Significance in the Dabaoshan Copper Deposit of Guangdong Province

铜矿床类型以斑岩型和矽卡岩型为主,英安斑岩是斑岩型铜钼金成矿系统中普遍发育的岩石类型,当发育良好的热液蚀变时即可作为找矿标志和成矿岩体。广东大宝山是华南目前最大的铜多金属矿,前人研究表明该矿区英安斑岩既是铜多金属矿的成矿母岩,又是有利的赋矿围岩,但关于该矿区英安斑岩与成矿的关系目前存在多种认识,随着近年来英安斑岩中厚大矿体的找矿突破,英安斑岩与成矿的关系再度成为关注的焦点。本文在野外工作和室内分析的基础上,采用X射线荧光光谱法(XRF)和电感耦合等离子体质谱法(ICP-MS)等技术测定大宝山英安斑岩样品中的铜、钨、硅、铝和钙等主次痕量元素的含量,重点研究了该矿区英安斑岩的同位素、金属元素组成与蚀变特征,探讨蚀变与找矿方向的关系。结果表明:①铜元素含量变化于107~6909μg/g之间,与矿区采场铜矿石品位0.3%~0.5%一致,烧失量(LOI)等可代表蚀变程度的地球化学指标与铜存在正相关性;②热液蚀变在英安斑岩中广泛发育,由早到晚依次为钾硅酸盐化蚀变、青磐岩化蚀变和绢英岩化蚀变;③各种蚀变中,绢英岩化蚀变中的铜含量要高出其他蚀变带2~5倍,表明绢英岩化热液蚀变与铜矿化关系密切,是重要的找矿标志;④英安斑岩深部(270m标高以下)蚀变强度没有减弱,仍然具有很大的找矿潜力;此外,英安斑岩与侏罗系接触带以及岩体中的围岩捕掳体也是重要的找矿方向,均可能赋存有矽卡岩型的富铜矿。

BACKGROUND:The main types of copper deposits are porphyry and skarn types.Dacite porphyry is a rock type commonly developed in the porphyry Cu-Mo-Au system.The rock can be used as a marker clue for ore prospecting and ore-forming rock when there is well-developed hydrothermal alteration.The Dabaoshan deposit is currently the largest copper polymetallic deposit in the South China Block.Previous studies have shown that the dacite porphyry of the mining area is not only the ore-forming parent rock,but also a favorable ore-bearing wall rock.However,there are some debates about the relationship between dacite porphyry and mineralization in this mining area.With the breakthrough of prospecting for medium and large dacite porphyry ore bodies in recent years,the relationship between dacite porphyry and mineralization has once again become a concern.OBJECTIVES:To understand the relationship between dacite porphyry alteration and mineralization of the Dabaoshan deposit.METHODS:Based on field work and indoor analysis,X-ray fluorescence spectroscopy(XRF)and inductively coupled plasma-mass spectrometry(ICP-MS)were used to determine copper,tungsten,silicon,aluminum and calcium in Dabaoshan dacite porphyry samples.The isotope composition,metal element contents and alteration characteristics of the dacite porphyry in the mining area were used to discuss the relationship between alteration and prospecting direction.RESULTS:The copper content ranged from 107 to 6909μg/g,which was consistent with the copper ore grade of 0.3%-0.5%in the mining area.The LOI and other geochemical indicators that can represent the degree of alteration were positively correlated with copper.Hydrothermal alteration was widely developed in dacite porphyry,including potassium,prophyliticition and quartz-sericitization from early to late stage.Among different stages of alterations,the copper content of quartz-sericitization was 2 to 5 times higher than other alteration types,indicating that quartz-sericitization was closely related to copper mineralization and was an important prospecting marker clue.CONCLUSIONS:The deep alteration intensity of the dacite porphyry(below the 270m elevation)has not been weakened,suggesting a great prospecting potential.In addition,the contact zone between dacite porphyry and the Jurassic strata,and the xenoliths in the dacite porphyry are also important prospecting directions,which may host skarn-type copper-rich deposits.