SEDIMENTARY ORESUnderstanding Economic Geology--Eamon McCarthy Earls

PLACER DEPOSITS “heavy” high density detrital crystals build-up in sediment

Wide range of minerals—cassiterite, native gold, uraninite, ilmenite, rutile & zircons

Both rivers & coastal systems Witwatersrand Basin—South African

Huronian Basin—Canada “black sand” beach deposits in Western Australia enriched in Zr, Th, Ti

So far sedimentology has not come up with comprehensive explanations of placer deposit creation

REYNOLD’S NUMBER Standard quantitative explanation of fluid flow

Re<1—laminar flow—parallel streams of fluid moving very slowly past one another

Re 1-10—transitional flow Re>10—turbulent flow—streams of fluid interfere with one another

OTHER SEDIMENTATION TERMS Saltation—particles move by skipping along

Settling—determined by a low Reynold’s number controlled by diameter & density

Estimated with Stoke’s law Entrainment—some grains are removed selectively from the bedload (barely suspended, larger particles moving new the bottom of the water column)

Transport sorting—different size grains are deposited differently, either by entrainment for large particles or settling for smaller particles

SEDIMENTARY ENVIRONMENTS

BEACHES (HIGH-ENERGY COASTAL MARINE) Most placer deposts related to swash zone—upper beach between backbeach & surf

Swash-broken wave that comes ashore and drives up the beach

Source fertility & swash processes important to ore formation

BEACHES Swash zones are not deep enough for settling grains

Little if any entrainment of individual grains

Shear sorting-upward pressure on dense grains more significant than on small grains

EOLIAN (WIND-BLOWN) SEDIMENTS How do you get a grain of sediment airborne? Entrainment follows a linear relationship to its size & density—Bagnold (1941)

Small particles are easier to transport with wind than water Very few studies about the role of wind in changing beach deposits

Ex) Richard’s Bay, KwaZulu-Natal, South Africa has beach swash deposits that were then mounded into dunes—dunes are enriched in Ti & Zr placers

CHEMICAL SEDIMENTATION (PRECIPITATION)

CHEMICAL SEDIMENTATION Some sedimentary rocks are precipitated as ions and minerals precipitate out of fluid—often seawater

Contrasts with large clastic particles moved by water or wind

Precipitation created most of the world’s large deposits of Mn, Fe & phosphates

Evaporites—precipitation following evaporation of water

Commonly form on continental shelf and shallow marine environments

BOG IRON Iron (Fe) commonly found in two oxidation states>Fe2+ --ferrous iron>Fe3+ --ferric iron In reduced environments, Fe2+ can oxidize to Fe3+ and precipitate out of solution forming lumps of oxyhydroxide compounds

>goethite>limonite Common in northern swamps & tundra—major source of iron in Iron Age Europe and colonial America

IRONSTONE Phanerozoic age (600 MYA-present) Major source of iron in Western Europe & eastern US Jurassic sediments—Britain & Alsace-Lorraine, France Silurian-Clinton ores—Kentucky & Alabama Found in remnant deltas & lagoons Goethite & hematite rolled in oolites (egg-like pellets) and combined with silicate matrix minerals like chamosite & glauconite

Related to tectonics—clustered in Ordovician/Silurian & Jurassic age rocks

IRONSTONE Also tied to high-sea level and warm, wet climate

Siehl & Thein (1989)—ironstone formed from heavily weathered Fe-rich soil

Insoluble Fe3+ laterites remained in place while other parts of the soil weathered away

Chemical reactions driven by biology form iron ooids (egg-shapes) in soil

Ooids & laterites transported into deltas by rivers

Ex) Lateritic soils in India at right

BIFS (BANDED IRON FORMATIONS) Chemical deposit rich in Fe & chert Usually has distinct layering Main source of iron ore for manufacturing Much older than ironstone & bog iron Dates to Archean & Proterozoic—3.5 BYA to 500 MYA

Multiple types of BIF

BIFS Algoma-type BIF—related to ancient volcanic arcs and usually found in metamorphosed greenstone belts—Archean-age

>Mines in Abitibi greenstone belt, northern Canada Superior-type BIF—found on stable continental cratons—Paleoproterozoic-age

>Western Australia>Transvaal Basin, South Africa>Ukraine & India Rapitan-type BIF—related to glacial sediments moved during global Neoproterozoic ice age

>northwestern Canada

BIF MINERALOGY Usually iron oxide mineral—magnetite or hematite

Other mineral arrangements Iron+chert Iron+carbon-rich shale Siderite (carbonate) Greenalite & minnesotaite (silicates) Pyrite (sulfide)

