Gossans and Leached Cappings: Field Assessment by Roger Taylor

By Roger Taylor

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

Although the appearance of recent multielement geochemical sampling and more uncomplicated mechanical excavation help significantly in subsurface interpretation, there are nonetheless many events the place the 1st remark and popularity are made via the lone box geologist. New exposures remain present in distant and sometimes tricky terrains, the place “on the spot” talents are of leading importance.

In common phrases the textual content has been prepared from the extensive scale to the categorical, and it's going to be realised that each one scales supply important enter for ultimate interpretation. the subjects lined include:

• Theoretical perspectives

• preliminary recognition

• normal box observations

• targeted box observations (secondary minerals, boxworks)

• Porphyry copper leached cappings

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Extra resources for Gossans and Leached Cappings: Field Assessment

Sample text

Fig. 11 Pitch limonite. 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).

Altered to red haematite (indigenous). Mt Surprise district, Queensland, Australia. Width of frame 10 cm. 6 SECONDARY MINERALS 46 Fig. 15 Chalcotrichite (Copper oxide). A variety of cuprite with rich red colouration occurring in both powder-like formats and as fine fibres (inset). Location (Powder format) – Cloncurry district, Queensland, Australia. Width of frame 8 cm. Location (Fibrous format) – (“Hair copper”) Ray Mine, Arizona, USA. Width of frame 2 cm.  Huehne Fig. 16 Malachite and azurite (Copper carbonates).

Siderite seems to commonly oxidise to a distinctive deep brown colour, and within any individual region an experienced prospector/geologists may gain enough confidence to interpret some of the colour variations. A further limonite feature worth both early and continuing inspection, concerns the interpreted origin in terms of source. 3 LIMONITE site is termed indigenous, whereas that fi xed away from the dissolution site is termed exotic. Limonite that can be interpreted as coming from a nearby ex-sulphide source is termed fringing (Figure 30), (Blanchard, 1968).

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