From a distance, the hills of Sengerema look ordinary: gently sloping ridges, cassava fields, and scattered homesteads overlooking the vast waters of Lake Victoria. But beneath these ridges lies a geological story that has shaped two decades of corporate boardrooms and government negotiations. It is called Nyanzaga.
Gold, unlike many resources, does not reveal itself in glittering veins. At Nyanzaga, it hides in subtle stockworks, thin, crisscrossing veins of quartz and sulphides spread through layers of ancient sediments. To a farmer, the rocks are stubborn obstacles in their fields. To a geologist, they are a coded map of an ore body that stretches for kilometers.
Understanding these rocks matters. They explain why Nyanzaga is a large-scale open pit rather than a high-grade underground mine; why the processing plant must include oxygen sparging and mercury scrubbers; why investors argue about strip ratios and cut-off grades; and why resettlement footprints are larger than at other sites. In other words, geology is not just science; it is the DNA of the mine’s economics, its politics, and its community impact.
Tusker: the engine of Nyanzaga
At the heart of Nyanzaga lies Tusker, a deposit that geologists describe as “bulk-tonnage, orogenic-style.” In plainer words, it is a vast, low-to-moderate grade body of rock carrying millions of ounces, not in single rich veins, but in a dense fabric of tiny ones.
Tusker sits within an anticline, an ancient fold in the Sukumaland Greenstone Belt, where gold-bearing fluids once circulated and precipitated along fractures. The ore zones are thick, often tens of metres across, with gold locked in pyrite and arsenopyrite sulphides. This geometry makes Tusker a natural open-pit candidate: broad, continuous zones that lend themselves to truck-and-shovel mining rather than narrow-vein underground stoping.
Tusker is also deep. Near the surface, it offers some transitional ores, but most of its metal is trapped in harder, sulphide-rich zones hundreds of metres down. That is why earlier designs flirted with underground mining. But for Perseus, the arithmetic pointed elsewhere: a larger pit, lower average grade, but simpler execution and faster returns. Today, Tusker accounts for roughly 80% of Nyanzaga’s reserves. It is the deposit that underwrites the mine’s 11-year life.
Kilimani: the cap and complement
Just 450 metres northeast of Tusker lies Kilimani, a smaller deposit but strategically crucial. Kilimani is shallow, oxide-rich, and metallurgically forgiving. Unlike Tusker’s sulphides, Kilimani’s ore is softer, requires less grinding, and releases gold more readily.
For a mine builder, this matters. Kilimani provides “sweetener ore” in the early years, higher recovery, lower processing costs, and faster cash flow to balance the heavier lift of Tusker’s sulphides. Though it holds a fraction of the ounces, Kilimani is Nyanzaga’s accelerator. Its early tonnes will stabilise the plant ramp-up, support more substantial margins in the first five years, and soften the risks of relying solely on Tusker.
In the local narrative, Kilimani also matters for scale. Its footprint expands the pit outline and resettlement zone, reminding communities that “small” deposits still carry visible social footprints. In boardrooms it is a rounding error; in villages it is the difference between one household displaced and ten.
Resource & reserve evolution: ounces in flux
Numbers tell a revealing story about how Nyanzaga’s geology has been interpreted over time:
- 2012 (African Barrick Gold): 4.6 Moz of resources declared, a mix of Indicated and Inferred. This was the “big picture” number, based on aggressive drilling and optimistic pit shells.
- 2017 (OreCorp): a JORC-compliant resource of ~3.7 Moz, with ~3.1 Moz Indicated. Stricter standards, more conservative cut-offs, and recognition that not all ounces were equal.
- 2022 DFS (OreCorp): 2.6 Moz of Probable reserves, using a combined open-pit and underground scenario. High grades deeper down kept the underground option alive.
- 2025 FS update (Perseus): 2.3 Moz of Probable reserves, open-pit only, 52 Mt @ 1.4 g/t. A simpler, lower-grade but larger tonnage plan that prioritises execution speed and capital certainty.
This evolution is not “gold disappearing.” It is the constant recalibration of what counts as economic under prevailing laws, prices, and designs. The rocks are the same; the economics are not. For Tanzania, the 2017 reforms, mandatory free-carry, higher royalties, raised the cost of capital. For OreCorp, that meant tightening estimates. For Perseus, it meant choosing a design that maximises certainty of delivery, even if it leaves some ounces for a later underground phase.
What these numbers show is how geology bends to governance. A 4.6 Moz resource in 2012 meant little without fiscal stability. A 2.3 Moz reserve in 2025 is far more valuable because it is banked under a licence, financed by a builder, and underwritten by the state’s equity.
