Guide to Collecting and Preparing Kimberlite Samples for Analysis

Kimberlite samples are the only direct window into the deep mantle that diamonds call home. A single 5 kg shard, if collected and preserved correctly, can reveal grade, mantle age, and indicator mineral chemistry worth millions of dollars in decision-making.

Yet most companies lose half the value of their drill core through sloppy protocols, and even the best geologists can miss a diamondiferous interval because the rock oxidized on the way to the lab. This guide walks you through field, lab, and legal steps that keep every grain of kimberlite trustworthy.

Field Recognition: Spotting Kimberlite Before You Waste a Drill Bit

Visual Signatures in Weathered Terrain

Fresh kimberlite weathers to a yellow-brown popcorn crust that traps surface moisture longer than surrounding country rock. In the Kalahari, this contrast shows up on 0.5 m-resolution Sentinel-2 SWIR imagery as a 3–5 % higher moisture index, flagging pipes weeks before ground truthing.

Look for circular depressions filled with darker, finer soil that supports thicker grass; cattle trails often ring the margin because the vegetation stays green deeper into the dry season. A hand lens on loose laterite will reveal purple garnet grains sitting in a pale green clay matrix—classic G10 pyrope that survives weathering.

Hard-Rock Exposure Tactics

In Canadian Shield outcrop, kimberlite dykes pinch and swell, so trench perpendicular to the strike every 25 m even when surface expression dies. A 40 m dyke that pinches to 10 cm can flare to 2 m at depth, carrying the highest diamond concentration in the swollen zone.

Carry a 405 nm UV flashlight at dusk; calcite-serpentine kimberlite fluoresces pale orange while granite stays dark, letting you trace the dyke under moss. Mark the contact with spray paint that survives −30 °C, then channel-sample 10 cm into the wall to avoid the blast-damaged skin.

Quick Indicator Test in the Field

Drop a 1 cm piece of suspected kimberlite into a 40 % HF bath for 30 s; true kimberlite foams vigorously because interstitial calcite reacts, while lamproite or carbonate stays quiet. Rinse immediately—this test destroys the sample, so use only discard chips.

Drill Planning: Designing Holes that Maximize Sample Value

Azimuth and Dip Math

Orient every drill hole to intersect the kimberlite at 70–80 ° to the contact; angles shallower than 60 ° smear the contact over metres and dilute the sample with wall rock. If the pipe plunges 75 ° southeast, drill northwest at −75 ° with a 5 ° left-hand twist to compensate for rod bend.

Triple-Tube vs. Double-Tube Core

Switch to triple-tube (HQ3 or PQ3) once the bit enters the crater zone; the inner split barrel prevents rotation bruising that liberates diamonds into the drilling fluid. A 2018 Lac de Gras program recovered 32 % more +1 mm stones after switching from HQ2 to PQ3 at 80 m depth, paying for the extra barrel cost in the first batch.

Fluid Chemistry to Preserve Surface Texture

Use a biodegradable ester-based polymer instead of bentonite mud; bentonite plates smother diamond surfaces and bias micro-texture analysis that predicts liberation behavior. Keep pH at 8.5 with potassium carbonate—any lower corrodes chrome diopside, any higher precipitates gypsum that locks diamonds in the core.

On-Site Core Handling: From Barrel to Freezer in 15 Minutes

Time-Temperature Thresholds

Serpentine begins to de-hydrate at 35 °C, turning glossy black kimberlite into powdery grey useless rock within hours. Wheel the core tray straight from the barrel into a shaded polystyrene box lined with frozen gel packs; the target is <15 °C within 10 min.

Photogrammetry Before Washing

Shoot 360 ° photos while the core is still wet with drilling fluid; the film records soft-clay infill that will wash away and eliminates arguments later about whether a contact was faulted or drilled through. Use a 24 MP camera on manual focus, f/8, with a colour checker card for white balance.

Splitting Protocol for Dual Sampling

Cut the core with an oil-free diamond saw, not a hammer, to avoid micro-fractures that drop diamonds into the saw slurry. Bag the “field” half in woven polypropylene and the “reject” half in plastic; write the drill metre on both bags with a lumber crayon that survives −40 °C storage.

Legal Chain of Custody: Paperwork that Wins Court Cases

Witnessed Transfers

Every hand-off gets a three-way signature: driller, site geologist, and truck driver. GPS the transfer point—courts love coordinates more than vague “loading bay” notes.

Sealing Technology

Fit 2 mm steel wire seals through the bag drawstring and crimp with a custom die that embosses the project logo; photos of the seal number go into the cloud within 60 s using a phone app that time-stamps to the second. A broken seal on arrival voids that batch and triggers a re-sampling program paid by the contractor.

