2003 Seattle Annual Meeting (November 25, 2003)
Paper No. 114-12
Presentation Time: 1:30 PM-5:30 PM


COOK, E.1, FRIEHAUF, K.1, GIBBINS, S.2, and TITLEY, S.2, (1) Dept. of Physical Sciences, Kutztown Univ, Kutztown, PA 19530, YxoRoxY@aol.com, (2) DEPARTMENT OF GEOSCIENCES, Univ OF ARIZONA, GOULD-SIMPSON BLDG, Tucson, AZ 85721

SEM/EDS analysis of samples from the gold-rich (>1 ppm Au) Ertsberg porphyry Cu-Au deposit demonstrate that important amounts gold were deposited in the stockwork ore zone by late stage sericitic fluids in addition to that deposited by early fluids associated with potassic alteration. Gold occurred in all three vein types studied: 1) early quartz-bornite chalcopyrite anhydrite veins, with potassic alteration (secondary biotite and K-feldspar), 2) quartz-bornite chalcopyrite veins with green sericite halos, and 3) late-stage pyrite chalcopyrite veins with broad, white sericite halos. Of the eight gold grains observed in the samples of the early-stage quartz-biotite-biotite veins (vein type 1), four grains were Au-Ag tellurides, one was a Au telluride, and three were electrum. Most gold mineral grains were between 1-3 microns, but one Au-Ag telluride grain was 20 microns. All of the electrum grains occurred on the margins of bornite grains, whereas the tellurides were associated with either bornite or chalcopyrite. Seven of the nine gold mineral grains observed in the green sericite veins (vein type 2), were electrum. Only one Au-Ag telluride grain and one native gold grain were observed in the green sericite vein samples. Gold mineral grains in the green sericite veins ranged in size from 2 to 6 microns. High fineness electrum grains (Au:Ag 20:1 to 40:1) were restricted on the margins of bornite grains associated with quartz, whereas lower-fineness electrum grains (Au:Ag 1:10 to 4:1) occurred exclusively as subequant blebs on and within chalcopyrite associated with orthoclase. Gold in the late-stage pyrite chalcopyrite-white sericite vein (vein type 3) occurred as electrum (2 grains) and one gold telluride. Electrum occurred with pyrite or chalcopyrite overgrowing pyrite, but the gold telluride grain was associated with the chalcopyrite overgrowths. All three gold mineral grains were located on the margins of sulfide grains, ranging from 5-20 microns.

2003 Seattle Annual Meeting (November 25, 2003)
Session No. 114--Booth# 12
Economic Geology (Posters) I: PGE, Magmatic, and Porphyry Deposits
Washington State Convention and Trade Center: Hall 4-F
1:30 PM-5:30 PM, Monday, November 3, 2003

Geological Society of America Abstracts with Programs, Vol. 35, No. 6, September 2003, p. 232

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