Metallurgy: the invisible challenge
In mining, it is not enough to find gold; you must free it. Nyanzaga’s metallurgy is the quiet fulcrum of its economics.
The headline number is ~88% recovery. That figure, repeated in feasibility studies and investor decks, is the product of years of testwork: drilling cores, crushing samples, running them through pilot circuits. What it hides is complexity. Tusker’s ore is sulphide-heavy; gold is locked in fine particles of pyrite and arsenopyrite. To liberate it, the plant must grind ore fine, inject oxygen, and hold slurry in tanks long enough for cyanide to work.
Complicating matters are minor but significant deleterious elements: arsenic and mercury. Arsenic requires precipitation into stable compounds; mercury must be captured in scrubbers and not released into air or water. These are not optional add-ons but engineered safeguards that make Nyanzaga politically viable near Lake Victoria.
Metallurgy is where geology meets chemistry, and chemistry meets ESG. If recoveries hold, Nyanzaga can produce ounces at ~US$1,200/oz AISC. If recoveries slip, margins shrink. This is why Perseus has opted for a robust, conventional flowsheet, less innovation, more certainty.
The pit vs. underground debate
For years, Nyanzaga’s engineers argued: should it be a hybrid mine (open pit feeding an underground) or a single large pit? OreCorp’s 2022 DFS leaned toward the hybrid. By year 5, once the open pit exhausted shallow ore, an underground decline would chase deeper high-grade zones.
Perseus chose differently. In its 2025 update, it abandoned the underground in favour of a larger pit. Why? Simplicity. A single open pit reduces capital complexity, avoids transition risk between methods, and accelerates schedule. The trade-off is grade: underground would have delivered higher grams per tonne; the larger pit dilutes grade but captures more tonnes.
This choice has consequences beyond the balance sheet. Open-pit sprawl consumes more land, displaces more households, and requires larger tailings capacity. Undergrounds are compact but technically riskier. In effect, Perseus traded smaller social footprint for execution certainty. For villagers, that means more visible impact. For investors, it means fewer sleepless nights about underground cost overruns.
Comparables and benchmarks
To understand Nyanzaga, it helps to hold it up to its peers.
- Vs. Geita (AngloGold Ashanti): Geita is higher-grade (~2.5 g/t) and longer-lived, but its older infrastructure is costly to maintain. Nyanzaga’s grade (~1.4 g/t) is lower, but its modern flowsheet and grid power should keep AISC competitive.
- Vs. Bulyanhulu (Barrick): Bulyanhulu is an underground mine, characterized by high-grade ore but technically complex, and has historically been troubled by social conflict. Nyanzaga’s pit model avoids some of those risks but acceptsa larger land-use impact.
- Vs. African greenstones (Yaouré, Ahafo, Sabodala): Nyanzaga’s metrics line up mid-pack: capital intensity (~US$250/oz annual production capacity) is reasonable; AISC is within the lowest quartile. Its differentiator is governance, being the first Tanzanian mine built under the 2017 reforms.
The comparison shows Nyanzaga is neither the cheapest nor the richest deposit in Africa. Its real value is strategic: a fresh build in a reformed jurisdiction, with fiscal terms and ESG standards that will be scrutinised regionally.
What geology teaches us about risk
The rocks themselves carry risks. Nyanzaga’s pit slopes are designed conservatively, but weak saprolite zones and tropical rains could destabilise walls. Groundwater inflows will need continuous pumping. The strip ratio is manageable but still means moving three to four tonnes of waste for every tonne of ore, a reminder that “grade” is not the only number that matters.
Orebody variability is another lesson. Kilimani oxides blend smoothly; Tusker sulphides can be stubborn. Managing plant feed to stabilise recoveries will be an operational art. Inferred material, ounces that exist geologically but not yet proven to mining confidence, sits under the pit, tempting future expansion but risky to count today.
What these risks teach is simple: geology is not static. Each blast, each bench, reveals surprises. Mines that respect that uncertainty, designing conservatively, blending wisely, and drilling continually, survive. Those who do not stumble.
Geology as politics
At Nyanzaga, geology is not just science; it is politics in rock. It dictates whether the mine is open-pit or underground, whether households must move, whether Lake Victoria is at risk, and whether investors see a viable project.
The Tusker anticline and the Kilimani oxide cap may look like technical jargon. Still, they are the reasons Tanzania can expect royalties, Mwanza can expect jobs, and villagers can expect new homes. The rocks set the parameters; governance and capital set the choices. Together, they will decide whether Nyanzaga becomes just another pit in Africa, or a case study of how geology and governance can finally align.