BIF FORMATION Pyrite & siderite need reducing conditions

Rare earth elements indicate different iron sources

>Algoma-type—hydrothermal vents (“black smokers”)/volcanoes>Superior-type??? (no one knows for sure) Upwelling currents carry ferrous iron out of the reduced deep ocean

In shallower depths, it oxidizes due to oxygen from photosynthesis or UV light & precipitates

Most BIFs date to Paleoproterozoic—growth of bacterial life, oxygenation of atmosphere & ocean, & flocculation of Fe-Si by microbes

Rapitan-type: Earth ices over & switches to slower reducing conditions

BEDDED MANGANESE DEPOSITS Manganese available in multiple oxidation states>Mn2+, Mn3+, Mn4+ Higher oxidation potentials needed to create the stabile form of Mn—MnO2 (pyrolusite)

Fe2+ oxidizes more quickly meaning that Mn & Fe are usually in separate locations or stratigraphic layers

Largest Mn-deposits in Transvaal Basin, South Africa Range in age from Paleoproterozoic to present>Groote Eylandt, Australia>Molango, Mexico—MnCO3 (rhodochrosite) ores>Black Sea—reduced waters accumulated sedimentary pyrolusite

BEDDED MANGANESE MINERALOGYPyrolusite (MnO2) Rhodochrosite (MnCO3)

PHOSPHORITES Phosphate is critically important to building proteins and it’s a main plant nutrient

Every year, the fertilizer industry needs 150 million tons of phosphate

Most comes from phosphorites—sedimentary rocks with 15-20% P2O5

Shallow marine deposition associated with upwelling deep ocean water

A.V. Kazakov first proposed upwelling deep ocean currents in the 1930s Soviet Union—only later was it used with BIFs and manganese

Many of the best fishing areas in the world are known to be current phosphorite formation zones—upwelling phosphorous aids ocean life

>Namibia>Chile

PHOSPHORITES (HPO4)2- most common phosphorus anion

(PO4)3- common under very basic conditions

(H2PO4)- common under very acidic conditions

Decaying organisms that sink to the seafloor release calcium phosphateundergoes conversion to apatite

Collophone is also a potential end-member

Relatively low P-concentrations in modern seawater—rare in deep ocean and even lower in coastal areas due to biological uptake

PHOSPHORITE FORMATION Sheldon (1980) suggests that Precambrian oceans would have been able to precipitate more phosphate due to CO2-rich atmosphere leading to acidic ocean water

After the ‘Cambrian explosion’ less phosphate precipitated, but large numbers of dying organisms accumulated phosphorus more easily in sediments to form phosphorites

Periods of high eustatic sea level=larger deposits, possibly because of more ocean upwelling

Permian Phosphoria Formation-western US, biggest phosphorus deposit tied to post-glacial ocean gyresupwelling

BLACK SHALES Organic-rich shales Often enriched in metals Ex) Cr, Co, V, Ni, Ti, Pb, Zn, Mo, U and many others Formed alongside ironstones Rarely mined but often source of hydrothermal ore enrichment Tied to high sea-level in Ordovician/Devonian & Jurassic/Paleogene Often related to Ocean Anoxic Events that helped to preserve organics

New Albany shale, Indiana—Pb Alum shale--Scandinavia

SEAFLOOR MANGANESE NODULES First discovered in the 1870s by HMS Challenger research mission

Found only in the deep ocean in temperate regions Rich in Mn, Co, Zn, Fe, and Ni Found in areas with little biological, chemical or gravitational sedimentation

Largest fields in Pacific Ocean—up 100 nodules/m Concentric internal layering—roughly potato sized

NODULE FORMATION Still not fully understood Bacterially driven oxidation? Direct precipitation? Metal concentration from seawater in fecal matter?

EVAPORITES Nitrates Borates Halite (rock salt) Potash Formed from drying of massive lagoons & lakes

Ex) Silurian Salina formation—massive salt deposits from Michigan to New York

Largest deposits span 2000 km—2km thick beneath the Mediterranean

MARINE EVAPORITES Usually contain same main minerals>halite>anhydrite & gypsum>sylvite Can also contain rarer salts as bitterns>epsomite>carnallite>kainite>borate

SABKHA PRECIPITATES Massive Mediterranean deposits date to the Miocene Groundwater precipitation Evaporation of groundwater out of the capillary zone

Indicates that the Mediterranean dried up entirely and lost most of its near-surface groundwater to evaporation

