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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

The Red Dog deposit is the second largest known zinc deposit in the world, with announced reserves of 77 million metric tons of 17.1 percent zinc, 5 percent lead, and 82 grams per metric ton of silver. When production begins in 1990, Red Dog will be the largest, predominantly basemetal lode mine operating in Alaska.

The discovery of the Red Dog deposit by I.L. Tailleur of the USGS was first published in 1970. The USGS, in cooperation with the operating partners of the Red Dog deposit, Cominco American, Inc., has been conducting detailed geologic studies in the area since 1984. The Red Dog deposit, located about 85 miles north of Kotzebue in the DeLong Mountains of Alaska, is owned by the Northwest Alaska Native Corporation.

Red Dog is a member of the "sediment-hosted massive sulfide" or "sedex" zinc-lead class of deposits. The ore is stratabound and hosted in black shales and subordinate black limestones of Mississippian to Pennsylvanian age (290 to 360 million years (m.y.) old). The deposit was subjected to intense faulting and moderate folding during the Jurassic to Early Cretaceous (63 to 205 m.y. ago) formation of the Brooks Range fold-and-thrust belt. Because of this tectonic setting, previous geologic investigations focused on identifying the local stratigraphic and structural complexities of the deposit and defining portions that are repeated by thrusting or removed by faulting and erosion. The USGS completed extensive relogging of

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, ore from the Red Dog Main deposit (fig. 2) is very rarely laminated, is low in pyrite, has very abundant finely dispersed silica gangue in a predominantly semimassive sulfide rock without significant sedimentary interbeds, and is overlain by a thick and extensive barite layer.

Classic sedex deposits are interpreted to form directly on the sea floor by the exhalation of basinal brines or convected hydrothermal fluids into seawater. Precipitation and gravity settling of sulfides are produced by the mixing of this fluid with seawater near the vent area and subsequent dilution, pH change, and cooling. Subsurface mineralization in the sedex model shown on figure 1 is limited to minor iron-sulfide-rich stringers and veins within the vent area and minor associated silica or carbonate alteration of wall rocks. In contrast, the Red Dog deposit contains few sedimentary/clastic or exhalative type ores, and contains very common replacement and multistage ore textures, in which earlier stages of sulfide mineralization are overprinted by sulfide veining and solution breccias. A zonation of ore types is observed in much of the main deposit, and a baritic cap, often containing sulfides in the lower portion, overlies siliceous semimassive to massive sulfides. These latter sulfides laterally grade into silica rock containing only disseminated sulfides and then outward into silicified shale. Diagenetic barite blades and nodules, carbonate rhombs, and radiolaria and sponge spicules (fossils) within the host shale are replaced

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

By Paul K. Sims

Background

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 segments 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

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.

Known and potential mineral resources

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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MIDDLE PROTEROZOIC (1600-900 million years old)

Midcontinent rift system (1.0-1.2 billion years old)—Favorable for copper-nickel-platinum deposits and petroleum

+ H Rhyolite and granite (1.35-1.48 billion years old)—Favorable for Olympic Dam- type iron-copperr. | uranium-gold deposits

Anorogenic anorthosite and rapakivi granite (1.48 billion years old}

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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)

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

, Limit of outcrop
- Fault or shear zone

Arkansas and Kansas. This area in particular and other areas in the granite-rhyolite terranes merit serious consideration for exploration.

Early Mesozoic Basins Workshop

By Gilpin R. Robinson, Jr., and Albert J. Froelich

Approximately 110 participants from government institutions, eastern universities, and the petroleum and mining industries gathered at a workshop on the geology of the early Mesozoic (180-230 million years (m.y.) old) basins of eastern North America, held at the USGS National Center during May 1987. The workshop was designed as a forum where scientists could meet to present results and exchange ideas, with the goal of developing a geologic framework to support mineral and energy resource studies. Significant results include the following:

Organic geochemical analyses and thermal maturity indices of Mesozoic organic-matter-rich shales show, in the Newark basin of New Jersey and Pennsylvania, that Triassic shales (approximately 225 m.y. old) are overmature with respect to petroleum generation but that Jurassic shales (approximately 195 m.y. old) contain both mature and immature sections. Triassic shales in the Taylorsville and Richmond basins of Virginia are mature with respect to hydrocarbon generation, and hydrocarbon shows are reported from oil test holes. Migration of liquid hydrocarbon in many other basins is directly indicated by its occurrence in fluid inclusions and as bitumen in veins. The basinwide continuity, thickness, and high organic-carbon content of the mature Triassic and Jurassic shales

indicate a petroleum source of potential commercial interest in some cases. Stratigraphic and structural traps for hydrocarbons may be present in early Mesozoic basins buried beneath the Coastal Plain and Atlantic Continental Shelf and in some exposed basins.

Platinum-group-element chemistry has been measured in a number of Jurassic

diabase sheets. Results indicate that, under silicate fractionation conditions, platinum (Pt) is enriched slightly relative to 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 diabase sheets in Pennsylvania and New Jersey show anomalous Pd and chlorine (CI) abundances that are an order of magnitude greater (as much as 207 parts per billion (ppb) Pd and 3,500 parts per million (ppm) CI) than values in most other diabase bodies.

Some copper-rich hornfels associated with diabase sheets in Pennsylvania are reported to be enriched in gold (as much as 7 ppm) and other rare metals such as molybdenum, arsenic, and bismuth.

Sediment-hosted, stratabound basemetal enrichment (as much as 2.8 percent copper, 6,000 ppm zinc, 1,000 ppm lead, and 40 ppm silver) is observed in black root-disrupted mudstones, which underlie laminated lacustrine shales in the Culpeper basin of Virginia. This mineralization appears to be largely restricted to specific sedimentary facies that occur repetitively as part of sedimentary cycles that are controlled, in part, by climatic variation. Four metal-enriched horizons have been identified from one locality in the Culpeper basin.

Epithermal base-metal and barite veins, which are associated with many of the early Mesozoic basins, are reported to have high silver contents. Fluid-inclusion, isotope, and other studies indicate that the veins are associated with the transport of moderate temperature (100-250 "Celsius) and salinity (10-16 weight percent sodium chloride) brines from within the basins and adjacent basement to shallow sites of mineral precipitation. Fluid migration may be due to a seismic pumping process that was active during the Middle Jurassic (about 175 m.y. ago) during the separation of North America from Africa.

40Ar/39Ar analyses of feldspar separates from diabase bodies in the Culpeper basin, Virginia, and Newark basin, New Jersey, give argon closure ages of approximately 175 m.y., which are at variance with the igneous crystallization age of approximately 200 m.y. The argon closure age represents the age at which all argon produced by radioactive decay of potassium remains trapped in the mineral and is not

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