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Field Map · Unit 3

Ore Deposits

One page to name any deposit. Almost every ore body is one of a few engines — hot fluids, cooling magma, mountain-building, or surface weathering — acting in a particular setting. Read the setting and the host rock, and you can usually name the deposit type and predict its metals. Everything below is a snapshot of the thirteen lessons.

A

The four engines

Hot fluids

Hydrothermal water dissolves metal and drops it where conditions change.

porphyry · skarn · CRD · epithermal · VMS · orogenic · IOCG · Carlin

Cooling magma

Metal separates straight out of a melt as it crystallises.

Ni-Cu-PGE · chromite · REE · pegmatite · kimberlite

Mountain-building

Collisions heat and squeeze the crust, driving gold up deep faults.

orogenic gold

Surface

Weathering and gravity concentrate metal at or near the surface.

laterite · placer · BIF · sediment-hosted Cu
B

The deposit models

Porphyry

intrusion · hot fluids

MetalsCu, Mo, Au
SignatureLow grade, giant tonnage; stockwork & disseminated
EXAMPLE  Escondida, Chile · Andean belt

Skarn & CRD

intrusion + limestone

MetalsCu, Fe, W, Zn, Au; Pb-Zn-Ag (CRD)
SignatureReplacement at and beyond the contact
EXAMPLE  Antamina, Peru

Epithermal

shallow volcanic · hot springs

MetalsAu, Ag
SignatureBonanza veins; the shallow top of a porphyry system
EXAMPLE  Yanacocha, Peru · Ring of Fire

VMS & SEDEX

seafloor hot springs

MetalsCu, Zn, Pb (+Au, Ag)
SignatureMassive-sulfide mounds; black smokers
EXAMPLE  Kidd Creek, Canada

Orogenic gold

mountain-building · metamorphic fluids

MetalsAu
SignatureGold-quartz veins along deep faults
EXAMPLE  Kalgoorlie, Australia · Abitibi belt

Magmatic

cooling magma

MetalsNi, Cu, PGE; Cr; REE; Li
SignatureMetal settles from the melt; reefs & layers
EXAMPLE  Bushveld, South Africa

IOCG & Kiruna

saline fluids · iron oxide

MetalsCu, Au (+U, REE); Fe
SignatureIron-oxide systems; not tied to one intrusion
EXAMPLE  Olympic Dam, Australia

Carlin-type gold

fluids in sedimentary rock

MetalsAu (often refractory)
SignatureInvisible, disseminated gold in favourable beds
EXAMPLE  Carlin Trend, Nevada

Uranium

groundwater · chemistry change

MetalsU
SignatureUnconformity (very rich) & roll-front (mined by ISR)
EXAMPLE  Athabasca Basin, Canada

Kimberlite

deep mantle magma

MetalsDiamond
SignatureFast carrot-shaped pipes; plus placers
EXAMPLE  Jwaneng, Botswana

Granite Sn-W & IRGS

cooling granite · late fluids

MetalsSn, W, Au
SignatureGreisen & sheeted veins at the cupola
EXAMPLE  Cornwall, England

Surface & sedimentary

weathering · gravity · chemical sediment

MetalsNi, Al, Au, Ti, Fe, Cu
SignatureLaterite, placer, banded iron, sediment-hosted Cu
EXAMPLE  Witwatersrand, South Africa
C

One crust, many deposits

metamorphic belt limestone sedimentary basin 1 2 3 4 5 6 7 8 9 10 11
  1. 1Magmatic — Ni-Cu-PGE & chromite in a deep magma chamber
  2. 2Porphyry — Cu-Mo-Au around the intrusion
  3. 3Skarn & CRD — at and beyond the limestone contact
  4. 4Epithermal — Au-Ag near surface, above the porphyry
  5. 5Granite Sn-W / IRGS — at the granite cupola
  6. 6Orogenic gold — quartz veins on a deep fault
  7. 7VMS — massive sulfide on an old seafloor
  8. 8Carlin / sediment-hosted — invisible gold in beds
  9. 9Uranium — at the unconformity (and roll-fronts)
  10. 10Kimberlite — diamonds in a pipe from the mantle
  11. 11Surface — laterite cap & river placer

Setting + host rock give each deposit away. IOCG sits apart — an iron-oxide system not tied to one intrusion.

D

World belts & giants

Andean copper belt
Porphyry Cu giants — Escondida, El Teniente (Chile–Peru)
Ring of Fire
Epithermal Au-Ag & porphyries (circum-Pacific)
Abitibi belt
Orogenic gold & VMS (Canada)
Athabasca Basin
Highest-grade uranium (Canada)
Bushveld Complex
Most of Earth's platinum & chromite (South Africa)
Witwatersrand
Largest gold ever mined — a paleoplacer (South Africa)
Iberian Pyrite Belt
VMS, mined since antiquity (Spain–Portugal)
Central African Copperbelt
Sediment-hosted copper (DRC–Zambia)
+

Field glossary

Hydrothermal
Involving hot, metal-carrying water.
Intrusion / pluton
A body of magma that cooled underground.
Stockwork
A dense mesh of tiny veins (porphyry).
Disseminated
Ore as tiny grains sprinkled through rock.
Alteration halo
Bullseye zones of changed rock guiding explorers.
Replacement
Ore that takes the place of dissolved host rock.
Bonanza vein
An unusually rich ore vein (epithermal).
Massive sulfide
Nearly solid sulfide ore (VMS).
Lode gold
Hard-rock vein gold — source of placers.
PGE / reef
Platinum-group elements in a thin rich layer.
Refractory gold
Gold locked in sulfides; resists normal leaching.
Unconformity
A buried ancient land surface (rich uranium).
Roll-front / ISR
Uranium in sandstone, mined by pumped solution.
Kimberlite
A fast deep magma pipe that carries diamonds up.
Greisen / cupola
Altered granite top hosting tin-tungsten veins.
Supergene
Weathering that enriches the top of a deposit.
Paleoplacer
An ancient, buried placer (Witwatersrand).
Metallogenic belt
A region where one deposit type repeats.