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Oxide Gold IndexPublished by Lannister Mining

Exploration methods

Near surface gold exploration: methods, order of work and what results mean

Last updated: Publisher: Lannister Mining

How do geologists explore for gold near the surface?

Geologists explore for near-surface gold by mapping and sampling outcrop and soil, trenching across anomalies, and then drilling to prove depth and continuity.

In the Oxide Gold Index, published by Lannister Mining, near surface gold exploration means testing lode mineralization that starts at or close to the ground. Each stage costs more than the last, so a company only advances a target when the cheaper stage justifies it. Long intervals from surface are the result explorers hope to see.

Near-surface exploration in brief

Definition

What counts as near-surface

The Oxide Gold Index uses near-surface for bedrock mineralization that starts at or close to surface and can be tested with trenches and shallow drilling. It is distinct from placer gold, which has been eroded and concentrated in stream gravels. Many popular guides to finding gold near the surface describe placer locations for panning; exploration companies look for the lode sources upslope.

Near-surface mineralization is often oxidized, which links this method guide to the oxide gold exploration guide.

Numbers

Real near-surface results

Highlighted historical intersections from surface at Basin Gulch, as published by Lannister Mining:

  • 1.8 g/t Au over 162 m from surface
  • 0.66 g/t Au over 326 m from surface
  • 3.2 g/t Au over 72 m from surface

For comparison, U.S. GoldMining reports historical rock grabs up to 26.9 g/t Au near its Whistler deposit in Alaska (U.S. GoldMining). Grab samples are selective and do not represent width.

The order of work, step by step

  1. Desk study and historical data

    Review old maps, workings, assays and drill logs. Basin Gulch, for instance, has more than 320 historical drill holes to compile and verify.

  2. Geological mapping and rock chips

    Map rock types, structures, veins and iron oxide alteration; collect rock chip and grab samples to find gold.

  3. Soil and stream geochemistry

    Grid soil sampling outlines gold and pathfinder anomalies under cover, narrowing where to dig and drill.

  4. Trenching and channel sampling

    Continuous cuts across an anomaly give grade over a measured surface length, ideal where mineralization outcrops.

  5. Shallow drilling

    RC drilling covers ground quickly; core drilling preserves structure and supplies metallurgical samples.

  6. Confirmation and step-out drilling

    Twin historical holes to verify them, then step out along strike and down dip to test extensions.

Which method answers which question?

Oxide Gold Index comparison of common near surface gold exploration methods, judged on what each one measures.
MethodWhat it tells youWhat it cannot tell youBest used when
Rock chip / grabWhether gold is present in a spotWidth or continuity; grabs are selectiveFirst pass over outcrop
Soil gridWhere anomalies sit under thin coverGrade in bedrockCovered or forested ground
TrenchGrade over a measured surface lengthWhat happens at depthMineralization reaches surface
RC drillingGrade with depth, quicklyDetailed structureShallow, broad targets
Core drillingGrade, structure and rock texturesFast, cheap coverageGeology and metallurgy samples

Reading near-surface drill results

A result such as 1.8 g/t Au over 162 meters means the hole averaged 1.8 grams of gold per tonne across 162 meters of drilled length. Three checks matter. First, is the interval from surface? Shallow starts mean less waste rock to move in any future open pit. Second, what is the true width? Radisson Mining estimates true widths at 30-80% of core length on its O'Brien drilling (Radisson), which shows how much angle can matter.

Third, is the result new or historical? Historical results guide targeting but typically need confirmation drilling before use in a resource estimate. The Basin Gulch case study includes a gram-metre calculator for comparing intercepts on one scale.

Part of the gold oxide mineralization guide.

Common questions

How do geologists explore for near-surface gold?
They start with geological mapping, rock chip sampling and soil geochemistry to find anomalies, then cut trenches to expose mineralized rock, and finally drill to test depth and continuity. Each step costs more than the last, so results at one stage decide whether the next stage is worth funding.
What is trenching in gold exploration?
Trenching means digging a shallow, continuous cut across mineralized ground and sampling it at regular intervals. It works best where mineralization reaches the surface, as oxide zones often do. Lannister Mining reports historical trench results at Basin Gulch including 4 g/t Au and 23 g/t Ag over 146 meters.
What is the difference between RC and core drilling?
Reverse circulation (RC) drilling returns crushed rock chips and is generally faster and cheaper, which suits shallow oxide targets. Diamond core drilling returns solid cylinders of rock that preserve structures, contacts and textures. Many programs use RC for coverage and core for geology and metallurgical samples.
What is true width versus drilled length?
Drilled length is the distance a hole travels through mineralization. True width is the actual thickness of the zone measured perpendicular to it. The two differ when a hole cuts a zone at an angle. Radisson Mining, for example, estimates true widths at 30-80% of core length for its O’Brien drilling.
Why do historical drill holes matter?
Historical holes show where mineralization was found before and help target new drilling. They usually cannot be used in a modern resource estimate until they are verified through confirmation drilling and checks of records and assay quality. Basin Gulch has more than 320 historical drill holes that Lannister Mining plans to use alongside new work.
What is the difference between placer and lode gold?
Lode gold is still in its host rock, such as an oxidized vein or breccia. Placer gold has been eroded from lode sources and concentrated in stream gravels. Hobby prospecting guides usually focus on placers, while oxide exploration companies explore lode deposits in bedrock.

Read all 44 questions in the oxide gold FAQ

Exploring near-surface oxide targets? Lannister Mining welcomes questions from investors and exploration partners about Basin Gulch.

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Published by Lannister Mining