Geologic setting and genesis of the Red Dog zinc-lead-silver deposit, western Brooks Range, Alaska

Abstract

Red Dog is a Mississippian- to Pennsylvanian-age black shale-hosted, zinc-lead-silver deposit in the De Long Mountains, western Brooks Range, Alaska. It contains drilling-indicated reserves of 77 million metric tons of 17.1 percent zinc, 5.0 percent lead, and 82 g per metric ton silver.The De Long Mountains are characterized by eight stacked and folded thrust allochthons. The six structurally lowest allochthons are composed of Devonian through Cretaceous clastic and chemical sedimentary rocks; differing lithologic assemblages of coeval units distinguish the allochthons. The two uppermost allochthons contain Jurassic or older mafic and ultramafic igneous sequences.The Red Dog deposit occurs in the second lowest allochthon, in black siliceous shale and chert of the Mississippian to Pennsylvanian Kuna Formation. A distinctive interbedded light gray calcarenite and dark gray calcareous shale termed the Kivalina unit is the stratigraphic footwall to mineralization and defines the second order sedimentary basin in which the deposit formed.The Red Dog deposit is a strata-bound accumulation of silica rock, barite, and sulfides. Silica rock consists dominantly of growth-zoned mosaic aggregates of translucent quartz grains with accessory sulfides and occurs within and peripheral to the main mass of sulfides. The barite facies contains accessory sulfides, silica, and rare calcite and is concentrated toward the top and periphery of the deposit. Barite is commonly coarse grained and massive to poorly bedded. Sulfide rock textures vary from massive, chaotic, or fragmental to poorly bedded. Major sulfides in decreasing order of abundance are sphalerite, pyrite, marcasite, and galena. Rare disseminated chalcopyrite and pyrrhotite occur in sphalerite; boulangerite occurs in galena. The dominant gangue constituents are quartz, barite, and minor shale. The sulfides are generally fine grained, although coarse-grained crustiform and comb-textured sphalerite occurs in feeder veins which are best developed at the base and on the periphery of the deposit. Thin pyrobitumen-calcite veins form the latest stage fracture filling. The shales which host the deposit appear to have been silicified, and carbonate, which is a common accessory in the remainder of the upper Kuna Formation, is absent in the vicinity of Red Dog. The deposit is capped by silica- and sulfide-poor barite of the Pennsylvanian to Permian(?) lower Siksikpuk Formation.Deposit-scale structure mimics the structure of the De Long Mountains, that is, stacked thrust slices which were subjected to at least two directions of compression. The Main deposit consists of two major mineralized plates and one minor one with their associated capping postore rocks, lying within the Red Dog sequence of the Brooks Range allochthon. The deposit lies on a tectonic melange zone which separates it from the Cretaceous Okpikruak Formation of the underlying allochthon. Restoration of the two main mineralized plates to their relative prethrust position indicates that the Red Dog deposit was composed of a central core enriched in zinc and lead with a relatively iron-enriched halo. The deposit is weakly enriched upward in lead relative to zinc and more strongly depleted upward in iron relative to both lead and zinc. Limited sulfur isotope determinations indicate a fairly wide range and relatively light values for sulfide sulfur at Red Dog. These light values contrast sharply with heavier sulfur in basinwide sedimentary pyrite. There is some evidence to suggest that Red Dog formed partially by sediment replacement perhaps at a time when conditions in the basin were gradually becoming more oxidizing. The observed sulfur isotope values in the synsedimentary portion of the Red Dog deposit may, therefore, have been produced by constant mixing of a buoyant hydrothermal plume with suboxic seawater.A well-developed vent biota is preserved at Red Dog, particularly in silica rock. Slightly curved to sinuous tubular structures with organic-rich siliceous walls bear similarities to the alvinellid worms observed around modern-day sea-floor hydrothermal vents. Associated with the tubes are oblong chert pellets that contain very small carbon spheres of probable biogenic origin.The Red Dog deposit formed during the early stages of a long-lived starved sedimentary basin. Whereas some textures at Red Dog can be interpreted to be replacement in origin, the presence of conglomerates and scour features in sulfides indicates that at least part of the deposit formed at or near the same time as the enclosing stratigraphy. The deposit probably formed by a combination of sediment replacement and deposition on the sea floor. Tectonic instability at the time of ore formation is indicated by abrupt lateral facies changes evident in the Mississippian to Pennsylvanian basin. Small igneous intrusions of possible Mississippian to Pennsylvanian age provide evidence for locally elevated heat flow. The underlying permeable Upper Devonian to Lower Mississippian(?) fiuvial-deltaic rocks may have provided a basin aquifer through which the mineralizing fluids moved prior to exhalation on the sea floor.Pennsylvanian-age change to oxidizing conditions in the basin terminated the sulfide depositional event of Red Dog, followed by the cessation of major barite accumulation in Pennsylvanian to Permian(?) time.