Accumulation of magnetite by flotation on bubbles during decompression of silicate magma

Abstract

Magnetite (Fe 3 O 4 ) is an iron ore mineral that is globally mined especially for steel production. It is denser (5.15 g/cm 3 ) than Earth’s crust (~2.7 g/cm 3 ) and is expected to accumulate at the bottom of melt-rich magma reservoirs. However, recent studies revealed heterogeneous fluid bubble nucleation on oxide minerals such as magnetite during fluid degassing in volcanic systems. To test if the attachment on fluid bubbles is strong enough to efficiently float magnetite in silicate magma, decompression experiments were conducted at geologically relevant magmatic conditions with subsequent annealing to simulate re-equilibration after decompression. The results demonstrate that magnetite-bubble pairs do ascend in silicate melt, accumulating in an upper layer that grows during re-equilibration. This outcome contradicts the paradigm that magnetite must settle gravitationally in silicate melt.

Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust

“…Our proposed "sulfide buoyancy aid" process, operating from the metasomatized lithospheric mantle through to base of the continental crust, is analogous to the established mechanism where aqueous or saline vapor bubbles are suggested to "float" sulfide and/or magnetite at mid-upper crustal depths 12,14,15,17 . However, the critical difference is the deeper lithospheric window where this process operates, which provides a first order mechanism to fertilize the continental crust with mantle-derived chalcophile and siderophile metals.…”

“…The question is whether the carbonate-sulfide association documented here is simply serendipitous, or if it reflects a C-driven physical mechanism to flux S and metals from the mantle into the lower continental crust, very much like water-dominated processes have been shown to play a fluxing role in the mid to upper crust [12][13][14] . To address this question, it is necessary to discuss the geochemical behavior of S-and C-bearing fluids in the mantle and in ascending silicate magmas.…”

“…In the upper crust, hydrous volatile phases have been demonstrated experimentally to provide a viable mechanism of upward physical transport for relatively dense sulfide liquid droplets 12 , 13 and magnetite crystals 14 . Empirically, this process is reflected in the recent identification of sulfides in magmatic systems associated with coarse-grained hydrous silicate caps 15 – 17 .…”

Silicate, Iron Oxide and Copper-Gold-Silver Microspherules in Ores and Pyroclastics of the Kostenginskoye Iron Ore Deposit (Russian Far East)

“…Современные петрологические и геохимические исследования показывают, что многие железо-оксидные месторождения типа Кируна являются продуктами эффузивного или эксплозивного вулканизма [79,103]. Их формирование связывается с селективной флотацией магнетита в верхние части магматической камеры газовыми пузырьками в сильно дифференцированном магматическом очаге в условиях декомпрессии, в ряде случаев (Эль Лако) заканчивающейся эксплозией [61,62].…”

Mineralogical and Geochemical Study on the Yaojiazhuang Ultrapotassic Complex, North China Craton: Constraints on the Magmatic Differentiation Processes and Genesis of Apatite Ores

“…Additionally, the widespread hydrated minerals like biotite and the occurrence of pegmatites which has not been reported in detail indicate that during the formation of Yaojiazhuang complex, the magma ought to be enriched in fluids. Recent studies had revealed that the interactions between fluids and minerals during the last magmatic stage could significantly modify rock structures and textures, and could also act as a key mechanism in the generation of economic ore deposits, especially apatite and IOA deposits (e.g., Roedder, 1992;Huber et al, 2012;Ballhaus et al, 2015;Guo and Audétat, 2017;La Cruz et al, 2019;Knipping et al, 2019). Therefore, a combined study involving mineralogy, petrology and geochemistry studies is required to elucidate these processes.…”