Best Methods to Identify Kimberlite in the Field

Kimberlite is a rare and valuable igneous rock known primarily for its association with diamonds. It forms deep within the Earth’s mantle and is brought to the surface through volcanic pipes. Identifying kimberlite in the field is crucial for geologists and prospectors seeking diamond deposits, as well as for academic researchers studying mantle processes. However, kimberlite can be challenging to recognize due to its varied appearance and alteration over time. This article delves into the best methods to identify kimberlite in the field, focusing on geological, mineralogical, geochemical, and geophysical techniques.

Understanding Kimberlite: A Brief Overview

Before exploring identification methods, it is essential to understand what kimberlite is and how it forms. Kimberlites are ultramafic, potassic igneous rocks originating from depths exceeding 150 kilometers in the mantle. They are typically erupted explosively, creating vertical pipes known as kimberlite pipes or diatremes.

Kimberlites often contain distinct minerals such as olivine, phlogopite, and a variety of mantle xenoliths. They may exhibit varying textures from coherent rocks to breccias, broken fragments cemented together by finer matrix material. Their unique petrology and geochemistry make them identifiable despite extensive weathering and alteration.

Visual Identification: What to Look For in the Field

1. Color and Texture

Kimberlite often displays a characteristic coloration but can vary significantly:
– Fresh kimberlite typically appears dark greenish to brownish due to abundant olivine.
– Weathered kimberlite may take on rusty hues from iron oxidation.
– Texture ranges from coarse-grained to fine-grained; many kimberlites show a porphyritic texture with large olivine phenocrysts embedded in a finer groundmass.

2. Presence of Olivine Phenocrysts

Olivine crystals are one of the best visual clues:
– Look for greenish, glassy olivine grains up to several millimeters in size.
– These crystals give the rock a distinctive spotted appearance.
– Olivine is often rounded or fractured due to transport within the magma.

3. Brecciated Nature

Many kimberlites present as breccias:
– Consist of angular rock fragments set in a finer matrix.
– Fragments may include xenoliths of mantle peridotite or crustal rocks.
– Such brecciation distinguishes kimberlite from other ultramafic rocks like peridotite or basalt.

4. Indicator Minerals

While not definitive alone, certain indicator minerals provide clues:
– Pyrope garnet (deep red), chrome diopside (green), chromite (black), ilmenite (black metallic).
– These minerals are common in kimberlite and mantle xenoliths.
– Finding these around suspected kimberlite pipes supports identification.

Field Sampling Strategies for Kimberlite Identification

To maximize reliable identification, proper sampling is critical:
– Collect fresh, unweathered samples when possible.
– Target outcrops showing brecciation or olivine-rich zones.
– Sample xenolith-bearing zones as these may be rich in diagnostic mantle minerals.
– Take multiple samples across different parts of suspected pipes to capture variability.

Handheld Tools and Techniques

1. Hand Lens Examination

A 10x hand lens can reveal mineral grains:
– Check for green olivine crystals embedded within the matrix.
– Identify shiny mica flakes (phlogopite).
– Look for small garnet grains embedded in rock fragments.

2. Acid Test

Kimberlites generally do not react strongly to dilute hydrochloric acid (HCl), unlike carbonates:
– Applying few drops of HCl will help distinguish from calcite or dolomite-bearing rocks.

3. Magnetic Susceptibility

Many kimberlites are weakly magnetic due to magnetite or ilmenite:
– Portable magnetic susceptibility meters can aid identification.
– Strongly magnetic readings indicate other ultramafic rocks; weak to moderate readings better match kimberlite.

Geochemical Field Testing Methods

Though laboratory analysis provides definitive results, some field-adapted geochemical tests assist identification:

Portable X-Ray Fluorescence (pXRF)

Portable XRF analyzers allow rapid elemental analysis:
– Kimberlites have distinctive chemical signatures: high MgO, K2O, Ni, Cr content.
– Elevated potassium levels differentiate them from other ultramafic rocks like peridotite.

pH and Conductivity Measurements

Fresh kimberlite tends toward neutral pH with low conductivity compared to soils or altered rocks.

Geophysical Survey Techniques for Kimberlite Detection

Because kimberlites form pipe-like intrusions deep underground, geophysical methods provide vital clues before excavation:

1. Magnetic Surveys

Kimberlite pipes often cause localized magnetic anomalies due to magnetite or ilmenite content:
– Magnetometer surveys detect these anomalies.
– Kimberlite signatures tend to be subtle compared with more magnetic ultramafics.

2. Gravity Surveys

Kimberlites generally have lower density than surrounding country rock:
– Gravity meters detect negative gravity anomalies indicating possible kimberlite pipes.

3. Seismic Methods

Seismic reflection and refraction can delineate pipe geometry at depth but require advanced equipment beyond typical field use.

Petrographic Analysis: Confirming Kimberlite Identity

Once samples are collected, thin-section petrography under a microscope confirms field identifications:

Key Features Under Microscope

Olivine with characteristic fracture patterns and alteration rims.
Phlogopite mica flakes with high birefringence colors.
Groundmass consisting of serpentine, calcite, carbonates formed by alteration.
Presence of xenoliths such as eclogite or peridotite fragments.

Microscopic confirmation helps distinguish between true kimberlite and similar ultramafic rocks.

Challenges in Field Identification of Kimberlite

Despite multiple indicators, identifying kimberlite remains challenging due to:

Weathering and Alteration

Kimberlites frequently alter into serpentine and carbonate minerals over time:
– Original textures may be obscured.

Similarity with Other Ultramafic Rocks

Differentiating between kimberlite, lamproite, lamprophyre, and other mafic-ultramafic rocks requires careful mineralogical study.

Limited Outcrop Exposure

Kimberlite pipes are often covered by soil or glacial deposits making surface sampling difficult.

Summary: Best Practices for Kimberlite Identification in the Field

Observe Color and Texture: Look for dark greenish-brown colors with coarse olivine crystals and brecciated textures.
Identify Mineral Indicators: Focus on olivine phenocrysts plus garnet, chrome diopside, chromite presence.
Use Handheld Tools: Hand lens examination combined with acid testing and magnetic susceptibility.
Apply Geochemical Techniques: Portable XRF helps discern unique elemental signatures on-site.
Conduct Geophysical Surveys: Magnetic and gravity surveys pinpoint subsurface pipe locations before drilling.
Collect Multiple Samples: Ensure representative sampling from various parts of suspected pipes for lab confirmation.
Follow Up with Petrography: Confirm field observations through microscopic analysis of thin sections.

Final Thoughts

Identifying kimberlite in the field demands an integrated approach combining visual observation, mineralogical inspection, simple geochemical tests, and geophysical indications. While no single method guarantees accurate recognition due to natural variability and weathering effects, employing multiple complementary techniques increases confidence significantly.

Successful detection of kimberlite not only advances diamond exploration efforts but also enriches our understanding of Earth’s deep mantle processes acting below our feet. Field geologists armed with these methods stand a better chance at unveiling these fascinating geological treasures hidden beneath the surface landscape.