Alkalic rocks

Figure 1.199. Plot of K2O versus Si02 for volcanic rocks thought to be genetically related to Au mineralization. High sulfidation (HS) deposits appear to be associated with a narrow range of igneous rock composition, dominated by dacite and andesite. Low sulfidation (LS) deposits are largely associated with andesite and rhyolite, but major deposits such as Ladolam, Porgera and Cripple Creek are associated with shoshonitic and alkalic rocks (Hedenquist et al., 1996).

Paone, A., Ayuso, R. A., and De Vivo, B. (2001). A metallogenic survey of alkalic rocks of Mt. Somma-Vesuvius volcano. Mineral. Petrol. 73, 201-233. 

Enriched in relatively insoluble form in silicates in pegmatites also enriched in some coals and alkalic rocks and their associated mineral deposits. Soils and waters affected by emissions from coal-fired power plants. 

Placer deposits of potential future value as lanthanide ores invariably have the lanthanides present in common lanthanide or lanthanide-concentrating minerals (e.g., apatite, allanite, monazite, xenotime, zircon). Most pegmatites of potential value as ore have the lanthanides as common lanthanide-bearing minerals too (those previously mentioned plus fluorite). Alkalic rock complexes may produce commercially useful concentrations of common lanthanide-bearing minerals (e.g., apatite, perovskite) or rare ones (e.g., bastnaesite). 

The bastnaesite-synchisite series of lanthanide minerals can be considered in terms of stoichiometric ratios of RCO3F to CaCOj in proportions 1 0, 1 1, 2 1, and 3 2. The minerals of this series are much more rare than allanite or apatite but are fairly common in alkalic rock suites and in some metamorphic terrains. This mineral occurs in commercially important amounts in carbonatite associated with other alkalic rocks at Mountain Pass, California. The mineral is usually light-lanthanide rich, although rare heavy-lanthanide rich examples are known (fig. 21.28). 

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). 

ALKALI ROCKS. Igneous rocks which contain a relatively high amount of alkalis in the form of soda amphiboles, soda pyroxenes, or felspathoids, are said to be alkaline, or alkalic. Igneous rocks in which the proportions of both lime and alkalis are high, as combined in tire minerals, feldspar, hornblende, and angile, are said to be calcalkalic. 

The nature of the suboceanic mantle is largely constrained from geochemical studies of its partial melts (see Chapter 2.08) because occurrences of mantle xenoliths in the ocean basins are much rarer than on the continents. The host rocks for these xenoliths are exclusively alkalic and... 

Petrologic and major element studies of Cen-ozoic continental basaltic rocks have dehned two primary magma series in intraplate basaltic igneous rocks alkalic, and tholeiitic (the latter comprising, along with calc-alkalic, the subalkalic series of Iddings (1892)). Alkalic... 

Even within either group of volcanic rocks the variety of possible xenolith types is great. Table 1 presents a summary of the most common mantle xenolith groups that are found in kimber-litic hosts and within the alkalic and potassic mafic magmas. The significance and abundance of these groups will be discussed below. 

Ever since the discovery of analcime in tuffaceous rocks, most workers have assumed that the analcime formed directly from vitric material. The presence of vitroclastic texture and pyrogenic crystals in some anal-cimic tuffs seemed sufficient evidence however, these criteria do not necessarily prove that the glass altered directly to analcime. Hay (48) and Sheppard and Gude (123) concluded from a study of tuffs in saline-lake deposits that analcime commonly formed from alkalic, silicic zeolite precursors. Formation of analcime from clinoptilolite and phillipsite was documented in tuffs of the Miocene Barstow Formation. Relict fresh... 

Figure 3,3 The chemcial classification and nomenclature of pluomic rodts using the total alkalis versus silica (TAS) diagram of Cox et ai (1979) adapted by Wilson (1989) for plutonic rocks. The curved solid line subdivides the alkalic from subalkaiic ro<. -...

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