Deep weathering around gold deposits of the Carlin Gold Province

Weathering in and around gold deposits of the Carlin Gold Province extends to
several hundred meters depth in many open pits and is easily recognizable as
bleached material in which the gold is free-milling (i.e. non-refractory). This interval
occurs in the upper section of open pits regardless of elevation, is highly porous due to
carbonate dissolution, and contains clay (kaolinite)-goethite-hematite assemblages
devoid of calcite or sulfides. As already concluded by others, this interval is not the
upper part of any conventional epithermal gold system.
Drilling indicates that very similar bleached material with similar mineralogy
extends to 800 m, and possibly 1500 m, depth in the immediate vicinity of several
deposits. The deeper extensions of the bleached material well below the base of open
pits is interpreted as part of the same weathering profile recorded on pit walls. The
extension of the weathering zone to such great depth has resulted from the coincidence
of the same factors necessary for deep weathering in karst systems: these are
the intensity of structures immediately around the gold deposits that influenced
groundwater movement both during mineralization and during weathering, elevation
and relief of north-central Nevada, abundant supply of groundwater, and the reactivity
of carbonate-bearing assemblages. The weathering process is enhanced near gold
deposits by the oxidation of pyrite to form acidic waters, and these waters then react
with carbonates to leave the pore spaces. The depth of weathering is predicted to be
shallower away from mineralization.
Beneath the bleached interval in many open pits is porous black material containing
a mixture of gold-bearing arsenian pyrite, carbonaceous material, realgar
and/or orpiment. Some of this black interval contains supergene sulfides, and there
has been extensive removal of calcite to generate 10–30% porosity. The boundary of
the bleached interval and porous black refractory interval is marked by a sharp but
irregular boundary that varies from being horizontal, to steeply-inclined and influenced
by structures, to overturned in places. This boundary defines what would be
known in regolith parlance as the base of complete oxidation (BOCO), i.e. the bleached
material, generally above, is oxidized by the weathering process, but the material
below contains at least some oxidizable minerals that are not totally oxidized. BOCO
is not synonymous with the base of weathering, and in terrains where it has been
mapped in detail, half of the weathered profile may be below BOCO.
The boundary of weathered and unweathered rock has not been systematically
mapped across the Carlin gold province, nor have criteria been established to identify
readily the base of weathering in the Carlin ore environment. However, the high degree
of porosity, deuterium isotope data demonstrating deep influx of meteoric waters, and
the great depth of the bleached material all suggest that the full weathered interval
extends well below the bleached material making much of the porous black refractory
ore, including its present mineralogy and textures, the product of weathering.