Introduction

Drilling is at the heart of every gold and silver mining operation, guiding both discovery and extraction. From early-stage surface exploration to underground production, each drilling method plays a critical role in mapping, evaluating, and accessing ore bodies. This article offers a detailed look at the diverse drilling techniques used in precious metal mining, highlighting both traditional practices and modern innovations. By examining the tools, technologies, and strategic decisions that drive drilling, it provides a comprehensive guide for understanding how mines operate efficiently and safely. Special attention is given to cutting-edge advancements such as automation, directional drilling, and AI-driven optimization that are reshaping the industry.

Surface Drilling Methods

Exploration Drilling (Surface)

Precious metal exploration from surface typically uses reverse circulation (RC) and diamond core drilling. RC drilling is favored for early-stage exploration and quick sampling – it uses a pneumatic hammer to pulverize rock and carries the chips to surface via air through dual-walled drill rods. RC rigs are mobile, fast, and cost-effective, often drilling a hole in 2–3 fewer days than core drilling for depths up to ~300 m. RC also returns a large sample volume: a typical RC hole yields ~127 mm diameter cuttings, compared to ~47 mm core diameter in diamond drilling (over 7× the sample cross-sectional area). This larger, homogenized sample reduces the "nugget effect" in coarse gold systems, improving grade representativity. RC chips can be reliably analyzed for geochemistry and quick assays, though they provide only fragmented rock data.

In contrast, diamond core drilling uses a diamond-tipped bit to cut a continuous rock cylinder (core) that preserves geology. Core drilling is slower and more expensive, but delivers intact core samples for detailed geological, structural, and geotechnical analysis. The core reveals mineral veins, textures, and orientation in situ, critical for mapping veins or structures that control gold and silver mineralization. Diamond drilling is also better for deeper targets (>300 m) where RC becomes impractical. Many exploration programs begin with faster RC or rotary air-blast (RAB) holes for broad geochemical screening, then advance to diamond coring on promising zones that need precise data. In practice, both methods are often combined (e.g. RC "pre-collars" through barren overburden, with diamond tails to reach target depth) to balance cost and information quality.

Comparison Table: Surface Exploration Drilling Methods

Feature	Reverse Circulation (RC)	Diamond Core Drilling
Sample type	Rock chips	Intact core
Speed	Fast	Slower
Depth capacity	< 300 m (typically)	Up to >1000 m
Cost	Lower	Higher
Geological detail	Limited	Excellent
Use cases	Early-stage exploration, grade control	Resource definition, structural analysis

Production Drilling (Open Pits)

In open-pit gold and silver mines, blasthole drilling is the primary production drilling method. Large rotary or down-the-hole (DTH) drill rigs (e.g. Epiroc Pit Viper series) bore regular patterns of holes in benches, which are filled with explosives for blasting rock fragmentation. Blast-hole drills in hard-rock mines produce holes commonly 6–12 inches (150–300 mm) in diameter, with single-pass depths of ~10–20 m for each bench blast. Drilling is conducted on a designed grid, where hole spacing and burden (distance to the next row) are engineered based on hole diameter and rock conditions. For example, an 8-inch hole might be spaced several meters apart to achieve optimal fragmentation.

Blasthole patterns and drilling accuracy directly affect rock breakage and downstream loading efficiency, so modern drills use GPS and Hole Navigation Systems to collar holes precisely as per design. Specialized production drilling includes presplit drilling at the pit walls – closely spaced, smaller-diameter holes fired just before the main blast – to create a controlled fracture plane and protect the final slope stability.

In addition to blast production holes, open-pit operations drill grade control holes on grid spacing (often 5–15 m) in ore zones. Many gold mines employ RC drills for grade control in the pit, as these holes provide samples for on-site assaying while double-serving as blast holes when appropriate. Grade control drilling densely samples the bench before extraction, enabling mine geologists to map ore boundaries and refine the short-term mine plan. The RC method is ideal here because it yields high-quality, uncontaminated samples rapidly and cheaply, allowing assays to be returned about 3× faster than core and keeping production on schedule.

Underground Drilling Methods

Exploration/Delineation Drilling (Underground)

As a mine develops, drilling is often done from underground workings to further explore or delineate ore. The preferred method is underground diamond core drilling using compact rigs that can be wheeled or skid-mounted in drifts or chambers. These drills bore long holes (often hundreds of meters) from drill bays to intercept extensions of the orebody at depth or along strike. Core obtained underground is critical for resource definition, providing direct evidence of vein thickness, grade continuity, and structure ahead of mining.

In narrow-vein gold mines, companies routinely drill fans of long diamond holes from footwall drifts to ensure no payable veins are missed and to map the vein geometry for mine planning. Compared to surface drilling, underground exploration drilling can achieve tighter angles and shorter distances to target, and it's less hindered by weather or long travel through barren rock.

