Core Sample

A core sample is a cylinder of solid rock cut from below the surface by a hollow drill bit. The U.S. Bureau of Mines' Dictionary of Mining, Mineral, and Related Terms (1968) defines drill core as "a solid, cylindrical sample of rock produced by an annular drill bit," and a core sample as "one or several pieces of whole or split parts of core selected as a sample for analysis or assay."

The USGS describes cores as "long cylinders of rock that are extracted from below the ground by a hollow drill," which "show the rock, layer by layer, as it exists hundreds or thousands of feet underground."

Core versus cuttings

Not all drilling produces core. Cuttings are, in USGS's words, "small chips and fragments of rock that come out of a borehole as it's being drilled." Core keeps the rock intact, so geologists can see structure, veins, contacts, and mineralization in place, and measure exactly where each interval came from. That makes it the most informative sample in exploration, and the most expensive to collect.

How core is used

• Logging: geologists record rock type, structure, alteration, and mineralization along the length of the hole
• Splitting and assaying: the core is usually cut lengthwise; half is sent for assay and half kept as a record
• Grade and tonnage estimates: assays from many drill holes are combined into mineral resource estimates
• Verification: BLM's handbook for mineral validity reports asks examiners to describe the "correlation between assays or company core splits and your core splits," and to explain how they know the splits "were not tampered with"

Where old core goes

Core is costly to replace, and much of it would be lost when companies close or projects end. The USGS Core Research Center in Denver, established in 1974, exists "to rescue rock cores threatened with destruction or disposal" and make them available for study. USGS says it holds more than 9,900 rock cores and 53,000 cuttings from 36 states, representing more than 252 million linear feet of drilling. Many of the holes were drilled for energy and mineral exploration.

Why it matters in research

Drill results are often the main evidence of what a tract holds. When mining claims are evaluated, past drilling, where the holes were collared, who held the ground at the time, and whether the core or assay data still exist are all part of the record worth finding.