镁铁质岩浆周期性补给对云南普朗斑岩Cu-Au矿床的制约:能量约束下热力学模拟

doi:10.19657/j.geoscience.1000-8527.2024.093

摘要/Abstract

摘要：

镁铁质岩浆周期性补给于硅酸质岩浆房是形成大型斑岩矿床的关键因素。本文以普朗超大型斑岩Cu-Au矿床为例,通过能量约束体系下的热力学方法模拟浅部硅酸质岩浆房中镁铁质岩浆周期性补给过程,定量评估该过程对形成大型斑岩矿床的控制作用。普朗矿床成矿前粗粒石英闪长玢岩(CQD)和成矿期石英二长斑岩(QMP)复式岩体中均普遍发育镁铁质暗色微粒包体(MMEs),岩相学特征显示斑岩体中角闪石和黑云母发育韵律环带结构或港湾状溶蚀结构以及针柱状磷灰石的存在均指示发生了镁铁质岩浆混合作用。与单一的分离结晶模型(FC)相比,多阶段岩浆补给-分离结晶模型(R₃FC)显示,镁铁质岩浆的补给一方面会抑制长石的结晶,另一方面会促进钙铁镁和铁镁等多类型角闪石的形成,并大幅提前黑云母的结晶次序。以硅酸质岩浆物质摩尔分数变化与挥发分之间相关性为参照,获得的熔体H₂O、SCSS(硫化物饱和时硅酸盐熔体中的S含量)和Cl溶解度显示,镁铁质岩浆补给将在岩浆演化早期提高而在晚期降低残余熔体H₂O含量(0.16%、0.04%、-0.30%),持续提高熔体SCSS(78.74×10^-6、94.44×10^-6和137.88×10^-6)和Cl溶解度(0.04%、0.10%和0.20%),但对Cu含量影响有限。结果表明,能量约束体系下的R₃FC和FC热力学模型不仅能够合理解释普朗复式斑岩体矿物结构特征,也定量验证了镁铁质岩浆的补给对成矿岩体异常高H₂O、S和Cl含量的贡献。

关键词: 岩浆混合, 热力学模拟, 挥发分, 普朗斑岩Cu-Au矿床

Abstract:

Periodic replenishment of mafic magma into silicic magma chambers is a critical factor in the formation of large-scale porphyry deposits.Using the Pulang giant porphyry copper-gold deposit as an example, this study aims to quantitatively assess the significance of the formation of process porphyry deposits by constructing an energy-constrained thermodynamic model.Mafic microgranular enclaves (MMEs)are widely developed in both the pre-ore coarse-grained quartz diorite porphyry (CQD)and the syn-ore quartz monzonite porphyry (QMP)in the Pulang deposit.Petrological characteristics with rhythmic zoning and resorption textures of amphibole and biotite, and the presence of elongated apatife, both indicate the replenishment of mafic magma.Compared to the fractional crystallization model (FC), the multi-stage replenishment-fractional model (R₃FC) indicates that the replenishment of mafic magma would suppress the crystallization of feldspar, and promote the formation of multiple types of amphiboles and significantly advance the crystallization time of magmatic biotite.Using the correlation between the molar fraction variations of siliceous magma materials and volatiles as a reference, the results indicate that the replenishment of mafic magma would increase the H₂O content of the residual melt in the early stages and decrease it in the late stage of magma evolution (0.16%, 0.04%, and -0.30%).It will continuously increase the SCSS (sulfur concentration at sulfide saturation in silicate melt; 78.74×10^-6, 94.44×10^-6 and 137.88×10^-6), and Cl solubility (0.04%, 0.10% and 0.20%), but the effect on Cu solubility is limited.The results indicated that the R₃FC and FC models under the energy-constrained system can not only explain the petrological characteristics of the Pulang porphyry intrusions but also quantitatively verify the contribution of mafic magma replenishment to the abnormally high H₂O, S, and Cl contents in the ore-forming rocks.

Key words: magma mixing, thermodynamic modeling, volatile, Pulang porphyry Cu-Au deposit

中图分类号:

张少颖, 和文言, 肖仪武. 镁铁质岩浆周期性补给对云南普朗斑岩Cu-Au矿床的制约:能量约束下热力学模拟[J]. 现代地质, 2024, 38(04): 922-933.

