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.
Figures & Tables
Seventy-Fifth Anniversary Volume
The first notions of a new journal came to J. E. Spurr during the closing days of 1904. When he shared his thoughts with friends in Washington, D. C., they were so enthusiastic about the suggestion that they formed themselves into an ad-hoc committee to seek ways to implement the idea. The ad-hoc group met informally for several months and by May of the following year was ready to announce the birth of an unusual new publishing company and the journal the company would produce. The first formal meeting of the Economic Geology Publishing Company took place on May 16, 1905. The first issue of the new journal appeared in October of the same year, and the first volume was completed in December 1906. The birthing was not easy, but it was successful because the founders provided much of the financing as well as the first papers. The story of those earliest days and the many struggles of the fledgling journal is engagingly recounted by Alan M. Bateman in an article published in the Fiftieth Anniversary volume.
From inception, management of the journal has differed from the management of most scientific journals. There was no sponsoring society, so the founders raised capital by incorporating and selling shares in the venture. The journal has been owned and published by the Economic Geology Publishing Company ever since. There is no record that the founders experienced difficulties in selling shares in the Company, but they must have had some because the Publishing Company had a goal that other corporations(and presumably many of the investors) would have found difficulty in understanding: the new corporation was committed to keeping the books balanced but not to making a profit.
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The one tie the Publishing Company has with a society was instituted many years after the journal. was founded—with the Society of Economic Geologists. When the Society was founded in 1920 it first considered publishing its own bulletin. Because the venture seemed financially questionable, and the coffers of the new society were bare, an arrangement was reached whereby members of the Society first received offPrints of papers written by its members and eventually Economic Geology as