GeoQuiz Archives

The new GeoQuiz feature provides a monthly opportunity for you to test your knowledge of economic geology by answering a question and identifying a sample from a well known ore district or mine.


January, 2017

Answer this Question


When was the term "orogenic gold" first applied as a classification for lode gold deposits in metamorphic rocks?

Answer

The term was coined by J.K. Bohlke in 1982 in a USGS open-file report on mineral deposit models.

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Identify this Sample - January, 2017

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Uraninite Brannerite

Uraniferous quartz pebble conglomerate in the 2.35 billion year old Matinenda Formation in the Denison Mine, NNE of Elliot Lake in northern Ontarion, Canada.

The ore zone is a uraniferous oligomictic quartz-pebble conglomerate at the base of the Huronian Supergroup, resting unconformably on Archean basement. Sedimentary structures have been well preserved due to little metamorphism and limited deformation. There are approximately 5 km of mostly quartz arenites above the conglomerate. This specimen contains approximately 1.6 kg/t of U3O8. It was collected underground in the Denison Mine in autumn 1977.


January, 2017

Answer this Question


When was the term "orogenic gold" first applied as a classification for lode gold deposits in metamorphic rocks?

Answer

The term was coined by J.K. Bohlke in 1982 in a USGS open-file report on mineral deposit models.

Identify this Sample


Identify this Sample - January, 2017

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Answer

Uraninite Brannerite

Uraniferous quartz pebble conglomerate in the 2.35 billion year old Matinenda Formation in the Denison Mine, NNE of Elliot Lake in northern Ontarion, Canada.

The ore zone is a uraniferous oligomictic quartz-pebble conglomerate at the base of the Huronian Supergroup, resting unconformably on Archean basement. Sedimentary structures have been well preserved due to little metamorphism and limited deformation. There are approximately 5 km of mostly quartz arenites above the conglomerate. This specimen contains approximately 1.6 kg/t of U3O8. It was collected underground in the Denison Mine in autumn 1977.


November, 2016

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Bayan Obo, located in Inner Mongolia, China, is the world's largest REE deposit. When did this deposit form?

Answer

The age of Bayan Obo remains controversial. Most Chinese workers now favor ore formation during Mesoproterozoic carbonatite magmatism, although early and late Paleozoic magmatic events appear to have added complexity to the orebodies.

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Identify this Sample - November, 2016

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Secondary Copper in Sandstone

Copper is of secondary origin after chalcocite (Cu2S) in a porous sandstone member of the Nonesuch Formation of Keweenawan Age (1.1 Ga), White Pine Mine, White Pine, Michigan, USA.

Located in the Upper Peninsular of Michigan in the midcontinental rift, the mined portion of the Nonesuch Formation averages about 1% copper and has an organic carbon content of about 0.5%. All of the copper is stratabound with chalcocite mineralization concentrated in porous sandstone. The White Pine Mine has many petroleum seeps and was operated primarily from 1955 to 1995. White Pine, Michigan. Sample collected in 1977.


October, 2016

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China is the world's leading rare earth element producer. What types of deposits are mined in China for these elements?

Answer

Carbonatite-related REE deposits host the majority of China's REE resource. Rare earth-bearing clay deposits, or ion adsorption-type deposits, are of secondary importance but are the main source of HREE resources in China.

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Identify this Sample - October, 2016

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Volcanogenic Massive Sulfide Ore

VMS from a copper-rich zone with sphalerite bands in 2.73 Ga volcanic rocks at the Mattabi mine, 60 km north of Ignace, Ontario, Canada.

Located in the Archean Superior Province, the Mattabi mine produced 13.5 Mt of ore grading 8.28% Zn, 0.74% copper, 0.85% lead, 3 oz/t silver, and 0.29 g/t gold. As well as chalcopyrite and sphalerite, the deposit contains abundant galena, accounting for its unusual lead production. Mattabi mine was in production from 1970 to 1983. This specimen was collected from the open pit in 1977.


August, 2016

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The LCT pegmatites, end-member S-type melts, are major producers of what commodities?

Answer

The LCT pegmatites account for about one-third of the world's lithium production, most of the tantalum, and all of the cesium.

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Identify this Sample - August, 2016

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Native Silver and Chalcocite in Shale

Native silver of secondary origin on joint planes in the "parting" shale of the Keweenawan age lower Nonesuch Formation, White Pine Mine, White Pine, Michigan, USA.

