Reconnaissance Oxygen Isotope Study of Gold-Bearing Quartz Veins Within Metasedimentary Rocks of Valdez Group, Alaska: ABSTRACT

Abstract

A reconnaissance oxygen-isotope study of gold-bearing quartz veins within the metasedimentary rocks of the Valdez Group from around Prince William Sound was conducted to examine the variations and similarities within and between the different gold districts, and to investigate the origin of the ore-forming solutions. Analyses were made of mineralized samples from mines and prospects in the Port Valdez, Port Wells, Girdwood, Hope-Sunrise, and Moose Pass gold districts. In the Hope-Sunrise district the ^dgr18O values of the gold-bearing veins are relatively constant, ranging from +16.0 to +16.7 ^pmil, whereas in the Port Valdez district, the vein values are more variable (+13.9 to +17.0 ^pmil), implying variations in temperature or fluid compositions. Within individual mines, the ^dgr18O values are constant to within 0.1 ^pmil along strike in continuous veins, and vary by a maximum of 0.2 ^pmil between bands of quartz in ribbon vein samples. Petrographic and fluid inclusion data suggest a temperature of approximately 325°C for formation of the mineralized quartz veins. Calculated ^dgr18O values for fluids in equilibrium with the veins range from +8 to +12 ^pmil. A quartz separate from an altered quartz diorite intrusion at the Rough Tough mine in the Port Valdez district has a ^dgr18O value of +14.0 ^pmil; fluid in equilibrium with the intrusive at a reasonable temperature End_Page 675------------------------------ for its emplacement would have had a ^dgr18O value of approximately +12 ^pmil. These data suggest that the mineralized quartz veins formed from fluids derived from the Valdez Group during metamorphic dewatering. End_of_Article - Last_Page 676------------