Constraints from Periodic Replenishment of Mafic Magma on Porphyry Mineralization in the Pulang Porphyry Cu-Au Deposit, Yunnan Province: Energy-constrained Thermodynamic Modeling

doi:10.19657/j.geoscience.1000-8527.2024.093

Abstract

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

CLC Number:

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.

Figures/Tables 7

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

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

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

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

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

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

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

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