Abstract

Geologic studies during recent open-pit mining at Summitville, Colorado, have provided new information on an epithermal acid sulfate Au-Ag-Cu deposit formed in a volcanic dome. Geologic mapping, geochemical studies of whole-rock samples from blast holes, and geologic and geochemical traverse studies refine the details of the evolution of the Summitville deposit. Six distinct events followed emplacement of the quartz latite volcanic dome and define the development of the Summitville deposit: (1) an early stage of acid sulfate alteration, (2) subsequent Cu sulfide and gold mineralization, (3) widespread hydrothermal brecciation, (4) volumetrically minor, base metal sulfide-bearing barite veining, (5) volumetrically minor, kaolinite matrix brecciation, and finally, (6) supergene oxidation. Events 1 and 2 were the most important for the formation of the Summitville deposit. Acid sulfate altered rock is primarily found in the quartz latite volcanic dome rock and consists of vuggy silica (central), quartz-alunite, quartz-kaolinite, argillic, and propylitic (distal) zones. Ore deposition (event 2) of enargite, luzonite, covellite, pyrite, native sulfur, marcasite, and minor sphalerite, native gold, + or - barite and galena overprinted the acid sulfate altered rocks. Events 3, 4, and 5 are geologically important but rarely formed units large enough to be considered mineable ore by open-pit mining. The final supergene oxidation event affected much of the near-surface portion of the deposit and leached copper and sulfide minerals forming oxide zones. Near-surface oxide zones contain the highest Au grades and decrease progressively with depth.Fractures were important fbr localizing both the acid sulfate altered rock and ore deposition at Summitville. Such fracture control is apparent despite a lack of distinct throughgoing open-space quartz veins; instead, laterally and vertically extensive ore zones are identified by their highly silicified character that can be traced laterally for up to 500 m and vertically up to 200 m. Ore zones strike parallel to regional faults related to rifting and basin and range development, suggesting that hydrothermal fluids followed preexisting fractures. Ore zones follow two northwest trends, a N 30 degrees W + or - 20 degrees trend, and a N 60 degrees W trend. Most fractures and faults in the open pit dip steeply from 65 degrees to vertical. Ore zones intersect near the center of the deposit to form a large zone approximately 150 by 400 m. Ore zones form a rough radial pattern located near the center of the deposit and a discontinuous arcuate feature on the northern side. This radial pattern, arcuate feature, and the intersection of ore zones near the center of the deposit are probably related to the intrusion of a porphyry below the deposit. The overall low Au-grade, high-tonnage character of the Summitville deposit is related to low permeability in the quartz latite volcanic dome rock and absence of well-developed open-space hydrothermal veins.Gold analyses of development drill holes and blast hole samples, in conjunction with geologic mapping in the open pit, indicate that vuggy silica, quartz-alunite, and quartz-kaolinite altered rock constitute most of the Au ore. Vuggy silica averaged 2.4 ppm Au, quartz-alunite averaged 1.2 ppm Au, and quartz-kaolinite averaged 0.86 ppm Au during open-pit mining. Grades for argillically altered rock were usually below the ore cutoff of 0.34 ppm Au. Propylitically altered rock rarely contained detectable Au concentrations (0.03 ppm Au) and constituted only waste rock. Although volumetrically minor, event 4 base metal sulfide-bearing barite veins and event 5 kaolinite matrix breccias contained high Au grades (up to about 800 ppm Au), producing approximately 5 to 15 percent of the total Au mined in the deposit.Geochemical studies of samples collected across traverses of altered and mineralized ore zones indicate that Te, Bi, and Pb are characteristic trace elements, in addition to Au, Ag, Cu, and As that are typical of acid sulfate mineral deposits. Anomalous Te is associated with Cu sulfides and gold in vuggy silica zones. Anomalous Bi and Pb concentrations correlate most consistently with quartz-alunite zones. Thallium is also anomalous at Summitville. Highest concentrations of Tl are found in argillic rock, whereas silicified zones are depleted in it. The anomalous Tl in distal zones is similar to observations in some porphyry Cu systems. Pronounced lateral zonation of Ag/Au ratios is present in the deposit. The Ag/Au ratios vary from about 2:1 near the center of the orebody to greater than 20:1 on the periphery.The Summitville deposit is located in the midlevel portion of a mineralized volcanic dome, with a porphyry system at depth, and a hot springs environment at the surface. Drill holes through the volcanic dome intersected a quartz monzonite porphyry approximately 600 m below the surface that is altered to sericite and pyrite similar to some porphyry deposits. Laterally extensive cristobalite and opal replacing quartz latite are found southwest of the deposit and represent a hot-spring environment. Bedded siliceous sinter, some of which contains plant debris, overlies the cristobalite and opal at some localities. These deposits may represent surface vents for the fluids responsible for acid sulfate alteration or mineralization of the volcanic dome.

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