First generation quartz cement is covered by chlorite
rims and filling primary pores (Figure5a).
The descriptions and conditions of these chlorites
have been published (see references in Table 1), and only the prominent features will be summarized and discussed here.
Precipitation occurs when the pore water reaches saturation and forms authigenic minerals, such as kaolinite, chlorite
, illite, and authigenic quartz, which can occupy pore space, block throats, and thus reduce the quality of the reservoir properties.
The Ah horizon contains kaolinite and illite, and corrensite appears at the expense of chlorite
. This supports one of the theories saying that corrensite originates from degradation of chlorite
(Briggati and Poppi, 1984).
Vermiculite is most often a pedogenic clay mineral that can be formed by the transformation on weathering of illite or chlorite
(Banfield and Murakami 1998; Bronger et al.
The by-products: chlorites
and chlorates were determined by ion chromatography with a DIONEX DX500 system with IonPac AS-9-HC analytical column (4 x 250 mm) and IonPac AG-9-HC guard column (4 x 50 mm) connected with conductivity detector CD-20 (Dionex, USA).
Estimates of the composition of the clays examined were based on the area of characteristic diffraction peaks, and were as follows: chlorite
, 20-30%; illite, 17-28%; kaolinite, 31-48%; smectite, 3-7%; and quartz, 3-7%.
The general alteration assemblages are molybdenite-rich quartz-sericite [+ or -] chlorite
[+ or -] epidote [+ or -] calcite veins (Figures 2(f)-2(h) and 3(d)-3(g)) or quartzmolybdenite-chlorite breccias (Figures 2(i) and 2(j)).
Determinations of chlorite
structural modifications were made based on the un-treated difffactograms.
An undetermined black to greenish black species of the chlorite
group forms very thin, tabular microcrystals comprising spherical clusters which line the walls of cavities, covering total surface areas up to 100 square meters.
identification is based on the 001 and 003 basal reflections at 14.0-14.2 [Angstrom] and 4.75 [Angstrom] respectively that are stable after ethylene glycol solvation and showing a slight contraction of the 001 peak to 13.8 [Angstrom] after 550[degrees]C treatment.