ZHANG Shaoying, HE Wenyan, XIAO Yiwu. Constraints from Periodic Replenishment of Mafic Magma on Porphyry Mineralization in the Pulang Porphyry Cu-Au Deposit, Yunnan Province: Energy-constrained Thermodynamic Modeling[J]. Geoscience, 2024, 38(04): 922-933.

图/表 7

图1 区域大地构造位置(a)、义敦岛弧构造地质简图(b)及中甸地区地质简图(c)(据文献[36]修改)

Fig.1 Geological map of regional tectonic position (a),Yidun island arc (b) and Zhongdian area (c)(modified after reference [36])

图2 普朗铜矿矿区地质图(据文献[23]修改)

Fig.2 Geological map of the Pulang porphyry deposit (modified after reference [23])

图3 普朗铜矿主要岩石单元手标本及显微矿物特征(引自文献[39]) Amp.角闪石;Bt.黑云母;Pl.斜长石;Kfs.钾长石;Py.黄铁矿

Fig.3 Hand specimens and microscopic mineral characteristics of main rock units in the Pulang copper deposit (from reference [39])

图4 普朗复式斑岩体中代表岩浆混合过程的矿物结构特征 (a)(b)CQD中具有正反韵律环带的角闪石;(c)CQD中细长针柱状磷灰石;(d)(e)QMP中具有溶蚀结构的角闪石,内部充填有黑云母、斜长石、榍石和黄铁矿等;(f)QMP中斜长石斑晶的韵律环带;(g)MMEs中细长柱状角闪石;(h)MMEs中具港湾状边缘的片状和细长片状黑云母;(i)MMEs中细长针柱状磷灰石;Amp.角闪石;Bt.黑云母;Fsp.长石;Qtz.石英;Ap.磷灰石;Py.黄铁矿

Fig.4 Mineral texture representing the magma mixing process in the Pulang porphyry intrusions

表1 模拟计算所需硅酸质岩浆和镁铁质岩浆组成

Table 1 Composition of silicate magma and mafic magma required for simulation calculations

组成	硅酸质岩浆端元(%)	镁铁质岩浆端元(%)
SiO₂	63.30	59.65
TiO₂	0.52	0.56
Al₂O₃	15.38	13.50
Fe₂O₃	1.18	1.61
FeO	3.55	4.42
MnO	0.07	0.06
MgO	2.81	5.25
CaO	3.99	4.66
Na₂O	3.06	2.72
K₂O	4.56	4.82
P₂O₅	0.33	0.77
H₂O	1.23	1.97

图5 分离结晶模型FC(a)和多阶段补给-分离结晶模型R3FC(b)的熔体挥发分和矿物组合变化(1 kbar=100 MPa) Fsp-1.长石1(K0.04Na0.48Ca0.48Al1.48Si2.52O8);Fsp-2.长石2(K0.65Na0.32Ca0.04Al1.04Si2.96O8);Amp-1.角闪石1(Ca0.30Fe2.96Mg3.73Si8O22(OH)2);Amp-2.角闪石2(Ca1.57Fe2.58Mg2.86Si8O22(OH)2);Bt.黑云母;Ap.磷灰石;Qtz.石英;Rt.金红石

Fig.5 Changes of melt volatiles and mineral assemblages under the fractional model (a) and multi-stage replenishment-fractional model (b)

图6 普朗铜矿镁铁质岩浆周期性补给过程及斑晶矿物特征 Fsp-1.长石1(K0.04Na0.48Ca0.48Al1.48Si2.52O8);Fsp-2.长石2(K0.65Na0.32Ca0.04Al1.04Si2.96O8);Amp-1.角闪石1(Ca0.30Fe2.96Mg3.73Si8O22(OH)2);Amp-2.角闪石2(Ca1.57Fe2.58Mg2.86Si8O22(OH)2);Bt.黑云母;Ap.磷灰石;Act.阳起石

Fig.6 Periodic recharge process of mafic magma and characteristics of phenocryst minerals in Pulang copper deposit

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