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2.2 Intrusive Alkaline Rocks <br />• Intrusive alkaline rocks in the Cripple Creek district are broadly divisible into four groups, based <br />L~ <br />upon chemistry, mineralogy and textures (see figure 1, groups A-D). Intrusions form stocks, <br />flows, sills, unroofed ]accoliths and dikes. Most are porphyritic, but aphanitic and equigranular <br />phaneritic varieties are also common. Timing and relative proportions are as follows: <br />Event Rock types/characteristics: Relative Timing: <br /> Proportions <br />A Phonolites / trachytic phonolites; 90-95% Most erupted as part of earliest <br /> present as dikes, sills, ]accoliths(?), intrusive event; smaller volumes <br /> flows and stocks emplaced at later times <br />B Intermediate intrusions 5-10% Intrusions emplaced after larger <br /> (tephriphonolites, phonotephrites, volumes of phonolite / phonolitic <br /> trachyandesites and phaneritic trachyte. Cut by mafic / <br /> equivalents); seen as small stocks and ultramafic intrusions (groups C <br /> dikes. Possibly gradational with and D), as well as a small <br /> phonolite suite. volume of late phonolite dikes. <br />C Mafic intrusions; lamprophyres, < 1% Late; postdate all known <br /> tephrites and trachybasalts. intrusions of phonolite and <br /> Commonly seen as narrow dikes or intermediate alkaline rocks, with <br /> small stocks, although one flow (or few exceptions <br /> unroofed laccolith) is seen. <br />D Ultramafic lamprophyres and ~ I-2% Youngest igneous intrusions seen <br /> silicocarbonati[es. in the district <br />2.3 Group A: Phonolites and Phonolitec Trachytes <br />This group comprises the greatest volume of intrusive rocks within the diatreme. As seen in <br />figure 1, they have relatively high silica concentrations (57-63 wt % SiOZ), and extremely high <br />concentrations of alkalis (> 12 wt. Na:O + KzO). The rocks are dark green when unaltered <br />(figure 2), and are chiefly composed of alkali feldspar (mol Na>K), with subordinate albitic <br />plagioclase (± anorthite), nepheline, aegerine-augite, magnetite and accessory apatite, sphene and <br />REE phases. Subsolidus minerals include analcime (replacing nepheline) and carbonate <br />minerals. The carbonate minerals are often seen filling vesicles and replacing mafic minerals', <br />apparently the products of magmatic degassing or deuteric/hydrothermal processes (discussed <br />below). These rocks crystallized from relatively anhydrous magmas with high COz <br />~ Where not removed by weathering processes or oxidation <br />Cripplr CreeA & r¢mr Gold mining Co. Shrphrrd Mil/rr.lnr. <br />~N/AdfONDIP~DRD¢1I°rfEMprortWmrAmrn~l Jor 4 Mnrrh 2000 <br />