Red Dog District Research Articles

Diagenetic Controls on the Formation of the Anarraaq Clastic-Dominated Zn-Pb-Ag Deposit, Red Dog District, Alaska

AbstractThe Anarraaq clastic-dominated (CD) Zn-Pb-Ag deposit (Red Dog district, Alaska, USA) has an inferred mineral resource of 19.4 Mt at 14.4% Zn, 4.2% Pb, and 73 g/t Ag and is spatially associated with a separate ~1 Gt barite body. This study presents new cross sections and petrographic evidence from the Anarraaq area. The barite body, previously shown to have formed in a shallow subsurface environment akin to a methane cold seep, contains multiple generations of barite with locally abundant calcite masses, which are discordant to sedimentary laminae, and is underlain by an interval of massive pyrite containing abundant framboids and radiolarians. Calcite and pyrite are interpreted to have formed by methane-driven diagenetic alteration of host sediment at the sulfate-methane transition (SMT). The sulfide deposit contains two zones of Zn-Pb mineralization bounded by faults of unknown displacement. The dominant hydrothermal minerals are marcasite, pyrite, sphalerite, quartz, and galena. The presence of hydrothermal pseudomorphs after barite, early pyrite resembling diagenetic pyrite associated with the barite body, and hydrothermal quartz and sphalerite filling voids formed by dissolution of carbonate all suggest that host sediment composition and origin was similar to that of the barite body prior to hydrothermal mineralization. Rhenium-osmium isochron ages of Ikalukrok mudstone (339.1 ± 8.3 Ma), diagenetic pyrite (333.0 ± 7.4 Ma), and hydrothermal pyrite (334.4 ± 5.3 Ma) at Anarraaq are all within uncertainty of one another and of an existing isochron age (~338 Ma) for the Main deposit in the Red Dog district. This indicates that the Anarraaq deposit formed soon after sedimentation and that hydrothermal activity was approximately synchronous in the district. The initial Os composition of the Anarraaq isochrons (0.375 ± 0.019–0.432 ± 0.025) is consistent with contemporaneous seawater, indicating that a mantle source was not involved in the hydrothermal system. This study highlights the underappreciated but important role of early, methane-driven diagenetic processes in the paragenesis of some CD deposits and has important implications for mineral exploration.

More than a trace of oxygen: Ichnological constraints on the formation of the giant Zn-Pb-Ag ± Ba deposits, Red Dog district, Alaska

Sediment-hosted massive sulfide (SHMS) deposits are an important source of global zinc resources, and the Red Dog Pb-Zn-Ag ± Ba district in Alaska (USA) contains giant deposits of this type. The existing model for ore formation at Red Dog involves early diagenetic replacement of sediment deposited in a restricted basin with stratified suboxic bottom waters. We present new observations of trace fossils Schaubcylindrichnus ichnospecies (isp.) and Chondrites isp. in several Red Dog deposits. The presence of the trace fossils, the size of the largest burrows, and the pervasiveness of the ichnofabric indicate that at least some intervals of the host sediment were deposited in an oxygenated middle to outer shelf environment. The burrow linings and infill are replaced by barite, hydrothermal quartz, and sulfide minerals, and the lack of compaction suggests that mineralization was diagenetically early. To reconcile these data with those from previous regional sedimentological and lithogeochemical studies, we propose a new model whereby the ore-hosting sediment was deposited in a shelfal setting in which redox conditions were affected by a fluctuating oxygen minimum zone. The strong spatial correlation between bioturbation and Red Dog SHMS deposits suggests that the presence of trace fossils may have played an important role in controlling the flow of ore-forming fluids by increasing host sediment permeability.