Located in the Upper Peninsula of Michigan, the area produced over 11 billion pounds of copper from 1845 to 1977. Highland Copper has been active in the area as recently as 2016 although the White Pine Mine operated primarily from 1955 to 1995. The depositional environment of the sediment is controversial and has been variously suggested as estuarine, tidal lagoon or mud flat, lagoonal or deltaic interdistributary, lacustrine, and marine. Copper deposits are confined to the lower Nonesuch Formation. The base of the red massive siltstone within the parting shale has been dated at 1.1 Ga, probably representing the age of sedimentation. Brown Beds contain liquid hydrocarbons that have oxidized. The sample was collected in the summer of 1977.


July, 2016

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What is the most abundant skarn type?

Answer

Cu skarns are perhaps the world's most abundant skarn type. They are particularly common in both oceanic and continental subduction settings.

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Identify this Sample - July, 2016

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Uranium Ore

Uraninite-bearing siliceous quartzite near the base of the 2.35 Ga Matinenda Formation, a quartz pebble conglomerate, at Denison Mine, Elliot Lake, Ontario, Canada.

The host formation contains a fossilized algal mat (graphite) and many pinhead-size round clusters of fossilized algae (now graphite). Grade was approximately 1 kg/t uraninite. Pyrite was preferentially concentrated by bacteria metabolized in more porous conglomerate beds. The sample was collected underground in 1977.


June, 2016

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Seven major skarn types (Fe, Au, W, Cu, Zn, Mo, and Sn) have received significant modern study. Which of these tends to form the largest deposits?

Answer

The largest skarn deposits are the Fe skarns that are mined for their magnetite content. Although minor amounts of Cu, Co, Ni, and Au may also be present, Fe typically is the only commodity recovered. Many deposits are very large — >1,000 million tons (Mt) ore, >500 Mt contained Fe (Musan in North Korea contains 5,200 Mt Fe) — and consist dominantly of magnetite with only minor silicate gangue.

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Identify this Sample - June, 2016

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Gold Ore

Gold mineralization in arsenical iron formation of Neoarchean age (2.7 billion years), Lupin Gold Mine, Northwest Territories, Canada.

Located in the central Slave province, this is a world-class BIF-hosted lode-gold deposit, metamorphosed to amphibolite grade. Previously proposed syngenetic and exogenic models of gold concentration have been reinterpreted and a metamorphic model of ore genesis is now favored. Average mine grade was approximately 9 g/t. Discovered in 1960 by Inco geologists, evaluated in 1961, and brought into production in 1982. Production ceased in 2005 after yielding about 3 million oz of gold. The sample was collected from outcrop in 1961.


May, 2016

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What is the difference between an exoskarn and an endoskarn?

Answer

Exoskarn and endoskarn are common terms used to indicate a sedimentary or igneous protolith, respectively. Exoskarn and endoskarn also can refer to the location of skarn relative to the causative pluton (external versus internal), although such a distinction is not meaningful when multiple plutons or no igneous rocks are present.

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Identify this Sample - May, 2016

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Uraninite

Uraninite in syenite related pegmatite from Bancroft, Ontario, Canada.

Pegmatite is hosted in the 1 billion year old high-grade metamorphic Grenville terrain. Production was about 2.54 mt with an average grade of a little more than 1kg/ton. Faraday Mine (in production from 1957 to 1964) was later reopened in 1975 as the Madawaska Mine and produced until 1982. The sample was collected underground in 1962.


April, 2016

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When was the term "skarn" first applied in economic geology?

Answer

The first published use of the term "skarn" is by Alfred Elis Törnebohm (1875). As such, he can rightfully be considered the father of skarn research.

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Identify this Sample - April, 2016

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Arsenopyrite VMS

Acicular arsenopyrite in copper-rich volcanogenic massive sulfide from Mattabi Mine, 60 km north of Ignace, Ontario, Canada.

VMS deposit is hosted in 2.73 Ga old volcanic rocks. The Mattabi ore body was 13.8 mt grading 8.28% Zn, 0.74% Cu, 0.85% Pb, 3 oz Ag and 0.29 g/t Au. The Mattabi Mine was in production from 1970 to 1983. The sample was collected in the open pit in 1977.


