The Tasmanian Geologist - Geological Society of Australia

gold, arsenic and vanadium in black shale
and turbidite-hosted gold deposits.
Ross R Large
Organic-rich mudstones or shales, within thick
packages of calcareous or siliciclastic turbidites,
have long been recognised as important host rocks
for major gold deposits (e.g. Victorian Goldfield,
Carlin District, Lena Gold Province, Otago Schist
belt). Many of the deposits in these districts are
referred to as orogenic gold deposits, emphasising
the tectonic and metamorphic processes involved in
the concentration of gold (Groves et al., 2003;
Goldfarb et al., 2005). The current genetic models
propose that the organic-rich sedimentary host rocks
are the trap-rock for gold precipitated during
deformation-related fluid flow, with the gold being
transported by deep-sourced fluids of mantle or
lower crustal origin, or in some cases from felsic
magmas. Recent research at CODES (Wood and
Large, 2007; Large et al., 2007) suggests the
possibility that organic-rich black shales are
potential source rocks for gold, arsenic and
vanadium, and that the gold was originally trapped
on seafloor organic material by chemical process
during sedimentation and concentrated in arsenian
pyrite during diagenesis.
Research on the giant black shale-hosted Sukhoi
Log deposit in the Lena gold province of Siberia
(Large et al. 2007, Scott et al, 2007) has revealed a
multi-stage process of gold concentration and
release that occurred throughout the 50 million year
period of sedimentation, diagenesis and
metamorphism in the Bodaibo trough. We have used
the rapidly developing laser ablation ICP-MS
technology to study the siting and partitioning of
gold, arsenic, vanadium and other metals during
diagenesis and metamorphism of the black shales.
During periods of extreme euxinic conditions on the
seafloor, gold and a range of other trace elements
(As, Zn, Ag, V, Ni, Mo, Pb, Se, Te, Cu, Cr, U) are
trapped on organic-surfaces forming strong organometallic
ligands, leading to a concentration of metals
in the most organic-rich facies of the sediments (e.g.
Algeo and Maynard, 2004; Rimmer, 2004). During
diagenesis, pyrite framboids, and clusters of pyrite
micro-crystals of various textural forms, grow
within the organic-rich sediments, commonly
mediated by the biological reduction of marine
sulfate to H 2 S. A portion of the gold and arsenic,
plus many other trace elements (particularly Ni, Mo,
Se, Pb, Ag, Te, Cu) are partitioned into the
diagenetic pyrite. Other trace elements remain in the
organics or form insoluble oxy-hydroxides
(particularly V, U, Cr, Algeo and Maynard, 2004).
The arsenic content of the diagenetic pyrite is a
critical control on the amount of gold that is trapped
within the sediment (Reich et al., 2005). The more
arsenic-rich the black shale package, the more gold
that is likely to be trapped during early diagenesis,
thus contributing to form a more suitable gold
source rock.
With the onset of late diagenesis, the early
forms of micro-crystalline gold-bearing arsenian
pyrite, recrystallise to larger crystals ( commonly <
0.2 mm) of subhedral to euhedral pyrite, or in some
cases pyrite nodules, developed parallel to bedding.
Some gold and selected other trace elements are
released during this process forming a gold-bearing
diagenetic fluid, that may migrate and deposit free
gold in late diagenetic structures. As the sediment
passes through the oil window, mobile organics
migrate along structures and may transport further
gold, dissolved within the organic-rich fluid
(William-Jones and Migdisov, 2007). These
processes have also been observed in the Carlin
district (Emsbo and Koenig, 2005).
Deformation and metamorphism is
accompanied by further recrystallisation and growth
of pyrite in the black shales. The syn-deformation
pyrite is commonly coarser grained (0.2 to 20 mm),
with less sediment inclusions, and a significantly
lower dissolved trace element content (Large et al.,
2007). Gold originally dissolved in the diagenetic
pyrite is released during metamorphism to form free
gold, which concentrates in preferred structural sites
(anticlinal zones or shears) associated with either
metamorphic pyrite or syn-deformation quartz veins.
Not only is the gold released from the diagenetic
pyrite during metamorphism, but other trace
elements, in particular, Cu, Zn, Pb, Ag, Bi, Te, are
also released to form minor discrete sulfides
(sphalerite, chalcopyrite, galena and various goldsilver-tellurides)
which commonly form inclusions
within the metamorphic pyrites. Certain elements,
that are strongly held within the structure of pyrite
(e.g. As, Ni, Co, Se), remain to become concentrated
in the metamorphic pyrite. Vanadium which was
originally concentrated in organic matter in the
sediments, by reduction of V 5+ to V 4+ (Wood, 1996),
is subsequently incorporated into gold-bearing
diagenetic pyrite as micro-inclusions of V-bearing
illite, and finally occurs as roscolite, intergrown with
pyrite and free gold, in metamorphic assemblages.
At higher metamorphic grades, above middle
greenschist facies, much of the early diagenetic
pyrite is converted to pyrrhotite. This is
accompanied by the complete release of gold to the
metamorphic fluid, as pyrrhotite, unlike pyrite, has a
low trace element content and does not
accommodate significant gold or arsenic within its
structure (Buryak, 1982; Large et al, in press). For
this reason pyrrhotite-rich black shales are unlikely
to be good host-rocks for gold.
In conclusion, organic rich black shales,
especially those enriched in As, Ni, Mo, Pb, Zn and
V are ideal source rocks for gold. Diagenetic and
metamorphic processes are critical to releasing the
gold, originally bound in organic-matter and
dissolved within early arsenian pyrite, to become
concentrated in later stages of syn-deformation
pyrite and associated quartz veins.
Full references are available from the editor or
author on request.
Abstracts from IAVCEI meeting
The GSA (Tasmanian Division) gave financial support to
assist two students attending the IAVCEI ( International
The Tasmanian Geologist, 10th February 2008