Depositional Conditions for the Kuna Formation, Red Dog Zn-Pb-Ag-Barite District, Alaska, Inferred from Isotopic and Chemical Proxies

Water column redox conditions, degree of restriction of the depositional basin, and other paleoenvironmental parameters have been determined for the Mississippian Kuna Formation of northwestern Alaska from stratigraphic profiles of Mo, Fe/Al, and S isotopes in pyrite, C isotopes in organic matter, and N isotopes in bulk rock. This unit is important because it hosts the Red Dog and Anarraaq Zn-Pb-Ag ± barite deposits, which together constitute one of the largest zinc resources in the world. The isotopic and chemical proxies record a deep basin environment that became isolated from the open ocean, became increasingly reducing, and ultimately became euxinic. The basin was ventilated briefly and then became isolated again just prior to its demise as a discrete depocenter with the transition to the overlying Siksikpuk Formation. Ventilation corresponded approximately to the initiation of bedded barite deposition in the district, whereas the demise of the basin corresponded approximately to the formation of the massive sulfide deposits. The changes in basin circulation during deposition of the upper Kuna Formation may have had multiple immediate causes, but the underlying driver was probably extensional tectonic activity that also facilitated fluid flow beneath the basin floor. Although the formation of sediment-hosted sulfide deposits is generally favored by highly reducing conditions, the Zn-Pb deposits of the Red Dog district are not found in the major euxinic facies of the Kuna basin, nor did they form during the main period of euxinia. Rather, the deposits occur where strata were permeable to migrating fluids and where excess H2S was available beyond what was produced in situ by decomposition of local sedimentary organic matter. The known deposits formed mainly by replacement of calcareous strata that gained H2S from nearby highly carbonaceous beds (Anarraaq deposit) or by fracturing and vein formation in strata that produced excess H2S by reductive dissolution of preexisting barite (Red Dog deposits).

ZINC ISOTOPES IN SPHALERITE FROM BASE METAL DEPOSITS IN THE RED DOG DISTRICT, NORTHERN ALASKA

Analyses of sphalerite samples from shale-hosted massive sulfide and stratigraphically underlying vein breccia deposits in the Red Dog district in northern Alaska show a range of δ 66Zn values from zero to 0.60 per mil. The lowest values are observed in the vein breccia deposits, and the stratigraphically overlying (but structurally displaced) shale-hosted massive sulfide deposits show a systematic trend of increasing δ 66Zn values from south to north (Main-Aqqaluk-Paalaaq-Anarraaq). The δ 66Zn values are inversely correlated with sphalerite Fe/Mn ratio and also tend to be higher in low Cu sphalerite, consistent with precipitation of lower δ 66Zn sphalerite closer to the principal hydrothermal fluid conduits. The most likely control on isotopic variation is Rayleigh fractionation during sulfide precipitation, with lighter zinc isotopes preferentially incorporated in the earliest sphalerite to precipitate from ore fluids at deeper levels (vein breccias) and close to the principal fluid conduits in the orebodies, followed by precipitation of sulfides with higher δ 66Zn values in shallower and/or more distal parts of the flow path. There is no systematic variation among the paragenetic stages of sphalerite from a single deposit, suggesting an isotopically homogeneous zinc source and consistent transport-deposition conditions and/or dissolution-reprecipitation of earlier sphalerite without significant fractionation. Decoupled Zn and S isotope compositions are best explained by mixing of separate metal- and sulfur-bearing fluids at the depositional site. The results confirm that Zn isotopes may be a useful tracer for distinguishing between the central and distal parts of large hydrothermal systems as previously suggested and could therefore be of use in exploration.

Reactive flow models of the Anarraaq Zn–Pb–Ag deposit, Red Dog district, Alaska

The Red Dog ore deposit district in the Brooks Range of northern Alaska is host to several high-grade, shale-hosted Zn + Pb deposits. Due to the complex history and deformation of these ore deposits, the geological and hydrological conditions at the time of formation are poorly understood. Using geological observations and fluid inclusion data as constraints, numerical heat and fluid flow simulations of the Anarraaq ore deposit environment and coupled reactive flow simulations of a section of the ore body were conducted to gain more insight into the conditions of ore body formation. Results suggest that the ore body and associated base metal zonation may have formed by the mixing of oxidized, saline, metal-bearing hydrothermal fluids (<200°C) with reducing, HS-rich pore fluids within radiolarite-rich host rocks. Sphalerite and galena concentrations and base metal sulfide distribution are primarily controlled by the nature of the pore fluids, i.e., the extent and duration of the HS− source. Forward modeling results also predict the distribution of pyrite and quartz in agreement with field observations and indicate a reaction front moving from the initial mixing interface into the radiolarite rocks. Heuristic mass calculations suggest that ore grades and base metal accumulation comparable to those found in the field (18% Zn, 5% Pb) are predicted to be reached after about 0.3 My for initial conditions (30 ppm Zn, 3 ppm Pb; 20% deposition efficiency).

