Request for technical advice: 300×150 m drill spacing to estimate resources in heterogeneous limesto
Hello everyone,
I would like to seek your technical advice regarding a limestone exploration permit located in a sedimentary environment composed of limestone, marl, and clay layers. Prior to my involvement, an initial geophysical survey had been conducted, and five core drillholes were installed across the area.
The results of these early drillings indicated that limestone occurs at greater depths than expected, around 7 meters below the overburden, contradicting the geophysical predictions. Three additional drillholes, spaced at least 70 meters apart, revealed significant lateral heterogeneity with no clear continuity: alternating limestone–calcareous sandstone, more massive limestone beds at another location, and alternating limestone–marl layers at a third point.
These observations clearly demonstrate the lack of geological continuity of the limestone at the local scale, raising important questions regarding the drilling strategy for the next phase.
We are planning a new geophysical survey covering two blocks of approximately 1 km² each, prior to a more extensive drilling campaign over a 6 km² area of interest. At this stage, I am considering a 300 m × 150 m drill spacing for the next phase.
My question is as follows: Given the observed lack of continuity in the initial drillholes, do you consider a 300 × 150 m spacing technically appropriate to support an estimate of Inferred Resources, and potentially Indicated Resources, in a heterogeneous limestone deposit intended for quarrying?
Specifically, I would greatly appreciate your technical insights regarding:
- the suitability of this spacing in a context where limestone shows significant variations from point to point;
- the potential conditions under which Indicated Resources could still be considered despite this heterogeneity;
- the geological or statistical criteria (variograms, sedimentary structures, geophysical signatures, etc.) that should be monitored to adjust the drilling pattern.
My aim is to design a drilling pattern that is credible and technically defensible, in accordance with best practices under JORC, NI 43-101, and CRIRSCO standards for bulk industrial minerals.
Your experience and advice will be invaluable in refining the planning of our next drilling campaign and ensuring that our resource estimates are robust and reliable.
Thank you in advance for your thoughtful feedback.