Hydrothermal deposit

Ocean Basins. Known consohdated mineral deposits in the deep ocean basins are limited to high cobalt metalliferous oxide cmsts precipitated from seawater and hydrothermal deposits of sulfide minerals which are being formed in the vicinity of ocean plate boundaries. Technology for drilling at depth in the seabeds is not advanced, and most deposits identified have been sampled only within a few centimeters of the surface. 

Deposits which are forming are frequentiy characterized by venting streams of hot (300°C) mineralized fluid known as smokers. These result in the local formation of metalliferous mud, rock chimneys, or mounds rich in sulfides. In the upper fractured zone or deep in the rock mass beneath the vents, vein or massive sulfide deposits may be formed by the ckculating fluids and preserved as the cmstal plates move across the oceans. These off-axis deposits are potentially the most significant resources of hydrothermal deposits, even though none has yet been located. 

This spatial difference is consistent with the distribution with that of hydrothermal deposits of middle Miocene (Kuroko and polymetallic vein-type deposits in Japan). 

Figure 1.170. Diagram showing the octahedral composition of chlorites from the subvolcanrc hydrothermal deposits, propylite, and Kuroko deposits in Japan (Nakamura, 1970). Chlorite occurring as a gangue mineral in the subvolcanic hydrothermal deposits Nos. 1, 2, 3 and 4 Chlorite from the Ashio copper mine. Nos. 5, 6, and 7 Chlorite from the Kishu mine. No. 8 Chlorite from the Arakawa mine. Nos. 9 and 10 Chlorite from the Ani mine. No. 11 Chlorite from the Osarizawa mine. Chlorite from the so-called propylite No. 12 Chlorite from the Yugashima mine. No. 13 Chlorite from the Budo mine. Chlorite from the Kuroko deposits No. 14 Chlorite from the Wanibuchi mine.

Yamaoka, K. (1969) Metallic minerals of the Kuroko deposits in Northeast Japan. Proc. Symp. Mineral Constituents of Sulfide Minerals from Hydrothermal Deposits, Morioka, pp. 1-38 (in Japanese). Yamaoka, K. (1976) On the genetical problems of the vein-type deposits of the Neogene age in the inner belt of Northeast Japan. Mining Geology Special Issue, 7, 59-74 (in Japanese). 

Peter, J.M. and Scott, S.D. (1988) Mineralogy, composition, and fluid inclusion microthermometry of seafloor hydrothermal deposits in the Southern Trough of Guaymas Basin, Gulf of California. Can. Mineral, 26, 567-587. 

During the last three decades, many hydrothermal deposits have been discovered at midoceanic ridges, back-arc basins and subaerial active geothermal systems. Characteristic features of back-arc deposits at the western Pacific region (e.g., Okinawa Trough, Izu Ogasawara, North Fiji and Mariana deposits) are very similar to those of Kuroko deposits. 

For the formation of hydrothermal deposits the following are essential (i) the availability of mineralising solutions capable of dissolving and transporting mineral matter, (ii) the availability of openings in rocks through which the solutions may be channelled, (iii) the availability of suitable sites for deposition and localisation of ore minerals, (iv) chemical reactions that result in deposition, and (v) sufficient concentration of mineral matter to constitute economic deposits. 

Deloule, E., 1982, The genesis of fluorspar hydrothermal deposits at Montroc and Le Bure, The Tam, as deduced from fluid inclusion analysis. Economic Geology, 77,1867-1874. 

Watanabe, Y., Murakami, H., Cengiz, i., Sari, R., KuguKEFE, S., ve Yildirim, S. 2003. Study on Hydrothermal Deposits and Metallogeny in Western Turkey MTA, Ankara. 

According to the processing characteristics of PGM-dominated deposits, they can be divided into the following three groups (a) Morensky type, (b) hydrothermal deposits and (c) placer deposits. Each type of deposit is briefly described below. 

An example of a hydrothermal deposit is the New Rambler deposit, described by McCal-lum et al. [4] in the Medicine Bow Mountains in south-western Wyoming, USA, which contains a significant amount of PGM. The ore occurs in irregular pods that are hydro-thermally decomposed into metadiorite and metagabbro zones. Pyroxenite and peridotite are reported to be intersected at a depth beneath the ore zone. All have been affected by supergene alteration. The main sulphides in the ore include pyrite, chalcopyrite, pyrrhotite, covellite and marcasite with associations of electrum, pentlandite and PGM. 

The most important tin deposits are hydrothermal deposits (hypothermal and mesothermal). The magmatic deposits do not often contain tin mineralization. Tin may also be present in pegmatitic ore bodies. However, tin found in pegmatitic deposits can be classified into two basic types (a) quartz-cassiterite lenses in granite, when cassiterite is associated with topaz, beryl and, to a lesser degree, sulphides (b) sulphide deposits, where tin is mainly cassiterite associated with arsenopyrite, pyrite, chalcopyrite and pyrrhotite. Such deposits are common in South America (Peru, Bolivia). 

Pneumatalitic-hydrothermal deposits contain Ta/Nb as isomorph impurities in cassiterite and wolframite. Ta/Nb from these ores is recovered in a tin and wolframite concentrate. 

Doe BR (1994) Zinc, copper, and lead in mid-ocean ridge basalts and the source rock control on Zn/Pb in ocean-ridge hydrothermal deposits. Geochim Cosmochim Acta 58 2215-2223 Ehrlich S, Butler I, Halicz L, Rickard D, Oldroyd A, Matthews A (submitted) Experimental study of copper isotope fractionation between aqueous Cu(II) and covellite, CuS. Chem Geol Finney LA, O Halloran TV (2003) Transition metal speciation in the cell insights from the chemistry of metal ion receptors. Science 300 931-936... 

Most of the solid matter found in the sediments of the open ocean was transported to the seafloor via the slow sinking of small particles through the water column. This process is termed pelagic sedimentation. Other types of sedimentation are discussed in the next chapter and include turbidity flows, hydrothermal deposits, and deposition of large animal carcasses, e.g. whales, squid, and fish. 

CHAPTER 19 Metalliferous Sediments and Other Hydrothermal Deposits... 

---