HISTORY OF THE MISSISSIPPIAN KUNA BASIN FROM STRATIGRAPHIC PROFILES OF C- AND N-ISOTOPES: IMPLICATIONS FOR SHALE-HOSTED ZINC-LEAD-SILVER ORES OF THE RED DOG DISTRICT, NORTHWEST ALASKA

Mudrocks from three sections through the upper Kuna Fm in the Red Dog district, northwest Alaska, have been analyzed for δ¹³C of organic matter, δ¹⁵N of total N, and total organic C (TOC) to examine the history of the depositional basin that hosts the Red Dog and Anarraaq sedex Zn-Pb-Ag deposits. The TOC and δ¹³C profiles are similar among the sections (total thicknesses: 300–600 ft, separated by 8 km), which allows the rocks to be correlated and supports the interpretation that the organic matter was derived from sinking phytoplankton. The δ¹³C values are –30±1 per mil—lower than normal marine organic matter during Mississippian time (–27.8±0.8 per mil; Hayes et al, Chem Geol 161:103)—except for three 30–70 ft thick intervals where δ¹³C values show positive excursions that overlap the normal marine range. The uppermost excursion is at the top of the Kuna Fm. Values of δ¹⁵N are generally high (8.6±0.8 per mil), but tend to be lower (4–7 per mil) where δ¹³C values are high. The isotopic profiles imply that the Kuna basin had restricted communication with the broader Panthalassic Ocean except during three discrete time periods when the basin was ventilated.

To examine the relationship between paleoceanography and hydrothermal history, the isotopic profiles were compared to profiles of metal concentrations. The profiles for Ag are most insightful. Concentrations are low (<0.5 ppm) in mudstones interbedded with the Anarraaq massive sulfide deposit, but elevated (2–35 ppm) through an interval 70–140 ft higher, both proximal and distal to the deposit. The enrichments may represent syngenetic deposition of exhaled Ag.

The interval of high Ag begins at the middle δ¹³C excursion—a basin ventilation event—and ends at the uppermost δ¹³C excursion—also a basin ventilation event that coincided with formation of the Red Dog sulfide deposits in the uppermost Kuna Fm. The correlation of hydrothermal venting and basin ventilations may reflect a shared driving mechanism: tectonic activity that involved extensional faulting and possibly magmatism. The stratigraphic extent of the Ag anomaly suggests that hydrothermal activity was protracted and episodic, so that the Anarraaq deposit, which is stratigraphically lower, and the Red Dog deposits, at the top of the Kuna Fm, could represent ore-forming events that were temporally distinct.