Page 144 - Handbook of Gold Exploration and Evaluation

122 Handbook of gold exploration and evaluation

· a geologically long active zone of crustal weakness originating along a
palaeo-continental margin, with development of major through-going fault
systems
· a regional environment of crustal thinning with multiple intrusive episodes
and sustained high heat flow
· multiple episodes of hydrothermal activity
· reactive and highly permeable carbonate host rocks.
Christenson (1993) summarises the salient alteration features that characterise
Carlin trend deposits:

· carbonate dissolution
· argillic alteration of primary silicate minerals
· silicification
· gold-enriched sulfidation of reactive iron in host rocks to form gold-bearing
sulphide (pyrite, arsenopyrite).
Teal and Jackson (1997) have adapted the alteration zonation pattern
proposed by Kuehn and Rose to include the following major distal-to-proximal
alteration assemblages:

· fresh silty limestone ± calcite + dolomite + illite + quartz + Kspar + pyrite
· weak to moderate decalcification (dolomite halo) ± dolomite calcite + quartz
+ illite + kaolinite + pyrite + gold
· strong decalcification ± dolomite + quartz + illite kaolinite + pyrite gold
· decarbonisation ± quartz + kaolinite/dickite + pyrite gold.
Carlin type gold is also being discovered elsewhere. The gold occurs as sub-
micron particles primarily within the lattices of pyrite and arsenopyrite. Fluid
inclusion studies suggest that the metal was transported as hydrogen bisulphide
complex (Kuehn, 1989) by gold-bearing fluids of mixed meteoric-magmatic
origin. Due to high CO 2 content in fluid inclusions, Kuehn estimated a depth of
formation of the Carlin trend deposits of 4:4 2:0 km within a temperature range
of 180±245 ëC. Deposits that appear to be similar to those documented for the
Carlin trend ores were discovered at Salaman (Leon, Spain) by BP Minera Espana
in 1985 (Paniagua et al., 1996). Hydrothermal alterations at the Salamon deposit
are fundamentally decarbonatisation-dolomitisation, silicification and
argillisation, and vary according to the type of host rock. The gold is refractory,
hosted by pyrite and arsenopyrite, crystals measuring a few dozen microns.
Mineralised bodies are made up with mineralised tectonic breccias, veins and
pockets, and disseminated sulphides in a quartz-carbonate gangue. The host rocks
consist primarily of carbonate rocks with very bituminous perlitic intercalations
from the Lena Group. Salamanca University, in collaboration with SIEMCALSA,
is carrying out further work (at Castilla-Leon Autonomy Community) in the
framework of the metallogenic research on hydrothermal gold deposits.