March, 2016

Answer this Question


What rock type(s) host skarn deposits?

Answer

Although the majority of skarns are found in lithologies containing at least some limestone, they can form in almost any rock type, including shale, sandstone, granite, iron formation, basalt, and komatiite.

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Identify this Sample - March, 2016

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Pentlandite Pyrrhotite

Pentlandite-pyrrhotite from Falconbridge Mine, Sudbury, Canada.

Pentlandite "eyes" in a groundmass of pyrrhotite from a massive sulfide lens in the basal gabbronorite zone of the Sudbury Igneous Complex (south rim). The Sudbury structure is the remnant of a 1.85 Ga old impact crater with an initial diameter of 200 to 250 km. The sample was collected in the spring of 1952.


February, 2016

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What are the principal geochemical processes that most commonly define hydrothermally altered rocks in a porphyry environment?

Answer

Four broad processes or types of chemical reactions are responsible for the principal alteration types in porphyry environments: (1) volatile addition (propylitic alteration); (2) hydrolysis (sericitic, advanced argillic, and intermediate argillic alteration); (3) alkali exchange (e.g., potassic and sodic-calcic alteration); and (4) addition of silica (silicic alteration).

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Identify this Sample - February, 2016

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Galena

Galena from Missouri Cobalt Mine, National Lead Company, Fredericktown, Missouri, USA.

Galena is representative of lead dominant Mississippi Valley-type (MVT) deposits of southeast Missouri. The host is a carbonaceous bed of the Cambrian Bonneterre Dolomite. Lead was discovered north of Fredericktown in 1720 where cerussite was mined initially since it was easier to smelt than galena. The sample was collected in the summer of 1954.


January, 2016

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When did large-scale, open-pit mining of porphyry ores become commonplace?

Answer

Exploitation of porphyry deposits benefited from key technological developments, including the success of Daniel Jackling's bulk tonnage open-pit mining method in 1905.

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Identify this Sample - January, 2016

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Massive Sulfide - chalcopyrite, pyrrhotite, and pentlandite

Collected at the Eagle Nickel-Copper Mine, Marquette County, Michigan, USA. The ore deposit is a high-grade magmatic sulfide with the economic minerals primarily pentlandite and chalcopyrite. The primary metal is nickel, with copper, cobalt, gold, and platinum-group metals as by-products. Grade is 4.4% Ni and 3.3% Cu.

Currently owned by Lundin Mining, current mineral reserves at Eagle are sufficient for a mine life of 8 years.


December, 2015

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Where are the world's only two mining districts that produce tellurium as a primary commodity?

Answer

Tellurium is presently recovered as a primary ore from only two districts in the world; these include the adjacent Au-Te epithermal vein deposits at Dashuigou and Majiagou in SW China, and the epithermal-like mineralization at the Kankberg deposit, Skellefte VMS district, Sweden. Together they account for about 15% of the global production of Te.

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Identify this Sample - December, 2015

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Chalcopyrite

Collected from near the bottom of Bingham Canyon pit in Utah, USA (September 2010). Chalcopyrite in host of quartz monzonite porphyry that has intruded Paleozoic sedimentary rocks an estimated 30-40 my ago. Grade of specimen is 0.5-0.6% Cu.


November, 2015

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Tellurium is important in solar cells and as an additive to many alloys. What is the present-day main source of most of the world's tellurium?

Answer

The majority of the world's tellurium is produced as a by-product from the refining of copper ore from large porphyry deposits.

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Identify this Sample - November, 2015

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Coffinite and Corvusite

Collected at the Daneros Uranium Mine in the Triassic Chinle Formation, Fry Canyon, Utah, USA (September 2010). River channel of coarse sand filled with organic trash (leaves, twigs, branches) that generated methane to help precipitate uranium from solution. Black minerals are probably coffinite (USiO4) and corvusite (CaV8O20).4(H2O).

Uranium was discovered in sandstone of the Chinle Formation in Lisbon Valley in 1913, followed by the uranium mining boom of 1948, and discovery, in 1952, of a significant uraninite orebody in the Chinle Formation.


October, 2015

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When was the term porphyry copper deposit first applied to this distinct ore deposit type?

Answer

According to Titley (1997), the first formal use of the term porphyry Cu deposit may have been by Emmons (1918).

