Significant boron isotopic fractionation in the magmatic evolution of Himalayan leucogranite recorded in multiple generations of tourmaline

Abstract Niobium and tantalum, rare metals and high field strength elements (HFSEs) that are essential to modern technologies, are concentrated among others in lithium-cesium-tantalum (LCT) pegmatites and rare metal granites. The most important hosts for Nb-Ta in these types of deposits are the columbite group minerals (columbite-tantalite), but at some ore deposits significant Ta is also contained in wodginite, microlite, and tapiolite. Previous solubility experiments of HFSE minerals have been limited to high temperatures because of the slow diffusivities of HFSEs in granitic melts. An experiment protocol is described herein that allows HFSE mineral solubilities to be determined at lower temperatures, more in line with the estimated solidus temperatures of LCT pegmatites and rare metal granites. This is achieved through the interaction of a melt that is enriched in high field strength elements (e.g., P and Nb or Ta) with a fluid enriched in a fluid-mobile element (FME, e. g., Mn). A starting glass enriched in a slow diffusing HFSE was synthesized, and HFSE mineral saturation is obtained via the diffusion of a FME into the melt via interaction with a fluid. This interaction can occur at much lower temperatures in reasonable experimental durations than for experiments that require diffusion of niobium and tantalum. The solubility product of columbite-(Mn) from the fluid-melt interaction experiment in a highly fluxed granitic melt at 700 °C is the same as those from dissolution and crystallization (reversal) experiments at the same P-T conditions. Thus, both methods produce reliable measurements of mineral solubility, and the differences in the metal concentrations in the quenched melts indicates that the solubility of columbite-(Mn) follows Henry’s Law. Results show that columbite-(Mn) saturation can be reached at geologically reasonable concentrations of niobium in melts and manganese in hydrothermal fluids. This experimental protocol also allows the investigation of HFSE mineral crystallization by fluid-melt interactions in rare-metal pegmatites. Magmatic origins for columbite group minerals are well constrained, but hydrothermal Nb-Ta mineralization has also been proposed for pegmatite-hosted deposits such as Tanco, Greenbushes, and granite-hosted deposits such as Cínovec/Zinnwald, Dajishan, and Yichun. This study shows that columbite-(Mn), lithiophilite, and a Ca-Ta oxide mineral (that is likely microlite) crystallized from experiments in fluid-melt systems at temperatures as low as 650 °C at 200 MPa. It is important to note that HFSE minerals that crystallize from fluid-melt interactions texturally occur as euhedral crystals as phenocrysts in glass, i.e., are purely magmatic textures. Therefore, crystallization of HFSE minerals from fluid-melt interactions in rare metal granites and pegmatite deposits may be more widespread than previously recognized. This is significant because the formation of these deposits may require magmatic-hydrothermal interaction to explain the textures present in deposits worldwide, rather than always being the result of a single melt or fluid phase.

An experimental approach to examine fluid-melt interaction and mineralization in rare-metal pegmatites


 Sulfide-rich hydrous arc cumulates in the lower crust are considered a likely source for porphyry Cu-forming magmas in post-subduction settings. However, it remains controversial whether or not post-subduction porphyry Cu magmas can be produced by partial melting of lower crustal arc cumulates. Here we performed detailed petrographic and geochemical studies on exhumed lower to middle crustal cumulates from the Gangdese and Kohistan arc sections, and then conducted partial melting experiments of the Gangdese sulfide-rich lithologies at 1.5 GPa and 800-1150 °C using a piston cylinder press. Both the Gangdese and the Kohistan arc-root samples formed as cumulates in the lower to middle crust, but the Kohistan samples were subsequently brought into the garnet stability field during the India–Eurasia continental collision. Most of the Gangdese samples contain relatively high abundances of magmatic sulfides preserved in the form of Cu-poor monosulfide solid solution (MSS). In contrast, most of the Kohistan samples contain very low abundances of Cu-rich sulfides preserved as inclusions in garnet, probably due to intense metamorphic dehydration and up to 2 wt.% partial melting in the garnet stability field. The original magmatic sulfides at Kohistan appear to have been removed mainly by metamorphic fluids rather than by partial melts, as the arc-root samples could have experienced only very low degree of partial melting and the sulfur solubility in experimentally produced partial melts is low (≤0.1 wt.%) even at oxygen fugacities two log-units above the fayalite-magnetite-quartz buffer. Furthermore, the experimental partial melts contain much less K2O, MgO, Cl and ±S than typical post-subduction porphyry Cu-forming magmas. Hence, we conclude that the role of sulfide-rich arc cumulates in the formation of post-subduction porphyry Cu magmas has been severely overestimated.

