Rocks, felsic

Figure 2. Chondrite-normalized REE-HFSE diagram of Archean F-type rhyolites against the pattern of the LaRonde host felsic rocks. From Mercier-Langevin ef a/.. 2007b).

Abstract Two distinct mineralizing fluids formed the Hollinger-Mcintyre-Coniaurum (HMC) deposit. The earliest fluid was associated with emplacement of a disseminated Cu-Au-Mo zone in the Pearl Lake Porphyry (PLP). The alteration pattern of the felsic rocks in the PLP is characterized by increased concentrations of K2O, Au, Cu, Mo, W, and Sn, and K/AI, Sericite / Chlorite (SCI) and Sericite Alteration Indexes and the removal of CaO, relative to nearby unaltered rocks. The H20-C02-NaCI mineralizing fluid that altered the PLP had a temperature between 340° and 390°C, and a 5 Owater composition of 11.7 to 12.7 %o. 

The association of the SCI alteration pattern with the Pearl Lake Porphyry and other felsic rocks is interpreted to indicate that they acted as a fluid conduit and permitted the vertical movement of the fluids through the fractured rocks in order to form the Cu-Au-Mo deposit. 

The C02/Ca0 anomaly appears to indicate that Au deposits associated with the HSZ were formed by fluid that moved up along the HSZ, rather than by fluid moving through the felsic rocks. 

The felsic rocks have negative Sr and Eu anomalies suggesting plagioclase fractionation. Zr/AbOs appears to be the best criteria for the discrimination of volcanic rocks and related volcaniclastic sedimentary rocks, because the rocks have been hydrothemally altered AI2O3 is the most conserved. The mafic rocks are dominantly of transitional alkaline to calc-alkaline affinity with high Cr and Ni... 

The primitive mantle-normalized trace-element spider diagram of felsic rocks shows negative Sr and Eu anomalies that are indicative of either plagioclase restite or plagioclase fractionation resulting from a combination of the partial melting and fractional crystallization processes (Fig. 5), and later changed by hydrothermal alteration. 

An idealized water, derived from the attack of C02-bearing water on a typical felsic rock to produce only kaolinite, should have the following characteristics, expressed in terms of molar concentrations ... 

Table V compares the Sierra waters with some waters from other felsic rocks. The fundamental similarities are clearly apparent as well as the minor variations that show the imprint of the details of rock mineralogy and small additions of constituents from various other sources.

Rehnements of the Taylor and McLennan (1985) model are provided by McLennan and Taylor (1996) and McLennan (2001b). The latter is a modihcation of several trace-element abundances in the upper crust and as such, should not affect their compositional model for the bulk crust, which does not rely on their upper crustal composition. Nevertheless, McLennan (2001b) does provide modihed bulk-crust estimates for niobium, rubidium, caesium, and tantalum (and these are dealt with in the footnotes of Table 9). McLennan and Taylor (1996) revisited the heat-flow constraints on the proportions of mahc and felsic rocks in the Archean crust and revised the proportion of Archean-aged crust to propose a more evolved bulk crust composition. This revised composition is derived from a mixture of 60% Archean cmst (which is a 50 50 mixture of mahc and felsic end-member lithologies), and 40% average-andesite cmst of Taylor (1977). McLennan and Taylor (1996) focused on potassium, thorium, and uranium, and did not provide amended values for other elements, although other incompatible elements will be higher (e.g., rubidium, barium, LREEs) and compatible elements lower in a cmst composition so revised. 

Piccoh P. M. and Candela P. A. (1994) Apatite in felsic rocks a model for the estimation of initial halogen contents in the Bishop Tuff (Long Valley) and Tuolumne Intrusive Suite (SierraNevada Batholith) Magmas. Am. J. Sci. 294,92-135. 

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