A Special Issue Devoted to Barite and Zn-Pb-Ag Deposits in the Red Dog District, Western Brooks Range, Northern Alaska

A Special Issue Devoted to Barite and Zn-Pb-Ag Deposits in the Red Dog District, Western Brooks Range, Northern Alaska

Structure of the Red Dog District, Western Brooks Range, Alaska

Structure of the Red Dog District, Western Brooks Range, Alaska

Origin of the Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska: Evidence from Regional Pb and Sr Isotope Sources

Origin of the Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska: Evidence from Regional Pb and Sr Isotope Sources

Structure of the Red Dog District, Western Brooks Range, Alaska

The Red Dog district of the western Brooks Range of northern Alaska, which includes the sediment-hosted Zn-Pb-Ag ± Ba deposits at Red Dog, Su-Lik, and Anarraaq, contains one of the world’s largest reserves of zinc. This paper presents a new model for the structural development of the area and shows that understanding the structure is crucial for future exploration efforts and new mineral discoveries in the district. In the Red Dog district, a telescoped Late Devonian through Jurassic continental passive margin is exposed in a series of subhorizontally stacked, internally imbricated, and regionally folded thrust sheets. These sheets were emplaced during the Middle Jurassic to Late Cretaceous Brookian orogeny and subsequently were uplifted by late tectonic activity in the Tertiary. The thrust sheet stack comprises seven tectonostratigraphically distinct allochthonous sheets, three of which have been subject to regional and detailed structural analysis. The lowermost of these is the Endicott Mountains allochthon, which is overlain by the structurally higher Picnic Creek and Kelly River allochthons. Each individual allochthon is itself internally imbricated into a series of tectonostratigraphically coherent and distinct thrust plates and subplates. This structural style gives rise to duplex development and imbrication at a range of scales, from a few meters to tens of kilometers. The variable mechanical properties of the lithologic units of the ancient passive margin resulted in changes in structural styles and scales of structures across allochthon boundaries. Structural mapping and analysis of the district indicate a dominant northwest to west-northwest direction of regional tectonic transport. Local north to north-northeast transport of thrust sheets is interpreted to reflect the influence of underlying lateral and/or oblique ramps, which may have been controlled by inherited basin margin structures. Some thrust-sheet stacking patterns suggest out-of-sequence thrusting. The west-northwest–east-southeast–trending Wrench Creek and Sivukat Mountain faults were previously interpreted to be strike-slip faults, but this study shows that they are Tertiary (Eocene?) late extensional faults with little or no lateral displacement.

Depositional Settings, Correlation, and Age of Carboniferous Rocks in the Western Brooks Range, Alaska

