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Significant Accomplishments of Research Programs: 1987

Variability Within
Ore-Deposit Models -
An Example From the
Red Dog Zinc Deposit,
Northwestern Alaska
By Jeanine M. Schmidt

drill cores from the deposit in fiscal year 1987; petrographic, fluid-inclusion, and stable-isotope studies of the sulfide mineralization and host rocks in order to determine the mineralogy, textures, and genesis of the deposit are ongoing.

These new data suggest that Red Dog differs in many ways from a classic sedex model. In the classic model (fig. 1), sheetlike bodies of laminated, fine-grained and pyrite-rich massive sulfides, often with monomineralic layers, are usually thinly

interbedded with sediment. In contrast, The Red Dog deposit is the second ore from the Red Dog Main deposit (fig. 2) largest known zinc deposit in the world, is very rarely laminated, is low in pyrite, with announced reserves of 77 million met has very abundant finely dispersed silica ric tons of 17.1 percent zinc, 5 percent lead, gangue in a predominantly semimassive and 82 grams per metric ton of silver. sulfide rock without significant sedimenWhen production begins in 1990, Red Dog tary interbeds, and is overlain by a thick will be the largest, predominantly base- and extensive barite layer. metal lode mine operating in Alaska.

Classic sedex deposits are interpreted The discovery of the Red Dog deposit to form directly on the sea floor by the by I.L. Tailleur of the USGS was first exhalation of basinal brines or convected published in 1970. The USGS, in coopera hydrothermal fluids into seawater. Preciption with the operating partners of the Reditation and gravity settling of sulfides are Dog deposit, Cominco American, Inc., has produced by the mixing of this fluid with been conducting detailed geologic studies seawater near the vent area and subsein the area since 1984. The Red Dog quent dilution, pH change, and cooling. deposit, located about 85 miles north of Subsurface mineralization in the sedex Kotzebue in the DeLong Mountains of model shown on figure 1 is limited to minor Alaska, is owned by the Northwest Alaska iron-sulfide-rich stringers and veins within Native Corporation.

the vent area and minor associated silica or Red Dog is a member of the carbonate alteration of wall rocks. In con“sediment-hosted massive sulfide” or trast, the Red Dog deposit contains few "sedex” zinc-lead class of deposits. The ore sedimentary/clastic or exhalative type is stratabound and hosted in black shales ores, and contains very common replaceand subordinate black limestones of Missis ment and multistage ore textures, in which sippian to Pennsylvanian age (290 to 360 earlier stages of sulfide mineralization are million years (m.y.) old). The deposit was overprinted by sulfide veining and solution subjected to intense faulting and moderate breccias. A zonation of ore types is folding during the Jurassic to Early Creta- observed in much of the main deposit, and ceous (63 to 205 m.y. ago) formation a baritic cap, often containing sulfides in of the Brooks Range fold-and-thrust belt. the lower portion, overlies siliceous semiBecause of this tectonic setting, previous massive to massive sulfides. These latter geologic investigations focused on identify- sulfides laterally grade into silica rock coning the local stratigraphic and structural taining only disseminated sulfides and then complexities of the deposit and defining outward into silicified shale. Diagenetic portions that are repeated by thrusting or barite blades and nodules, carbonate removed by faulting and erosion. The rhombs, and radiolaria and sponge spicules USGS completed extensive relogging of (fossils) within the host shale are replaced

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New Midcontinent
Basement Geologic
Map Provides Basis for Strue
Evaluation of Hidden
Mineral Resources

encountered by 1,500 drill holes that penetrated the Precambrian basement. Goodquality aeromagnetic and gravity data, which generally were not available until fairly recently, were used to infer the structural trends, extent, and boundaries of the rock bodies identified in the drill holes. The geology was interpreted in terms of the tectonostratigraphic terrane concept, which has been successfully applied to delineate groups of closely related rocks in exposed areas in the Lake Superior region.

By Paul K. Sims

Background

Known and potential mineral resources

The midcontinent region of the United States is renowned for its world-class basemetal mining districts, including the Upper Mississippi Valley lead-zinc district, the Illinois-Kentucky fluorspar district, the Southeast Missouri lead-zinc district, and the Tri-State (Missouri-Kansas-Oklahoma) lead-zinc district. All these districts are hosted by flat-lying carbonate rocks of Paleozoic age (as old as 570 million years).

Hidden beneath these well-known mines may be mineral resources of comparable value. The complex Precambrian rocks (more than 570 million years old), which come to the surface in Minnesota and Wisconsin and in the St. Francois Mountains of southeastern Missouri, are now being investigated for their resource potential. The Precambrian rocks exposed in Minnesota and Wisconsin contain the famed iron ores of the Lake Superior district and other valuable unmined metal deposits; those exposed in Missouri host significant iron ore bodies, some of which have associated base metals, principally copper. The rocks in these exposed seg ments extend into the subsurface. Drilling also has shown that the basement contains several other Precambrian terranes that have potential for other kinds of mineral deposits.

As part of the USGS Midcontinent Strategic and Critical Minerals Project, a Precambrian basement map of the northern midcontinent was compiled in cooperation with the Geological Surveys of Arkansas, Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, Oklahoma, South Dakota, Tennessee, and Wisconsin (fig. 3). The map is based primarily on rock types

The Precambrian basement of the northern midcontinent has a high potential for undiscovered mineral resources because many of the terranes are favorable for ore generation. Exposed parts of the region—the Lake Superior district in the north and the southwest Missouri district in the southeast-have a combined production of iron and copper ores valued at several tens of billions of dollars. Recent discoveries of as yet unmined deposits that contain large zinc-copper and coppernickel-cobalt-platinum resources in the Lake Superior region are further encouragement for exploration in this area. The relatively shallow depth of much of the buried basement-in about two-thirds of the midcontinent area the basement is less than 3,000 feet below the surface-is another positive factor.

