热带地区红土型镍矿风化壳元素迁移富集规律研究——以菲律宾南部苏里高Pili镍矿为例

Migration and enrichment of elements in the nickel laterite weathering crust of the tropical region:A case study of the Pili Nickel deposit in the Surigao district,southern Philippines

位于东南亚热带地区的菲律宾和印度尼西亚是世界上主要的红土型镍矿产区。在菲律宾南部的苏里高(Surigao)地区广泛分布红土型镍矿床,矿床类型属于含水镁硅酸盐型。笔者等主要通过对菲律宾苏里高地区Pili红土型镍矿详细的野外地质调查,结合红土风化壳剖面的矿物学组成及地球化学特征来探讨热带地区红土风化壳的分层特征及元素迁移富集规律。根据矿物组成特征将Pili红土型镍矿风化壳剖面自上而下分为四层:黏土层、红土层、腐岩层以及基岩。其中腐岩层镍含量可达1%以上,是主要的镍含矿层。红土剖面地球化学特征显示红土层呈现富Fe、Mn贫Si、Mg的特征。上部黏土层相对红土层反而出现富Al、Si、Cr、Ni贫Fe、Mn的特征。区别于常见的红土型镍矿,Pili镍矿红土层反而呈现贫镍的特征。剖面元素迁移率特征显示,黏土层经历了强烈的Si、Mg、Fe、Mn流失,红土层经历了Mg、Si等元素的强烈流失以及Fe、Mn等元素中等程度的流失。下层腐岩层中经历了Fe、Mn、Cr等元素的富集以及镍的强烈富集。由此我们提出三阶段过程来解释Pili红土型镍矿的元素迁移和富集:腐岩发育阶段、红土发育阶段以及晚期溶解再沉淀阶段。腐岩初始发育阶段未发生明显的元素迁移流失。红土初始发育阶段受地表流体的影响发生了强烈的Mg、Si等元素的流失。红土初始形成以后,受热带地区强降水作用影响,后期还原性地表流体不断淋滤使其上部还经历了强烈的Fe、Mn流失而形成黏土层。中部的红土层也相应发生了强烈的镍流失和中等程度的Fe、Mn流失。该阶段强烈的元素迁移可能是热带地区红土型镍矿风化壳所特有的特征。流体淋滤携带的镍逐渐由红土层向下迁移,并以含镍镁硅酸盐的形式最终富集在腐岩层中。本研究的发现对该区红土型镍矿找矿勘查工作具有重要指示意义。

Objectives:The Philippines and Indonesia in tropical Southeast Asia are the world’ s major producers of laterite nickel deposits.Laterite nickel deposits are widely distributed in the Surigao district of the southern Philippines and are generally hydrous magnesium-silicate type.In this study,we discuss the zonation characteristics and element migration-enrichment of the laterite nickel weathering crust in the tropical region.Methods:We studied the detailed field geology of the Pili laterite nickel deposit in the Surigao district,Philippines,combining mineralogical compositions and geochemical characteristics of the laterite nickel weathering crust profile.Results:According to the mineral compositions,the weathering crust profile of Pili laterite nickel ore is divided into four layers from top to bottom:clay layer,laterite layer,saprolite layer,and bedrock.Saprolite is the main Ni ore-bearing layer of the deposit,which contains about 1 wt% nickel.Geochemical characteristics of the laterite profile show that the laterite is rich in Fe and Mn,and poor in Si and Mg.The upper clay layer is rich in Al,Si,Cr,and Ni,and poor in Fe and Mn compared with the laterite layer.Different from common laterite nickel deposits,the laterite of the Pili nickel deposit is quite Ni-poor.The element migration ratios in the profile show that the clay layer has experienced strong losses of Si,Mg,Fe,and Mn,and the red soil has experienced strong losses of Mg and Si and a moderate loss of Fe and Mn.The lower saprolite layer experienced the enrichment of Fe,Mn,Cr,and Ni.Conclusions:Based on the above findings,we propose three stages of processes to explain the element migration and enrichment of Pili laterite nickel deposits:the saprolite development stage,laterite development stage,and late dissolution and precipitation stage.There was no obvious element migration or loss during the initial development stage of saprolite.At the initial development stage of laterite,the loss of Mg,Si,and other elements occurred under the influence of surface fluids.After the initial formation of laterite,the upper part of laterite also experienced strong losses of Fe and Mn and formed the clay layer due to the continuous leaching of reducing surface fluids,which was probably affected by heavy rainfalls in the tropical region.In the middle laterite layer,there were strong Ni loss and moderate Fe and Mn losses.The intense element migration at this stage may be the specific characteristic of nickel laterite weathering crust in the tropical region.The fluid-leached nickel gradually migrates down from the laterite layer following the fluid and eventually accumulates in the saprolite in the form of nickelbearing magnesium silicate.The findings of this study provide significant indications for the exploration of laterite nickel deposits in the district.