A gossan in dolomite: one drill hole, where?

First target: the IP chargeability high 100 m east of the gossan.

• The gossan is an oxidized cap. Drilling it first risks hitting leached, low-grade rock. Malachite and boxwork show that copper was mobilized and may have been redeposited below the water table, not in the gossan itself.
• The IP high is the best geophysical expression of disseminated or vein sulfides. Its offset east of the gossan means the primary sulfide zone may be displaced by the fault, not directly beneath the oxidation.
• The resistivity low beneath the gossan is consistent with clay-sericite alteration and with supergene leaching. It is a vector, not the ore target.

Second target: the fault contact. It is a plausible fluid pathway and may host vein or replacement sulfide. But the contact is long and the soil anomaly is broad; drilling it without a geophysical focus would be a scattergun approach.

Third target: the gossan core. Useful for geological logging and for testing the supergene profile, but it is the most likely to disappoint as a grade target.

Ranking: IP high > fault contact > gossan core. The hole should still start near the gossan so that the oxidation profile is logged, then angle east to intersect the IP high.

The geochemistry points to a copper-rich sulfide system, not a lead-zinc SEDEX deposit.

• Cu median 800 ppm with a 500 m core above 1500 ppm is a strong copper anomaly. Zn at 220 ppm and Pb at 150 ppm are low by comparison, giving a Cu/Zn ratio of about 3.6. SEDEX deposits are typically Pb-Zn rich with low Cu, so the nearby barite-galena occurrence is more likely a distal or separate mineralizing event.
• Gold at 20 ppb is modest but not background. Combined with Cu-Zn and a faulted dolomite contact, the best-fit models are a volcanic-hosted massive sulfide or a carbonate-replacement copper system. A porphyry copper is less likely because there is no described porphyry intrusion or potassic alteration.
• The gossan minerals show that the upper zone is oxidized and leached. Copper released from primary chalcopyrite may have been reprecipitated as malachite near the surface and as chalcocite at the water table. That means surface grades can be higher or lower than primary grades, and the drill hole must test below the oxidation zone.

Judgement: the anomaly is primary, but its surface expression is supergene-modified. The drill should target primary sulfides at depth, not the malachite stain.

Caution: without water, the hole should be planned to avoid excessive use; a single deep hole is better than several shallow ones.

Hole design: collar about 50 m east of the gossan, angled west at 60 degrees from horizontal, to 250 m.

• The collar should be on the east side so that the hole crosses the fault contact and the IP high, not just the oxidized gossan. The 60 degree angle gives a reasonable intersection with a steeply dipping fault and a sulfide zone at 100-200 m depth.
• At 250 m, the hole should pass through the gossan, the fault zone and the chargeability high. If the IP high is shallower than expected, the hole can be stopped early; if it is deeper, the hole still tests the contact.

Logging requirements:

• Oxidation profile: limonite, malachite, chalcocite and the depth of the water table.
• Sulfide species and abundance: pyrite, chalcopyrite, sphalerite, galena; note whether sulfides are disseminated, vein-hosted or massive.
• Alteration: quartz-sericite, dolomite, chlorite and clay; record overprinting relationships.
• Structure: fault orientation, vein orientation and shear sense.
• Petrophysics: magnetic susceptibility and conductivity to explain the IP response.

False-negative check: graphite or pyrite alone can cause a chargeability high. If no copper sulfide is found, the IP anomaly may be barren. The log must distinguish sulfide from graphite, and the assay should include Cu, Zn, Pb and Au.