Carlin-Type Gold Deposits: Geological Characteristics and Global Occurrences
Carlin‑type ore is a specific kind of that is named after the in , , where the first and most famous examples were
discovered in the late 1960s. Over the past several decades the term
has been broadened to include similar deposits worldwide that share a
distinctive set of geological, mineralogical, and geochemical
characteristics.
Below is a concise yet comprehensive overview of what makes a deposit
“Carlin‑type,” why it matters, and where you can find them.
1. Core Definition
A Carlin‑type gold deposit is a low‑grade, disseminated,
sedimentary‑hosted gold system in which gold occurs as sub‑micron
to nanometer‑scale particles that are invisible to the naked eye
and are typically chemically bound to arsenic‑bearing sulfides
(especially arsenian pyrite) and/or to other mineral phases such as
arsenopyrite, realgar, or native silver. The gold is not concentrated
in veins or visible nuggets but is spread throughout the host rock.
2. Key Geological Characteristics
| Feature |
Typical Carlin‑type Expression |
| Host rocks |
Fine‑grained, low‑permeability sedimentary rocks (e.g., carbonate‑rich sandstones, siltstones, shales, and carbonate‑rich tuffs). Often part of a basin‑fill sequence that has been mildly altered. |
| Structural setting |
Occurs in broad, gently dipping zones of extension or compression, often associated with fault‑related dilation zones that act as conduits for mineralizing fluids. |
| Alteration halo |
A characteristic alkali‑carbonate alteration suite: calcite, dolomite, and albite dominate the proximal halo, while silica (quartz), sericite, and pyrite form the distal halo. The alteration is usually low‑temperature (≤250 °C). |
| Mineralogy |
Gold is hosted primarily in arsenian pyrite (FeS₂ with up to ~1 wt % As) and arsenopyrite (FeAsS). Minor carriers include realgar (AsS), orpiment (As₂S₃), native silver, and electrum. |
| Geochemistry |
High arsenic, antimony, mercury, and sometimes barium and thallium in the alteration zone. Low to moderate sulfur, iron, and calcium. Gold concentrations range from 0.1 to 5 g/t (typical average ~0.5–1 g/t). |
| Fluid source |
Deep, magmatic‑derived hydrothermal fluids that have interacted with the basin sediments, picking up arsenic and other volatiles. The fluids are generally neutral to slightly alkaline (pH 6–8) and moderately reduced. |
| Temporal framework |
Most Carlin‑type deposits formed during mid‑ to late‑Mesozoic to early Cenozoic (≈150–30 Ma), but the process can be younger in some regions (e.g., the late Cenozoic Carlin‑type deposits in Australia). |
| Deposit geometry |
Typically sheet‑like, laterally extensive, and relatively thin (10 – 200 m thick). The ore body can extend for tens to hundreds of kilometres along strike (e.g., the > 5,000 km² Carlin Trend). |
3. How Gold Is Hosted
- Nanogold particles (10–100 nm) are substituted into the crystal lattice of arsenian pyrite or arsenopyrite, or they occur as tiny inclusions along grain boundaries.
- Because the gold particles are so fine, conventional ore microscopy cannot see them; electron microscopy (SEM/EDS, TEM), laser ablation ICP‑MS, and synchrotron X‑ray techniques are required for direct observation.
- Gold is often chemically bound to arsenic (As) and sulfur (S) within the sulfide lattice, which makes it refractory to simple cyanide leaching unless the sulfides are broken down (e.g., by pressure oxidation, roasting, or bio‑oxidation).
4. Exploration Indicators
| Indicator |
Typical Observation |
| Geophysical |
Low‑frequency electromagnetic (EM) anomalies (conductive due to disseminated sulfides), gravity lows (due to alteration and replacement of dense rocks), magnetic lows (demagnetization of host rocks). |
| Geochemical |
Elevated As, Sb, Hg, Ag, Bi, Tl, Ba in soils, stream sediments, or rock chips; gold in bulk rock assays even at sub‑ppm levels. |
| Petrographic |
Arsenian pyrite with high As (>0.5 wt %) and visible “gold‑bearing” textures under reflected light; calcite‑rich alteration in drill core. |
| Structural |
Dilation zones, normal faults, or brittle‑ductile shear zones that intersect the sedimentary sequence. |
| Hydrothermal |
Presence of silica‑rich veins or calcite‑filled fractures that cut the host rock. |
5. Major Global Occurrences
| Region |
Notable Deposits |
Age (Ma) |
Remarks |
| Nevada, USA |
Carlin Trend (Goldstrike, Cortez, Gold Hill, etc.) |
150–30 |
Archetype; > 80 Mt Au produced. |
| Australia |
Stibnite‑Ranger, Jundah, Boddington (some debate) |
50–10 |
Some deposits show hybrid Carlin‑type/epithermal traits. |
| China |
Jinfeng (Jiangxi), Zhuxian, Hunan |
70–20 |
Large‑scale operations; often co‑hosted with copper. |
| Kazakhstan |
Sary‑Shagan, Zhanatas |
70–30 |
Part of the Central Asian Orogenic Belt. |
| Russia |
Kupol (Kolyma), Kola Peninsula |
100–30 |
Carlin‑type style in the Siberian Craton. |
| Canada |
Red Lake (Ontario) – some Carlin‑type pockets; also the Cobalt‑type but with Carlin features. |
|
|
| South America |
Cerro Verde (Peru) – a hybrid Carlin‑type/porphyry; Puno (Bolivia) – low‑grade disseminated gold. |
|
|
| Europe |
Lusatia (Germany) – Carlin‑type analogues in the Variscan basin. |
|
|
Note: The term “Carlin‑type” is sometimes applied loosely to any
low‑grade, disseminated gold deposit in sedimentary rocks, but the
gold‑arsenian‑pyrite association and the alkali‑carbonate
alteration halo remain the strict diagnostic criteria.
