Sericite origin

Ore deposits associated with volcanic rocks generally exhibit polymetallic (Cu, Pb, Zn, Sn, W, Au, Ag, Mo, Bi, Sb, As and In) mineralization. Sulfur isotopic values of sulfides from these deposits are close to 0%o, suggesting a deep-seated origin of the sulfide sulfur. Clay deposits (pyrophyllite, sericite and kaolinite) are associated with both felsic volcanic rocks and ilmenite-series granitic rocks of late Cretaceous age in the San-yo Belt. 

Calculate how much Si02 and A1203 are contributed by sericite and subtract from original analysis %oSi02 %A1203 %K20 %MgO %Na20... 

Eberl, D.D., Srodon, J., Lee, M., Nadeau, P.H., Northrop, H.R. 1987. Sericite from the Silverton caldera, Colorado Correlation among structure, composition, origin and particle thickness. American Mineralogist, 72, 914-934. 

Probably the most passionately debated mineral (if one might use this adverb in a discussion of clays) found in argillaceous sediments, rivalling perhaps the enigmatic dolomite and dolomitization in the realm of sedimentary rocks, is the mineral or group name illite. Defined and redefined by its originator, R. E. Crim debated and further redefined, denied a proper existence and reprieved, this species has attracted the attention of clay mineralogists for the past two decades. It represents, in fact, the dominantly potassic, dioctahedral, aluminous, mica-like fraction of clay-size materials. Known as sericite or hydro-mica in studies of hydrothermal alterations, soil mica or illite in soils and illite... 

Studies of hydrothermal alteration products associated with ore mineralization in acidic rocks have established the general propensity for the original minerals to be replaced by illite, sericite or hydromica in the innermost zone near the source of hydrothermal fluids and by kaolinite or expandable minerals further from the vein or center of fluid emanation. The newly-formed "mica" can be 2M, 1M, or lMd in polymorph and range compositionally from muscovite to a low potassium, silicic species which can be assimilated in the term illite (Lowell and Guilbert, 1970 Schoen and White, 1966, 1965 Kelly and Kerr, 1957 Bonorino, 1959 Tomita, e al., 1969 Yoder and Eugster, 1955 Meyer and Hemley, 1959, among many authors). 

The quartz monzonite samples (U15E-7 and U15E-7a) were originally composed of 70-80 percent feldspar, 10-15 percent biotite, 3-8 percent quartz, and 2-8 percent opaques. Both samples have been hydrothermally altered U15E-7a being the more altered. The original feldspars in both samples have been sericitized and/or altered to clinozoisite. Secondary cal cite also occurs in both samples. Pyrite is an abundant secondary mineral and minor amounts of both epidote and chlorite replace biotite and fill fractures. 

The term sericite is frequently used to describe fine-grained dioctahedral micas. This material is usually coarser than illites and often hydrothermal in origin. Sericites... 

The mineral composition (mode) of the Wyatt Formation in the Scott-Glacier area was determined by Minshew (1967) and is listed in Appendix 6.7.3.1 followed by chemical analyses in Appendix 6.7.3.2. The rocks of the Wyatt Formation in the La Gorce Mountains were altered either by the original magmatic fluid or during later metamorphism. In extreme cases, plagioclase was completely altered to sericite or saussurite (Stump et al. 1986). Saussurite is a mineral aggregate composed of albite and zoisite or epidote with variable amounts of calcite, sericite, and prehnite (Jackson 1997). 

The mechanism of mica formation from feldspars is not yet completely understood. Obviously, part of the illite produced by weathering of feldspar originates from the finegrained sericites developed by metamorphic processes during the late phases of rock formation (Muller [1966a]). By mechanical or chemical action, feldspars are dispersed, and the included sericites released. Illites formed by this process are, thus, detrital in character. On the other hand, two mechanisms of pedogenic formation of mica from feldspar are discussed. 

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