Belingwe-West chain: a first free-EO read

A third case study, and the newest. A holder handed over corner pegs for eight adjacent claim blocks in the Midlands greenstone belt of Zimbabwe, walked on the ground the day before, and asked the same question the Zvishavane and Kwekwe holders did: how does it look on free satellite data?

The pegs arrived as bare eastings and northings with no zone label. Read as UTM 36S they land in Mozambique; read as UTM 35S (EPSG:32735) they form a tight 3.0 × 3.2 km chain at ~28.82° E, −19.78° in central Zimbabwe. 35S is the only consistent read, the same label-free correction the other two sites needed. That single interpretation, shown in the open, is the difference between walking the right ground and being 600 km off.

Bottom line (iron-oxide arm only, so far): a real but ambiguous ferric signal. The chain is modestly iron-stained relative to its surroundings (claim median at the 65th percentile of local background, vs Kwekwe's 40th). But the honest test fails: the 38 known gold mines within 25 km sample at −0.08σ ferric, essentially background, so on this belt red-oxide staining does not mark where gold is. The elevated ratio is real; it is not by itself a gold vector. Every reading is at 10 m pixel scale, so this speaks to block-scale ground, not a narrow vein or a small outcrop. The spots below are best-case corners to walk first, never gold. The structure, DEM-terrain, embedding and EMIT arms (the stronger vectors on a shear-hosted belt) are pending for this site.

The ferric spots, as a list (UTM 35S, the holder's grid):

The eight blocks

The pegs form a chain, not a cluster: ordered as supplied they march north-west to south-east across ~3.2 km, the signature of pegging along a structure (a reef or shear strike) rather than gridding an area. Total ~26 ha over eight blocks, each triangle or quad 0.5 to 5.7 ha. The ferric best-case spots concentrate in the south-east half of the chain (blocks 4 to 7), where it bends SE; the northern blocks (1 to 3) produce no top spots.

What the map shows (iron-oxide arm)

Ferric ratio (Sentinel-2 B04/B02) is the holder's red-oxide thesis, computed on a 0%-cloud dry-season scene with vegetation and cloud masked. The ground is fully exposed (0% vegetation-masked), so the optical read is clean. Over the chain it is modestly elevated, 65th percentile of the surrounding terrain.
The mine calibration is the honest test. The ferric ratio is sampled at the 38 known gold mines within 25 km on the same scene. They sit at −0.08σ, indistinguishable from background, so on this belt a bright ferric patch is weak evidence for gold. A bright chain does not inherit meaning it hasn't earned.
Gold mines sized by production give the district context.
Spots are the ferric arm's best-case ground inside the pegs, coordinates in the table. None is a gold indication; all are walk-first-with-low-expectations points.

How this was made

One command per site, over free data, no Earth Engine. The claim pegs become a standardized AOI (build_aoi.py), then every arm runs through one driver (run_site.py belingwe-west). Recipes and the reproducible outputs live in research/mineral-prospecting/ (arms/iron_oxide.py, sites/belingwe-west/). The same pipeline produced the Zvishavane and Kwekwe studies; adding this site was two commands.

The single biggest lever from here is running the structure and DEM-terrain arms (the better gold vectors on a shear-hosted belt) and one real ground observation from the chain, which wires straight into the ferric calibration and re-weights every layer against real data.