Deposits related to clastic sedimentation - Gold Deposits. GLY 361 Lecture 16

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1 Deposits related to clastic sedimentation - Gold Deposits GLY 361 Lecture 16

2 A change so unexpected and a development never known before was due to the discovery in 1886 of the greatest gold mines of all history, ancient and modern. From 1886 [until 1940] the story of South Africa is the story of gold. - C.W. de Kiewiet,

3 Gold production in 2011/2012

4 World s biggest buyers and sellers

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6 Facts about Gold (Au) Gold Physical Characteristics: - Density: g/cm 3 - Melting point: 1,064 C Gold Production / Reserves: - Total amount of Gold ever produced: 160,000 Metric Tons (est. as of 2009) - Total volume of Gold ever produced: 8,282 Cubic Meters (A cube Meters in size) - Total Gold reserves left (not mined) 83,000 Metric Tons (est. as of 2009)

7 Types of Gold Deposits Epithermal vein (lode) deposits Intrusion-related breccia pipes, Mesothermal turbidite- and greenstone-hosted deposits Contact deposits (skarns) Archean Banded Iron Formation deposits Placer deposits

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10 Types of Gold Deposits Small bits of gold are scattered through this piece of mesothermal vein quartz from the mother lode region of California. The gold is mostly concentrated around the edges of dark colored spots of iron oxide that were likely originally clots of pyrite. This is very rich gold ore.

11 Types of Gold Deposits

12 Types of Gold Deposits There are several types of placer gold: 1. Residual placers - This type of placer occurs directly at the site of the original gold vein. As the vein erodes gold accumulates near the surface. 2. Eluvial placers - The material weathered from the vein has now been carried away from the original site, usually by gravity as material works its way down a hill. Also known as ''hillside placers''.

13 Types of Gold Deposits 3. Alluvial placers - The most common type of placer deposit. Gold that has been deposited through the action of water. Often called ''stream placers'' but applies to any situation where running water has deposited the gold (or other heavy minerals). 4. Eolian placers - Winds carry away surface sand and dust in a process known as ''deflation''. Heavy, resistant materials such as gold can accumulate at the surface. This process is most common in desert areas, particularly in Australia.

14 Types of Gold Deposits 5. Beach placers - The concentration of heavy minerals by wave action. The most famous would be the gold deposit on the beaches of Nome, Alaska.

15 Types of Gold Deposits

16 Primary Gold Deposits from the Precambrian

17 Primary Gold Deposits of Mesozoic age

18 Primary Gold Deposits of Cenozoic age

19 Areas of alluvial (placer) gold deposits

20 Placer Deposits Surficial mineral deposits formed by mechanical concentration, commonly by alluvial but also by marine, aeolian, lacustrine, or glacial agents, of heavy mineral particles such as gold from weathered debris (Hails, 1976). Quartz pebble conglomerate: Placer Au, U and PGE in ancient conglomerate. The quartz-pebble conglomerate deposits supply 50% or more of the world's annual gold production.

21 Quartz pebble conglomerate Au-U Age Range: Major deposits are Archaean to early Proterozoic ( Ga). Rock types: Oligomictic, mature conglomerate beds in thick sequences of less mature conglomerate and sandstone deposited on Archaean basement (granites, greenstones). Locally basal volcanic rocks.

22 Quartz pebble conglomerate Au-U Sedimentary features: Bimodal clast-size distribution with well-sorted pebbles and well-sorted matrix. Well-rounded, well-packed pebbles of vein-quartz, chert and pyrite. Matrix = quartz, mica, chlorite, pyrite and fuchsite. Granite clasts are absent. Pyrite may occur as rounded grains and concentrically layered concretions. Gold occurs as small angular grains (0.005 to 0.1 mm in diameter).

23 Quartz pebble conglomerate Au-U Depositional Environment: Onlapping sedimentary deposits in elongate epicontinental basins or half-grabens. Middle and basal reaches of alluvial fans deposited on steeper side of basins. Braided river channels in alluvial fans. Gold concentrated at base of mature conglomerate beds deposited on an erosion surface. Carbonaceous layers resembling algal mats deposited at lowenergy base of fan containing U and fine Au. Reducing atmosphere believed to be necessary to preserve detrital pyrite and uraninite.

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26 Quartz pebble conglomerate Tectonic Setting: Au-U Slow subsidence of Archaean Craton. Later moderate uplift and erosion to remove Phanerozoic strata and retain Early Proterozoic rocks.