In certain cases, mines have employed underground RC drilling for delineation or grade control, particularly in highly nuggety gold deposits. For example, a pioneer program at Sunrise Dam (Australia) introduced underground RC rigs that drilled 115 mm holes in ore blocks to obtain large samples for grade modeling. The RC method underground, though requiring specialized equipment and mine services, delivered a 7-fold larger sample that dramatically improved the confidence in local grade estimates.

Production Drilling (Underground)

In underground gold and silver mines, production drilling refers to drilling the holes for blasting ore from stopes or development headings. A common scenario is long-hole stoping in steeply dipping orebodies. Here, drillers use long-hole production rigs to drill a fan or ring of parallel holes into the ore zone from a drilling drift. These holes might be 64–165 mm in diameter and can extend 20–50+ meters in length.

Long-hole drilling demands accuracy. Modern long-hole rigs use automated positioning and alignment systems to ensure parallel holes and correct inclination. For narrow vein gold mines, accuracy is essential to prevent over-breaking. Additional underground production drilling includes development face drilling using tunneling jumbos equipped with multiple booms, and raise drilling for vertical openings using raise bore machines.

Comparison Table: Underground Drilling Methods

Method	Use Case	Hole Diameter	Sample Type	Key Advantages
Underground Diamond Drilling	Exploration, delineation	47.6–63.5 mm	Core	Geological detail, depth reach
Underground RC Drilling	Grade control, modeling	115 mm	Rock chips	Large sample volume, reduced nugget effect
Long-Hole Production Drilling	Stope blasting	64–165 mm	N/A (blast hole)	Accurate, deep blasts
Jumbo Drilling	Drift development	~45–51 mm	N/A (blast hole)	Fast, mechanized development
Raise Drilling	Shafts, slots	Variable	N/A	Safe vertical excavation

 

Drilling Technologies and Equipment

Diamond Core Drills

Used for exploration and resource definition. Employs diamond-impregnated bits to retrieve continuous rock cores. Core size ranges from NQ (47.6 mm) to HQ (63.5 mm) and larger. Key advantages include preservation of geological structures and ability to drill deep holes.

Reverse Circulation (RC) Drills

Used widely in exploration and grade control. Employs a pneumatic hammer and dual-walled rods to bring chips to surface. Advantages include speed, cost-effectiveness, and large sample volume.

Rotary Blasthole Drills

Heavy-duty machines for drilling large-diameter holes in open-pit benches. Commonly use tricone bits or DTH hammers. Fitted with GPS, automation features, and sometimes electric powertrains.

Underground Jumbos and Long-Hole Rigs

Jumbos drill development faces using multiple booms. Long-hole rigs drill production holes in stopes. Include both top-hammer and ITH variants, with computer-controlled precision.

Directional Drilling Tools

Enable steering of boreholes and creation of branch holes. Used for deep targeting, minimizing drilling meters. Tools like Devico’s DeviDrill are widely used.

Ancillary Tools

Raiseboring machines, roof bolt drills, sonic and air core rigs for overburden, and modern support tools (dust collectors, electric drivetrains).

Modern Innovations in Precious Metal Drilling

Automation and Remote Operation

Autonomous rigs are increasingly deployed in both surface and underground operations. Rio Tinto, for example, operates remote-controlled drills from centralized hubs. Underground rigs offer tele-remote operation to increase safety and utilization.

Data Integration and AI Optimization

Modern rigs are equipped with sensors to feed into centralized mine planning systems. AI is used for predictive maintenance, pattern optimization, and automated decision-making based on drilling feedback. These systems improve rock fragmentation, grade targeting, and drill utilization.

Automated Exploration Drilling

Automated rod handlers and core tube systems (e.g., Boart Longyear LF160) reduce manual labor and increase safety. Start-ups like Tribe Tech are testing fully autonomous RC exploration rigs.

Directional and Smart Drilling Techniques

Steerable core barrels and MWD (Measurement While Drilling) techniques allow real-time sensing. These systems can detect rock hardness, ore/waste boundaries, and structural features during drilling, improving real-time modeling.

Electrification and Sustainability

Battery-electric rigs reduce diesel use and improve underground working conditions. Epiroc and Sandvik now offer electric jumbos and long-hole rigs.

Geophysical Integration

Use of downhole imaging, XRF analyzers, and seismic sensors integrated into drilling systems enables better real-time geological understanding.

Conclusion

Drilling remains one of the most crucial and technically diverse operations in the gold and silver mining industry. The thoughtful selection of methods, equipment, and innovations, tailored to geological settings and production demands, ensures that ore is located, defined, and extracted efficiently and responsibly. As mining evolves, automation, AI, and sustainable technologies are transforming the way drills operate and how decisions are made. By mastering the balance between cost, data quality, and operational flexibility, modern mining companies can continue to unlock new resources and improve profitability while minimizing environmental and safety risks.