deposits related to clastic sedimentation - gold deposits · deposits related to clastic...

Deposits related to

clastic sedimentation -

Gold Deposits

GLY 361

Lecture 16

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, 1941 -

Gold production in 2011/2012

World’s biggest buyers and sellers

Gold Physical Characteristics:

- Density: 19.32 g/cm3

- 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 20.23 Meters in size)

- Total Gold reserves left (not mined) 83,000 Metric Tons (est.

as of 2009)

Facts about Gold (Au)

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

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.



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''.


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.


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.

Primary Gold Deposits from

the Precambrian

Primary Gold Deposits of

Mesozoic age

Primary Gold Deposits of

Cenozoic age

Areas of alluvial (placer) gold

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.

Placer Deposits

Age Range: Major deposits are Archaean to early Proterozoic

(3.1 – 2.2 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.

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).


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 low-energy base of fan containing U and fine Au.

Reducing atmosphere believed to be necessary to preserve detrital pyrite and uraninite.


Au-U

Tectonic Setting:

Slow subsidence of Archaean Craton.

Later moderate uplift and erosion to remove

Phanerozoic strata and retain Early

Proterozoic rocks.


Au-U

Mineralogy:

Quartz, gold, pyrite, uraninite, zircon,

chromite, monazite, osmium-iridium alloys,

isoferro platinum.

By-product = Ag.

Middle Proterozoic and Phanerozoic

occurrences have only traces of pyrite and

no uraninite.


Au-U

Examples:

Witwatersrand, South Africa.

Elliot Lake, Canada.

Jacobina, Brazil.

Tarkwa, Ghana.


Au-U

Witwatersrand Supergroup

6 km thick

coarse-grained

sedimentary

sequence on the

Kaapvaal Craton.

Late Archaean:

± 2.8 – 3.1 Ga.

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.

G. Davidson

Witwatersrand

Supergroup

Miners in a Witwatersrand mineshaft

„Wits“ conglomerates

Handsample of „Wits“

conglomerate

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)


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.


G. Davidson


Sedimentary processes: Mechanical concentration/ selective sorting

Size/ volume

Density

Characteristics of placer minerals: High density

High hardness

Chemical stability (atmospheric conditions)

density hardness

Ilmenite 4.7 6

Rutile 4.2 6

Zircon 4.7 7

Tourmaline 3.0 7

Diamond 3.5 10

Cassiterite 7 6

Magnetite 5.2 6

Gold 19.3 3

Platinum 21.5 3.5

PHYSICAL CHARACTERISTICS OF SOME HEAVY

MINERALS

Geologic sites for gold extraction in the goldfields: In the fan head or midfan:

In the sandy or sandy-pebbly matrix at and near the bases of fluvial pebble-supported conglomerates in channel fills, where open-framework conglomerates trapped and retained sand-sized heavy minerals from the sediments flux.


Geologic sites for gold

extraction in the

goldfields:

In the midfan:

In pyritic through cross-

bedded sands in erosion-

deposition channels with

gold, uranite, and pyrite

particles on foresets, in

bottom-set spoons and

scours, and in basal lag

sands or gravels.


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.


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.


Schematic diagram illustrating depositional

environment for the Wits Basin and the 3 main

types of conglomerates described in the gold

fields

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 up-slope migration of the sea level.

Leader Reef type sheet-like conglomerates: well-defined fluvial channels.

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.

What is the source of gold?

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