东天山地区多头山铁铜矿床磁铁矿化学成分及其对成矿流体演化的指示

多头山矿床位于阿齐山-雅满苏成矿带西段，是东天山地区海相火山岩型铁铜矿床的代表，但目前缺乏对其矿石矿物的直接研究.磁铁矿是一种常见的矿石矿物，其化学成分可以用于指示成矿演化过程.在详细划分磁铁矿形成期次的基础上，对东天山地区的多头山矿床展开磁铁矿化学成分研究.结果表明按照磁铁矿的生成顺序和共生矿物组合的不同，多头山铁铜矿床中的磁铁矿从早期到晚期可以划分为M_1a、M_1b和M₂型.其中，M_1a型磁铁矿为粒状结构，与绿帘石-角闪石-黄铁矿共生；M_1b型磁铁矿也为粒状结构，与石英-绿帘石-角闪石-黄铁矿共生；M₂型磁铁矿则呈长条状产出，与角闪石共生.这3类磁铁矿都有较低含量的Ti（84×10-6~1 117×10-6）、Al（417×10-6~5 273×10-6）和高场强元素，属于热液型磁铁矿.与M₂型磁铁矿相比，前两类磁铁矿具有较高含量的Si、Ca、Al和Mn，可能受到微细包体的影响.从M_1a型到M₂型磁铁矿，Ti含量呈现逐渐降低的趋势，可能与结晶温度逐渐降低有关；V和Cr含量表现出先升高后降低的变化规律，暗示成矿流体的氧逸度先降低后升高.综合考虑区域地质特征及M₂型磁铁矿更加富Mg，表明有一定比例的海水参与到多头山矿床中磁铁矿形成的晚期阶段. 

Mineral Chemistry of Magnetite from the Duotoushan Deposit in the Eastern Tianshan: Constraints on the Evolution of Ore-Forming Fluids

The Duotoushan Fe-Cu deposit is a typical submarine volcanic rock-hosted iron oxide deposit and situated in the western margin of the Aqishan-Yamansu belt, eastern Tianshan. Nevertheless, studies on its ore minerals are absent. As a common mineral in many types of hydrothermal deposits, mineral chemistry of magnetite can reveal the ore-forming processes in mineralization systems. In this paper, we present integrated study on paragenesis and mineral chemistry data of magnetite at the Duotoushan deposit, eastern Tianshan. Based on the mineral paragenesis and mineral assemblages, it is found that there are three representative magnetite types in the Duotoushan deposit. The granular M_1a type magnetite grains coexist with epidote, amphibole and pyrite, whereas the M_1b magnetite is intergrown with quartz, epidote, amphibole and pyrite. The branch shape magnetite grains of M₂ type coexist with amphibole only. Due to depletion in Ti (84×10-6-1 117×10-6), Al (417×10-6-5 273×10-6) and high field-strength elements, all the magnetite grains are identified as a hydrothermal origin. Compared with the M₂ magnetite, M_1a and M_1b magnetite are enriched in Si, Ca, Al and Mn, which can be attributed to the influence of micro-scale inclusions. The gradually reduced concentrations of titanium in the magnetite samples from M_1a to M₂ may be attributed to the decreasing crystallization temperatures. In addition, the variable compositions of vanadium and chromium suggest oxygen fugacity decreased first and then increased during the fluids evolution. Given that M₂ magnetite contains higher magnesium contents and geological constrains, we propose that seawater may have contributed to the hydrothermal system during late magnetite mineralization stage.