Volcanic chert

Po22olans iaclude natural materials such as diatomaceous earths (see Diatomite), opaline cherts, and shales, tuffs, and volcanic ashes or pumicites, and calciaed materials such as some clays and shales. By-products such as fly ashes and siUca fume are also employed. In the United States the proportion of po22olan iaterground with clinker has varied from 15 to over 30%, whereas ia Italy, cements with a 30—40% po22olan content are produced. 

In the Chichibu Zone, the intimate association of abundant strata-bound Mn-Fe deposits, limestone-dolomite and silica (chert) with basic volcanic rocks suggests an ocean-ridge hydrothermal origin. 

Heavy Rare Earth Element). Therefore, it is considered that negative Ce and positive Eu anomalies in hydrothermally altered volcanic rocks, Kuroko ores, and ferruginous chert and LREE enrichment in the Kuroko ores have been caused by hydrothermal alteration and precipitations of minerals from hydrothermal solution responsible for sulfides-sulfate (barite) mineralization. 

Geology of the province is composed of Paleozoic basements. Tertiary altered submarine volcanic and sedimentary rocks (Green tuff) and Quaternary volcanic rocks. The basements are shale, tuff, limestone and chert of unknown ages. A simplified geologic map is shown in Fig. 1.148. 

The basement rock of the area is composed of the Mesozoic (limestone, chert, and late Tertiary volcanic formation. Osorezan is a composite volcano with caldera structure and post caldera domes. The K-Ar age of strata volcano in the earliest stage and the latest domes are 1 Ma and 0.2 Ma, respectively (Aoki, 1991). 

The best sealed-in minerals are zircons, zirconium silicate minerals which are formed when melted lava on the flanks of volcanoes solidifies. When the zircons crystallize out, they incorporate radioactive uranium (in particular 238U), which decays in several steps, leading Anally to the lead isotope 208Pb. The rate of decay is very low, as the half-life of uranium-238 is 4.5 x 109 years. Thus, the U-Pb-zircon method for age determination of Precambrian rock is very important. The fossils studied by Schopf were sandwiched between two lava layers (Schopf, 1999). The volcanic layers were dated to 3.458 0.0019 x 109 years and 3.471 0.005 x 109 years the age of the fossil layer (Apex chert) was thus determined to be about 3.465xlO9 years. 

The Boucher Brook Formation is the upper-most unit in the California Lake Group and consists of grey shale, siltstone, chert, alkalic basalt, and minor peralkaline felsic volcanic rocks including the alkaline basalt of the Camel Back Member (Fig. 1). 

Curve Pond Zone, and Boomerang all hosted within felsic volcanic rocks and spatially associated with black, locally graphitic shale and chert. (McKenzie et al. 1993 Evans Kean 2002 Rogers van Staal 2002, Squires Moore 2004) however, the Boomerang deposit is spatially associated with significant volumes of volcaniclastic rocks of intermediate composition. 

Geological Setting The area is underlain by Birimian meta-volcanic and meta-sedimentary rocks that have been intruded by granites. The meta-volcanic rocks are of basaltic and gabbroic compositions and most have been altered to various schists. The metasedimentary rocks consist of sandstones, siltstones, tuffs, carbonaceous phyllites, tuffaceous phyllites, cherts and maganeferous rocks (Leube et al. 1990). 

The Tyrone Volcanic Group sequence comprises basaltic pillow lavas, tuffs of basic to intermediate composition, rhyolites, cherts, siltstones and dark grey mudstones representing up to three volcanic cycles. From base to top of each cycle and through the sequence as a whole, the Tyrone Volcanic Group becomes progressively more acidic in composition. 

Sedimentary rocks in the upper (mainly mafic) part of the pile are predominantly mudrocks, but they have an exhalative component. Maroon shale and chert are present in the BB, LR, and SK formations (Fig. 1). Notably, maroon shale and chert are abundant in the CLL Formation and also occur locally near the top of the FLB and SR formations. Caradocian black shale and pelagic chert of the BB, LR, and SK formations mark the end of volcanic activity in the BMC. In places, these rocks grade upward into flysch of the M or T formations (Fig. 1). 

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. 

