That was confirmed through "field mapping", which is when a geologist walks around, picks up a rock, and describes it...maybe even has it analyzed by a lab for detailed analysis. A great deal of this type of geologic mapping can be done in the mid-west of the USA by looking at contact zones between strata layers or by looking closely at outcroppings. Think about the grand canyon and how you can see the different "cake layers" of the strata. the areas between the different layers are the contacts and represent a period of nondeposition. You can find these contact zones or "cake layers" laying vertically across the ground you walk on as well. This comes from uplifting and folding of the strata, maybe where there were prior mountain uplifts that have eroded away over millions of years, or even where you have an igneous intrusion...like you see on Sarissa's property...where a bubble of magma came up through the ground like a cork.
The areas of interest for Sarissa are the contact zones where the native strata was contacted and cooked by that igneous, magmatic, intrusion. At the right temperature and pressure, an array of different elements precipitate...the one of interest for Sarissa is...Niobium.
It's not confirming drill cores by having them analyzed in a laboratory. It's done by walking around, picking up rocks, breaking them with a rock hammer, then describing what you see.
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