The Granitoid Series and Mineralization
Authors: Ishihara Shunso 
Published: May 2021

Granitoids can be classified into either a magnetite-bearing magnetite series or a magnetite-free ilmenite series, a classification also applicable to their effusive equivalents. Magnetite series granitoids are associated with major sulfide mineralization, whereas ilmenite series granitoids are related to cassiterite and wolframite mineralization. The magnetite series has a magnetite content more than 0.1 vol percent; a magnetic susceptibility higher than 1 × 10‒4emu/g; a bulk Fe2O3/FeO ratio higher than 0.5; a positive δ34S value and a low δ18O value; depletion in lithophile elements; accessory magnetite, (0.1–2 vol %), ilmenite, hematite, pyrite, and chalcopyrite; biotite with high Fe2O3/FeO and low refractive index; and intrusive sequences in which Fe/(Fe + Mg) for amphiboles and biotites decreases with increasing SiO2 content of host rocks. The ilmenite series has a magnetite and ilmenite content less than 0.1 vol percent; a magnetic susceptibility less than 1 × 10‒4emu/g; a bulk Fe2O3/FeO ratio lower than 0.5; a negative δ34S value and a high δ18O value; enrichment in lithophile elements; accessory ilmenite, pyrrhotite, graphite, (monazite, garnet), and muscovite; biotite with low Fe2O3/FeO and high refractive index; and intrusive sequences in which Fe/(Fe + Mg) for amphiboles and biotites increases with increasing SiO2 content of host rocks.

The two series of granitoids are considered to have resulted from the prevalence of different oxygen fugacities during evolution of the granitic magmas, in which dissociation of water in the hydrous magmas is a main oxidizing agent for magnetite series magmas and incorporation of crustal carbon is the most essential reducing media for ilmenite series magmas. The magnetite series can be correlated with I-type granitoids; the ilmenite series is composed of both I-type and S-type granitoids.

The distribution of magnetite series-ilmenite series granitoids varies in time and space in the circum-Pacific orogenic belt. In the continental margin of eastern Asia, magnetite series granitoids are typically younger than ilmenite series granitoids; which may also be true in the continental margin of western America. However, the spatial distribution of the two granitoid types differs on each side of the Pacific Ocean. In eastern Asia, the volcano-plutonic belt facing the marginal sea is generally composed of the magnetite series, implying that the magnetite series rocks tend to occur along the coast in the continent. In western America, however, ilmenite series granitoids are distributed along the coast. In island arcs such as Japan, ilmenite series granitoids are found on the oceanic side and magnetite series granitoids become predominant toward the back-arc basin.

These petrogenetic provinces strongly affect metallogenic provinces of volcano-plutonic affinity. Where both series occur together in one geotectonic unit, the magnetite series granitoids are characterized by sulfide mineralization and the ilmenite series granitoids by sulfide- free mineralization. In solely magnetite series or ilmenite series terrains, the metallogenic zoning is unclear, but the mineralization seems to be controlled to some degree by the temperature and pressure of the granitic magmas, as well as by their oxygen fugacity. Nearly 100 percent of the sulfur, base metal sulfides, and gold-silver, and a large part of molybdenum, including that from porphyry-type and Kuroko-type deposits, is related to magnetite series magmatism; whereas cassiterite, wolframite, beryl, and fluorite are largely associated with ilmenite series magmatism. Scheelite mineralization, including a porphyry type, appears to be related to magnetite series magmatism.

Magnetite series magmatism is capable of supplying abundant sulfur and ore metals that tend to combine with sulfur (and chlorine); these magmas may originally be enriched in these components, which are expelled into the aqueous phase during fractionation and crystallization throughout a long journey to the surface in a tensional tectonic setting. Some of the ore components may have been brought to the site of magma generation by subduction. Differences in metal ratios such as Cu/Pb may be attributed to differing compositions of the source materials as well as different transport media (S, Cl) for each component. Ilmenite series magmas may have originated in much shallower levels than magnetite series magmas under a compressional tectonic setting. Thus, magnetite-free magmatism may be strongly affected by crustal materials which contain carbon, even if the magmatism is triggered by injection of mafic magmas from much deeper levels. Sulfur in this magma would have been crystallized in granitoids as rock-forming minerals, thus forming no significant sulfide deposits. Most of the ore metals such as tin appear to have been concentrated in granitoids by recycling of crustal materials and were transported to ore deposits as fluorides. Recognition of the two series of granitoids is an important first step in mineral exploration.