Customs and Kimberley Compliance

Even if no diamonds are visible, declare the core as “potentially diamondiferous kimberlite” on the export permit; customs officers have X-ray machines and will confiscate undeclared samples. Attach a pro-forma invoice showing $0 value but listing exact weight to the gram—rounding invites fines.

Transport: Keeping Samples Cold, Dry, and Legal

Phase-Change Coolers

Pack field halves in coolers with PCM-5 panels that hold exactly 5 °C for 96 h; dry ice cracks serpentine, so avoid it unless the shipment reaches the lab within 24 h. Slip a Bluetooth temp logger between the bags—if the curve ever exceeds 8 °C, the lab rejects the batch and you re-drill.

Shock Isolation

Line the cooler with 50 mm closed-cell foam; kimberlite rich in monticellite fractures at 3 g shock, common on gravel runways. Orient bags so the core axis is horizontal; vertical stacking concentrates impact on the end pieces that usually carry the most diamonds.

Quarantine Regulations

Australia treats kimberlite as “basic igneous rock” exempt from fumigation, but Chile requires 48 h methyl bromide if the core came from a region with fruit fly; schedule the lab arrival after the fumigation window or risk a 30-day customs hold that oxidizes your samples.

Lab Reception: First 60 Minutes Decide Data Quality

Micro-CT Before Anything Else

Run the whole core through a 5 µm medical CT scanner while still sealed in the bag; the scan locates diamonds >0.3 mm and maps internal fractures so the sawyer avoids them. One 30 s pass costs $12 and saves $2 000 in saw-blade damage when a hidden 2 mm diamond hits the blade.

Controlled Thawing

Move bags from 5 °C to 20 °C over 8 h; rapid thaw sucks condensation into the rock and mobilizes water-soluble phases like nahcolite that cement the matrix. Place bags on wire racks with 10 cm spacing so air circulates evenly.

Subsample Division

Use a stainless-steel riffle splitter with 20 chutes; plastic splits build static that attracts garnet dust and biases the heavy mineral count. Split twice to get a 250 g lab split and a 250 g referee split, then seal the remainder for future metallurgical test work.

Crushing and Liberation: Freeing Diamonds Without Breaking Them

Low-Impact Jaw Choice

Fit the crusher with Mn-steel jaws ground to a 5 mm gap; tighter gaps chip diamonds, wider gaps leave them locked in silicate. Check the gap weekly with a feeler gauge—operators sneak it wider to reduce blockages.

Pre-screening at 3 mm

Pass the crushed product over a 3 mm wet screen; diamonds larger than the screen ride over and bypass the cone crusher that follows, reducing fracture risk. Capture the -3 mm for dense media separation; this split usually carries 60 % of the total stone count but only 20 % of the carats.

Water Chemistry Again

Recirculate process water through a 1 µm bag filter to remove garnet chips that scratch diamonds. Maintain 200 ppm Ca to suppress clay dispersion, but keep chloride below 50 ppm to avoid pitting stainless steel spirals.

Dense Media Separation: Floating Diamonds the Right Way

Ferrosilicon Grade

Use 150-grade atomized FeSi with 15 % Si; rounded particles sink slower and reduce viscosity, letting 2.65 g/cm³ kimberlite float while 3.5 g/cm³ diamonds sink. Angular 75-grade FeSi scours diamonds and raises media viscosity 8 %, losing 5 % of 1 mm stones to the floats.

Media Stability Test

Measure rheology every 30 min with a Marsh funnel; 40 s efflux time gives the sharp density cut you need. Add 0.1 % bentonite if the time drops below 35 s, but never more—bentonite entrains diamonds in the froth.

Sink Product Handling

Collect sinks in a 20 L plastic pail with 5 cm of water on the bottom; dry dumping creates static that flings 0.5 mm diamonds onto the floor, never to be found again. Label the pail with a QR code linked to the original drill metre and crusher batch.

Microdiamond Recovery: Catching Stones Below 0.5 mm

Grease Table Setup

Condition the grease with 5 % lanolin to raise tack at 4 °C; colder grease peels off the belt cleanly, carrying diamonds to the recovery scraper. Run the belt at 0.2 m/s—faster throws small crystals into the tailings, slower lets wet silica build up and bury diamonds.

Acid Cleaning Loop

Soak the grease concentrate in 50 °C 30 % HCl for 10 min to dissolve silicate, then rinse with 40 % NaOH to saponify the grease; diamonds drop out clean and ready for hand-picking. Use Teflon beakers—HF attacks glass and contaminates the sample with Al.

Hand-Picking Microscope

Use a 10–40× zoom stereo with LED ring light at 5 500 K; this temperature renders diamond’s adamantine luster distinct from quartz’s glassy flash. Pick with a 000 sable brush dipped in ethanol; static-free brushes flick stones into oblivion.