The Kuna Formation (Lisburne Group) in northwest Alaska hosts the Red Dog and other Zn-Pb-Ag massive sulfide deposits in the Red Dog district. New studies of the sedimentology and paleontology of the Lisburne Group constrain the setting, age, and thermal history of these deposits. In the western and west-central Brooks Range, the Lisburne Group includes both deep- and shallow-water sedimentary facies and local volcanic rocks that are exposed in a series of thrust sheets or allochthons. Deep-water facies in the Red Dog area (i.e., the Kuna Formation and related rocks) are found chiefly in the Endicott Mountains and structurally higher Picnic Creek allochthons. In the Red Dog plate of the Endicott Mountains allochthon, the Kuna consists of at least 122 m of thinly interbedded calcareous shale, calcareous spiculite, and bioclastic supportstone (Kivalina unit) overlain by 30 to 240 m of siliceous shale, mudstone, calcareous radiolarite, and calcareous lithic turbidite (Ikalukrok unit). The Ikalukrok unit in the Red Dog plate hosts all massive sulfide deposits in the area. It is notably carbonaceous, is generally finely laminated, and contains siliceous sponge spicules and radiolarians. The Kuna Formation in the Key Creek plate of the Endicott Mountains allochthon (60–110 m) resembles the Ikalukrok unit but is unmineralized and has thinner carbonate layers that are mainly organic-rich dolostone. Correlative strata in the Picnic Creek allochthon include less shale and mudstone and more carbonate (mostly calcareous spiculite). Conodonts and radiolarians indicate an age range of Osagean to early Chesterian (late Early to Late Mississippian) for the Kuna in the Red Dog area. Sedimentologic, faunal, and geochemical data imply that most of the Kuna formed in slope and basin settings characterized by anoxic or dysoxic bottom water and by local high productivity. Shallow-water facies of the Lisburne Group in the Red Dog area are present locally in the Endicott Mountains allochthon and throughout the Kelly River allochthon and consist of the Utukok and Kogruk Formations. The Utukok Formation is an impure limestone with disseminated and interbedded noncarbonate mud and quartz-rich silt and sand. Clean carbonate predominates in the overlying Kogruk Formation but has been widely altered to dolostone and chert. Deep-water strata, similar to that in the Kuna Formation, overlie and grade laterally into shallow-water rocks in the Endicott Mountains allochthon and compose the uppermost Lisburne in the Kelly River allochthon. The Utukok Formation is mainly Osagean but its base is Kinderhookian (early Early Mississippian) in the highest plates of the Kelly River allochthon. The Kogruk Formation is mostly Meramecian but partly Osagean and early Chesterian in some areas. We interpret inner to middle platform settings for most of the Utukok and Kogruk Formations. Deep-water, locally phosphatic facies of the uppermost Lisburne Group formed during a platform-drowning event of regional extent that began in the late Meramecian. In the Howard Pass area, ~120 to 250 km to the east, the Lisburne Group includes an array of shallow- and deep-water facies that correspond well in age and general depositional environment to those in the Red Dog area but differ in some details. Deep-water strata are generally thinner, include less carbonate, and formed in settings that were deeper and/or had less detrital input. Shallow-water deposits are less extensive, contain less quartz silt and sand, and formed chiefly in middle and outer platform settings. Paleogeographic reconstructions imply that carbonate platforms flanked the Kuna basin to the north and south in the west but flourished chiefly north of the basin in the east. These platforms provided carbonate and perhaps some siliciclastic detritus to the basin; siliciclastic detritus could also have come from older strata underlying the platforms and/or from the Endicott delta to the north. Carbonate turbidite deposition in the Kuna basin occurred chiefly during middle Osagean and late Meramecian-early Chesterian times and ceased when adjacent platforms drowned. A combination of local and eustatic factors probably controlled the timing of detrital input to the basin and the demise of the adjoining carbonate platforms. Platform inundation is roughly coeval with the onset of barite precipitation and sulfide mineralization, implying that regional extension was a significant component of all three events. High productivity, locally thick calcareous turbidite fill, and proximity to restricted, shallow-water carbonate platform environments are aspects of the Kuna basin that appear to have been important in forming the giant Red Dog deposits. Conodont color alteration indices of the Lisburne Group in the Red Dog area and much of the Howard Pass area are mostly 2.5 to 3.5 and indicate temperatures of 110° to 200°C. Such temperatures are typically produced by burial depths of 3,500 to 5,500 m. Because Pennsylvanian-Cretaceous strata above the Lisburne Group are less than 1,500 m thick, tectonic burial and/or an elevated geothermal gradient are needed to explain the conodont color alteration index data.

Sulfur and Oxygen Isotopes in Barite Deposits of the Western Brooks Range, Alaska, and Implications for the Origin of the Red Dog Massive Sulfide Deposits

