Post: Below the Oxide Cap - Where Cyanide-Based Mining Applications Start to Strain

Below the Oxide Cap - Where cyanide starts to strain — and why low-grade copper heaps, copper-gold concentrates, and sulfide-based ores and tailings may represent one of gold mining’s most important next opportunities

For more than a century, cyanide has been the dominant chemistry of gold extraction because, in the right ore, it works. But the easiest ore is usually found near the top of the deposit: oxidized, weathered, and comparatively simple to leach. The harder material lies below. That is where the real cyanide failure window begins — not necessarily where cyanide stops working altogether, but where it becomes slower, more reagent-intensive, less selective, and increasingly dependent on workarounds to stay economic.

This window is not confined to one district or one country. It usually appears in a position within an ore system. It is commonly found below oxide caps, in mixed oxide-sulfide transition zones, and in primary sulfide ore associated with porphyry copper-gold deposits and high-sulfidation gold systems. Reviews of gold processing note that many orebodies have an outer oxide layer that is highly amenable to cyanidation, while the precious metals underneath are increasingly tied up in sulfide minerals that are much less responsive to direct cyanide leaching. The geological literature places these settings in major camps such as Morenci, Arizona, Chuquicamata, Chile, Yanacocha, Peru, and El Indio, Chile — all examples of large hydrothermal systems where oxidation, supergene enrichment, and deeper sulfide mineralization create very different metallurgical domains within the same broader deposit.

That matters because “transition ore” is not just a geological label. It is often the point where a mine stops behaving like a simple oxide heap-leach operation and starts behaving like a mixed mineral system. The oxide portion may still leach well, but deeper down the ore can shift into mineral assemblages containing chalcopyrite, pyrite, arsenopyrite, covellite, chalcocite, and other sulfides that consume reagent, narrow the operating window, and interfere with gold recovery. In sulfide-rich or refractory material, gold may also be physically locked within sulfide minerals, forcing operators to add pretreatment or alternative downstream steps just to regain access to the metal.

You can see this clearly in low-grade copper heap leach applications. Heap leaching is meant to be a low-cost way to recover gold from large volumes of marginal ore. But once even modest amounts of cyanide-soluble copper are present, the chemistry becomes more complicated very quickly. In the published Kinross/SGS Maricunga case study in Chile, heap-leach ore grading roughly 0.6–1.0 g/t gold with 0.05%–0.1% copper generated pregnant solutions carrying more than 300 ppm copper, enough to justify evaluating a SART circuit to recover cyanide and precipitate copper sulfide. In other words, even relatively low copper content was enough to turn what should have been a straightforward gold heap into a more complex and more expensive recovery system.

Copper-gold ores and concentrates intensify the same problem. The technical literature is clear that copper-bearing gold systems can materially increase cyanide consumption and reduce process selectivity, which is why the industry has developed add-on routes such as SART, ammonia-cyanide systems, and other copper-cyanide recovery strategies. A major review from Colorado School of Mines describes copper-gold ores and concentrates as a distinct challenge because copper minerals compete for cyanide, complicating conventional gold leaching and often forcing different flowsheet choices altogether.

That challenge becomes even more concentrated in copper-gold flotation concentrates. Flotation does not remove complexity. It compresses it. Industry guidance notes that copper concentrate typically contains about 20% to 35% copper, meaning that copper, sulfur, and gold are all concentrated into a smaller, higher-value stream that can be harder — not easier — to process hydrometallurgically. In one published study on a high-copper gold concentrate, flotation upgraded one stream to 545.62 g/t gold, and ammonia pretreatment ahead of cyanidation significantly improved gold dissolution while cutting cyanide consumption by more than 10 kg/t. The takeaway is not that ammonia is the long-term answer. It is that once gold is travelling with copper and sulfides in concentrate, conventional cyanidation often needs help: more chemistry, more control, and more cost.

Then there are sulfide tailings, which may be the largest overlooked part of the opportunity set. Public datasets do not cleanly isolate only the gold-bearing subset, but the broader scale is enormous. UNDRR cites estimates of 8.85 to 14.4 billion tonnes of tailings generated globally each year, while the Global Tailings Review reports that 46% of global tailings volume comes from copper mining — the best large-scale public proxy for sulfide tailings. UNDRR also notes that, across 1,743 disclosed facilities, total stored tailings volume had reached about 45.5 billion cubic metres by 2020. A recent review in Minerals Engineering describes sulfide tailings as secondary resources of valuable metals and notes that reprocessing them can both recover value and help reduce acid mine drainage risk. These are not just waste materials. They are above-ground inventories of difficult mineralization that earlier circuits did not fully unlock.