Indicator Mineral Separation: Garnet, Chrome Diopside, Ilmenite

Magnetic Fractionation

Pass the 0.3–2 mm fraction through a Frantz isodynamic separator at 1.0 A and 10° side tilt; ilmenite jumps to the magnetic chute while G10 garnet stays non-magnetic. Repeat at 0.5 A to split high-Cr diopside from low-Cr enstatite, a proxy for diamond fertility.

Heavy Liquid Last Step

p>Sink the non-magnetic fraction in 3.32 g/cm³ di-iodomethane; diamonds float, garnets sink, giving a diamond concentrate free of coloured silicates that confuse automated scanners. Neutralize the liquid with activated copper turnings after each run to keep the density stable.

Grain Mounting for EPMA

Mount 100 grains per 1 cm epoxy puck, polish with 0.25 µm diamond paste, then carbon-coat 20 nm thick; thicker films absorb the Ca Kα signal and drop chrome diopside totals below 98 %. Orient each grain so the 100 plane is perpendicular to the beam—this reduces clinopyroxite pleochroism errors.

Geochemical Aliquots: Preparing Powder for XRF and ICP-MS

Contamination Avoidance

Powder 10 g of reject chips in a tungsten carbide ring mill for 40 s; tungsten adds only 1 ppm W, well below ICP-MS detection, while steel adds 500 ppm Fe and ruins trace-element balance. Pass the powder through a 75 µm nylon sieve; metal sieves shed Co and Ni.

Flux Ratio for XRF Beads

Mix 0.9 g sample with 9.0 g of 66 : 34 LiT : LiM flux; this ratio dissolves kimberlite completely at 1 050 °C and keeps Cr2O3 in solution. Add 0.5 g NH4NO3 oxidizer to prevent FeS prills that scratch the platinum mold.

Acid Digestion for REE

Dissolve 50 mg powder in 5 mL 49 % HF + 2 mL HNO3 at 160 °C for 24 h in a PFA bomb; shorter times leave perovskite untouched and bias Nb-Ta ratios. Evaporate to dryness, re-dissolve in 2 % HNO3 + 0.05 % HF to keep Nb and Ta stable as fluorides.

Quality Assurance: Blind Duplicates and Certified Reference Materials

Duplicate Insertion Rate

Insert a 1 : 20 coarse reject duplicate and a 1 : 30 pulp duplicate; kimberlite is heterogenous at 1 m scale, so duplicates must come from the same crusher batch. Flag any duplicate pair with >10 % relative difference for Cr2O3—this catches mill cross-contamination faster than diamond assays.

Certified Reference Selection

Run NIST-2780 (hard rock mine waste) for trace metals and SY-4 (diorite gneiss) for major elements; neither is kimberlite, but both bracket the compositional range and force the lab to meet matrix-matched accuracy. Do not use SARM-1 (South African reference kimberlite) for every batch—its garnet abundance is atypical and can mask bias.

External Lab Round Robin

Ship 5 % of rejects to a second ISO-17025 lab every quarter; disagreement on CaO above 0.5 % absolute triggers an audit of fusion temperature and bead homogeneity. Keep the second lab blind to the first’s results to avoid subconscious curve-fitting.

Data Handoff: From Assay Spreadsheet to 3D Model

Field-to-File Naming

Code every sample as ProjectHoleFrom-To-Zone, e.g., “MKR21_345.3-345.6_KIMB”; underscores survive Excel auto-correct and keep the string sortable. Never embed spaces or commas—they break GSLIB and SGeMS import parsers.

Validation Rules

Reject any row where the sum of major oxides falls outside 98.5–101 %; kimberlite has 5–8 % LOI, so totals below 98 % usually mean incomplete digestion. Flag Cr2O3 >1.5 % outside logged garnet zones—the assay probably sampled a hidden pegmatite vein.

3D Block Model Feed

Import assays into Leapfrog with 0.5 m composites capped at 25 ppb Au and 2 000 ppm Cr; kimberlite can carry micron gold that skews economic models if left uncapped. Export as CSV with east-north-elevation in GDA2020 to match government topography.

Archive Strategy: Storing Rejects for 30 Years

Climate-Controlled Warehouse

Store reject bags at 4 °C and 40 % RH; higher humidity grows mold that binds fines into concrete lumps, lower humidity electro-statically separates garnet from diamonds. Use pallet racking with 150 mm gaps between bags so air circulates and inspectors can pull any bag without restacking.

Sample Selection Map

Link each warehouse location to a QR code that opens a 3D viewer showing the exact drill trace and assay interval; auditors can scan and verify in seconds instead of opening 50 kg bags. Back up the database on three continents—kimberlite archives have outlived more than one mining company.

Destructive Retest Rights

Write into every joint venture that either partner can demand a 1 kg re-split for metallurgical testing up to 10 years after mine closure; this clause prevents future disputes if diamond breakage rises in the plant. Keep 2 kg extra in a separate locked cage to satisfy the clause without touching the main archive.