Disseminated-type deposits

Estimated total productions of Au, Ag and other metals and Ag/Au total production ratio (Ag/Au, by weight ratio) from the individual vein-type and disseminated-type deposits in Japan (Shikazono, 1986). Type 1-A gold-silver-rich deposits, Type 1-B base-metal-rich deposits, Type 2 disseminated-type deposits... 

Figure 1.62. Location of epithermal-type deposits in Japan (Shikazono and Shimizu, 1988a). 1 Green tuff and subaerial volcanic region of Tertiary/Quaternary ages, 2 Main Paleozoic/Mesozoic sedimentary terranes, 3 Main metamorphic terranes. TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median tectonic line. Open circle epithermal Au-Ag vein-type deposits, solid circle epithermal base metal vein-type deposits, open triangle epithermal Au disseminated-type deposits.

Figure 1.109. Sulfur isotopic compositions of Neogene Au-Ag vein-type and disseminated-type deposits. Sulfur isotopic compositions on the samples from the Yatani deposits (Sample No. YT26 from Zn-Pb vein S S = -)-3.3%o), and HS72050305-YT1, YT24 and NS-3 from Au-Ag vein (average S S = +3.3%c)) by Shikazono and Shimazaki (1985) are also plotted. Base-metal rich implies the sample containing abundant sulfide minerals but no Ag-Au minerals from base-metal rich deposits and also from Ginguro-type deposits (Shikazono, 1987b).

Figure 1.124. Ag/Au total production ratio from each mine and Ag content of electrum. Solid circle epithermal Au-Ag vein-type deposits. Open circle epithermal base metal vein-type deposits. Solid square hypo/mesothermal polymetallic vein-type deposits. Open. square epithermal Au disseminated-type deposits. I Tada, 2 Toyoha, 3 Omidani, 4 Innai, 5 Ikuno, Oe-Inakuraishi, 7 Nebazawa, 8 Kawazu, 9 Todoroki, 10 Yatani, 11 Seigoshi, 12 Sado, 13 Takeno, 14 , awaji, 15 Yugashima, 16 Takadama, 17 Handa, 18 Konomai, 19 Sakoshi-Odomari, 20 Toi, 21 Sanru, 22 Arakawa, 23 Taio, 24 Chitose, 25 Hokuryu, 26 Okuchi, 27 Fuke, 28 Yamagano, 29 Akeshi, 30 Kasuga (Shikazono, 1986).

Figure 1.194. Frequency histogram for the Ag content of electrum from epithermal Au disseminated-type deposits in Japan (Iwato and Kasuga). (Shikazono and Shimizu, 1987).

Shikazono, N. (1986) Ag/Au total production ratio and Au-Ag minerals from vein-type and disseminated-type deposits in Japan. Mining Geology, 36, 411-424. 

Almost two-thirds of the world s copper resources are porphyry deposits. The term porphyry is generally appUed to a type of disseminated copper deposit that is hydrothermal in origin and characterized by a large proportion of minerals uniformly distributed as disseminations or in fractures and small veins. Copper contents are generally 1% or less. The most extensive porphyry deposits are located in western Canada, the southwestern United States, Mexico, and western South America. In addition to the porphyrys, there are large bedded copper deposits in Germany, Poland, the CIS, AustraUa, and central Africa. 

A third type is a disseminated fibrin deposit in very small blood vessels or capillaries. 

Figure 1.89. Activity of 82(052 )-temperature diagram showing possible as and temperature ranges for epithermal Au disseminated-type (hot spring type), epithermal Au-Ag vein-type and epithermal base metal vein-type deposits in Japan (Shikazono 1986 Shikazono and Shimizu, 1988b).

Hot spring-type gold deposits (Nansatsu-type by Urashima et al. (1981,1987), high sulfidation-type by Hedenquist (1987), epithermal Au disseminated-type) are distributed in the Nansatsu district of southern Kyushu (Fig. 1.193). The deposits (Kasuga, Akeshi, Iwato) were formed at Pliocene age (5.5-3.7 m.y.) in the calc-alkaline volcanic rocks of nearly same age (Togashi and Shibata, 1984). The deposits, which are similar to Nansatsu-type deposits, occur in Southwest Hokkaido (Date, Hakurhu). 

These opaque minerals are common in Pb-Zn vein-type deposits in Japan Opaque minerals identified from silica sinter containing gold and mercury are krennerite, col-oradoite and metacinnabar (Osorezan, Japan Waiotapu, New Zealand). These minerals are not found in low sulfidation-type Au-Ag deposits in Japan but are reported from Kobechizawa, and Date in south Hokkaido which are massive and disseminated types and similar to hot spring type deposits. 

