广东河台金矿方解石地球化学特征与地质意义

Geochemical characteristics and geological significances of calcite in the Hetai gold deposit, Guangdong Province, China

广东河台金矿田位于钦州湾—杭州湾(钦—杭)结合带南段的云开地区,是华南地区规模最大的金产地之一。矿区内已发现的金矿床都产于韧性剪切带中,被认为是典型的剪切带型金矿。方解石是河台金矿常见的脉石矿物之一,矿相学观察发现矿石中矿物具有明显的分带特征,硫化物向外依次为方解石、绢云母和石英。为了阐明成矿热液的来源及演化过程,本文对河台金矿田2类方解石(矿石中方解石和成矿后节理裂隙中方解石)和围岩糜棱岩进行了微量元素和Sr-Nd同位素分析。结果显示,方解石相对于PAAS的REE配分曲线都呈明显的左倾特征,而糜棱岩则呈右倾,暗示流体并非来自围岩。矿石中的方解石ΣLREE/ΣHREE>1,具有明显的Eu正异常(δEu_(SN)为1.32~1.88),Y/Ho值为32.08~40.33,(^(87)Sr/^(86)Sr)_(i)初始比值为0.72593~0.73293,ε_(Nd(t))为-14.90~-12.40,具有中-古元古代变质基底特征。因此,推测成矿流体主要为燕山期重熔型花岗岩浆流体(伍村岩体)且明显混入浅表部大气降水,而与围岩之间的水-岩反应相对有限。随着深度增加,矿石中方解石δEu_(SN)及REE含量均明显增高,即深度越大成矿环境越偏还原;而后期无矿节理裂隙中方解石具明显的δEuSN负异常(0.53和0.58),且Sr、Ba、Y和Yb含量明显高于矿石中的方解石,可能是在相对开放的氧化环境下结晶形成的。

The Hetai goldfield in Guangdong Province,located in the Yunkai area of the southern section of the QinzhouHangzhou Bay(Qin-Hang)junction zone,is one of the largest gold mining districts in South China.In the orefield,the gold deposits hosted in ductile shear zones were believed as the typical shear zone type gold deposits.Calcite is one of the common gangue minerals in the goldfield.Mineralography observation shows that the minerals in ores are distributed with characteristics of remarkable zonation varying from sulfides to calcite,sericite,and quartz in outward sequence.In order to clarify the origin and evolution process of ore-forming hydrothermal fluids,trace elements and Sr-Nd isotopic analysis of two types of calcite(calcite in ore and calcite in post-ore-stage barren joints and fissures)and mylonite of the wallrock in the Hetai goldfield have been carried out in this paper.The results show that the PAAS-normalized rare-earth elements patterns of calcite and mylonite samples are characterized by left inclined and right inclined REE patterns,respectively,suggesting that the ore-forming fluid was not derived from the wallrock.The calcites in ores have∑LREE/∑HREE>1,distinct positive Eu anomaly(δEu_(SN)=1.32-1.88),Y/Ho ratios of 32.08~40.33,initial(^(87)Sr/^(86)Sr)_(i)ratios of 0.72593-0.73293,and ε_(Nd(t))values varying from-14.90 to-12.40,showing the characteristics of Meso-Paleoproterozoic metamorphic basement.Therefore,it is inferred that the ore-forming fluid was likely a mixing fluid of the major magmatic fluid,derived from Yanshanian granitic magma(Wucun pluton)formed by the remelting of Meso-Paleoproterozoic metamorphic basement,with minor shallow surface meteoric water.However,the water-rock reaction between the fluid and wallrocks is relatively limited.TheδEuSNvalues and REE contents of calcites from ores are obviously increased with the increase of sampling depth,indicating that the greater the depth,the more reductive the mineralization environment.By contrast,the calcites in late-stage barren joints and fissures have negative Eu anomalies(with δEuSNvalues of 0.53 and0.58),and obviously higher contents of Sr,Ba,Y and Yb than the calcites in ores,indicating that they were probably formed in a relatively open oxidizing environment.