Sulfur and oxygen isotope analyses have been obtained for barite samples from the giant stratiform sulfide-barite deposits at Red Dog in the western Brooks Range of Alaska, from stratiform barite deposits elsewhere in the Red Dog district, and from stratiform and vein and breccia barite occurrences in the central Brooks Range. Twelve of the 15 deposits studied lie within middle to Upper Mississippian black shale and chert units. The data reveal two different patterns on δ 34 S versus δ 18 O plots. The first, which is best illustrated by the barite deposit at Anarraaq, shows linear trends with slopes that vary with barite texture. For most samples, δ 34 S and δ 18 O values are both higher than the values characteristic of Mississippian marine sulfate. The second pattern, which is evident at the Red Dog deposits, shows no correlation between δ 34 S and δ 18 O. In most samples, δ 18 O is below the value for Mississippian marine sulfate. Comparisons with sulfate in modern marine environments suggest a possible model for the mineralizing process. Anarraaq-type barite formed at sea-floor vents where ascending fluids carrying barium and methane encountered sulfate-bearing pore waters or bottom waters. Barite deposition was accompanied by the reduction of sulfate to H 2 S by means of microbially mediated anaerobic methane oxidation. Red Dog-type barite was formed in a manner similar to Anarraaq-type barite but was overprinted by a massive sulfide-forming event. Red Dog sulfides precipitated where metal-bearing hydrothermal fluids encountered pore waters that had been charged with H 2 S by anaerobic methane oxidation. Textural and isotopic evidence indicates that the sulfide bodies grew by consuming the available H 2 S and then by reductively dissolving barite. Dissolution of barite caused barium to be released to higher stratigraphic levels where it was reprecipitated on encountering sulfate. Isotopic evidence is presented for a link between methane venting and barite formation and raises the possibility that the coexistence of barite and sulfide at Red Dog, and the occurrence elsewhere in the district of barite-only and sulfide-only deposits, can be explained by a spectrum of vent types in the Mississippian basin analogous to the spectrum that is observed today along the modern continental margins. Authigenic barite formed at some but not all methane seeps, perhaps owing to differences in the barium content of vent fluids, differences in the relative proportion of aqueous fluid and gas emanating from vents, or differences in sulfate availability in local bottom waters. Some barite-forming seeps were later replaced by sulfides (Red Dog deposits) whereas others were not (e.g., Anarraaq barite horizon, Gull Creek, Moil). At sulfide occurrences where there is little evidence of preexisting barite (e.g., Anarraaq, Wulik, Suds), methane venting is indicated by fossils suggestive of chemosynthetic fauna. Mammiform sedimentary structures that are widespread in black chert at the top of the Kuna Formation may represent seeps that supported neither authigenic mineral formation nor chemosynthetic megafauna.

Multistage Hydrothermal Silicification and Fe-Tl-As-Sb-Ge-REE Enrichment in the Red Dog Zn-Pb-Ag District, Northern Alaska: Geochemistry, Origin, and Exploration Applications