Another intriguing possibility is that the Middle Proterozoic anorogenic St. Francois and Spavinaw granite-rhyolite terranes may contain deposits of the Olympic Dam type. The Olympic Dam deposit, at Roxby Downs, South Australia, is one of the world's premier metal deposits, which is now being developed for production. It is reported to contain at least 2,000 million metric tons of ore that has an average grade of 1.6 percent copper, 0.06 percent uranium oxide, 0.6 grams per metric ton of gold, and 3.5 grams per ton of silver. The deposit occurs within an area dominated by Middle Proterozoic anorogenic magmatism, and the ores are hosted by coarse clastic sedimentary rocks, which are interpreted mainly as breccias. Such an ore-forming environment could be present in southwestern Missouri and adjacent parts of

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EARLY PROTEROZOIC (2500–1600 million years old)
Quartzite of “Baraboo interval”—Favorable for unconformity-type uranium deposits
Metamorphic and granitoid rocks of Central Plains orogen
Rhyolite and granite(1.76 billion years old)
Granite and associated rocks (age uncertain)

Granite (1.8 billion years old)

9 Wisconsin magmatic terrane of Penokean orogen (1.8-2.1 billion years old)—Favorable for copper-zinc boood deposits

ARCHEAN (2500 million years old and older)
Greenstone-granite terrane of Superior craton (2.6–2.75 billion years old)

Gneiss of central Wisconsin (2.8–3.0 billion years old)
“. Gneiss terrane (3.0-3.6 billion years old ); includes granite (2.6 billion years old)

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------ Limit of outcrop

Fault or shear zone

Arkansas and Kansas. This area in partic- diabase sheets. Results indicate that, ular and other areas in the granite-rhyolite under silicate fractionation conditions, platterranes merit serious consideration for inum (Pt) is enriched slightly relative to exploration.

palladium (Pd) in the early cumulate stage, whereas Pd is enriched relative to Pt in late-stage differentiates. Late-stage

ferrogabbro-ferrodiorite bodies in two diaEarly Mesozoic Basins

base sheets in Pennsylvania and New Jer

sey show anomalous Pd and chlorine (CI) Workshop

abundances that are an order of magnitude greater (as much as 207 parts per billion

(ppb) Pd and 3,500 parts per million (ppm) By Gilpin R. Robinson, Jr.,

Cl) than values in most other diabase and Albert J. Froelich

bodies.

Some copper-rich hornfels associated

with diabase sheets in Pennsylvania are Approximately 110 participants from

reported to be enriched in gold (as much as government institutions, eastern universi

7 ppm) and other rare metals such as ties, and the petroleum and mining indus

molybdenum, arsenic, and bismuth. tries gathered at a workshop on the geol

Sediment-hosted, stratabound baseogy of the early Mesozoic (180–230 million

metal enrichment (as much as 2.8 percent years (m.y.) old) basins of eastern North

copper, 6,000 ppm zinc, 1,000 ppm lead, America, held at the USGS National Cen

and 40 ppm silver) is observed in black ter during May 1987. The workshop was

root-disrupted mudstones, which underlie designed as a forum where scientists could

laminated lacustrine shales in the Culpeper meet to present results and exchange

basin of Virginia. ideas, with the goal of developing a geo

This mineralization logic framework to support mineral and

appears to be largely restricted to specific energy resource studies. Significant results

sedimentary facies that occur repetitively include the following:

as part of sedimentary cycles that are conOrganic geochemical analyses and

trolled, in part, by climatic variation. Four thermal maturity indices of Mesozoic metal-enriched horizons have been identiorganic-matter-rich shales show, in the fied from one locality in the Culpeper basin. Newark basin of New Jersey and Pennsyl Epithermal base-metal and barite vania, that Triassic shales (approximately veins, which are associated with many of 225 m.y. old) are overmature with respect the early Mesozoic basins, are reported to to petroleum generation but that Jurassic have high silver contents. Fluid-inclusion, shales (approximately 195 m.y. old) contain isotope, and other studies indicate that the both mature and immature sections. Trias veins are associated with the transport of sic shales in the Taylorsville and Richmond moderate temperature (100—250 °Celsius) basins of Virginia are mature with respect and salinity (10–16 weight percent sodium to hydrocarbon generation, and hydrocar- chloride) brines from within the basins and bon shows are reported from oil test holes. adjacent basement to shallow sites of minMigration of liquid hydrocarbon in manyeral precipitation. Fluid migration may be other basins is directly indicated by its due to a seismic pumping process that was occurrence in fluid inclusions and as bitu- active during the Middle Jurassic (about men in veins. The basinwide continuity, 175 m.y. ago) during the separation of thickness, and high organic-carbon content North America from Africa. of the mature Triassic and Jurassic shales 40 Ar/9Ar analyses of feldspar sepaindicate a petroleum source of potential rates from diabase bodies in the Culpeper commercial interest in some cases. Strati basin, Virginia, and Newark basin, New graphic and structural traps for hydrocar Jersey, give argon closure ages of approxbons may be present in early Mesozoic imately 175 m.y., which are at variance basins buried beneath the Coastal Plain with the igneous crystallization age of and Atlantic Continental Shelf and in some approximately 200 m.y. The argon closure exposed basins.

age represents the age at which all argon Platinum-group-element chemistry produced by radioactive decay of potassium has been measured in a number of Jurassic remains trapped in the mineral and is not

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