Abstract

The Cripple Creek district (653 metric tons (t) of Au) consists of Au-Te veins and disseminated gold deposits that are spatially related to alkaline igneous rocks in an Oligocene intrusive complex. Vein paragenesis includes quartz-biotite-K feldspar-fluorite-pyrite followed by base metal sulfides and telluride minerals. Disseminated deposits consist of microcrystalline native gold with pyrite that are associated with zones of pervasive adularia.New 40 Ar/ 39 Ar dates indicate that there was a complex magmatic and hydrothermal history. Relatively felsic rocks (tephriphonolite, trachyandesite, and phonolite) were emplaced into the complex over about 1 m.y., from 32.5 + or - 0.1 (1Sigma ) to 31.5 + or - 0.1 Ma. A younger episode of phonolite emplacement outside of the complex is indicated by an age of 30.9 + or - 0.1 Ma. Field relationships suggest that at least one episode of mafic and ultramafic dike emplacement occurred after relatively more felsic rocks and prior to the main gold mineralizing event. Only a single whole-rock date for mafic phonolite (which indicated a maximum age of 28.7 Ma) was obtained. However, constraints on the timing of mineralization are provided by paragenetically early vein minerals and K feldspar from the disseminated gold pyrite deposits. Early vein minerals (31.3 + or - 0.1-29.6 + or - 0.1 Ma) and K feldspar (29.8 + or - 0.1 Ma) from the Cresson disseminated deposit, together with potassically altered phonolite adjacent to the Pharmacist vein (28.8 and 28.2 + 0.1 Ma), suggest there was a protracted history of hydrothermal activity that began during the waning stages of phonolite and early mafic-ultramafic activity and continued, perhaps intermittently, for at least 2 m.y.Estimated whole-rock delta 18 O values of the alkaline igneous rocks range from 6.4 to 8.2 per mil. K feldspar and albite separates from igneous rocks have lead isotope compositions of 206 Pb/ 204 Pb = 17.90 to 18.10, 207 Pb/ 204 Pb = 15.51 to 15.53, and 208 Pb/ 204 Pb = 38.35 to 38.56. These isotopic compositions, together with major and trace element data, indicate that the phonolitic magmas probably evolved by fractional crystallization of an alkali basalt that assimilated lower crustal material. Upper crustal contamination of the magmas was not significant The 206 Pb/ 204 Pb compositions of vein galenas almost entirely overlap those of phonolites suggesting a genetic relationship between alkaline magmatism and mineralization. However, a trend toward higher 207 Pb/ 204 Pb (15.57-15.60) and a 208 Pb/ 204 Pb ratios (38.94-39.48) of some galenas suggests a contribution to the ore fluid from surrounding Early Proterozoic rocks, probably through leaching by mineralizing fluids. Limited stable isotope compositions of quartz, K feldspar, and biotite from this and previous studies support a largely magmatic origin for the early vein fluids.It is suggested that three features were collectively responsible for generating alkaline magmas and associated mineral deposits: (1) the timing of magmatism and mineralization, which coincided with the transition between subduction-related compression and extension related to continental rifting; (2) the location of Cripple Creek at the junction of four major Precambrian units and at the intersection of major northeast-trending regional structures with northwest-trending faults, which served as conduits for magmas and subsequent hydrothermal fluids; and (3) the complex magmatic history which included emplacement of relatively felsic magmas followed by successively more mafic magmas with time.

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