Diamond (D50) described the types of silica that can take part in ASR. They include quartz if sufficiently strained or microcrystalline, tridymite, cristobalite and glass or other amorphous forms, which occur in varying combinations in opals, flints, cherts and other rock types. Opals are especially reactive. Macroscopic, unstrained crystals of quartz appear to be Linreactive but are possibly not completely inert. Some silicate minerals and volcanic glasses may undergo reactions similar to ASR. 

In Chapter 2, Hancock, Pavlish, and Sheppard give an example of a case in which visual examination of stone tools was not adequate to differentiate between lithic artifacts that were produced from rocks that were very different in their origins. During the Mesolithic and early Neolithic times, the inhabitants of what is now Portugal used a variety of materials. Although most of the stone tools were classified by the archaeologists as sedimentary cherts, Hancock concluded that many tools were made of volcanic rhyolite. 

The majority of cherts from the inland site of Fiais were also from calcareous deposits. Figure 4 shows that Al and Ti increase at the interior site, which is buried in a clay-rich soil. It is admittedly difficult to determine whether the increases in such elemental concentrations are caused by di-agenetic factors or by inhomogeneity of the materials. These changes stand out in sharp contrast to the majority of cherts from Samouqueira, which on the basis of thin-section analyses, are of a meta-volcanic origin. These cherts are less susceptible to weathering and appear to have experienced little... 

The cherty iron-metabasite formation (CIM) was formed in regions of submarine volcanism of basic composition. The iron cherts in the formation are not uniform in facies and amount to 10-30% of the total thickness of the sequence. The formation is extensively developed in the lower metabasic series of the Konka, Belozerka, Verkhovtsevo, and Sura synclines. 

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 ... 

Semenenko et al. (1959, 1967) assigned a large role in the formation of the iron cherts of the Ukrainian shield to processes of volcanism. According to his formational scheme, a direct relationship to submarine volcanism is established for the CIM, CIU, and CIK, from the paragenetic association of iron cherts with volcanic rocks. 

However, Rozanov (1971) relates the accumulation of iron cherts to volcanic processes and believes that a lateral series can be made of the known cherty.iron formations, in which the order of formation reflects the distance of the regions of iron accumulation from regions of intensive manifestation of volcanism. This relationship will be expressed in a gradual reduction in the role of solid products of volcanism in the composition of the formations. In that arbitrary lateral series the Kursk and Krivoy Rog formations occupy an end position and will correspond to Shatskiy s remote chert formations. 

Chernov et al. (1970), who studied the cherty iron-formations of Karelia, concluded that they are related to volcanism not only of basic, but mainly also of acid composition. In turn, on the basis of the composition of the parent lavas, they distinguished spilite-diabase and leptite-porphyry formations among those of eugeosynclinal type, formed simultaneously but in different paleotectonic conditions. A large part of the formations of the spilite-diabase series of the Baltic shield is confined to the junctions between geosynclinal depressions and central massifs. The leptite-porphyry series of geosynclinal formations is characterized by a close association of acid and basic volcanics with iron cherts and less often with limestones and clastic sediments (Fig. 9). 

Fig. 9. Sketch of facies relationships of the paragenetic associations of rocks of the leptite-porphyry cherty iron-formation (after Chernov et al.) / = plagioporphyries. halleflintas, volcanic breccias 2 = quartz-biotite tuffogenic schists with intercalations of amphibole-gamet and biotite-garnet schists 3 = graphitic quartz-biotite schists rich in sulfides 4 = iron cherts. Numerals on map—rock associations /= porphyry -iron chert //= tuff-porphyry paragenetic [11= tuff aluminous-iron chert paragenetic [V= tuff schist-iron chert paragenetic.

Investigators in foreign regions where Precambrian iron cherts are developed have likewise established their close relationship to volcanics of different composition, but they also have distinguished regions in which cherty iron-formations are developed where this relationship is less obvious, and therefore may be debatable. 

On the basis of the review of present ideas on the relationship of the processes of volcanism and sedimentation in the formation of the Pre-cambrian iron cherts, several general conclusions can be drawn ... 

The nature of that relationship varies and depends on proximity of the volcanic centers to the basins of deposition of the iron cherts. Even in those cases where the relationship is believed to be very remote, manifestations of volcanism are a necessary condition for the accumulation of iron-formation. 

The question whether the role of volcanic processes in the formation of the iron cherts diminished from Archean to Proterozoic, and correspondingly the role of products of subaerial weathering increased, remains controversial. 

---