Can post-subduction porphyry Cu magmas form by partial melting of typical lower crustal amphibole-rich cumulates? Petrographic and experimental constraints from samples of the Kohistan and Gangdese arc roots

The dynamic evolutions of fluid-mineral systems driving large-scale geochemical transformations in the Earth’s crust remain poorly understood. We observed experimentally that successive sodic and potassic alterations of feldspar can occur via a single self-evolved, originally Na-only, hydrothermal fluid. At 600 °C, 2 kbar, sanidine ((K,Na)AlSi3O8) reacted rapidly with a NaCl fluid to form albite (NaAlSi3O8); over time, some of this albite was replaced by K-feldspar (KAlSi3O8), in contrast to predictions from equilibrium reaction modelling. Fluorine accelerated the process, resulting in near-complete back-replacement of albite within 1 day. These findings reveal that potassic alteration can be triggered by Na-rich fluids, indicating that pervasive sequential sodic and potassic alterations associated with mineralization in some of the world’s largest ore deposits may not necessarily reflect externally-driven changes in fluid alkali contents. Here, we show that these reactions are promoted at the micro-scale by a self-evolving, kinetically-driven process; such positive feedbacks between equilibrium and kinetic factors may be essential in driving pervasive mineral transformations. This paper reveals that potassic alteration can be triggered by Na-rich fluids, indicating that pervasive sequential sodic and potassic alterations associated with mineralization in some of the world’s largest ore deposits may not necessarily reflect externally-driven changes in fluid alkali contents.

/pdf/kinetically-driven-successive-sodic-and-potassic-alteration-10sxl9g393.pdf

Kinetically driven successive sodic and potassic alteration of feldspar.

Petrogenesis of deformed tourmaline leucogranite in the Gurla Mandhata metamorphic core complex, Southwestern Tibet

The genetic relationship between different types of granite is critical for understanding the formation and evolution of granitic magma. Fluid-rock interaction experiments between two-mica leucogranite and boron-rich fluids were carried out at 600–700°C and 200 MPa to investigate the effects of boron content in fluid and temperature on the reaction products. Our experimental results show that tourmaline granite can be produced by reactions between boron-rich fluid and two-mica granite. At 700°C, the addition of boron-rich fluid resulted in partial melting of two-mica granite and crystallization of tourmaline from the boron-rich partial melt. Increasing boron concentration in fluid promotes the melting of two-mica granite and the growth of tourmaline. No melt was produced in experiments at 600°C, in which Fe, Mg and Al released from biotite decomposition combined with boron from the fluid to form tourmaline under subsolidus conditions. The Na required for tourmaline crystallization derived from Na/K exchange between feldspar and the K released by biotite decomposition. The produced tourmaline generally has core-rim structures, indicating that the composition of melt or fluid evolved during tourmaline crystallization. Based on the experimental results, we propose that tourmaline granite veins or dikes can be formed by the reactions between boron-rich fluids, presumably produced by devolatilization of boron-bearing granitic magma, and incompletely crystallized granite at the top of the magma chamber. This “self-metasomatism” involving boron-rich fluid in the late stage of magma crystallization could be an important mechanism for the formation of tourmaline granite.

