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Bulletin 59—Wall-rock alteration in the Cochiti mining district, New Mexico

By W. M. Bundy, 1958, 71 pp, 7 tables, 34 figs., 2 plates, 1 index.

Gold and silver mining in the Cochiti mining district began in 1889 and reached its peak in 1893. Operations continued until 1902 and have been intermittent to the present time. Production to 1915 was $1,168,772. Limited geologic information about the district was available; hence, general geologic study was made of the mineralized areas in Bland and Calla Canyons. The project included geologic mapping, a study of the quartz veins and associated ore, and a detailed investigation of wall-rock alteration. Emphasis was placed upon the relation of alteration to lithology and ore deposits. Particular attention was directed to the genesis of clay minerals and their significance in relation to hydrothermal processes.

Numerous mines in the district, most of which had not operated for several years, were examined. Extensive caving in the mines prevented observations below the zone of oxidation; however, the degree of oxidation varies considerably in the accessible mines, permitting a limited comparison between supergene and hydrothermal alteration. Such comparison, however, is limited by the pervasive character of hypogene alteration.

Hydrothermally altered volcanic and intrusive rocks of Tertiary age are associated with gold- and silver-bearing quartz veins. Two stages of epithermal quartz veins occur, but only the first stage is associated with economic mineralization. Alteration zones, named with respect to clay minerals present in the greatest abundance, are: (1) dickite, (2) illite-kaolinite, (3) vermiculite-halloysite, and (4) chlorite-montmorillonite.

Interpretation of X-ray spectrometer traces indicate that chlorites become enriched in magnesium in a veinward direction. Montmorillonites become progressively enriched in calcium toward the vein; this generalization is corroborated by chemical analyses. Illite is present in intense zones of alteration as a 1Md polymorph.

Base exchange, with resultant mixed-layer clay minerals as intermediate phases, is an important mechanism for transitions between 2:1 clay minerals. Transformation from 2:1 to 1:1 clay minerals has involved, at least partially, selective solution and recrystallization, with amorphous aluminum silicate as an intermediate phase. Order of formation of 1:1 clay minerals is apparently allophane to halloysite to kaolinite to dickite.  

Chemical analyses of the altered rocks show a marked veinward decrease in basic ions; less significant changes are shown by acidic ions. Analyses of relatively pure illites indicate a general veinward decrease in basic cations. Alteration associated with the two stages of quartz veins proceeded as acid-alkaline fronts, the second stage being more acidic because of previous leaching by first-stage solutions. Increasing intensity of alteration is regarded as a guide to quartz veins and possible ore deposits.

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