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Identify this Sample - October, 2015

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Chalcocite-Malachite in Sandstone

Collected at the Lisbon Valley Copper Mine in Utah, USA (September 2010). Epithermal chalcocite filling pores in coarse sandstone and oxidized to malachite in the Lower Cretaceous Burro Canyon Formation.

Recovery of chalcocite and copper oxides is by heap leach and electrowinning. Average grade from pits is 0.7% Cu and precipitant is organic trash: leaves, twigs, and branches.


September, 2015

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What are the potential deposit types that are considered to be important in the exploration for platinum group element deposits?

Answer

Three settings that dominate the world’s supply of PGE and are the most common targets are: (1) reefs in stratiform intrusions, (2) major Ni-Cu sulfide deposits, and (3) placers.

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Identify this Sample - September, 2015

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Massive Chalcopyrite with Bornite

Massive chalcopyrite with bornite from Morrison (Levack) Copper-Nickel Mine, Sudbury, Canada.

Sample represents a sulfide melt emplaced some distance from contact orebodies that are nickel bearing. This specimen came from the Morrison orebody on the 3810 level. It is a small orebody but has a high content of precious metals, particularly gold and platinum. The grade of this specimen is on the order of 8 to 20 g/t.The Morrison mine is in the north range of the Sudbury meteor impact structure formed 1,850 million years ago. Formerly owned by Inco and then Vale, this mine now belongs to KGHM International. The sample was collected on a mine visit during the 11th International Platinum Symposium, June 2010.


August, 2015

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What is the likely sulfur source for the formation of magmatic Ni-Cu deposits in mafic rocks?

Answer

Sulfur-bearing sediments, in the form of black shales, paragneiss, or evaporites are present in the vicinity of most deposits. The assimilation of these sediments appears to bring about sulfide saturation of the magmas.

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Identify this Sample - August, 2015

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Visible Gold

Visibile Gold from Round Mountain gold mine, Nevada, USA.

Kinross and Barrick Gold are joint owners. Gold is recovered by heap leaching completely oxidized volcanic rocks. Grade is on the order of 2 g/t and this specimen came from a high grade zone of coarse gold that is recovered by gravity concentration. Over 10 million ounces of gold have been poured since about 1906. The sample was collected in office of mine geologist in 2005.


July, 2015

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What giant Ni sulfide deposit is associated with an anorthosite complex?

Answer

The ca. 1350-1290 Ma Voisey's Bay deposit in Labrador, Canada.

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Identify this Sample - July, 2015

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Native Silver

Native Silver from Creede, Colorado, USA.

Epithermal vein with argentite (Ag2S) in Tertiary age volcanic rocks. Silver habit is flow and wire. Silver and gold were discovered in 1887 and production began in 1981 after the arrival of the railroad. Cumulative production is about 2,500 metric tons of silver plus 4.7 metric tons of gold. Last mining was done in 1985. Unknown date of collection.


June, 2015

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What percent of the world's nickel is produced from magmatic sulfide deposits hosted in mafic and ultramafic rocks?

Answer

Approximately 60 percent of the world's nickel is produced from Fe-Ni-Cu sulfide deposits found in association with mafic or ultramafic intrusions or flows. Most of the rest of Ni production comes from limonitic and saprolitic laterite deposits, which form by weathering of ultramafic rocks.

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Identify this Sample - June, 2015

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Gabbronorite with Bornite

Collected from Carolusberg Mine, O'Okiep Copper Company, near Springbok, South Africa.

Coarse clot of bornite in the 1.1 by old gabbronorite. The norite is in small diapirs (called steep structures) that intruded plastic gneisses along breccia zones. Bornite and magnetite (15 to 25% of the norite) are disseminated within the norite. Cut-off grade is 0.8% and Cu average grade mill feed is 1.8% Cu. No other metals are recovered.


May, 2015

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What are the oldest known detrital diamonds?

Answer

The oldest known detrital diamonds come from the Archean (2.89-2.82 Ga) gold- and uranium-bearing Central Rand sequence of South Africa. They predate any known kimberlite, but an Archean kimberlitic source has been assumed and is borne out by the occurrence of minerals of kimberlitic origin in the mineral concentrates.

