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Geology, mineralization, and fluid inclusion analysis of the Ajax vein system, Cripple Creek, Colorado




Dwelley, Peter C., author

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The Ajax mine is located in the Cripple Creek mining district. Teller County, Colorado. Mine workings extend through a vertical range of 3363 feet (1025 m), from 10,105 feet (3081 m) to 6742 feet (2055 m) elevation. Production is from gold-telluride veins hosted in Precambrian granite and Tertiary breccia. Mine production amounts to more than 700,000 oz. of gold at average grades of 0.60 to 1.00 oz. gold per ton. The mine is situated on the southern margin of a Tertiary volcanic complex composed of highly differentiated alkaline rocks that intrude fine-grained breccia and minor sediment. Most of the mineralization in the Ajax is hosted by Precambrian granite which surrounds the complex. Complex formation began about 34 m.y. ago. Five stages of vein mineralization have been recognized. Vein content is dependent on the relative time at which the structure was receptive to ore-forming fluids. Vein minerals, in order of decreasing volume, consist of quartz, fluorite, pyrite, adularia, dolomite, rutile, sphalerite, hematite, galena, marcasite, calaverite, chalcopyrite, pyrrhotite, and acanthite. The Au/Ag ratio varies from 20:1 to 1:1 and is controlled by grade; higher Au/Ag ratios correspond to greater gold values. No consistent vertical trend in Au/Ag ratio has been recognized. A subtle increase in base metal content with depth may reflect initial development of a weak zonation pattern. Vein-related alteration is fracture controlled and poorly developed. Two lateral zones of alteration were defined. Inner zone alteration varies from one to three times vein width and consists of the following: complete replacement of biotite and plagioclase, quartz recrystallization, and microcline rimmed and veined by adularia. Outer-zone alteration varies from two to five times vein width and consists of partial replacement of biotite and plagioclase, unaltered quartz, and mostly unaltered microcline that may be weakly veined by adularia and quartz. Fluid inclusion analyses of quartz, fluorite, and sphalerite from stages 1 through 4 define a complex fluid evolution. Filling temperatures ranged from 206 to 510°C during stage 1 mineralization and from 123 to 350°C during stage 3. Stage 4 fluid deposited calaverite and quartz at temperatures ranging from 105 to 159°C. Salinity of the ore fluid ranged from 30 to 47 wt% eNaCl during stage 1 and decreased to between 0 and 8.3 wt% eNaCl during stage 3. Stage 4 fluid salinity was approximately equal to stage 3. Initial temperature and salinity decrease was caused by mixing of stage 1 magmatic fluid with meteoric water. Additional temperature decrease resulted from cooling of the magmatic heat source(s). The irregular vertical thermal gradient present in the vein system may be the result of lateral fluid flow caused by intersection of veins with the breccia complex and/or presence of intrusive heat sources within the breccia. CO2 has been recognized in the fluids of stages 1, 3, and 4. Subtle boiling occurred in all stages over a great vertical range, consisting primarily of CO2 effervescence. The presence of CO2 greatly increases the estimated maximimi depth at which boiling can occur. Based on stability of alteration minerals, pH is estimated to have a minimum value of 5.5 at 300°C. Opaque mineral relationships indicate oxygen fugacity ranged from -36 to -28.4 log f02. physical-chemical character of the ore fluid indicates the most amenaible gold transport mechanism. Base metals and gold were originally transported as chloride complexes. Decreasing salinity lowered chlorine iocn activity which, in conjunction with temperature decrease, decomposed the chloride complexes. Gold remained in solution by forming migration complexes with tellurium. Sulfide deposition increased the H2Te:H2S ratio, causing greater gold-tellurium complex stability. Gold remained in solution until decreasing temperature in stage 4 deposited calaverite and quartz. Character of the ore-forming fluid and close relationship of the veins to intrusive activity, among other evidence, indicate a magmatic source for the ore metals.


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Geology -- Colorado -- Cripple Creek
Mineralogy -- Colorado -- Cripple Creek
Mines and mineral resources -- Colorado -- Cripple Creek


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