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-- T-4518 78 <br />apatite; 2) fine-grained (<100 µm) secondary quartz with minor subhedral to euht:dral <br />adularia (20 to 100 µm), sometimes replacing primary quartz from grain edges; 3;i sericite, <br />chlorite, phengite and minor secondary adularia replacing plagioclase; 4) an inter~;rowth of <br />muscovite and ankerite and minor adularia replacing biodte; and 5) ankerite replacing <br />homblende, augite, and, partially, magnetite. <br />The central zone is identical to the inner zone except that it does not contain <br />secondary quartz. Because virtually all mafic minerals have been altered, this zom: <br />appears "bleached" with a pale greenish tint. In contrast, the outer zone still cons:uns <br />some mafic minerals and therefore appears only pamally bleached (Fig. 46d). It c~ansists <br />of fresh orthoclase, quartz, plagioclase, apatite, and magnetite. Biotite is partially altered <br />to adularia while homblende and augite are partially altered to ankerite. <br />Distribution of MSPC. Carbonate. and MSPCAA Alteration <br />Because MSPC, MSPCAA, and carbonate alteration look similaz and normally <br />occur around the same structures, they were mapped as a single alteration type (P:iates 1 <br />through 6). In the mica schist unit intense alteration halos tend to be narrow, generally <br />less than about 6 inches aoound discrete veins but wider in densely veined or stockwork- <br />veined zones. Beyond the intense alteration halo only plagioclase is altered, possit~ly due <br />to supergene processes. Fresh plagioclase is uncommon. Alteration is not easily <br />recognized macroscopically in migmatite zones due to a lack of biotite. <br />Intense alteration halos in the monzonite aze wider, but plagioclase beyond the <br />alteration halos is fresh. In the monzonite, the alteration envelope width is related to the <br />abundance of base-metal minerals in the associated vein, a further indication of <br />