Common in many desert areas

COAL

COALIFICATION Coal/lignite remain the main sources of heat & electrical energy for the planet

Coal & oil form from kerogen—a disordered mess of different organic compounds from different sources

Coal is mainly from Type III: humic kerogen—kerogen from woody plants that usually includes terpenes, lignin, cellulose and plant waxes

Marine, marsh & freshwater sources Ex) mangroves, brackish water plants, waterlilies

Coal blurs the line between sedimentary and metamorphic processes under the pressure of overlying strata

COALIFICATION Begins as peat McCabe (1984)—must contain more than 50% carbon

Peat can also be burned—widely abundant in northern climates like Russia & Ireland

Peatlignitesub-bituminous (brown)bituminousanthracite (hard coal)

COAL FORMATION Formed of maceral groups—different plant materials

Vitrinite—branches, leaves, & other woody material

Exinite—waxes, algae, resins, & other lipids

Inertinite—remnant fungus & oxidized material

Sapropelic—rare coals formed exclusively from spores, algae and other microscopic material

Hard coals expel CO2 & CH4 gas—believed to be source beds for overlying North Sea gasfields

SURFICIAL DEPOSITS

LATERITES We saw these earlier, with ironstone formation

Laterites form from percolation of tropical rains through soil & sediment leaving behind insoluble oxides of Fe & Al

Often enriched in Cu, PGE, Au, and Ni Common in Australia, South America, & Africa 100 Million years of sediment weathering has created 150 m profiles

Regolith no longer resembles parent rocks Illuvation—particles/ions accumulate at greater depths

Eluvation—clays and soluble molecules removed

SAPROLITH ZONE Very heavily weathered rock Preserves some original texture/structure Acidic—leaching of chalcophile metals & REEs Most heavily weathered regolith near the surface enriched in kaolinite & Fe in the form of hematite & goethite

Medium zones, grading into saprock weather into high expansion chlorite & smectite

BAUXITE ORES Main sources of Al are near surface in laterite deposits

Enriched in minerals boehmite, diaspore, gibbsite

High humidity with alternating wet & dry seasons

Gibbsite is hydrated Boehmite dehydrated Fe & silicates must leach out

JAMAICAN & VENEZUELAN BAUXITE Jamaica has some of the biggest reserves because it fits the bill climatically

Bentonite volcanic ash atop limestone (karst) basement

Rapid breakdown of silica in volcanic glass

Good drainage through karst system

Formation of Gibbsite In Venezuela, tectonic changes created the Guyana Uplift—granites undergoing slow, long-term weathering

NICKEL LATERITES Formed from ultramafic rocks Rich in olivine & orthopyroxene Rainwater changes olivine to serpentine, limonite or unstructured silica

Often requires low-grade metamorphism or seawater

Serpentinized bedrock with increasingly neutral pH soil & groundwater overlying it

Olivine degrades to smectite & goethite

Cation exchange with Mg2+ leaves rare Ni-rich minerals like nepouite, pimelite, and kerolite

GOLD-ENRICHED LATERITES Small crystals to nuggets Africa, Brazil & Australia Main source in Yilgarn Craton, Western Australia

Weathering removes silver contaminents Acidic, oxidizing groundwater, low-salinity, chloride-ion ligands in play

Reducing conditions favor silver mobility

High-concentrations of Mn2+ & Fe2+ at depth removes gold from solution and drives it to precipitate

Poorly understood role of bacteria

CLAY DEPOSITS Not ores in the traditional sense

Widely used for filtration, ceramics & making shiny magazine paper

Smectite, kaolinite, & illite among most common clays

Acid hydrolysis of feldspars and other minerals

Kaolinitewet conditions Smectitearid conditions (common in Western Australia and Texas, incredible shrink swell capacity)

KAOLINITE IN CORNWALL High quality kaolin used to make porcelain (“china clay”) & add shine to magazines

Best deposits in Cornwall, UK Hydrothermal breakdown of batholith or rainwater?

Fe leached out Intermediate clays between muscovite & smectite Formed from plagioclase feldspar in granite

CALCRETE Calcified soil Calcite builds up in vadose zone—zone of sediment above water table

Host uranium deposits in Namibia & Australia

Carnotite—a K & U enriched vanadate mineral

Calcified drainage channels U & alkali elements from weathered granites

~neutral groundwater Carnotite generates hydrogen ions that break down calcite and make U more mobile

Aridity in the Pliocene

REFERENCES Robb, L. (2005). Introduction to ore-forming processes. Blackwell Publishing.