Chert sources

Figure 6. Bivariate plot of chromium and sodium log-concentrations for chert source groups analyzed by LA-ICP-MS. LA-ICP-MS data are nommlized to silicon. Ellipses represent 90% confidence levels for group membership.

Figure 1.46. REE patterns of the altered volcanogenic rocks and Kuroko ores. Data sources Shikazono (1999a). (A) Hydrothermally altered dacite and anhydrite underlying the Kuroko ores. (B) Barite, Kuroko ore and ferruginous chert. (C) Hydrothermally altered basalt overlying the Kuroko ores (Shikazono, 1999a).

The source of the large amounts of silica present in Precambrian cherts is unclear. Since few siliceous organisms (Klemm, 1979)8> have been reported to occur so early in sediments, it is assumed that most or all of the cherts were of non-biological origin and derived as precipitates from silica-saturated... 

A source of error in chemical analyses of montmorillonites (and in other clays) that is not commonly checked is the presence of amorphous material, particularly Si and Al. Table XXXII lists structural formulas given by Osthaus (1955) for montmorillonites which were purified by size fraction and by extraction with 0.5 N NaOH to remove amorphous Si and Al. In six analyses dissolved silica ranged from 3.6 to 8.4% and alumina from 0.6 to 2.25%. Amorphous silicon dioxide should be expected in most montmorillonites derived from volcanic material. The source glass has more Si than is required for the 2 1 layer and the excess must be leached from the glass. Much of the Si is deposited in the sediments underlying the bentonite bed in the form of chert but it is to be expected that the extraction would not be complete and a portion of the colloidal Si would remain in the bentonite bed. 

The graywacke and quartzite cobbles used for tool making were probably obtained locally from the storm beaches immediately below the coastal sites. Conversely, chert or quartz crystals have not been found on coastal beaches or adjacent regions, despite intensive survey. Although potential chert-bearing calcareous sediments are located inland, no source is known to exist in this region of southern Portugal (5). 

Study of specific regions in which iron cherts and volcanic rocks are spatially unconnected naturally led to the development of hypotheses of an exogene source of the iron, unrelated to volcanism. Thus there arose the hypothesis that the iron cherts were formed from material supplied to the sedimentary basin in the course of intensive subaerial weathering. Svital skiy (1924), Piatnitskiy (1924), Strakhov (1947), James (1954), White (1954), Belevtsev (1957) and Plaksenko (1966) shared this hypothesis. 

These two types of hypotheses characterize the extreme points of view. However, detailed study has shown that there often are transitions between cherty iron-formation related to volcanogenic complexes and cherty iron-formation which at first glance has no such relationship. The existence of transitions has made it possible to postulate a remote indirect relationship to volcanism and to presume a volcanogenic source of the material even for those iron-chert complexes in which there are no volcanics at all. This point of view borders on the first group of hypotheses and requires an answer to a number of controversial partial questions of the problem how is the formation of the iron cherts related to volcanism in space does this relationship change in time, does the character of volcanism itself change, and how does this affect the accumulation of cherty iron sediments ... 

As has already been mentioned. Gross (1965, 1973) distinguished two types of iron cherts on the Canadian shield, differing in textural-structural features, paragenetic associations, and also in sources of the material. 

There are two common stratigraphic occurrences of chert as bedded cherts associated with shales or iron formations and as nodules in carbonate rocks (Blatt et al., 1980). The bedded cherts are predominant in Precambrian time, reaching a maximum extent 2-3 Ga, when they represented as much as 15% of the sedimentary record. The Precambrian bedded cherts contain microspheres of quartz, suggesting that they may have precipitated inorganically. Commonly, bedded cherts are associated with ophiolite sequences, which may have hydrothermal or metasomatic sources of silicate. The co-occurrence of bedded cherts and shales (typically dark in color) suggests that many cherts form in a hemi-pelagic or deep-sea, open-ocean setting, far from sources of coarse clastic material. In... 

The most widespread fill material is reddish brown (2.5 YR 4/4, 5 YR 4/4) loam with a minor admixture of relatively large oolitic bauxite pebbles (derived from the Late Triassic - Camian - beds) and coarse clasts of black chert. Pilot X-ray diffraction analysis revealed mostly muscovite/illite, plus mixed-layer clay minerals of illite/montmorillonite type, chlorite plus mixed-layer clay minerals of chlorite/montmorillonite type, calcium montmorillonite, and diaspore plus gibbsite, or just traces of bauxite minerals (Misic, 2000). The mineral composition is not as uniform as might be expected, and further research, intended for application of factorial analysis, is in progress. A potential sediment source area in the present Cerkniscica River basin (Fig. 1) appears obvious at first glance, but similar outcrops of bauxite and chert do also appear at other sites that are not much more remote. 

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