The other important deposit types of the Neogene age are polymetallic (Cu, Pb, Zn, Sn, W, Au, Ag) (xenothermal or subvolcanic type deposits) vein-type deposits, hypo/mesothermal Au vein-type deposits, disseminated-type (hot spring type) Au deposits, and Hg-Sb disseminated and vein-type deposits. 

As a specific illustration reference may be drawn to molybdenum reserve scenario in the United States. The reserves are mainly grouped under five categories (i) primary, (ii) byproduct of copper ores, (iii) co-product of copper-molybdenum ores, (iv) by-product of tungsten ores, and (v) by-product of uranium ores. These have been presented and briefly elaborated in Table 1.14. It may finally be recorded by way of summary that the present day molybdenum sources in the world today seem to be principally of two main kinds first, the large-tonnage, low-grade, disseminated type of deposit in which molybdenite is the principal economic mineral second, the deposits in which molybdenite occurs as a by-product in... 

Oceanic hydrothermal metal deposits have been classified into four categories by Bonatti (1983). Pre-discharge deposits form by reaction of the hydrothermal solutions with igneous crust prior to discharge of the solutions at the seafloor. These deposits include massive and disseminated metal sulfides and oxides, and are not considered further here, as they do not form sedimentary deposits. Syn-discharge deposits form at the sites of hydrothermal discharge at the seafloor and include high-temperature smoker-type deposits as well as more disseminated deposits at the locations of... 

Maeda, H. (1997) K-Ar age of mercury mineralization and related volcanic activity in Kitami metallogenic province, Hokkaido, Japan Specimens from Asahino disseminated and Tokoro vein-type mercury deposits. Resource Geology, 47, 11-20. 

Miscellaneous deposits - there are a number of deposits around the world (USA, Canada, Brazil, Chile, etc.) with a variety of ore types, some of which have been extensively studied. Such deposits include (a) deposits of ilmenite disseminated in schist, (b) complex deposits of apatite-ilmenite (Canada) and (c) deposits of rutile, anastase and brookite in a pegmatic phase of alkaline rocks (USA, Chile). 

This deposit is located in the north-east of Russia and belongs to a gold-arsenic type of low-sulphide formation (Abramson et al, 1980). It lies within a carbonaceous terrigenous rock of Triassic age and is associated with a dome-shaped uplift in a node of intersecting faults of various directions. The ore bodies consist of zones of silicification and kaolinization with veinlet-disseminated sulphide mineralisation. Gold is present in the form of finely-dispersed dissemination in arsenopyrite and pyrite. As an example. Figure 1 illustrates the distribution of Au and Mn in connection with commercial ore... 

Five different vein phases (Types i to V) are recognized at both deposits, aii have variabie amounts of carbonates and quartz gangue. Type i veins contain oniy brecciated quartz and carbonate minerals and at ED are spatially associated with disseminated arsenopyrite, chalcopyrite, pyrrhotite, and pyrite in the mafic host rock. Type II veins in both deposits are partly brecciated and contain 5-80% sulfides of dominantly pyrite, arsenopyrite, and at GB chalcopyrite. Type III veins are quartz-calcite-tetrahedrite-bismuthinite microveins that cut both Types I and II veins. The fine-grained sulfides replace and enclose arsenopyrite and pyrite in Type II veins and are also visible in microfractures within the Type II sulfides. Type IV veins are base-metal rich and characterized by galena, sphalerite, chalcopyrite, pyrite, and stibnite with a maximum width of 20 cm. The Type V veins are late barren-carbonate veins cutting all previous veins and textural features. 

Metal contents of the sulfide samples collected from the Key Anacon deposits have been classified using the Cu and Zn ratios (Fig. 5). The majority of the massive sulfide samples plot in the Zn-Pb-Cu type (of. Large 1992 Fig. 5) however, the semi-massive to disseminated sulfides in the footwall are enriched in Cu, Co, and Bi and lower in Zn and Pb, typical of stringer zone mineralization. These plots in the Cu type field and one sample contains enough zinc to be placed in the Zn-Cu group (Fig. 5). 

The U.S. Geological Survey was scheduled to publish a revised study of U.S. thorium resources in August 1979. Partial results of this study, which cover most of these resources but do not include the beach placers of Florida, Georgia, and the Carolinas, were presented orally by Staatz [S5] of the U.S. Geological Survey in 1978. Table 6.14 lists the types of deposit, the principal districts in which potentially economic thorium-bearing deposits have been found, the principal thorium minerals, and estimates of thorium reserves and resources. Thorium from the vein deposits, the first type, could be produced for less than 30/lb. Thorium is the principal salable product in these deposits. Thorium could be coproduced with other elements from disseminated deposits, massive carbonatites, and placers the amount of thorium that might be produced from them, and its cost, depends on the marketability of the other minerals that occur with the thorium. 

Two small but high-grade deposits in the western United States are possibly young examples of the unconformity-related type. Disseminated pitchblende mineralization at Pitch mine. 

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