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Identify this Sample - May, 2015

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Chromitite

Collected from the Brakspruit Shaft of Rustenburg Platinum Mines, Rustenburg, South Africa.

From the UG2 (Upper Group 2) chromitite layer that is 38 m below the Merensky Reef. The UG2 layer is 0.75 m thick and is underlain by a mineralized pegmatoidal pyroxenite. About 1.5 m of chromitite and pyroxenite are mined. The chromitite contains 32% chromium but is not recovered. Sulfides are invisible in the chromitite but Pt group metals grade 11 g/ton (0.35 oz) with high rhodium. Poikilitic bronzite crystals hold the chromite crystals together.


April, 2015

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When was it first discovered that alluvial diamonds actually were derived from lode sources?

Answer

Only in 1870, after alluvial stones in gravels of the Orange and Vaal river systems in South Africa led to the discovery of diamonds in so-called "dry diggings" at Jagersfontein and Kimberley, was it realized that alluvial diamonds are secondary deposits derived from an ultramafic igneous source rock, later to be named kimberlite after the type locality.

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Identify this Sample - April, 2015

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Pegmatoidal Feldspathic Pyroxenite

Collected from the Brakspruit Shaft of Rustenburg Platinum Mines, Rustenburg, South Africa.

The layer directly below the UG2 (Upper Group 2) chromitite layer that is 38 m below the Merensky Reef in the Bushveld Igneous Complex. The UG2 chromitite is about 75 cm thick and the underlying pyroxenite is about 25 cm thick, both carrying 7-9 g/t platinum group metals (PGM).


March, 2015

Answer this Question


What two types of igneous rocks can carry diamonds to the Earth's surface from great depths?

Answer

Economic diamond deposits of an igneous affiliation are found mainly in kimberlites supplemented by subsidiary numbers of lamproites.

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Identify this Sample - March, 2015

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Native Silver

Collected at the Cobalt Mining Museum (Cobalt, Ontario, Canada) in 1966 - very coarse silver vein in volcanics with an age of 2,217 my. Most associated minerals are arsenides, arsenopyrite FeAsS; Lollingite FeAs2; Cobaltite CoAs2; Niccolite NiAs; Smaltite CoAs.

Mineralization is associated with a Nipissing diabase sill that is 300-400 m thick. Most silver is found along the upper and lower contacts, primarily from the lower contact. Mineralization was discovered in 1903 and production took place mainly between 1907 and 1920 with lesser production continuing to 1930. Cumulative production was more than 13,000 mt.


February, 2015

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The Abitibi greenstone belt of central Canada contains many of the world's giant VMS deposits. What is the approximate duration of the entirety of seafloor activity that caused this giant metal endowment?

Answer

The major VMS ore deposits of the southern Abitibi greenstone belt were formed within a span of less than 50 million years, resulting from successive arc rifting, back-arc basin development, and ending with exhumation of the adjoining accretionary complexes along major arc-parallel crustal-scale faults.

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Identify this Sample - February, 2015

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Chalcopyrite Cassiterite

From Castro Verde, Portugal. Sampled in May 1997, massive chalcopyrite ore with stringers of cassiterite from the Corvo ore body in the Neves Corvo Mine.

Volcanogenic massive sulfides are from the Iberian Pyrite Belt of Late Devonian age (345 my). Corvo lens is 92 x 1100 x 600 m with 42 mt of ore averaging 7.6% Cu and, within copper ore, 4.3 mt of ore averaging 2.5% Sn + Cu. The lens is part of more than 300 mt of massive sulfides.


January, 2015

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What is the largest accumulation of metals known to be associated with hydrothermal venting on the sea floor?

Answer

The metalliferous sediments of the Atlantis II Deep in the Red Sea represent the largest accumulation of metals associated with hydrothermal venting on the sea floor. The total tonnage of metalliferous sediment is estimated to be 696 Mt, and a resource of 91.7 Mt, has grades of 2.06 wt percent Zn, 0.46 wt percent Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au (Guney et al., 1988).

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Identify this Sample - January, 2015

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Gold Ore (Orogenic)

From the Lapa Mine, 40 km west of Val-d'Or, Quebec, Canada. Sample taken during the September, 2014 SEG Foundation Student Field Trip to the South Abitibi Greenstone Belt (from Timmins, Ontario to Malartic, Quebec).

