Graduate Thesis Or Dissertation
 

The geology, igneous petrology, and mineral deposits of the Ataspaca mining district, Department of Tacna, Peru

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  • The Ataspaca prospect is in the Cordillera Occidental, 45 km northwest of Tacna, Peru. Epizonal intrusions satellitic to the Caplina- Ataspaca pluton were emplaced 41 m.y. ago into limestones of the Lower Jurassic Pelado Formation along the northeast-trending Chucchuco fault system. Depths of emplacement ranged from 4.5 to 6.0 km. The intrusions record the differentiation of a calc-alkalic, I-type magma system from diorite to monzogranite by fractionation of augite, plagioclase feldspar, hornblende, and biotite, and by mixing of granitic and dioritic magmas. This magmatic system is compositionally similar to the Arequipa segment of the Coastal Batholith of Peru. Seven hydrothermal systems correlate temporally and spatially with the emplacement of granodiorite and monzogranite stocks, and two magmatic-hydrothermal breccias. Replacement lodes, fissure veins, and disseminations of base metal sulfides and molybdenite are hosted by compositionally zoned, porphyry-related garnet-diopside skarn that formed within and adjacent to an aureole of potassic alteration. Base metal sulfide fissure veins formed at temperatures of 170-280 °C, based on sulfur isotopic calculations. Sulfur isotopic values (-3.3 to +1.1 permil) are consistent with a magmatic source. Elsewhere, marmatite-arsenopyrite mineralization is hosted by unzoned idocrase-garnet skarn adjacent to a shattered stock of pyrite-bearing, seriate-textured monzogranite. Most intrusive rocks have been altered to endoskarn or Ca-Na assemblages. Endoskarn is characterized by the progressive replace ment of ferromagnesian minerals by actinolite, tremolite, and diopside and formed by reciprocal exchange of Fe for Ca derived from nearby silicated limestone. Ca-Na alteration is characterized by similar calc-silicate minerals, but is distinguished by the replacement of orthoclase by plagioclase feldspar, and (or) magmatic andesine by oligoclase. Alteration temperatures are estimated from recent data (Carten, 1986) to have been between 400 and 500 °C, and fluid salinities were in excess of 23 % NaCl-equivalent. Ca-Na and potassic alteration constitute a metasomatic couplet whereby alkalies were exchanged as fluids passed through thermal gradients adjacent to intrusive contacts. Regions of inflow and out flow are characterized by Ca-Na and potassic alteration, respectively. The distribution of these alteration-types indicates that the tops and bottoms of several porphyry-type alteration systems are exposed at Ataspaca. Additional base metal lodes may be localized by northeast-trend ing faults within and near the aureole of potassic alteration, and at depth in skarn-type deposits where these structures cut limestone of the middle Pelado Formation. Regional exploration potential for porphyry- and skarn-type mineralization is high.
  • The Ataspaca prospect is in the Cordillera Occidental
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