wAbstract
@The Drenchwater shale-hosted Zn-Pb-Ag deposit and the immediate
vicinity, on the northern flank of the Brooks Range in north-central
Alaska, is an ideal example of a naturally low pH system. The
two drainages, Drenchwater and False Wager Creeks, which bound
the deposit, differ in their acidity and metal contents. Moderately
acidic waters with elevated concentrations of metals (pH†4.3,
Zn…1400ƒÊg/L) in the Drenchwater Creek drainage basin are attributed
to weathering of an exposed base-metal-rich massive sulfide occurrence.
Stream sediment and water chemistry data collected from False
Wager Creek suggest that an unexposed base-metal sulfide occurrence
may account for the lower pH (2.7-3.1) and very metal-rich waters
(up to 2600ƒÊg/L Zn, …260ƒÊg/L Cu and …89ƒÊg/L Tl) collected at least
2 km upstream of known mineralized exposures. These more acidic
conditions produce jarosite, schwertmannite and Fe-hydroxides
commonly associated with acid-mine drainage. The high metal concentrations
in some water samples from both streams naturally exceed Alaska
state regulatory limits for freshwater aquatic life, affirming
the importance of establishing base-line conditions in the event
of human land development. The studies at the Drenchwater deposit
demonstrate that poor water quality can be generated through entirely
natural weathering of base-metal occurrences, and, possibly unmineralized
black shale.x
1. Introduction
2. Physiography
3. Local geology
@3.1. Geologic setting
@3.2. Known mineralized zones
@3.3. Alteration
4. Methods
5. Results
@5.1. Rock and sediment geochemical signatures
@5.2. Water chemistry
@5.3. Precipitates
@5.4. Vegetation
6. Discussion
@6.1. Mechanisms of acid generation
@6.2. Acid-sources - Drenchwater Creek
@6.3. Acid-sources - False Wager Creek
@6.4. Comparison to other weathering deposits and sedimentary
rocks
@6.5. Relationship to water-quality standards
7. Conclusions
Acknowledgements
References