Geochemical analyses of major, trace, and rare earth elements (REE) in more than 200 samples of variably silicified and altered wall rocks, massive and banded sulfide, silica rock, and sulfide-rich and unmineralized barite were obtained from the Main, Aqqaluk, and Anarraaq deposits in the Red Dog Zn-Pb-Ag district of northern Alaska. Detailed lithogeochemical profiles for two drill cores at Aqqaluk display an antithetic relationship between SiO2/Al2O3 and TiO2/Zr which, together with textural information, suggest preferential silicification of carbonate-bearing sediments. Data for both drill cores also show generally high Tl, Sb, As, and Ge and uniformly positive Eu anomalies (Eu/Eu* >1.0). Similar high Tl, Sb, As, Ge, and Eu/Eu* values are present in the footwall and shallow hanging wall of Zn-Pb-Ag sulfide intervals at Anarraaq but are not as widely dispersed. Net chemical changes for altered wall rocks in the district, on the basis of average Al-normalized data relative to unaltered black shales of the host Kuna Formation, include large enrichments (>50%) of Fe, Ba, Eu, V, S, Co, Zn, Pb, Tl, As, Sb, and Ge at both Red Dog and Anarraaq, Si at Red Dog, and Sr, U, and Se at Anarraaq. Large depletions (>50%) are evident for Ca at both Red Dog and Anarraaq, for Mg, P, and Y at Red Dog, and for Na at Anarraaq. At both Red Dog and Anarraaq, wall-rock alteration removed calcite and minor dolomite during hydrothermal decarbonation reactions and introduced Si, Eu, and Ge during silicification. Sulfidation reactions deposited Fe, S, Co, Zn, Pb, Tl, As, and Sb; barite mineralization introduced Ba, S, and Sr. Light REE and U were mobilized locally. This alteration and mineralization occurred during Mississippian hydrothermal events that predated the Middle Jurassic-Cretaceous Brookian orogeny. Early hydrothermal silicification at Red Dog took place prior to or during massive sulfide mineralization, on the basis of the dominantly planar nature of Zn-Pb veins, which suggests filling of fractures that developed in previously lithified rock. Uniformly low Ca and Mg and uniformly negative Ce anomalies in highly siliceous Red Dog wall rocks reflect hydrothermal decarbonation reactions and pervasive silicification owing to conductive cooling of oxidized metalliferous fluids. Similar Ca and Mg depletions are evident at Anarraaq but generally lack associated silicification, possibly because temperatures of the hydrothermal fluids were too low (<180°C) or because the thermal contrast between the fluids and wall rocks was smaller owing to the greater depth of alteration and mineralization there, compared with Red Dog. Chalcophile element anomalies (Fe, Zn, Pb, Tl, As, Sb) in wall rocks at both Red Dog and Anarraq are attributed to sulfidation reactions, coeval with subsurface Zn-Pb-Ag mineralization, during the mixing of oxidized metalliferous fluids with H2S-rich fluids derived locally within the Kuna Formation. Sedimentary wall rocks in the Red Dog district are characterized by a distinctive suite of geochemical anomalies, especially for Zn, Pb, Tl, As, Sb, Ge, and Eu/Eu*. At the Aqqaluk deposit, wall rocks without visible sphalerite or galena (<300 ppm Zn + Pb) have anomalous Eu/Eu*, Tl, Sb, and As for up to ~100 m stratigraphically below Zn-rich silica rock. At Anarraaq, the Tl anomaly is most extensively developed, and enrichment relative to unaltered black shale of the Kuna Formation is present up to 62 m above the highest Zn-Pb sulfide zones. The magnitude of the enrichment and systematic behavior of Tl in the district make Tl a promising geochemical exploration guide for Red Dog-type Zn-Pb-Ag deposits elsewhere.