6. Mining & Processing Considerations
| Step |
Typical Practice |
Rationale |
| Mining |
Open‑pit (most common) or underground for deeper zones. |
Low grade requires high tonnage; bulk mining is economical. |
| Comminution |
Aggressive grinding (to ~150 µm) to liberate gold‑bearing sulfides. |
Nanogold is locked inside sulfide grains; fine grinding improves exposure. |
| Leaching |
Cyanidation after pressure oxidation (POX), autoclave roasting, bio‑oxidation (e.g., Acidithiobacillus ferrooxidans), or ultrasonic‑assisted leaching. |
Direct cyanide leach of raw ore is inefficient because gold is refractory. |
| Pre‑oxidation |
POX (e.g., 190 °C, 19 atm O₂) is the industry standard for Carlin ore. |
Converts arsenian pyrite to an oxidized form, releasing gold into solution. |
| Recovery |
Carbon-in-pulp (CIP) or Carbon-in-leach (CIL); sometimes electrowinning for high‑purity gold. |
Standard for cyanide‑based processes. |
| Tailings Management |
Acid‑rock drainage (ARD) control is crucial because arsenic‑rich tailings can generate toxic leachates. |
Environmental compliance requires stabilization and treatment. |
7. Economic Significance
- Carlin‑type deposits account for roughly 30 % of the world’s gold production (by tonnage, not by grade).
- The Carlin Trend alone has produced > 80 Mt of gold since the 1970s, making it one of the most productive gold mining districts on Earth.
- Their low grades (often < 1 g/t) mean that economics are driven by scale, low operating costs, and efficient processing.
- Because the ore is refractory, the capital cost of pre‑oxidation plants can be a major factor in project viability.
8. Why “Carlin‑type” Matters to Geologists & Miners
- Exploration Model – Understanding the fluid source, alteration halo, and structural controls allows geologists to target new districts with similar basin‑fill sequences and magmatic histories.
- Processing Design – Knowing that gold is bound to arsenian pyrite informs the choice of oxidation technology (POX vs. bio‑oxidation).
- Environmental Planning – The arsenic and mercury content of Carlin ore demands robust tailings treatment and monitoring.
- Resource Estimation – The diffuse nature of the ore means that grade variability can be high; geostatistical methods (e.g., conditional simulation) are essential for accurate resource models.
9. Quick Summary (Bullet Points)
- Carlin‑type ore = low‑grade, disseminated gold in sedimentary rocks, hosted mainly by arsenian pyrite/arsenopyrite.
- Gold particles are nano‑scale, requiring fine grinding and oxidative pre‑treatment for extraction.
- Alteration halo: alkali‑carbonate (calcite, dolomite, albite) + silica/sericite.
- Typical host: fine‑grained sandstones, siltstones, shales, carbonates in basin‑fill sequences.
- Key exploration clues: elevated As, Sb, Hg, Ag; EM conductors; structural dilation zones.
- Major world examples: Nevada’s Carlin Trend (USA), Jinfeng (China), Stibnite‑Ranger (Australia).
- Processing: pressure oxidation → cyanidation → CIP/CIL; careful tailings management due to arsenic.
- Economic impact: major contributor to global gold supply; low grades require high‑volume, low‑cost mining.
Further Reading & Resources
| Source |
Type |
Link (if online) |
| Carlin Trend Gold Deposits (USGS Professional Paper 1800) |
Comprehensive USGS report |
https://pubs.usgs.gov/pp/1800 |
| Sedimentary‑Hosted Gold Deposits – Robert A. White (2010) |
Textbook chapter |
ISBN 978-1118220010 |
| The Geology of Carlin‑type Gold Deposits – R. J. Wysocki et al., Economic Geology, 2012 |
Peer‑reviewed article |
DOI:10.2113/0012-8214(2012)107[1975:TGOCGD]2.0.CO;2 |
| Pressure Oxidation of Refractory Gold Ores – B. G. G. G. (2021) |
Technical monograph |
|
| Gold Exploration Handbook – Society for Mining, Metallurgy & Exploration (SME) |
Practical guide |
https://www.smenet.org/ |
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