Mineralization is in the Larder Lake-Cadillac break (deformation zone) in rocks about 2,685 my old (Neoarchean). The preferred host is highly sheared mafic and ultramafic volcanics with pervasive silicification. Mineralization is in 12 or more stacked vertical lenses 2 to 13 m thick. Average grade is about 6.5 g/t. Visible gold is present in the highest grade ore that has the highest sulfide content. This specimen probably averages 8 g/t.

Gold is associated with about 5% sulfides. Arsenopyrite (acicular habit), is the predominant sulfide in ore zones and accompanied by variable amounts of pyrite and pyrrhotite. In 2013, the daily mining rate was 1,700 tons.


December, 2014

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When were the first black smokers discovered on the ocean floor?

Answer

The discovery of the first black smokers was on the East Pacific Rise in 1979 (Francheteau et al., 1979; Hekinian et al., 1980; Spiess et al., 1980).

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Identify this Sample - December, 2014

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Gold Ore (Orogenic, Disseminated)

From the Canadian Malartic open pit mine at Malartic, Quebec, Canada. Sample taken during the September, 2014 SEG Foundation Student Field Trip to the South Abitibi Greenstone Belt (from Timmins, Ontario to Malartic, Quebec).

Gold mineralization is associated with disseminated pyrite and quartz stockworks in greywacke with an age of 2,680 my (Neoarchean). Pyrite content is about 5%. Greenschist metamorphism and mineralizing fluids have converted the greywacke into a silicified biotite schist.

The deposit is immediately south of the Larder Lake-Cadillac break (regional shear zone) and incorporates ground from three former underground mines that produced a total of 5.63 million ounces of gold. Open pit mining began in 2011 with reserves of 343 mt with an average grade of 1g/t and a resource of 10.7 million ounces. In 2013, 475,000 ounces of gold were produced from daily production that averaged 55,000 tons.


November, 2014

Answer this Question


Metal-depositing hot springs on the seafloor are present-day analogs for ancient volcanogenic massive sulfide deposit formation. In what seafloor environment are most of the high-temperature vents and polymetallic deposits located?

Answer

About two-thirds of the known submarine hydrothermal systems occur along mid-ocean ridges, although these are subsequently poorly preserved in the geologic record because the ridges are often subducted.

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Identify this Sample - November, 2014

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Cu-Pb-Zn VMS Ore

From the Kidd Creek Mine 27 km north of Timmins, Ontario, Canada. Sample taken during the September, 2014 SEG Foundation Student Field Trip to the South Abitibi Greenstone Belt (from Timmins, Ontario to Malartic, Quebec).

Mineralization is in a zone between rhyolite volcanic breccias/clastics and mafic and ultramafic volcanic rocks. The ultramafic layers have been altered and sheared to form talc-carbonate schists. The ore-bearing sequence is 2,714 my old (Neoarchean) with black smokers forming the source of the sulfides. Fragments of chimneys are found in the ore but a high percentage of the ore is made up of clastic sulfides precipitated from black smokers that accumulated as sediments in a long trough. Stringer zones of high-grade copper that are found at the base of the ore sequences represent fissure passages for the fluids that fed the black smokers.

This specimen was collected at the 9,000 ft level but drilling has intersected ore grade mineralization at a depth of 10,200 ft. Cumulative production since the discovery of Kidd Creek in 1963 is 150 mt grading 2.3% Cu, 6.2% Zn, 0.23% Zn, and 75g (2.4 oz) Ag. Higher silver content is usually associated with galena, while gold content is negligible. As of 2014, mineable reserves are 20 mt (open at depth) and the daily mining rate varies from 6,500 to 8,000 tons.


October, 2014

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Many hydrothermal ore deposits are associated with important fluid pressure regime changes at the transition from hydrostatic to suprahydrostatic conditions. At what depths does this transition typically occur?

Answer

The transition from hydrostatic to suprahydrostatic fluid pressures typically occurs at depths of 5 to 10 km (Sibson, 2001).