The gold context makes this even more important. McKinsey has estimated that refractory gold accounts for about 24% of current gold reserves and 22% of gold resources worldwide, which underscores how meaningful difficult sulfide-related gold already is within the industry. The U.S. Geological Survey estimates 2025 global gold mine production at 3,300 tonnes, and notes that nearly one-quarter of U.S. undiscovered gold resources are estimated to occur in porphyry copper deposits. Public reporting does not provide a clean standalone number for only the gold-bearing fraction of sulfide tailings or copper-gold concentrates, but it clearly shows that this broader mineral window is large, strategic, and far from niche.

This is where RZOLV becomes relevant. On its technology page, the Company describes RZOLV as a water-based, non-cyanide extraction platform designed for low-grade ore, concentrates, sulfide ores, complex ores, concentrates, and tailings, with compatibility across heap, vat, and tank leaching systems. The Company also states that it is reporting promising recovery from low-grade copper/gold ores and positions the platform around reagent efficiency, selectivity, and reusability in more difficult mineral settings. If that performance continues to hold in copper-bearing, sulfide-rich, and legacy materials, then RZOLV is not simply a cleaner replacement chemistry. It becomes a targeted solution for one of the most commercially important gaps in conventional gold processing.

Why This Window Matters

Segment	Public scale / indicator	Why it matters for gold
Low-grade copper heap leach ore	At Maricunga, published heap-leach ore ran about 0.6–1.0 g/t Au with 0.05%–0.1% Cu, and pregnant solutions carried >300 ppm Cu.	Shows that even modest copper can materially burden a gold cyanide circuit and justify add-on recovery steps like SART.
Copper-gold concentrates	Copper concentrate typically contains about 20%–35% Cu. Using USGS 2025 world mine copper output of about 23 million tonnes Cu as an order-of-magnitude proxy, the physical sulfide concentrate stream is on the order of tens of millions of tonnes per year.	Indicates a very large annual concentrate stream where gold can travel with copper and sulfur into more complex downstream treatment routes.
Sulfide tailings	Global tailings generation is estimated at 8.85–14.4 billion tonnes/year. Copper mining accounts for about 46% of global tailings volume, implying roughly 4.1–6.6 billion tonnes/year of copper-linked tailings as a reasonable sulfide-tailings proxy.	Highlights the sheer scale of above-ground material that may still contain unrecovered gold and other payable metals.
Stored tailings inventory	For 1,743 sites with available data, combined stored tailings volume reached about 45.5 billion m³ in 2020.	Reinforces that legacy tailings are not a niche issue but a very large long-term inventory.
Refractory / sulfide gold share	Refractory gold accounts for about 24% of current reserves and 22% of gold resources worldwide.	Confirms that difficult sulfide-related gold is a meaningful share of the global gold opportunity.
Global gold context	USGS estimates 2025 world gold mine production at 3,300 tonnes. It also notes that nearly one-quarter of U.S. undiscovered gold resources are estimated to occur in porphyry copper deposits.	Supports the point that a meaningful portion of gold is geologically tied to copper systems before it ever reaches the plant.

 That is the strategic opening. The next enabling technology in gold is unlikely to win first in the easiest ore, where the incumbent already works well. It is more likely to win below the oxide cap — where copper starts stealing cyanide, where sulfides narrow the process window, where concentrates become harder to treat, and where tailings still hold value the first flowsheet left behind. That is where cyanide starts to strain. And it may be exactly where RZOLV has the most to offer.

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Disclosure and Cautionary Note

This article is provided for general informational purposes only and does not constitute investment advice or a recommendation to buy or sell securities. References to third-party technical literature and public data are included solely as contextual background and should not be interpreted as a claim that RZOLV will achieve similar results on any specific ore, concentrate, or tailings material. Public datasets do not cleanly isolate the gold-bearing subset of sulfide tailings or copper-gold concentrates; accordingly, references to scale in this article should be understood as indicators of the broader mineral windows in which gold commonly occurs, rather than precise market measurements for RZOLV. Forward-looking statements regarding the potential applicability of RZOLV technology, future testing, commercialization opportunities, and expected performance are subject to metallurgical variability, site-specific operating conditions, regulatory considerations, market factors, and other risks and uncertainties.