金川岩体母岩浆成分及其分离结晶过程的熔浆热力学模拟

Parental Magma Compositions of the Jinchuan Intrusion,Gansu Province and MELTS Thermodynamic Modelling of Fractional Crystallization

金川铜镍硫化物矿床是仅次于加拿大Sudbury和俄罗斯Noril’sk-Talnakh的世界第三大在采镍矿床。金川岩体主要由含二辉橄榄岩、二辉橄榄岩、斜长二辉橄榄岩和橄榄辉石岩组成,岩相学观察表明主要造岩矿物的结晶顺序为:橄榄石→斜方辉石→单斜辉石→斜长石。为了进一步探讨金川岩体母岩浆成分及其分离结晶过程,本文在前人工作基础上根据主要造岩矿物结晶顺序及其电子探针成分,借助熔浆热力学软件"MELTS"的计算,获得金川岩体更为准确的母岩浆成分为:SiO248.2%,TiO21.00%,Al2O311.3%,FeO12.9%,Fe2O31.30%,MgO12.6%,CaO10.1%,Na2O1.51%,K2O0.72%,NiO0.04%。MELTS模拟计算表明金川岩体母岩浆的分离结晶经历了两个阶段,在深度约为10.9～12.5km的深部岩浆房经历了约5%的橄榄石以及约4%的斜方辉石分离结晶,并伴随硫化物熔离。在重力作用的影响下,橄榄石、斜方辉石和硫化物向下沉降,形成由下至上的分层:橄榄石-斜方辉石-硫化物-硅酸盐熔浆层,橄榄石-斜方辉石-硅酸盐熔浆层和硅酸盐熔浆层。硅酸盐熔浆首先挤出形成贫硫化物的岩体或喷出地表,之后橄榄石-斜方辉石-硅酸盐熔浆被挤入到7.6～9.2km的浅部岩浆房,经重力分异形成金川I号岩体的上部岩相带和II号岩体顶部的含辉橄榄岩。橄榄石-斜方辉石-硫化物-硅酸盐熔浆层最后被挤入金川岩体,并再次结晶出橄榄石、单斜辉石、斜长石,形成金川岩体的主要岩相和硫化物矿体。这些计算结果不仅与野外和室内岩相学观察吻合,也与硫化物熔离过程的最新研究结果相一致。

The Jinchuan intrusion consists of pyroxene dunite, lherzolite, plagioelase lherzolite and olivine pyroxenite and hosts the third largest magmatic Ni-Cu-（PGE） sulfide deposit in the world. Petrography indicates that the crystallization sequence of the main rock-forming minerals of the Jinchuan intrusion was olivine → orthopyroxene →clinopyroxene → plagioclase. Based on rock textures and compositions of the dominant rock-forming minerals, the more precise composition of the parental magma of the Jinchuan intrusion have been obtained by using the thermodynamic code ＂MELTS＂. 48.2% SiO2, 1.00% TiO2, 11. 3% Al2O3, 12.9% FeO2 1.30% Fe2O3, 12.6% MgO, 10.1% CaO, 1.51% Na2O, 0.72% K2O, 0.04% NiO. The thermodynamic simulation also indicated that the parental magma experienced two stages of fractional crystallization. In the first stage, crystallization of about 5% olivine and 4% orthopyroxene as well as sulfide liquid immiscibility occurred in a magma chamber at a depth of 10.9~12.5km （pressure is 3.3~ 3.8kbar）. Stratigraphic zones from the base to the top of the deep staging magma chamber were formed due to gravity segregation, including olivine-orthopyroxene-sulfide-silicate liquid mush, olivine- orthopyroxene-silicate liquid mush, silicate magma. The silicate magma was squeezed out firstly to form some sulfide poor mafic intrusions or erupted to the surface elsewhere. The olivine-orthopyroxene-silicate liquid mush was squeezed to the shallower magma chamber at a depth of about 7.6~9.2km and formed the upper sequences of the Segment I and the upper pyroxene dunite in the Segment II of the Jinchuan intrusion. The olivine-orthopyroxene-sulfide-silicate liquid mush lastly intruded into the shallow magma chamber, where further crystallization of olivine, clinopyroxene, and plagioclase occurred and formed the main rocks and sulfide ore bodies of the Jinchuan intrusion. These results are consistent with the petrology and petrography of the Jinchuan intrusion, and also are consistent with the results of recent studies on the formation of the Ni-Cu sulfides.