27 Quartz pebble conglomerate Mineralogy: Quartz, gold, pyrite, uraninite, zircon, chromite, monazite, osmium-iridium alloys, isoferro platinum. By-product = Ag. Au-U Middle Proterozoic and Phanerozoic occurrences have only traces of pyrite and no uraninite.

28 Quartz pebble conglomerate Au-U Examples: Witwatersrand, South Africa. Elliot Lake, Canada. Jacobina, Brazil. Tarkwa, Ghana.

29 Witwatersrand Supergroup 6 km thick coarse-grained sedimentary sequence on the Kaapvaal Craton. Late Archaean: ± Ga.

30 Witwatersrand Supergroup

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32 Nearly 40% of all gold ever mined originates from the Wits gold deposits (35 million kg), enough to make a golden sphere 16.6 m high and worth around $60 billion.

33 Witwatersrand Supergroup G. Davidson

34 Witwatersrand Supergroup

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38 Miners in a Witwatersrand mineshaft Wits conglomerates Handsample of Wits conglomerate

39 Witwatersrand Supergroup Six separate major goldfield mining areas, each involving several auriferous conglomerate horizons: Welkom goldfield (4 auriferous-uranite horizons) Klerksdorp (7 horizons) Carletonville (3 horizons) West Rand (10 horizons) East Rand (9 horizons) Evander (1 horizon)

40 Witwatersrand Supergroup Gold occurs in laterally extensive quartz pebble conglomerate horizons or reefs, that are generally less than 2 m thick and are widely considered to represent laterally extensive braided river deposits. Gold generally occurs in native form often associated with pyrite and carbon, with quartz being the main gangue mineral. Separate alluvial fan systems were developed at different entry points and these are preserved as distinct goldfields.

41 Witwatersrand Supergroup Sedimentary processes: Mechanical concentration/ selective sorting Size/ volume Density Characteristics of placer minerals: High density High hardness Chemical stability (atmospheric conditions) G. Davidson

42 PHYSICAL CHARACTERISTICS OF SOME HEAVY MINERALS density hardness Ilmenite Rutile Zircon Tourmaline Diamond Cassiterite 7 6 Magnetite Gold Platinum

43 Witwatersrand Supergroup

44 Witwatersrand Supergroup

45 Witwatersrand Supergroup Geologic sites for gold extraction in the goldfields: In the fan head or midfan: In the sandy or sandypebbly matrix at and near the bases of fluvial pebblesupported conglomerates in channel fills, where open-framework conglomerates trapped and retained sand-sized heavy minerals from the sediments flux.

46 Witwatersrand Supergroup Geologic sites for gold extraction in the goldfields: In the midfan: In pyritic through crossbedded sands in erosiondeposition channels with gold, uranite, and pyrite particles on foresets, in bottom-set spoons and scours, and in basal lag sands or gravels.

47 Witwatersrand Supergroup Geologic sites for gold extraction in the goldfields: In the midfan or upper fan base: In sheets of cross-bedded sands by winnowing of quartz grains, leaving thin layers of heavy minerals as lag deposits with or without pebbles, and thus also as the lag sands and gravels along unconformities at the bases of sedimentary units.

48 Witwatersrand Supergroup Geologic sites for gold extraction in the goldfields: In the fan base: In carbonaceous layers on unconformities, in scour pools, and in algal mats, which acted both as mechanical riffle traps and as chemical traps.

49 Schematic diagram illustrating depositional environment for the Wits Basin and the 3 main types of conglomerates described in the gold fields

50 Three types of gold placer conglomerates Ventersdorp Contact Reef-type placers: separates the predominantly sedimentary lithologies of the Witwatersrand from the volcanic Ventersdorp Supergroup differing in various aspects, especially its lava hangingwall and pronounced, undulating footwall from the older Witwatersrand orebodies. Indicate pronounced palaeorelief in high energy fluvial environment. Vaal Reef sheet-like conglomerates: uniform distribution of Au and U grade. Formed by reworking of previous delta sediments caused by upslope migration of the sea level. Leader Reef type sheet-like conglomerates: well-defined fluvial channels.

51 What is the source of gold? Detrital grains (quartz, gold, uranite, pyrite, etc.) were transported into the Wiwatersrand basin from N, NW, SW. Archean granite basement domes and greenstone belts surrounding the basin. Comparison of Pb isotopes of sulfides with greenstone belts and radiometric ages of radiogenic Pb in galena agrees with this theory.

52 What is the source of gold?