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Oxide Gold IndexPublished by Lannister Mining

Exploration methods

Oxide gold exploration: logging the oxide zone and testing recovery

Last updated: Publisher: Lannister Mining

What does oxide gold exploration look for?

Oxide gold exploration looks for gold in weathered, iron-stained rock near surface and then tests whether that gold can be recovered cheaply, usually by heap leaching.

The Oxide Gold Index, published by Lannister Mining, treats oxide gold exploration as two jobs run together: a geology job that measures grade, thickness and the depth of oxidation, and a metallurgy job that measures how much of that gold a leach solution actually dissolves. Both decide whether a deposit can work.

Oxide exploration in brief

Definition

The oxide zone, in exploration terms

For the Oxide Gold Index, the oxide zone is the upper part of a gold system where sulfides have weathered to iron oxides. Explorers log each drill interval as oxide, transition or fresh, map the oxidation front in three dimensions, and treat each domain separately in metallurgy and resource models.

Field signs include red, orange and brown staining, boxwork cavities where pyrite dissolved, and soft, porous rock. Assays and leach tests, not color, confirm the result.

Numbers

Published oxide recovery results

  • Richmond Hill: twelve oxide column tests ranged from 92.0% to 57.3% recovery, unweighted average 74.2% (Dakota Gold).
  • Richmond Hill breccia oxide material: 92.0% to 90.7% gold extraction (same source).
  • Lab cyanide leaching of gold-bearing iron oxide minerals: 88.69% within 7 hours, final 94.77% (Springer Nature).

Field column results vary more than lab tests on single samples.

How oxide samples are tested, step by step

  1. Log oxidation in every interval

    Record oxide, transition or fresh state alongside rock type and alteration.

  2. Define metallurgical domains

    Group samples by lithology, grade and oxidation, the way Dakota Gold built 24 master composites.

  3. Run bottle-roll tests

    Fine-ground samples in cyanide solution give a fast first read on leachability.

  4. Run column leach tests

    Crushed ore in columns imitates a heap; Dakota Gold used 0.5-inch and 1-inch crush sizes and 21 to 47 day leach times.

  5. Feed results into the resource

    Recovery by domain informs cut-off grades and pit shells in the resource estimate.

Lab chemistry versus field columns

Oxide Gold Index comparison of published oxide gold recovery tests, each stated with its own conditions.
TestMaterialConditionsReported result
Dakota Gold, Richmond HillTwelve oxide column tests0.5-inch and 1-inch crush; 21 to 47 days92.0% to 57.3%; average 74.2%
Dakota Gold, Richmond HillBreccia oxideColumn tests, same program92.0% to 90.7%
Springer Nature studyGold-bearing iron oxide mineralsCyanide dissolution, laboratory88.69% at 7 hours; 94.77% final
Liberty Gold, Black PineOxide resource model0.10 g/t Au cut-off; US$2,800/ozVariable leach recovery model

Why cyanide chemistry matters in oxide ore

Heap leach operations keep solutions alkaline for a reason. The Springer Nature study notes that at pH 10.2 over 90% of cyanide exists as free cyanide, while at pH 8.4 over 90% exists as HCN (source). Free cyanide is what dissolves gold, which is why pH control is part of every leach test.

Oxidation helps because it opens the rock. Gold once sealed inside pyrite is exposed along cracks and pores, so solution can reach it without fine grinding. Where oxidation is partial, recoveries can fall, which is why the transition zone is tested separately. Compare the two end states in oxide vs sulfide gold.

Part of the gold oxide mineralization guide.

Common questions

What gold recovery do oxide heap leach tests show?
Results vary by deposit and rock type. Dakota Gold reported twelve oxide column tests at Richmond Hill ranging from 92.0% to 57.3% recovery, with an unweighted average of 74.2%, using 0.5-inch and 1-inch crush sizes and 21 to 47 day leach times. Every deposit needs its own testing.
Why is oxide gold often cheaper to process?
Oxidation breaks down the sulfides that lock up gold, so oxide ore often responds to heap leaching, where crushed or run-of-mine ore is stacked and irrigated with a dilute cyanide solution. Sulfide ore more often needs fine grinding, flotation or pre-oxidation, which adds capital and operating cost.
Heap leach or mill: which suits oxide ore?
Heap leaching suits many oxide ores because it needs less capital, though recovery is often lower than in a mill. Mills grind ore finely and can reach higher recoveries on harder or sulfide ore. The choice depends on grade, metallurgy and tonnage, and is confirmed by column and bottle-roll tests.
What is a cut-off grade?
A cut-off grade is the minimum grade at which rock is counted in a resource. Open-pit leach deposits often use low cut-offs: Liberty Gold reports Black Pine resources at 0.10 g/t Au using US$2,800 per ounce, while Radisson’s underground O’Brien estimate uses a 2.20 g/t Au bottom cut-off at US$2,500.
Can you see gold in oxide ore?
Usually not. Most gold in oxide deposits is fine grained and invisible to the eye. Rusty red, orange or brown iron staining, boxwork textures where sulfides once sat, and quartz veining are the visual clues geologists look for, but only laboratory assays tell you the actual grade.
Does silver occur in oxide zones too?
Yes. Silver often occurs with gold in oxidized zones, and many oxide projects report both metals. At Lannister Mining’s Basin Gulch Property in Montana, published trench highlights report gold and silver together, for example 5 g/t Au and 54 g/t Ag over 34 meters, according to the company’s property page.

Read all 44 questions in the oxide gold FAQ

Lannister Mining is exploring near-surface gold and silver oxide mineralization at Basin Gulch, Montana. Investors and partners can contact the team.

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