An overview of vis-nir-swir field spectroscopy - Spectral International

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Definition of deposit type
HIGH SULFIDATION SYSTEMS
High sulphidation Au + Cu ore systems develop from the reaction with host rocks
of hot acidic magmatic fluids to produce characteristic zoned alteration and later
sulphide and Au + Cu + Ag deposition. Ore systems display permeability controls
governed by lithology, structure and breccias and changes in wall rock alteration
and ore mineralogy with depth of formation. One of the challenges is to
distinguish the mineralized systems from a group of generally non-economic
acidic alteration styles including lithocaps or barren shoulders, steam heated,
magmatic solfatara and acid sulphate alteration.
Original definitions (Heald et al.,1987) suggest that epithermal deposits formed at
temperatures < 300°C and therefore at shallow depths, typically < 1 km.
The term epithermal is therefore used in field exploration studies to describe Au ±
Ag ± Cu deposits formed in magmatic arc environments (including rifts) at
shallow depths, most typically above the level of formation of porphyry Cu-Au
deposits (typically < 1 km), although in many instances associated with
subvolcanic intrusions. Higher level epithermal deposits are commonly formed
later in a deposit paragenesis, and may be telescoped upon deeper earlier
formed deposits, including in some instances porphyry systems which are older,
or part of the same overall magmatic event (e.g., Lihir, Corbett et al., 2001b). In
strongly dilational structural environments porphyry-related systems may be
telescoped outwards into the deeper “epithermal” environment (e.g., Cadia,
Maricunga Belt).
In brief, high sulphidation deposits develop in settings where volatile rich
magmatic fluids rise to higher crustal levels without significant interaction
with the host rocks or ground waters. Volatiles (SO2) evolved from the
depressurising fluids oxidize to form a two stage hot acid fluid, the initial stage
of which reacts with the host rocks to produce the characteristic zoned acidic
alteration at epithermal crustal levels (Corbett and Leach, 1998). A later liquiddominated
fluid phase deposits sulphides which are characterised by pyrite
with enargite, or the latter’s low temperature polymorph luzonite.
Magmatic rocks are interpreted to represent the ultimate source of ore fluids as
demonstrated by the commonly association of HS deposits with felsic domes
and phreatomagmatic breccias within flow-dome complexes. Many HS deposits
display associations with porphyry Cu-Au systems of similar ages (Nena), or are
collapsed upon and form part of the porphyry ores. Fluids responsible for the
formation of HS deposits rarely evolve to Au- rich lower sulphidation style ores.
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