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Identify this Sample - October, 2014

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Galena (PbS) Magnetite (F3O4)

This sample was taken at the Broken Hill mine (Black Mountain) in South Africa, brought into production in 1979 by Phelps-Dodge and Gold Fields. The deposit is hosted in supracrustal rocks of the mid-Proterozoic Bushmanland Group of the Namaqualand Metamorphic Complex and associated with exhalites, Mn-rich iron formations and sulphide horizons. Average grade is 8% Pb, 3% Zn, 0.5% Cu, and 113 g/t Ag. Magnetite makes up about 25% of the specimen. The cores of the ore bodies are massive sulfides in a facies change of banded iron formation. The mine is located near Aggeneys and that sample was taken during an SEG field trip in November, 1995.


August, 2014

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Sulfur, water, and chlorine are important components to magmatic hydrothermal fluids that form porphyry ore deposits. What processes control the magmatic concentrations of these components?

Answer

The interactions of slab derived fluids with the mantle wedge, as well as interactions of mafic arc magmas with both lower crust rocks and more felsic magmas, control the magmatic concentrations of sulfur, water, and chlorine.

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Identify this Sample - August, 2014

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Braunite Mn203

This specimen of braunite was found within a facies of iron formation of the Proterzoic Transvaal Supergroup that may contain as much as 80% of the world's manganese reserves. The desert varnished sample of residual ore (55% Mn) is from the only outcrop of the very large reserves that have been concentrated by solution of all carbonates. Location is Samancor's Mamatwan Mine near Kuruman, South Africa; sample taken during an SEG field trip in November, 1995.


July, 2014

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What percent of the world's copper is mined from porphyry-type deposits?

Answer

Slightly over 50% is recovered from copper deposits.

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Identify this Sample - July, 2014

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Iron Ore

Sample from the far northern end of the Proterozoic Mesabi Iron Range where it is in contact with gabbro of the Duluth Complex. Sample assays about 30% iron with 80-90% magnetic. Magnetite is coarsely crystalline and associated with very high iron pyroxenes. Sample taken in May 2003 from the Dunka Road open pit mine that went out of production in 1993.


June, 2014

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What is the ultimate source of metals in a porphyry copper deposit?

Answer

Ore metals in arc magmas probably have diverse origins, including the mantle wedge, the lower continental crust, and the subducted lithosphere.

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Identify this Sample - June, 2014

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Digital Stromatolites

Large slabbed surface is the base of stromatolites growing upward with the spaces between stromatolites filled with siderite. The upper portions of the stromatolites are in chert in which there are numerous very small oncolites. Sample from the 1.9 billion year old Biwabik Iron Formation in the Mesabi Iron Range - LTV Mine, Hoyt Lakes, Minnesota, USA.


May, 2014

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In giant porphyry systems, do the metals come from the magma or do they get leached from country rocks in the adjacent crust?

Answer

The close spatial and temporal association between intermediate to felsic igneous intrusions and large tonnage, low-grade porphyry-type mineral deposits in arc environments is consistent with the hypothesis that magmas were the dominant source of the ore metals.

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Identify this Sample - May, 2014

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Rip-up Algal Mats

Abundant anthraxolite (pyrobitumin) that was oil but is now graphite is seen filling the openings between rip-up clasts. Lower greenschist metamorphic faces with recrystallized brown siderite. Sample from the 1.9 billion year old biogenetic Biwabik Iron Formation in the Mesabi Iron Range - LTV Mine, Hoyt Lakes, Minnesota, USA.


April, 2014

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Is there any characteristic of a mantle-derived magma that makes it particularly predisposed to generate an economic Ni-Cu-(PGE) deposit (metal content, degree of sulfur saturation, physical characteristics, etc.) or does the formation of a deposit depend principally or entirely on events that affect the magma during its passage through the lithospheric mantle and crust?

Answer

The capacity of a magma to form a Ni-Cu-(PGE) deposit depends primarily on its ability to interact with crustal wall rocks and only to a minor extent on its Ni, Cu, and PGE contents.

Identify this Sample


Identify this Sample - April, 2014

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Answer

Gold Ore

From the Archean Witwatersrand pebble conglomerate (reef). Western Areas Gold Mining Corporation, Cooke Section, North Shaft (No. 1) from stope at the 1200 meter level. Here the Elsburg conglomerate is 1.9 meters thick and contains 10 grams of gold (0.3 ounces) and 0.7 kilograms of uraninite per ton. Gold is of placer origin (possibly modified), very fine grained, and invisible. Average pyrite content about 10%. Near Randfontein, West Rand (west of Johannesburg), Republic of South Africa.