Experimental investigation of reactions between two-mica granite and boron-rich fluids: Implications for the formation of tourmaline granite

Abstract Transport and deposition of copper in the Earth’s crust are mainly controlled by the solubility of Cu-bearing phases and the speciation of Cu in magmatic-hydrothermal fluids. To improve our understanding of copper mobilization by hydrothermal fluids, we conducted an experimental study on the interaction between Cu-bearing phases (metallic copper, Cu2O, CuCl) and aqueous chloride solutions (H2O ± NaCl ± HCl; with Cl concentrations of 0 to 4.3 mol kg-1). The experiments were run in rapid heat/rapid quench cold-seal pressure vessels at 800 °C, 200 MPa, and logfO2 ~ NNO+2.3. Either Cu or Au capsules were used as containers. The reaction products were sampled in situ by the entrapment of synthetic fluid inclusions in quartz. Fluid composition was subsequently determined by analyzing individual fluid inclusions using a freezing cell and laser ablation inductively coupled plasma-mass spectrometry. Our results show that large isolated and isometric inclusions, free of late-stage modifications, can be preserved after the experiment even when using a high cooling rate of 25 K s-1. The obtained results demonstrate that: (1) reaction between native Cu, NaCl solution, and quartz (± silica gel) leads to the coexistence of fluid inclusions and Na-bearing silicate melt inclusions. Micrometer- to submicrometer-sized cuprite (Cu2O) crystals have been observed in both types of the inclusions, and they are formed most probably due to the dissociation of CuOH. (2) When Cu0 reacts with HCl and CuCl solutions, or Cu+ reacts with NaCl solution, nantokite (CuCl) formed due to oversaturation has been found in fluid inclusion. Copper concentration in the fluid shows a strong positive dependence on the initial chlorine content, with Cu/Cl molal ratios varying from 1:9 to 1:1 in case 1 and case 2, respectively. When Cl is fixed to 1.5 m, initial fluid acidity has a major control on the Cu content, i.e., 0.17 ± 0.09 and 1.29 ± 0.57 m Cu were measured in fluids of case 1 and case 2, respectively. Cu solubility in pure water and in 1.5 m NaCl solutions are 0.004 ± 0.002 and 0.16 ± 0.07 m, respectively. The main responsible Cu-bearing complexes are CuOH(H2O)x in water, NaCuCl2 in NaCl solutions and HCuCl2 in alkali-free solutions. These results provide quantitative constraints on the mobility of Cu in hydrothermal solutions and confirm that Cl is a very important ligand responsible for Cu transport. The first observation that silicate melt can be generated in the fluid-dominated and native-copper-bearing system implies that transitional thermosilicate liquids can coexist with metal-rich fluids and may enhance Cu mobility in magmatic-hydrothermal systems. This may have important implications for the formation of Cu deposits in the systems with low S activities.

Reaction between Cu-bearing minerals and hydrothermal fluids at 800 °C and 200 MPa: Constraints from synthetic fluid inclusions

The solubility of scheelite in evolved granitic magmas (Qz-Ab-Or-An system with minor FeOtotal, TiO2, and CaO added) was studied experimentally at 200 MPa, 750–850 °C and relatively oxidizing condition (logfO2 = NNO + 2.3, where NNO is Ni-NiO oxygen buffer). Water-saturated granitic melts have been equilibrated with seeds of scheelite crystals. The resulted WO3 contents in the melts vary only slightly from 0.21 to 0.32 wt.% WO3 over the investigated temperature and compositional range (0.7 to 1.4 wt.% CaO) but tends to increase with increasing temperature and decreasing CaO concentration. One important message conveyed from the study is that WO3 concentrations at scheelite saturation are more dependent on temperature in evolved Ca-poor melts than in Ca-rich melts. Natural granitic rocks associated with scheelite mineralization and associated melt inclusions hosted in quartz have much lower W contents than the experimental melts equilibrated with scheelite. This implies that enrichment of tungsten (W) at magmatic stages is not sufficient to produce significant scheelite mineralization and confirms the important role of W mobilization by magmatic-hydrothermal fluids in the formation of scheelite deposits.

Experimental evidence for a protracted enrichment of tungsten in evolving granitic melts: implications for scheelite mineralization

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Experimental investigation of reactions between two-mica granite and boron-rich fluids: Implications for the formation of tourmaline granite

Reaction between Cu-bearing minerals and hydrothermal fluids at 800 °C and 200 MPa: Constraints from synthetic fluid inclusions

Experimental evidence for a protracted enrichment of tungsten in evolving granitic melts: implications for scheelite mineralization