A short explanation why optical petrography knowledge is the key to preliminary identify hydrothermal magmatic processes

November 28, 2023

by Paula Montoya-Lopera, Postdoctoral Research Fellow in Economic Geology at CODES, University of Tasmania

Ore deposits are developed on specific magmato-tectonic environments, on this subject exploration geologists should identify features that help to elucidate those specific zones. This short article describes the importance of understanding textural and mineralogical associations as a key exploration tool.

Most of the hydrothermal processes start in magmatic chambers at different depths, so ‘How could we identify those processes by field or optical petrography inspection?’. Well, to answer this question let’s start with the ‘beginning’, when mafic magmas cool down slowly, crystallizing and solidifying anhydrous crystals such as calcic plagioclases, pyroxenes or olivines (Bowen 1956) (Fig. 1A, B, C). Consequently, as crystallization progresses, high-temperature magmas dehydrate the host rock through metamorphic processes, liberating volatiles such as water which is increasingly concentrated in the residual melt. Then, when crystallization progresses to some point, the residual melt is saturated with water, silica, cations and anions changing the magma initial chemistry (Fig. 1D to J). The supersaturated magma by further crystallization results in vesiculation, bubble formation and magma fractionation (Toramaru 2022). Such vesiculation triggered by the cooling crystallization of melt is known as secondary boiling (Melnik and Sparks 1999; Toramaru 2022). At this point, supersaturated magmas are developing (Fig. 1D to I).

Figure 1: A Ophitic texture;  B, C Subophitic textures;  D Myrmekite texture;  E, F Graphic texture;  G, H Hypogene chlorite in matrix in andesitic porphyry;  I Crowded sphene in andesitic porphyry;  J, K Sieve textures in micas;  L Sieve textures in feldspar
M–P Reabsorption texture in quartz;  O, P Silica recrystallization around broken reabsorbed quartz;  Q–U Porphyritic intrusive textures;  Q–R Anhedral quartz; 
T Glomerophyric texture in plagioclases; U Broken quartz in phaneritic textures in plutonic rock
The development of each texture is based on the thermodynamic and chemical conditions of the magma, e.g. subophitic textures indicate high temperature-pressure and Ca excess, meanwhile ophitic is contrary to the latter; myrmekite and graphic texture are associated to silica saturation in the system, same as crowded or higher amounts of sphene or zircons; sieve, reabsorb and broken crystals textures imply chamber decompression, however those textures should be analyzed in detail with other microtextures in the same sample to obtain an accurate description, as those also could be interpreted as magma mixing; hypogene, basic hydrous aluminum-silicate minerals in matrix indicate hotter magma saturation.
Photo A and B source: www.alexstrekeisen.it; C to U: by the author at Sinaloa and Durango State, Sierra Madre Occidental, Mexico). Microphotographs are in Cross Polarized Light (XPL).

In the above example for magmas located in the deep crust, the temperature inevitably decreases due to e.g., host rock interaction, and water gets gradually concentrated in the melt because of crystallization. But ‘What would have happened if the magma started ascending?’. So, continuing with the latter magma example, which is phasing secondary boiling, bubbles were formed somewhere in the magma chamber, because bubble formation causes volume expansion; the magma chamber is about to expand and exerts a force on the surrounding (Melnik and Sparks 1999; Toramaru 2022). However, the magma chamber cannot easily expand because it is surrounded by rock. Both cooling and crystallization unavoidably progress and magma vesiculation and bubble formation inevitably proceed. In this manner, the pressure of the magma chamber is highly increased, and at a point (the critical state; Wilson et al. 1980; Toramaru 2022) where the surrounding rock cannot withstand the pressure with all its strength, the surrounding rock cracks, and the magma flows out through the crack (Toramaru 2022). The magma that allows to somewhat increase its volume is decompressed (i.e., Solubility decreases and a degree of supersaturation increases; Fig. 1J to P) and increasingly vesiculates, leading to the increase in the fractionation of a gas phase (Melnik and Sparks 1999; Toramaru 2022). The fractionated and vesiculated magma decreases its density, expands the crack, and ascends towards the surface by buoyancy at an accelerated pace (Fig. 2A to D). In this way, second boiling can be considered as an overpressure that not only explains the dyke formation (Melnik and Sparks 1999; Toramaru 2022) but also triggers eruptions (Bowen 1956).

