6.3 Diamond Drilling, Core Logging & QA/QC Protocols

Exploration drilling provides physical subsurface samples required for geological modeling, geomechanical evaluations, and mineral resource estimation. To convert raw drill core into compliant mineral resource estimates under the Philippine Mineral Reporting Code (PMRC) or JORC code, exploration companies must execute rigorous core recovery procedures, standardized geological and geotechnical logging, and robust Quality Assurance / Quality Control (QA/QC) protocols.

Exploration Drilling Methodologies

The two primary drilling methods utilized in mineral exploration are Diamond Core Drilling (DDH) and Reverse Circulation (RC) drilling.

1. Diamond Core Drilling (DDH)

Diamond drilling utilizes a hollow, diamond-impregnated drill bit attached to rotating drill rods to cut a continuous cylindrical core of solid rock. Wireline core retrieval systems allow the inner core barrel containing the sample to be hoisted to the surface through the drill string without pulling the entire drill rod assembly.

• Standard Wireline Core Diameters:
  • PQ Core: $85.0\text{ mm}$ core diameter ($122.6\text{ mm}$ hole diameter). Preferred for highly fractured ground, weathered saprolite, and metallurgical bulk sampling.
  • HQ Core: $63.5\text{ mm}$ core diameter ($96.0\text{ mm}$ hole diameter). Standard size for detailed resource definition drilling.
  • NQ Core: $47.6\text{ mm}$ core diameter ($75.7\text{ mm}$ hole diameter). Applied in deep exploration holes ($> 500\text{ m}$) or tight budget scout drilling.
• Triple Tube Barrel Systems (HQ3 / NQ3): Incorporates a split inner tube inside the barrel to preserve friable, soft, or highly fractured rock cores without mechanical disturbance during extraction.

2. Reverse Circulation (RC) Drilling

RC drilling utilizes a dual-wall drill pipe and a pneumatic hammer bit. Compressed air is forced down the outer annulus of the drill string, driving the hammer and blowing rock cuttings upward through the center inner tube to a cyclone separator at the surface.

• Advantages: Rapid penetration rates ($100\text{--}200\text{ m/day}$), lower cost per meter, and elimination of sample cross-contamination.
• Limitations: Produces fine rock chips rather than intact core; cannot provide structural orientation measurements for fault or vein dip analysis.

Core Recovery & Rock Quality Designation (RQD)

Before geological logging, geotechnical engineers measure Core Recovery and calculate Rock Quality Designation (RQD) over each drill run (typically $1.5\text{ m}$ or $3.0\text{ m}$).

1. Core Recovery Percentage ($CR%$)

Core Recovery quantifies the percentage of intact rock length recovered relative to the total length of the drill run: $\text{Core Recovery } (CR\%) = \left( \frac{\sum L_{\text{recovered}}}{L_{\text{run}}} \right) \times 100\%$

Where $\sum L_{\text{recovered}}$ is the total cumulative length of all recovered core pieces laid end-to-end in the core trough, and $L_{\text{run}}$ is the length drilled (e.g., $3.0\text{ m}$). Core recovery below $95%$ indicates core loss in crushed fault zones or soluble horizons, which can introduce sampling bias into grade calculations.

2. Rock Quality Designation ($RQD%$)

Developed by Deere (1964), RQD measures the structural soundness and joint spacing of the rock mass: $RQD\% = \left( \frac{\sum L_{\sum L_{\text{sound pieces } \ge 10\text{ cm}}}}{L_{\text{run}}} \right) \times 100\%$

Measurement Rules:

• Sum only hard, sound core pieces that are $10\text{ cm}$ (4 inches) or longer measured along the core centerline.
• Mechanical Breaks: Fractures clearly caused by drilling, core handling, or box packing must be fitted back together and counted as single, unbroken sound pieces.
• Soft, highly weathered rock pieces that crumble by hand are assigned a length of zero regardless of physical length.

Geomechanical Classification Table:

Geological & Geotechnical Core Logging

Core logging systematically records geological, structural, and geotechnical observations into standardized digital databases.

1. Geological Logging Parameters

• Lithology: Primary rock type, igneous texture, sedimentary bedding, or metamorphic fabric.
• Alteration: Hydrothermal mineral assemblies (e.g., potassic, phyllic, argillic, propylitic) and alteration intensity (weak, moderate, intense, pervasive).
• Mineralization: Ore mineral species (chalcopyrite, bornite, galena, sphalerite, native gold), vein textures (crustiform, comb, stockwork, breccia), and visual percentage estimates.
• Structural Orientation: Measuring alpha ($\alpha$) and beta ($\beta$) angles of planar structures (veins, faults, joints) on oriented core marked with a bottom-of-hole ellipse line.

2. Geotechnical Logging & Core Splitting

• Geotechnical Parameters: Record Joint Roughness Number ($J_r$), Joint Alteration Number ($J_a$), Fracture Frequency per meter ($FF/m$), and Point Load Index ($I_{s(50)}$) for Rock Mass Rating (RMR) and Q-system slope stability designs.
• Core Splitting: Intact drill core is cut longitudinally using an automated diamond wire saw. One half-core is bagged for laboratory assay, while the remaining half-core is permanently retained in wooden or plastic core trays as an archival reference.

Quality Assurance & Quality Control (QA/QC) Protocols

QA/QC programs test sampling precision, preparation contamination, analytical accuracy, and laboratory precision. PMRC/JORC-style reporting requires the Competent Person to establish and disclose fit-for-purpose quality controls, but it does not prescribe one universal 15%–20% insertion rate or one set of control limits. Frequency and placement follow commodity, grade range, nugget effect, batch size, laboratory risk, decision consequence, and prior performance.

1. Control Sample Types and Objectives

1. Certified Reference Materials (CRMs): Matrix- and concentration-appropriate materials test analytical accuracy. Plot results against certified values and uncertainty. Define warning, failure, affected-batch, investigation, and release rules before results arrive; one $3\sigma$ or two same-side $2\sigma$ results is a common project rule, not a reporting-code command.
2. Blanks: Material with concentrations suitably below the decision level tests contamination during crushing, pulverizing, and analysis. Insert blanks at routine and strategically selected positions, including after expected high grades where this will not reveal sample identity improperly.
3. Field Duplicates: A second field sample or an appropriate half- or quarter-core split estimates primary sampling and short-range geological variability. The split method must preserve enough core for records and suit the mineralization.
4. Coarse Duplicates: Independent splits after crushing test preparation and splitting variance.
5. Pulp Duplicates / Laboratory Repeats: Repeat aliquots from a pulverized sample test analytical precision. A separate umpire-laboratory program uses selected pulps or rejects to assess inter-laboratory bias; it is not the definition of every pulp duplicate.

2. Statistical Precision Evaluation

Use time-ordered control charts, bias plots, paired scatterplots, absolute and relative difference, quantile plots, and duplicate precision methods suited to heteroscedastic data. Investigate trends by batch, grade, material, instrument, and laboratory. Fixed RPD limits can be misleading near detection limits or in nuggety mineralization, so acceptance criteria must be justified from fitness for purpose rather than copied from a generic table.