No abstract

There have been two primary sources for industrial indium; one from massive sulfides, while the other is dissemination-veins and skarns, related to felsic igneous rocks. The latter group of the In-bearing deposits is abundant in the Japanese Islands. Indium occurs as In-minerals such as sakuraiite, roquesite, laforetite and many unidentified minerals, but the majority is contained as an impurity in sphalerite, and tin and copper sulfides. Average grades of the ores from which indium has been extracted vary from a few ppm (e.g., Kosaka mine) to more than 300 ppm (Toyoha mine). The amount of indium in all the major basemetal deposits is estimated by analyzing representative samples. The main indium deposits are subvolcanic and tin-polymetallic vein types. The largest one is Toyoha mine (4,700 tons In) and the Ashio mine (ca. 1,200 tons In) was found to be the second largest. Many small occurrences, were recognized in the Miocene magnetite-series belt, besides the classic occurrences in the ilmenite-series granitic terrains of SW Japan, including the Ikuno and Akenobe tin(-tungsten) polymetallic veins, located in the northern margin of the late Cretaceous Sanyo ilmenite-series province. Magnetite-series magmas with deep source are necessary to concentrate sulfur in the magma chamber but sedimentary source rocks and their reducing agents are needed to collect and to precipitate indium. The Japanese islands are essentially accretionary terrains intruded by various deep oxidized magmas; thus forming magnetite/ilmenite-series paired belts, which are sometimes mixed. This unique geologic setting may be the most fundamental reason why indium is rich in vein-type deposits of the Japanese Island arcs.

Studies of Mesozoic granites associated with rare earth element (REE)-rich weathered crust deposits in southernmost Jiangxi Province indicate that they have high-K to shoshonite compositions and belong to ilmenite-series I-type granites. Of the studied rocks at 59 -292 ppm of bulk REE content, the highest are seen in the biotite granites of Dingnan (358, 429 ppm) and mafi c biotite granite of the Wuliting Granite (344 ppm) near the Dajishan tungsten mine, both areas where weathered-crust REE deposits occur. REE-bearing accessory minerals in these granites are mainly zircon, apatite and allanite, and REE-fl uorocarbonates are common. REE enrichment occurs in the rims of apatite crystals, and in fl uorocarbonates that occur along grain boundaries of and cracks in major silicate minerals, and in fl uorocarbonates that replaced altered biotite. It is therefore thought that a major part of the REE content of these granites was concentrated during deuteric activity, rather than during magmatic crystallization. The crack-fi lling REE-fl uorocarbonates could subsequently have been easily leached out and deposited in weathered crust developed during a long period of exposure.

Tin-polymetallic base metal deposits of Miocene age in the Eastern Cordillera in Bolivia were studied by ICP/MS and EPMA for major and minor elements, paying an attention to indium concentration of the ore deposits. The highest indium content and 1000 In/Zn ratio of individual ore deposits are 5,740 ppm and 22.2 for the Potosi deposits, 2,730 ppm and 7.4 for Bolivar deposit, 2,510 ppm and 17.5 for Siete Suyos-Animas deposits, and 1,290 ppm and 3.3 for San Vicente deposit.The same content and ratio of composite samples of the studied deposits are up to 292 ppm and 4.0 for Potosi deposits, 3,080 ppm and 11.3 for Huari Huari deposit, 100 ppm and 0.3 for Tuntoco deposit, 152 ppm and 1.8 for Porco deposit, 103 ppm and 59.2 for Animas deposit, and 1,160 ppm and 3.7 for Pirquitas deposit. Those of zinc concentrates are as follows: 1,080 ppm and 2.1 at San Lorenzo; 584 ppm and 1.7 at Bolivar; 499 ppm and 1.23 at Porco; 449 ppm and 1.21 at Reserva, and 213 ppm and 0.61 at Colquiri deposit. Indium occurs mostly in dark colored sphalerite and that of the Potosi deposit was found to have one of the highest concentrations, containing up to 1.27 wt% In. Petrukite was discovered in the Potosi deposit, and indium minerals are expected to occur in the Huari Huari deposit and others with the high 1000 In/Zn ratios.The indium contents of the zinc concentrates and composite samples were applied to the produced and remaining ores, then the total amounts of indium in the Bolivian tin-polymetallic base metal deposits are speculated to be more than 12,000 tons In, which is bigger than that of South China (11,000 tons) and the Japanese Islands (9,000 tons). Sphalerites of the Potosi deposit have one of the highest ranges of indium, similarly to those of the San Vicente deposit. Both the San Vicente and Potosi deposits are rich in silver, implying significance of both silver-polymetallic and tin-polymetallic environments for the concentration of trace amounts of indium.