Figure 2:  A Panoramic basalt dykes hosted in granodiorite;  B Basalt dykes hosted in granodiorite;  C Basalt dykes hosted in lava sequence;  D Dacite porphyry with crowded and anhedral pyroxenes overimposed;  E Diorite xenoliths in granite;  F Classic sinuous quartz veinlets in andesitic porphyry rock. Source: taken by the author at Sinaloa and Durango State, Sierra Madre Occidental, Mexico

So, let’s complicate the processes a little bit more. As it is known, most of the ore deposits are associated with continental subduction zones; in that area magma chambers can recharge it by crust assimilation (e.g., thin or thick crust), multiples magmatic chambers, asthenosphere, upper mantle, oceanic slab dehydration or even from continental sediments as a part of accretion wedge (Fig. 3), and other. If the magmatic chamber is recharged by hotter magma (e.g., evidenced by mafic xenoliths; Fig. 2G to P), and supersaturated with water, silica (e.g., assimilation of asthenosphere, slab dehydration, continental sediments; Fig. 1D to U, Fig. 2F), it would reach the second boiling phase faster (no matter the depth of the chamber, commonly at shallow depths, Fig. 1Q to U). In addition, if the chamber goes through faster devolatilization processes releasing the volatiles richer in water, silica, Mg, Fe, Ca, cations and anions (e.g. hotter hydrothermal fluids, Fig. 1D to I), the dehydrated magma will be represented by porphyritic felsic hypabyssal dykes with euhedral pyroxenes overimposed to the general texture (Fig. 2D; Montoya-Lopera et al. 2019, 2020), even hydrothermal veins will be formed following the same dyke structural controls (Montoya-Lopera et al. 2020).

Figure 3 – Schematic of a subduction zone. Source: Frisch et al. (2022).

On the other hand, if the saturated magma preserves the volatiles (e.g., water, silica, Mg, Fe, Ca, cations and anions) and crystallizes faster in structural traps, a most common product will be made, e.g., porphyry type deposits.

So, ‘Is there evidence that supersaturated fluids, secondary boiling occurred in a magma chamber, remain in crystalized rock or even eruptive materials?’. Well, the answer is yes, the most common evidence of those processes is rock, mineral texture, microtextures and mineral assemblage. As was cited through the above paragraphs, there are some key textures that help us to identify evidence of specific elementary processes that a geologist may take into account in order to create their own hypothesis, which should be supported later with more advanced experimental laboratory tests. It is highly important to observe those textures by thinking that something happened there when a magma chamber crystallized.


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Melnik, O., and Sparks, R.S.J. (1999). Nonlinear dynamics of lava dome extrusion. Nature. V 402 : 37-41

Montoya-Lopera, P., Ferrari, L., Levresse, G., Abdullin, F., Mata, L. (2019). New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico. Ore Geology Reviews. https://doi.org/10.1016/j.oregeorev.2018.12.020

Montoya-Lopera, P., Levresse, G., Ferrari, L., Hernández, G., Orozco, T., Abdullin, F., Mata, L. (2020). New geological, geochronological and geochemical characterization of the San Dimas mineral system: Evidence for a telescoped Eocene-Oligocene Ag/Au deposit in the Sierra Madre Occidental, Mexico. Ore Geology Reviews. https://doi.org/10.1016/j.oregeorev.2019.103195

Neukirchen, F. (2022). The formation of mountains. Springer. https://doi.org/10.1007/978-3-031-11385-7

Toramaru, A. (2022). Vesiculation and crystallization pf magma, Fundamentals of the volcanic eruption process. Springer. https://doi.org/10.1007/978-981-16-4209-8

Wilson, L., Sparks, R.S.J., Walter, G.P.L. (1980). Explosive volcanic eruptions – IV. The control of magma properties and conduit geometry on eruption column behavior. Geophys. J. R. astr. Soc. V63: 117-148

For more information contact Paula via email at: paula.montoyalopera@utas.edu.au