A different class of deposit runs the other way. In tropical settings, prolonged weathering can break granite down into clay and concentrate certain metals in material beginning at surface. The first drill sample does not come from hundreds of meters underground. It starts at zero.
What Depth Costs
Freeport-McMoRan Inc. (NYSE: FCX) offers one of the clearest current examples. Its Grasberg Block Cave in Indonesia is in a phased ramp-up following the September 2025 mud rush, with management targeting full capacity by the end of 2027. Behind it sits the next underground deposit, Kucing Liar, where Freeport had incurred approximately $1.4 billion of development capital through June and expects to spend roughly $4 billion more through 2033 while production begins ramping up around 2030. That is the timeline for a world-class ore body when the ore body is deep.
Harmony Gold Mining Company Limited (NYSE: HMY) operates Mponeng, widely recognized as the world’s deepest mine. In fiscal 2026 the group met gold production guidance for an eleventh consecutive year, with underground recovered grade of 5.83 grams per tonne and group all-in sustaining costs of US$2,195 per ounce, up about 22 percent in dollar terms. Harmony’s risk disclosures cite geotechnical challenges from ageing mines and a trend toward deeper, more complex deposits. Mponeng has nonetheless produced since 1986 and yielded millions of ounces at those depths.
Cameco Corporation (NYSE: CCJ) shows that the pattern is not confined to gold and copper. McArthur River in northern Saskatchewan is the world's largest high-grade uranium mine, and Cameco describes mining there as taking place between 530 and 640 meters below surface. Underground equipment circulates brine that keeps the porous rock surrounding the ore body frozen, while ore slurry is transported 80 kilometers to the Key Lake mill. Second-quarter packaged production from the operation was 3.3 million pounds of uranium oxide, and Cameco expects between 14.0 million and 16.5 million pounds this year.
Three major miners, each confronting what depth demands. What if the target metals never left the surface?
Thirty-Three Holes, Every One Starting at Zero
Spark Energy Minerals Inc. (CSE: SPRK) (OTCPK: SPARF) (FSE: 8PC) has been exploring the other kind of target. At its approximately 91,900-hectare Arapaima Project in Minas Gerais, Brazil, every one of the 33 reported reverse-circulation holes has returned gallium mineralization from surface, with rare earth mineralization beneath it. Not from 900 meters. From zero.
The holes are vertical and shallow. The 37-hole follow-up program totaled 2,024 meters, roughly 55 meters per hole, and the deepest hole on the project reaches 94 meters. The best gallium result to date, 114.3 g/t gallium oxide over two meters in ARA-RC-007 at Boa Vista, was the first two meters of the hole. At Cruzeta, ARA-RC-021 carried 32 meters from surface at 73.9 g/t gallium oxide, then 32 meters at 1,947 ppm total rare earth oxides deeper in the same profile. Ten holes across the two centers returned scandium from surface as well.
Spark interprets the mineralization as an ionic-adsorption clay style system developed over the Caladão Granite, a weathering-hosted deposit type recognized globally as a source of heavy rare earths. In that model, prolonged weathering created the vertical mineralized profile Spark is now drilling, with gallium and scandium in the iron-titanium oxide fraction near surface and adsorbed rare earths in the clay beneath.
A Second Center in Just 216 Meters
Boa Vista, roughly seven to eight kilometers south of Cruzeta, shows how a compact program can test that kind of target. It was ranked on stream-sediment, soil and rock geochemistry plus terrain analysis, then drilled with five holes totaling 216 meters. All five returned the same vertical sequence seen at Cruzeta. Spark is careful to say that the results do not establish continuity between the two areas; it takes more exploration to try and prove that. What they do, though, is establish a second mineralized center inside the same land package, found with just 216 meters of drilling.
Shallow Is Not the Same as Simple
None of this settles the questions that decide whether a deposit becomes a mine. Arapaima has no estimated mineral resource or reserve, and metallurgical recovery and economic viability have not been established. Shallow holes can reduce drilling time and cost, but important questions remain. ANSTO’s test work is evaluating metallurgical recovery, while planned infill drilling at Cruzeta is intended to assess grade continuity and support a future mineral resource estimate. Processing costs and project economics remain to be established.
What the depth comparison clarifies is why this style of discovery draws attention early. The examples above show the time, capital and specialized engineering that deep ore bodies can demand. At Arapaima, the metals governments now list as critical begin in the first few meters, across two centers, on ground Spark already controls. Drilling has established that mineralization begins at surface. The remaining questions are its extent, continuity and whether the metals can be recovered economically.
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