Drillhole Spacing for Exploration vs Infill: A Field Guide for Indonesian Deposits

I’ve sat through too many drilling budget meetings where the question “how close do we need to drill?” gets answered with “let’s just do 40m grids and see.” Sometimes 40m is right. Often it’s either overkill that burns the exploration budget on Inferred material, or undershoot that leaves you with a resource an auditor won’t sign off on. The honest answer depends on the deposit type, the variogram range, the classification you’re targeting, and — in Indonesia specifically — what the regulator expects to see before you move a project forward.

This is a field guide, not a textbook. I’ll cover how I think about drill spacing for the four deposit types I work on most in Indonesia (epithermal gold, porphyry Cu-Au, laterite Ni, sedimentary-hosted), when infill drilling is justified versus waste, and how to defend your spacing decision to a Competent Person and to the KCMI committee.

Why spacing matters more than people admit

Drillhole density is the single biggest driver of resource classification. Get it wrong in either direction and you pay:

• Too wide: your variogram is unconstrained at short lags, kriging extrapolates, blocks are over-classified, and the auditor downgrades everything to Inferred. You’ve spent the drilling budget and still don’t have a mineable resource.
• Too tight: you’ve spent money drilling material that was already Inferred or Indicated, the variogram doesn’t actually improve because you’ve added redundant information, and you’ve delayed the project timeline for no classification gain.

The cost asymmetry isn’t symmetric. Drilling too wide and getting downgraded is recoverable — drill more later. Drilling too tight and burning the budget is not. So the bias should be toward defensible wider spacing on the first pass, then targeted infill where the variogram and the economics actually justify it.

This is also why data validation before estimation matters so much — wide spacing with bad data is the worst of both worlds.

Spacing is driven by the variogram range

The geological rule of thumb: a drillhole spacing of roughly half the variogram range in the direction of maximum continuity is the sweet spot for Indicated resources. For Measured, you want closer to a third of the range. For Inferred, you can stretch to the full range or beyond, as long as you’re honest about the classification.

This isn’t a regulatory rule — it’s a geostatistical one. The variogram range tells you the distance within which samples are spatially correlated. Drilling closer than the range gives you information that improves the estimate. Drilling closer than half the range gives you information that meaningfully improves the estimate of the estimate (i.e. the kriging variance, which drives classification). Drilling closer than a quarter of the range gives you almost nothing new — the samples are so highly correlated that the additional information is largely redundant.

If you haven’t run the variogram yet — and on early-stage projects you often haven’t — you can use deposit-type analogs to set the first-pass spacing. But the moment you have enough data, you replace the analog with the actual variogram. For a refresher on reading variograms without the math panic, see variography for geologists who hate math.

Spacing by deposit type — Indonesian analogs

Epithermal gold (Sumatra, Sulawesi)

Low-sulfidation and high-sulfidation epithermal systems are the deposit type where spacing decisions bite hardest. The ore is structurally controlled, narrow, and grade can change over meters. A 60m grid that looks reasonable on the plan view can completely miss a vein that pinches and swells between sections.

First-pass exploration spacing: 80–120m along strike, 40–60m across. This gives you enough to define the vein geometry and a rough grade shell. Expect Inferred classification only.

Infill for Indicated: 30–50m along strike, 20–40m across. The along-strike spacing should be roughly half your variogram range — for most Sumatran low-sulfidation veins I’ve worked on, that’s 40–60m.

Infill for Measured: rarely justified at the resource stage. Measured usually only gets drilled at the grade-control stage or for the first year of mining reserves. I’ve seen too many projects burn money drilling 15m grids on resource definition that adds nothing to classification.

Halmahera and Sulawesi high-sulfidation systems tend to have broader alteration halos and more disseminated mineralization — spacing can relax by ~30% compared to narrow-vein Sumatran systems.

Porphyry Cu-Au (Papua, Sumbawa)

Porphyries are the friendliest deposit type for wide spacing. Grade is disseminated, continuity is excellent, and variogram ranges of 150–300m are common. The grade shell is large and forgiving.

First-pass exploration: 100–200m grids. Enough to define the footprint and a preliminary grade shell.

Infill for Indicated: 60–100m. Most Indonesian porphyry projects I’ve reviewed hit Indicated at 80m spacing without issue.

Infill for Measured: 40–60m. Worth it for the early production years, especially on deposits where the Cu-Au ratio varies and you want to control the mill feed.

The trap on porphyries is the late-stage vein stockwork. If the high-grade shell is driven by a sheeted vein system rather than disseminated mineralization, the variogram range in the vein direction drops dramatically and you need tighter spacing — closer to epithermal spacing — to classify the high-grade portion correctly.

Laterite nickel (Halmahera, Sulawesi, Papua)

Laterites are the special case. Mineralization is essentially 2D — a blanket at the base of the weathering profile. The grade continuity is high but the thickness continuity is what really drives the resource. You’re estimating Ni, Co, Fe, and the thickness of each horizon (limonite, saprolite, bedrock).

First-pass exploration: 100–200m grids. Rotary air blast (RAB) or auger is common, cheap, and adequate for footprint definition.

Infill for Indicated: 50–100m. The key here is regular grid — laterites are one of the few deposit types where a perfectly regular square grid is genuinely optimal, because the variogram is typically isotropic in the horizontal plane.

Infill for Measured: 25–50m. Often justified because laterite mining is selective and the thickness variability directly affects the mining schedule.

