Geologic Studies in Alaska by the U.S. Geological Survey, 1992

More documents

Recommendations

Info

RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMATICS OF VEIN MINERALS IN THE JUNEAU GOLD BELT, ALASKA By Ronald W. Kistler, Rainer J. Newberry, and David A. Brew INTRODUCTION ANALYTICAL TECHNIQUES Potassium-argon and rubidium-strontium techniques have been used to determine the ages of Early Cretaceous gold-bearing quartz veins in the Mother Lode (Kistler and others, 1983) and in the northern Sierra Nevada foothills metamorphic belt (Bohlke and Kistler, 1986) in California. The ages of individual veins determined by the two techniques were the same, but the Rb-Sr isochrons yielded additional information about sources of the ore-bearing fluids by virtue of the initial 87~r/86~r value of the vein minerals that was calculated in the age determination. In fact, vein carbonate, albite, chlorite, or quartz generally have such low Rb abundances that their measured 87~r/86~r values are close approximations of the initial 87~r/86~r value of mineralizing fluids. The Juneau gold belt extends for about 200 km within 5 km of the Coast Range megalineament of Brew and Ford (1978) in southeastern Alaska (fig. 1). Oxygen and hydrogen isotope compositions of vein minerals along the belt indicate that the ore fluids had a deep crustal source, most likely of metamorphic origin, whereas values of 634~ have a provinciality that suggests that sulfur in the vein minerals was derived mostly from rocks near the sites of ore deposition (Goldfarb and others, 1991a). A K-Ar age of hydrothermal muscovite from the Alaska-Juneau deposit is 55.3k2.1 Ma (Goldfarb and others, 1988) and a Rb-Sr isochron on vein alteration minerals indicated an age of 56.7k3.7 Ma for the same deposit (Newberry and Brew, 1988). This report documents Rb-Sr abundances and isochron ages of quartz, carbonate, albite, phlogopite, chlorite, and white mica from gold-bearing veins from three mines along the northern half of the gold belt (fig. 1): the Alaska-Juneau deposit in the Wrangellia terrane (reported by Newberry and Brew, 1988), the Treadwell deposit in the Gravina overlap assemblage, and the Kensington deposit in the Wrangellia terrane adjacent to Gravina overlap assemblage rocks. Oxygen, hydrogen, and carbon isotope data also are reported for the minerals from the vein at the Alaska-Juneau mine. Veins were crushed and essentially pure mineral species were separated by standard heavy liquid and magnetic techniques. Pure mica from the Kensington specimens could not be separated from included carbonate by the standard techniques. These micas were treated by attempting to remove the carbonate in 1N HCI. This exercise was successful in cleaning only one of the two mica specimens to a sufficient purity to be geochronologically useful. Rb and Sr concentrations were determined by standard isotope dilution techniques. Sr was loaded in a double rhenium filament mode and its isotopic composition was determined by multiple collection in a 90" sector mass spectrometer. Oxygen was liberated by reaction of minerals with ClF3 in nickel bombs at 550°C with the technique of Borthwick and Harmon (1982). Hydrogen isotopic composition of phlogopite was determined using the method of Godfrey (1962). Analytical results are given in table 1. All strontium isotope ratios are normalized to 86~r/88~r=0. 1 194. The decay constant for 87~b=1 .42x 10-"~r-'. The 6 values for oxygen and hydrogen are given in per mil relative to SMOW (standard mean ocean water), and the value for carbon is given in per mil relative to the PDB (Pee Dee belemnite) standard. AGE AND STRONTIUM ISOTOPE RESULTS The rubidium-strontium ages for the Alaska-Juneau, Treadwell, and Kensington vein minerals are 56.7k3.7 Ma, 56.1k3.1 Ma, and 47.6k1.6 Ma, respectively. The data are plotted on a strontium evolution diagram in figure 2. The Treadwell and Kensington deposits have similar initial 87~r/86~r values of 0.70357k13 and 0.70327k8, respectively. The Alaska-Juneau vein has a much more radiogenic initial 87~r/86~r value of 0.70822230. Figure 3, an expanded view of the Treadwell and Alaska-Juneau data, emphasizes that the quartz and chlorite from the Treadwell
RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMATICS OF VEIN MINERALS, JUNEAU GOLD BELT locality are not in isotopic equilibrium with the coexisting ankerite, albite, and white mica. DISCUSSION The Rb-Sr mineral-isochron ages of the Alaska-Juneau and Treadwell deposits are the same within experimental error, and they are also the same as the vein muscovite K- Ar age from the Alaska Juneau mine (Newberry and Brew, 1988) and the 40~r/39~r mica ages (Goldfarb and others, 1991b) for both of these deposits. However, the rubidiumstrontium data yield a significantly younger age (47 Ma) for our Kensington sample, which is not in agreement with a 55-Ma age of mica from the Kensington deposit by the 40~r/39Ar technique (Goldfarb and others, 1991b). Our Kensington sample (90RN102, table 1) is from a musco- 237 vite-rich vein about 15 m away from the ore zone, and on structural grounds is thought to have formed during the same event as the ore sample. However, on existing time scales (for example, Palmer, 1983) it formed in the middle Eocene rather than in the early Eocene, the common date obtained from all the other specimens dated in the Juneau gold belt. Goldfarb and others (1991a) noted a strong provinciality in sulfur isotope values of sulfide minerals from the mines along the Juneau gold belt. They interpreted this to be due to a regional metamorphic mineralizing fluid with a uniform sulfur isotope composition that acquired much of its sulfur from rocks near the sites of ore deposition. The strontium isotope values reported here have a provinciality that correlates positively with that observed for sulfur isotopes from the same three mines. The lightest (most Figure 1. Mineral occurrences within Juneau gold belt that were sampled for Rb-Sr mineral isochron studies. Terrane boundaries and designations from Brew and others (1991).
Page 1 and 2:
Geologic Studies in Alaska by the U
Page 3 and 4:
CONTENTS Introduction Cynthia Dusel
Page 5 and 6:
CONTENTS CONTRIBUTORS TO THIS BULLE
Page 7 and 8:
GEOLOGIC STUDIES IN ALASKA BY THE U
Page 9 and 10:
LATE HOLOCENE LONGITUDINAL AND PARA
Page 11 and 12:
Page 13 and 14:
Page 15 and 16:
Page 17 and 18:
Page 19 and 20:
DEEP-WATER LITHOFACIES AND CONODONT
Page 21 and 22:
Page 23 and 24:
Page 25 and 26:
Table 1. Locality register for key
Page 27 and 28:
Table 1. Locality register for key
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
LITHOFACIES AND CONODONTS OF CARBON
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
LITHOFACES AND CONODONTS OF CARBONI
Page 51 and 52:
Table 1. Conodont faunules and lith
Page 53 and 54:
Page 55 and 56:
DEPOSITIONAL SEQUENCES IN ATIGUN SY
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
U-Pb AGES OF ZIRCON, MONAZITE, AND
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
APPEAL FOR NONPROLIFERATION OF ESCA
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
FAVORABLE AREAS FOR METALLIC MINERA
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
GOLD AND CINNABAR IN HEAVY-MINERAL
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
EARLY CENOZOIC DEPOSITIONAL SYSTEMS
Page 109 and 110:
Page 111 and 112:
EARLY CENOZOIC DEPOSITIONAL S YSTEM
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
RESERVOIR FRAMEWORK ARCHITECTURE, C
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137:
GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
Page 140 and 141:
134 GEOLOGIC STUDIES IN ALASKA BY '
Page 142 and 143:
136 GEOLOGIC STUDIES IN ALASKA BY T
Page 144 and 145:
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155:
GEOLOGIC STUDIES IN ALASKA BY THE U
Page 156 and 157:
1 150 GEOLOGIC STUDIES IN ALASKA BY
Page 158 and 159:
Page 160 and 161:
Page 162 and 163:
Page 164 and 165:
Page 166 and 167:
Page 168 and 169:
Page 170 and 171:
1 64 GEOLOGIC STUDlES IN ALASKA BY
Page 172 and 173:
166 GEOLOGIC STUDES IN ALASKA BY TH
Page 174 and 175:
Page 176 and 177:
Page 178 and 179:
Page 180 and 181:
Table 1. Isotopic ages of intrusive
Page 182 and 183:
Table 1. Isotopic ages of intrusive
Page 184 and 185:
GEOCHEMICAL EVALUATION OF STREAM-SE
Page 186 and 187:
Page 188 and 189:
Page 190 and 191:
Page 192 and 193: 186 GEOLOGIC STUDIES IN ALASKA BY T
Page 206 and 207: 20 GEOLOGIC STUDIES IN ALASKA BY TH
Page 212 and 213: Table 3. Summary of geochemical sig
Page 216: 210 GEOLOGIC STUDIES IN ALASKA BY T
Page 219 and 220: GEOCHEMICAL CHARACTER OF UPPER PALE
Page 225 and 226: RECONNAISSANCE GEOCHEMISTRY OF BASA
Page 235 and 236: OSTRACODE ASSEMBLAGES FROM MODERN B
Page 241: OSTRACODE ASSEMBLAGES FROM MODERN B
Page 245 and 246: RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
Page 247 and 248: US. GEOLOGICAL SURVEY REPORTS ON AL
Page 249 and 250: U.S. GEOLOGICAL SURVEY REPORTS ON A
Page 251 and 252: U.S. GEOLOGICAL SURVEY REPOR TS ON
Page 253 and 254: REPORTS ABOUT ALASKA IN NON-USGS PU
Page 255 and 256: REPORTS ABOUT ALASKA IN NON-USGS PU
show all

Geologic Studies in Alaska by the U.S. Geological Survey, 1992

Create successful ePaper yourself

Delete template?

Save as template?