Mineralogia

Economic Geology
Vol. 69, 1974, pp. 1078-1085

SomeMineralogical Characteristics the Silver Deposits of in the Guanaiuato Mining District, Mexico*
W. PETRUK AND D. Owens
Abstract

The silver ores in the Guanajuato mining district in Mexico consistof disseminated silver sulfides,selenides, and electrum in quartz-calciteveins and occur in three different ore types. The mineralogical andchemical characteristicsof the ore types were
defined and the chemical compositionsof some of the minerals were determined. It was

found that the different ore types contain approximatelythe same minerals but in different proportionsand specific mineralsoccurringin different ore types contain the same minor elements but in different quantities.
Introduction

Tt•. ores in the Guanajuatominingdistrict, Guanajuato, Mexico, occur as mineralized quartz-calcite veinsand stockworks Tertiary sediments, in volcanics, and acid intrusives(Wandke and Martinez, 1928; Echagary, 1964). The veins vary from narrow

stringers twenty metersin width and the stockto works occurin three vein systems;(1) the Sierra spots to several up centivein system,(2) the Veta Madre vein system,and (Fig. 2A),andasirregu!ar meters in diameter. It consists gray quartz, calof (3) the La Luz vein system(Fig. 1). Three types cite, nontronite,chlorite, and adularia, and contains of ores and an assemblage carbonateand silicate of disseminated grains of pyrite, acanthite, polybasite, minerals occur in ore shoots in the Sierra and Veta pyrargyrite, chalcopyrite, sphalerite, galena, silver, Madre vein systems. The oretypes were recog(Fig. 2B). The disseminated nized by W. H. Gross, President of Pure Silver gold, and marcasite grainsare between and 500 microns diameter 0.5 in Mines Limited (pers. commun.), who is currently
investigating their characteristics has tentatively and classifiedthem accordingto indicated relative ages show that much of the gray quartz is fine-grained and contains cloudsof minuteinclusions (Fig. 2C). and to elevationof ore shootsabovesea level. Type
1 is the oldest and occurs at the lowest elevation,
but most are smaller than 20 microns. Thin sections

occurrence the ores in the Guanajuatomining for camp. The mineralized parts of the veins in the Peregrinamine and the hand specimens type 2 of and type 3 oressupplied W. H. Gross by consist of mineralizeddark gray materialin relativelybarren white quartz and calcite. The mineralized dark gray material is present as crenulatedhair-like stringers, poorlydefined as bands to 20 cm wide up

The composition the inclusions not determined of was because electronmicroprobe and X-ray diffraction type 2 is intermediate,and type 3 is the youngest did not detect them. Nevertheless, is it and occursat the highestelevation. Onesampleof analyses assumedthat these inclusionsin quartz, as well as type 1 ore, two of type 2 ore, and two of type 3 ore the disseminated grains of ore minerals impart the were providedby W. H. Gross,and the writer colgray colorto the dark gray material. The samples lectedfour samples from four veinsin the Peregrina from the Cebadamine are specimens high-grade of mine (two of type 2 and two oftype 3 ore in the silver ore and consistof mineralized vugs in ameSierra vein system), two from the Cebada mine thyst. The vugsare lined with concretionary quartz (type 2 ore in the Veta Madre vein system),and layersand containmasses mineralized of dark gray ten silicateand carbonatesamplesfrom the Valenmaterialand crystals ore minerals. The ore minof cianamine (Veta Madre vein system). The sampleserals in the dark gray material of the vugs are the were studiedby microscopical methods, and by Xsameas in the dark gray material described above. ray diffraction,electronmicroprobe, and chemical In some places, rhombohedral terminations be can analyses define the mineralogical to characteristics recognized large quartz crystalswhich occurbeon of eachore type and of the minerals. tween the...