Paleozoic Sedimentary Rocks in the Red Dog Zn-Pb-Ag District and Vicinity, Western Brooks Range, Alaska: Provenance, Deposition, and Metallogenic Significance

Geochemical analyses of Paleozoic sedimentary rocks in the western Brooks Range reveal a complex evolutionary history for strata surrounding the large Zn-Pb-Ag deposits of the Red Dog district. Data for major elements, trace elements, and rare earth elements (REE) were obtained on 220 samples of unaltered and unmineralized siliciclastic rocks from the Upper Devonian-Lower Mississippian Endicott Group (Hunt Fork Shale, Noatak Sandstone, Kanayut Conglomerate, Kayak Shale), the overlying Carboniferous Lisburne Group (Kuna Formation, unnamed drowned shelf facies), and the Pennsylvanian-Permian Siksikpuk Formation. Major base metal sulfide deposits of the region are present only in the Kuna Formation, which in the Red Dog district comprises siliceous black shale and black chert, minor limestone (calcareous radiolarite), and sparse lithic turbidite and bedded siliceous rock. Gray and rare black shales of the Kayak Shale and common black shales of the Kuna Formation are anomalously low in iron (avg Fe/Ti = 6.25 and 6.34, respectively) relative to other Paleozoic shales in the region (9.58–10.6) and to average shales worldwide (10.1–10.5). In contrast, the bedded siliceous rocks contain appreciable hematite (avg Fe/Ti = 35.0) and high U/Ti and REE/Ti ratios that are interpreted to reflect low amounts of detrital material and a major Fe-rich eolian component. Geochemical data (e.g., MnO <0.01 wt %; avg Cr = 317 ppm), sizes of framboidal pyrite grains, and limited bioturbation suggest anoxic and denitrifying depositional conditions for most black shales of the Kuna Formation; low Mo/Ti ratios argue against euxinic (sulfate-reducing) conditions. Organic-rich black shales of the Kuna Formation with up to 8.4 wt percent Corganic and gray to black shales of the Kayak Shale with up to 4.1 wt percent Corganic typically have only sparse pyrite (<1 wt % S) and very low iron-limited S/C ratios (mostly <0.2). Immobile element plots (e.g., Th-Zr/10-Sc) suggest that source terranes for all of the formations were dominated by one or more felsic-rich continental arcs; a small proportion of recycled sediments is present locally. A minor mafic igneous component also occurs in several shales of the Kuna and Siksikpuk Formations. High average values for the chemical index of alteration [Al2O3/(Al2O3 + CaO + Na2O + K2O)] ∞ 100 for shales of the Endicott Group (76.4–81.5) imply moderate to intense chemical weathering in source areas of these sediments. A lower average for black shales of the Kuna Formation (73.7) does not require such weathering. Textural and geochemical data record the effects of diagenetic and/or hydrothermal fluid flow in some of the Paleozoic rocks. Mobility of P, F, U, and light REE is documented in black shales of the Kuna Formation by phosphate replacements of carbonate clasts and of matrix material surrounding the clasts. A relatively low average Ce/Ce* value of 0.73 for P-poor black shales of the Kuna Formation (<0.05 wt % P2O5) and a similar Ce/Ce* value of 0.78 for a siderite concretion in Kayak Shale suggest that these diagenetic fluids were oxidizing. Many shales of the Kuna Formation have high (K2O ∞ 100)/(K2O + Al2O3) ratios of 21.0 to 23.0, which contrast with low ratios of generally <18.0 for shales of the underlying Endicott Group. The high ratios in shales of the Kuna Formation reflect preferential reaction of smectite to illite during the Jurassic-Cretaceous Brookian orogeny, owing to high silica activities in pore fluids that were generated by the dissolution of abundant biogenic silica. The distribution and composition of Paleozoic strata in the western Brooks Range may have played a fundamental role in Zn-Pb mineralization of the Red Dog district. In our model, deposition and early lithification of biogenic chert and bedded siliceous rocks in the upper part of the Kuna Formation served as a regional hydrologic seal, acting as a cap rock to heat and hydrothermal fluids during Late Mississippian base-metal mineralization. Equally important was the iron-poor composition of black shales of the Kuna Formation (i.e., low Fe/Ti ratios), which limited synsedimentary pyrite formation in precursor sediments, resulting in significant H2S production in pore waters through the interaction of aqueous sulfate with abundant organic matter. This H2S may have been critical to the subsurface deposition of the huge quantities of Zn and Pb in the district. On the basis of this model, we propose that low Fe/Ti and S/C ratios in black shale sequences are potential basin-scale exploration guides for giant sediment-hosted, stratiform Zn-Pb-Ag deposits.

A Special Issue Devoted to Barite and Zn-Pb-Ag Deposits in the Red Dog District, Western Brooks Range, Northern Alaska

The Brooks Range in northern Alaska (Fig. 1) contains an enormous accumulation of zinc and barite. The four deposits at Red Dog (Qanaiyaq, Main, Aqqaluk, and Paalaaq) have reserves and resources totaling 140.6 Mt at 16.6 percent Zn and 4.6 percent Pb (Jennings and King, 2002), which surpass most deposits worldwide in terms of grade and size (Fig. 2). Including resources of the unmined Su-Lik (38 Mt at 9% Zn + Pb; Jennings and King, 2002) and Anarraaq deposits (18 Mt at 23% Zn + Pb; King et al., 2002) northwest of Red Dog, the district has about 40 Mt of contained Zn + Pb (Fig. 2). For comparison, both the Broken Hill deposit and the Mt. Isa district (including the George Fisher, Hilton, and Mt. Isa deposits) in Australia contain more than 50 Mt Zn + Pb (Large et al., 2002). The Australian deposits are lead rich compared with the Brooks Range deposits. In terms of total zinc content, the combined deposits of the Red Dog district (~31 Mt) rival that of the Mt. Isa district (~36 Mt) and surpass Broken Hill (~25 Mt). In terms of zinc concentrate production, the Red Dog mine has the world’s largest—almost 600,000 tonnes (t) per year. The Brooks Range barite deposits are extraordinarily large. The barite body at the Anarraaq deposit alone is estimated to be as much as 1 billion metric tonnes (King et al., 2002). The Wulik, Moil, and Gull barite deposits (Fig. 1) are interpreted to be the lateral equivalents of the Anarraaq barite body (J. Clark, Teck Cominco, pers. commun., 2003), making the total area underlain by these deposits a minimum of 20 km2. Owing to a lack of published information, it is uncertain how this deposit compares with the giant barite bodies in south China, but …