Formation of clay minerals under hydrothermal influence is the result of rock alteration by circulating hot water in the Earth’s crust. A pre-existing rock-forming mineral assemblage is altered to a new set of minerals which are more stable under the hydrothermal conditions of temperature, pressure, and fluid composition. The interaction of hot water and rocks forms a spatially and temporally regular zonal pattern of new clay minerals, as the fluid with cooling temperature moves through the surrounding rock mass. This chapter discusses the formation of clay minerals in such dynamic processes of hydrothermal alteration. The approach is one of clay-mineral facies formed under conditions of massive alteration in the rocks. The chemical and mineralogical changes which occur on the scale of a rock or rock mass are considered to have been dealt with in the preceding chapter. The exact process of change via local, vein-influenced exchange processes is ignored for simplicity (see Chap. 6).

Formation of Clay Minerals in Hydrothermal Environments

Many chloritic minerals in low-grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties, expand slightly upon glycolation (“expandable chlorite”) and/or have excess AlVI relative to AlIV, as well as significant Ca, K and Na contents. Chloritic minerals with these properties fill vesicles and interstitial void space in low-grade metabasalt from northern Taiwan and have been studied with a combination of TEM/AEM, EMPA, XRD, and optical microscopy. The chloritic minerals include corrensite, which is an ordered 1:1 mixed-layer chlorite/smectite, and “expandable chlorite”, which is shown to be a mixed-layer chlorite/corrensite. Corrensite and some mixed-layer chlorite/corrensite occur as rims of vesicles and other cavities, while later-formed mixed-layer chlorite/corrensite occupies the vesicle cores. The TEM observations show that the mixed-layer chlorite/corrensite has ca. 20%, and the corrensite has ca. 50% expandable smectite-like layers, consistent with XRD observations and with their abnormal optical properties. The AEM analyses show that high Si and Ca contents, high AlVI/AlIV and low FeVI/(Fe+Mg)VI ratios of “chlorites” are correlated with interstratification of corrensite (or smectite-like) layers in chlorite. The AEM analyses obtained from 200–500 A thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low AlIV/SiIV and low FeVI/(Fe+Mg)VI, while chlorite has high AlIV/SiIV and high FeVI/(Fe+Mg)VI. This implies that the trioctahedral smectite-like component of corrensite has significantly lower AlIV/SiIV and FeVI/(Fe+Mg)VI. The ratios of FeVI/(Fe+Mg)VI and AlIV/SiIV thus decrease in the order chlorite, corrensite, smectite. The proportions of corrensite (or smectite-like) layers relative to chlorite layers in low-grade rocks are inferred to be controlled principally by Fe/Mg ratio in the fluid or the bulk rock and by temperature. Compositional variations of “chlorites” in low-grade rocks, which appear to correlate with temperature or metamorphic grade, more likely reflect variable proportions of mixed-layered components. The assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases, such as chlorite-corrensite, corrensite-smectite, or chlorite-corrensite-smectite. A model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1:1 ordered mixed-layer chlorite/smectite.

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Corrensite and mixed-layer chlorite/corrensite in metabasalt from northern Taiwan: TEM/AEM, EMPA, XRD, and optical studies

We report on the composition of chlorite in altered rocks from active and fossil gmthermal systems, and from Neogene hydrothermal Cu-Pb-Zn vein-type and Kuroko deposits of Japan Values of Fd+ /(F&+ + Mg) in chlorite from the ore deposits are more variable than in chlorite from the altered rocks of the geothermal systems The, value Fd+/Mg of chlorite from the latter is positively correlated with that of the fresh rock, and thus seems controlled by the composition of the original rock The variability in F&+ /Mg of chlorite from the Kuroko and Neogene vein-type deposits is attributed to a control by the fluid phase (Fea+/Mg, pH, ES, temperature) The influence of movement and mixing of fluid phases may also be important

Compositional differences in chlorite from hydrothermally altered rocks and hydrothermal ore deposits

Mg enrichment in diffusely bounded, schistose alteration zones occurring in Proterozoic submarine felsic volcanics NE of Hjulsjo, W. Bergslagen is shown to result from felsic rock-seawater interaction in a sub-seafloor hydrothermal system. The alteration is apparently in two stages, feldspar being altered first to a kaolinite-type clay which is then replaced by Mg-chlorite.

Proterozoic Seawater- Felsic Volcanics Interaction W. Bergslagen, Sweden. Evidence for High REE Mobility and Implications for 1.8 Ga Seawater Compositions

Statistical analyses of chemical data from the literature of corrensite minerals suggest a large compositional variability, more evident in octahedral than in tetrahedral coordination. Mg occupies 40–80% of the octahedral sites, with Al and Fe2+ making up the remainder. Approximately 15–30% of the tetrahedral sites are filled by Al. Despite this compositional variability, distinct fields for the several types of mixed-layer trioctahedral chlorite/trioctahedral swelling layer are not apparent. Statistical analyses of the composition of corrensite compared with saponite, vermiculite, and chlorite suggest that corrensite is an intermediate between trioctahedral chlorite and trioctahedral smectite. If Fe/(Fe + Mg) > 50%, chlorite alone is favored, but with increasing Mg, chlorite appears to transform into corrensite and then, by iron oxidation, into trioctahedral smectite. Despite the chemical variability between corrensite, chlorite, and saponite, corrensite appears chemically to be a well-defined species. On the other hand, corrensite cannot be characterized chemically on the basis of its swelling component. Thus, the current definition of corrensite as a regular 1:1 interstratification of trioctahedral chlorite and either trioctahedral smectite or vermiculite is appropriate.

Masaharu Ozaki

Papers

Mg-CHLORITE AND INTERSTRATIFIED Mg-CHLORITE/SAPONITE ASSOCIATED WITH KUROKO DEPOSITS