Gossans and Leached Cappings: Field Assessment by Roger Taylor

By Roger Taylor

This textual content concentrates upon box observations bearing on leached cappings and gossans, happening as oxidised floor expressions of underlying ore zones.

Although the arrival of contemporary multielement geochemical sampling and more straightforward mechanical excavation help significantly in subsurface interpretation, there are nonetheless many events the place the 1st statement and popularity are made via the lone box geologist. New exposures remain present in distant and infrequently tricky terrains, the place “on the spot” abilities are of leading importance.

In common phrases the textual content has been prepared from the large scale to the categorical, and it may be realised that each one scales offer helpful enter for ultimate interpretation. the themes coated include:

• Theoretical perspectives

• preliminary recognition

• common box observations

• exact box observations (secondary minerals, boxworks)

• Porphyry copper leached cappings

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Deep red brown oxidation product of chalcopyrite? With conchoidal fracture and resinous lustre. Encased in green garnet–skarn. Pukakaka district, Peru. Width of frame 2 cm. SECONDARY COPPER MINERALS SECONDARY COPPER MINERALS 45 6 Fig. 12 Cuprite-tenorite (Copper oxide). Maroon coloured cuprite with rims and partial coatings of tenorite (black). Cavities infilled with malachite (dull green) and chrysocolla (emerald green). Cloncurry region, Australia. 5 cm. Fig. 13 Cuprite (Copper oxide). Well formed, deep red – black crystals precipitating in a cavity.

Width of inset frame 8 cm. El Abra (porphyry copper) Chile. Width of main frame about 6 m. 1 General aspects Group Name Formula Element Copper (native copper) Cu Cuprite Oxides Tenorite CuO Malachite Cu2CO3(OH)2 Azurite Cu3(CO3)2(OH)2 Chalcanthite CuSO4 · 5H2O Brochantite CuSO4(OH)6 Antlerite Cu3SO4 (OH)4 Linarite PbCuSO4(OH)6 · H2O Atacamite Cu2Cl(OH)3 Chrysocolla (mineraloid) (Cu, Al)2H2Si2O5 · n(H2O). Dioptase CuSiO2(OH)2 Pseudomalachite Cu5(PO4)2 (OH)4 Turquoise CuAl6(PO4)4(OH)8· 4H2O Chalcosiderite Cu(Fe, Al)6(OH4)2 · 4H2O Olivenite Cu2AsO4OH Chalcocite CuS Covellite Cu2S Sulphates Silicates Phosphates Arsenates CuO Chalcotrichite (hair copper) Carbonates Chlorides Cu2O Sulphides The secondary copper minerals constitute one of the most abundant and conspicuous secondary mineral groupings, and with their predominately green-blue and red colours are easily noticed.

Fig. 18 Quartz – newly exposed. Boulders of low sulphidation epithermal quartz revealed via land clearing. 19). Kerta district, West Java, Indonesia.  Prihatmoko. 3 18 Fig. 19 Quartz dominant – hidden. Surface profile of leached/ weathered quartz vein. Quartz is visible as grey soil coated particles adjacent to the green fern (lower mid section) Inset illustrates classic epithermal layering within the drill core which is also visible in broken /slabbed surficial samples. Kerta district, West Java, Indonesia.

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