One Indonesian-specific note: the Sulawesi ultramafic complexes have a strong redox-driven Ni enrichment that can create local high-grade saprolite pods. If you’re working one of these, treat the high-grade pod domain like a separate deposit with tighter spacing — don’t let it ride on the broad grid.

Sedimentary-hosted (Java polymetallic, coal, manganese)

Sedimentary deposits have the highest continuity of all. Variogram ranges of 200–500m are normal, and the geometry is layer-cake.

First-pass exploration: 200–400m.

Infill for Indicated: 100–200m.

Infill for Measured: 50–100m.

Coal deposits in particular can be classified Indicated at 200m spacing if the seam is consistent. Polymetallic replacement deposits in Java are more variable and tend to need 100m for Indicated.

When infill drilling is justified

Infill drilling is justified when all three of these conditions hold:

1. The variogram shows you’re below the inflection point. If your current spacing is already less than half the variogram range, infill won’t materially improve the kriging variance. You’re already getting diminishing returns.
2. The economics work at the expected classification upgrade. Calculate the NPV uplift from moving X tonnes from Inferred to Indicated. If the infill cost exceeds the NPV uplift, the drilling doesn’t pay for itself. This sounds obvious but I’ve seen projects spend $2M on infill that upgraded 500K tonnes — a project NPV gain of roughly zero.
3. The target area is actually going to be mined in the early years. Infill drilling on year-7 reserve material is a waste of working capital. Infill the first 2–3 years of production, then stop.

When infill drilling is waste

• When the variogram is already flat at your current spacing — you’re collecting redundant information.
• When the deposit is high-nugget and you’re trying to drill your way out of the nugget effect. Tighter spacing doesn’t fix a high nugget. It just costs more. The fix for a high-nugget deposit is larger samples (bulk sampling, trenching) and a more conservative classification, not 10m drill grids.
• When the infill is “to be safe” rather than to address a specific question. “To be safe” isn’t a drilling objective. If you can’t state the question the infill is answering, don’t drill it.

This is also one of the resource estimation mistakes that cost mining companies millions — drilling without a geostatistical justification.

Indonesian regulatory context

The KCMI 2017 guidelines don’t specify hard drill spacing numbers for classification. This is intentional — spacing alone doesn’t determine classification, the geostatistical confidence does. But the KCMI committee and the Indonesian auditors who sit on it have practical expectations, and falling too far outside these ranges invites questions:

Deposit type	Inferred	Indicated	Measured
Epithermal Au	80–120m	30–50m	15–25m (rare)
Porphyry Cu-Au	100–200m	60–100m	40–60m
Laterite Ni	100–200m	50–100m	25–50m
Sedimentary	200–400m	100–200m	50–100m

These are starting points, not rules. If you’re classifying Indicated at 80m on a porphyry, the auditor will accept it. If you’re classifying Indicated at 80m on a narrow-vein epithermal, you’d better have an exceptional variogram and a tight domain model — or expect a downgrade.

The KCMI committee also pays attention to drill orientation relative to mineralization. A 60m grid drilled parallel to the vein is worse than a 100m grid drilled perpendicular. Orientation beats spacing every time. If your holes are oblique to the structural trend, no amount of infill will save you.

A worked example — Sulawesi epithermal

A project I reviewed last year: high-sulfidation epithermal Au in North Sulawesi. Original drilling was 80m × 80m, 42 holes, all diamond. The team wanted to upgrade to Indicated and proposed an 80m × 40m infill — basically halving the cross-strike spacing.

I ran the variogram. Along-strike range was 120m. Across-strike range was 45m. The 80m × 80m grid was already at 0.67× the along-strike range and 1.78× the across-strike range. The along-strike direction was already fine for Indicated. The across-strike direction was the problem — they needed roughly 22m across-strike to hit half the range.

The proposed 40m across-strike infill would have left them still above half the range in the critical direction. They’d have spent the drilling budget and gotten an Indicated classification that an auditor would still downgrade.

The fix: 25m across-strike infill on the core of the deposit only (the first 2 years of mining), accept Inferred on the rest. Total drilling cost dropped 30%, the Indicated classification held up at audit, and the project moved forward.

How Orebit GeoSuite helps

The Orebit GeoSuite Resource Estimation module runs the variography and kriging variance analysis you need to make spacing decisions defensibly:

• Variogram ranges per domain per direction — the actual numbers you need to set spacing, not analog guesses
• Kriging variance maps — see where your estimate is confident and where it’s weak, before you commit to a drilling program
• “What-if” spacing scenarios — drop synthetic drillhole locations into the model and see the kriging variance reduction before you spend a meter of drilling
• Classification map output — visualizes which blocks meet Indicated vs Inferred based on your kriging variance threshold
• Audit-ready spacing justification report — documents the variogram range, the kriging variance, and the classification logic per domain

The “what-if” scenario alone has saved clients from at least three bad infill programs in the last 18 months. Run the synthetic first. Drill the real second.

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Bottom line

Drill spacing is a geostatistical decision dressed up as a budgeting one. The variogram range sets the spacing. The deposit type sets the analog when you don’t have a variogram yet. Infill drilling is justified by kriging variance reduction and NPV uplift, not by “to be safe.”

Get the orientation right first. Get the spacing right second. Get the infill targeted to the early mining years third. Everything else is drilling for drilling’s sake.

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Working through a drilling program design and want a second pair of eyes on the spacing logic? Email hello@orebit.id with the variogram and the deposit type — I’ll tell you what I’d do.

# From this article:
open geosuite.orebit.id
load(your_drillhole.csv)
apply(workflow_above)

# Done. Ship the report.