Granite 1-type

Radon gas is formed in the process of radioactive decay of uranium. The distribution of naturally occurring radon follows the distribution of uranium in geological formations. Elevated levels have been observed in certain granite-type minerals. Residences built in these areas have the potential for elevated indoor concentrations of radon from radon gas entering through cracks and crevices and from outgassing from well water. 

Table 3 Representative analyses of the granite types referred to throughout this chapter and plotted in Figure 5 (see Figure 5 caption for data sources). 

Figure 8 Plot comparing the average and range of measured Th/U ratios in the various Archean granitic types (shaded squares) with those of post-Archean granites of the same chemical group (open squares). All data are from Sylvester (1995), though the range of Th/U ratios indicated for the post-Archean granites incorporates data from the LFB (sources as for Figure 6).

Another curious feature of A-types is their uneven distribution throughout the geological record. Rocks of this nature are encountered in Archean cratons (Section 3.11.3.2.1), though they are far subordinate to the TTG and other granitic types. Similarly, A-type magmas are volumetri-cally minor in Phanerozoic orogenic belts they comprise only 0.6% of the vast granitic batholiths of the LFB (Chappell et al., 1991), discussed in Section 3.11.4.4. 

If the model outlined above is valid, sihcic magmatism in the LFB involved net crustal growth, as juvenile mantle-derived liquids, or their differentiates were instrumental in the formation and compositional evolution of hornblende granites, and, to a lesser extent, the cordierite granites. The amount of new crust generated is estimated by determining the overall mantle component present within both granitic types, and this is best done isotopically, since the trace-element ratios are poorly constrained for the potential basaltic end-members. 

Wyborn L. A. I., Wyborn D., Warren R. G., and Drummond B. J. (1992) Proterozoic granite types in Austraha impheations for lower crust composition, structure and evolution. Trans. Roy. Soc. Edinburgh Earth Sci 83, 201-209. 

TABLE 7.2 The composition of a granitic-type rock and of its weathering products ... 

Chappell BW, White AJR (1974) Two contrasting granite types. Pacific Geology 8 173-174. The Royal Society of New Zealand, Wellington... 

Quartz-Pebble Conglomerate Deposits. Known quartz-pebble conglomerate ores are restricted to a specific period of geologic time. These ore types occur in basal Lower Proterozoic beds unconformably situated above Archaean basement rocks composed of granitic and metamorphic strata. A number of commercial deposits are located in Canada and South Africa. Some subeconomic occurrences have been reported in Brazil and India... 

Intrusive Deposits. Deposits included in the intmsive deposit type are those associated with intmsive or anatectic rocks of different chemical composition, eg, alaskite, granite, monzonite, peralkaline syenite, carbonatite, and pegmatite. Examples include the uranium occurrences in the porphyry copper deposits such as Bingham Canyon and Twin Butte in the United States, the Rossing Deposit in Namibia, and Ilimaussaq deposit in Greenland, Palabora in South Africa, and the deposits in the Bancroft area, Canada (15). 

Blancaneaux, P. and Pouyllau, M. (1977). Formes d alteration pseudokarstiques en relation avec la geomorphologie des granites precambriens du type Rapakivi dans le territoire Federal de I Amazone, Venezuela. Cah. ORSTOM Ser. Pedol. 15,131-142. 

In Southwest Japan, two styles of vein-type mineralization (Hg and Sb) formed from middle Miocene to the present. Many Hg and Sb deposits are present along the Median Tectonic Line, associated with the Setouchi andesites and ilmenite-series granitic rocks (Fig. 1.3). 

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