Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. Baro-acoustic decrepitation analyses at the Kisladag mine, Turkey
Samples collected during ECROFI field trip visit, June 2013
The Study
After the ECROFI 22 conference (2013) the Kisladag Au mine was
visited. The opportunity was taken to collect samples for
baro-acoustic decrepitation analysis to compare the fluid
inclusion assemblages from this deposit with that of many other
gold deposits shown elsewhere on this website.
Conclusions
The samples from Kisladag show only very weak decrepitation,
probably because of sub-optimal samples with insufficient
quartz. The ore zone samples do show weak, narrow decrepitation
spikes at 400 - 500 C which may be related to either sulphides
or the pervasive silicification of the ore zone. The late stage
3 barren intrusive in the core has a broad and more intense
decrepitation due to feldspars.
The fluid inclusion decrepitation study at Kisladag has not
not clearly identified an ore-related fluid inclusion event, but
there is a suggestion that more carefully collected samples
might be able to recognize such an ore related event. Without
more carefully collected samples this study indicates that
decrepitation is difficult to apply in exploration for the
Kisladag deposit.
Geological Background
The following geological information is from the ECROFI 22 field
trip guide, 2013 by Huseyin Ozturk & Zeynep Cansu.
Location
The Kisladag mine is located in west-central Turkey.
Property Geology
According to Richard Sillitoe (2000): “Kisladag is confirmed to be a true porphyry gold deposit, albeit possessing several distinctive geological features. These include the paucity of quartz veinlets, the dominance of molybdenum over copper and the exceptionally high gold values. The Kisladag deposit is centred on a steep, multi-phase latite porphyry intrusion of alkaline affiliation. Younger intrusive phases were emplaced progressively nearer the centre of the stock and are characterized by increasingly weaker alteration of lower gold contents. The centrally positioned late-mineral phase is essentially barren.”
Within the deposit area, the main lithologies that have been
logged and mapped by Eldorado Gold Company are quartz—trachyte to
quartz-latite flows and volcanoclastic rocks intruded by a series
of nested subvolcanic porphyritic intrusives of alkalic
affinities. Based on the intensity of alteration, mineralization
and cross cutting relationships three mineralized intrusives
(Intrusions 1, 2 and 2A) and one post mineral body (Intrusion 3)
have been identified. West of the open pit the schistose basement
has been intersected in a number of drill holes and outcrops of
basement rocks have been mapped north and west of the leach pad
area.
Lithologies
Basement Schists
The basement schist varies from a fine grained mylonite to coarse
augen gneiss with rounded porphyroclasts of orthoclase. The schist
is composed dominantly of quartz, plagioclase/albite and white
mica and contains quartz veins that have been affected by the
intense deformation. In the vicinity of the open pit the foliation
has a shallow dip to the south—east. Elsewhere on the property the
dips are dominantly flat lying but may show a rapid transition to
steeply dipping.
Pyroclastics
The volcanic rocks on the upper benches of the pit and in drill
holes on the south side of the pit vary in texture from fine
grained fragmental ash fall tuffs with pumice fragments to
porphyritic flows with flow banding and auto brecciation. Eight
hundred metres east of the pit two diamond drill holes intersected
a number of fine grained plagioclase-hornblende porphyries with
interlayered ash fall tuffs. The porphyries have been
propylitically altered and it is not clear if they are intrusive
bodies or flows.
The Çukuralan Au deposit is in the Bergama (İzmir) region
of western Turkey. The deposit is a low sulphidation epithermal
deposit (similar to the Ovacık-Au deposit (40 km southeastof
Çukuralan) occurring within the porphyritic dykes that cut
the local metamorphic sequence known as Karakaya Complex. The gold
grade varies between 0.5 and 50g/T, with mining from an open pit.
Mineralisation at Cukuralan is related to porphyry dykes which
were formed at the same time as the tectonics which caused ductile
deformation. It is uncertain if the gold is derived by leaching of
the regional Permian aged metamorphics during later Miocene aged
magmatic events. It is thought that the mineralisation is related
to the intrusion of porphyry dykes. The gold deposition is
primarily lithologically controlled by reducing host rocks, but
deposition also occurs without lithological control. Quartz, and
calcite occur as gangue minerals. The mineralisation is
structurally controlled and occurs along 2 different fault systems
oriented NE-SW (younger) and NW-SE (older).
The contact between volcanoclastics and intrusive 1 is difficult
to define due to the intensity of alteration but is steeply
inclined and often marked by a tourmaline rich hydrothermal
breccia.
Intrusion 1
Intrusion 1 is the oldest, and generally, best mineralized
porphyritic intrusive phase. It forms the core of the system, and
is cross cut by the younger porphyritic intrusions. it forms a
slightly elliptical body approximately 800 m across, which was
exposed at the pre-mining surface. Recent drilling indicates the
roots of Intrusion 1 lie on the southeastern side of the pit. In
the western half of the deposit, the basement schist is
encountered in deeper holes, at depths of 200 m to 350 m, and to
the west of the proposThe Çukuralan Au deposit is in the
Bergama (İzmir) region of western Turkey. The deposit is a low
sulphidation epithermal deposit (similar to the Ovacık-Au deposit
(40 km southeastof Çukuralan) occurring within the
porphyritic dykes that cut the local metamorphic sequence known as
Karakaya Complex. The gold grade varies between 0.5 and 50g/T,
with mining from an open pit.
Mineralisation at Cukuralan is related to porphyry dykes which
were formed at the same time as the tectonics which caused ductile
deformation. It is uncertain if the gold is derived by leaching of
the regional Permian aged metamorphics during later Miocene aged
magmatic events. It is thought that the mineralisation is related
to the intrusion of porphyry dykes. The gold deposition is
primarily lithologically controlled by reducing host rocks, but
deposition also occurs without lithological control. Quartz, and
calcite occur as gangue minerals. The mineralisation is
structurally controlled and occurs along 2 different fault systems
oriented NE-SW (younger) and NW-SE (older).
ed pit outline, Intrusion 1 may become sill-like in form, lying
above the schist, and not coming to surface. Contacts between
Intrusion 1 and the surrounding volcanic rocks are generally
obscured by alteration. Contacts with younger intrusions,
particularly Intrusion 3 are better preserved. Intrusion 1
comprises abundant phenocrysts in a K-feldspar dominant
groundmass. Plagioclase is the dominant phenocryst phase,
comprising up to 30% of the rock by volume. It occurs as tabular
crystals ranging in size from <1 mm to 5 mm. Biotite is the
second most abundant phenocryst phase, comprising up to 10% of the
rock. It is typically finer grained than plagioclase. Blocky
megacrystic K- feldspar phenocrysts, up to 1 cm, are a
characteristic of this unit, but are low in abundance. Quartz
phenocrysts are rare. Based on the primary mineral assemblages
this rock would lie in the quartz-syenite to quartz-monzonite
fields.
Intrusion 2a
Intrusion 2A occurs in the southeast corner of the pit, where it
intrudes the margin of Intrusion 1. It forms a circular stock 250
m to 300 m across. It appears to taper at depth. This unit carries
economic gold grades, but is not as well mineralized as Intrusion
1. It is a fine to medium-grained porphyritic rock. intense
pervasive clay-quartz alteration appears to have selectively
overprinted this unit. The unit contains a significant amount of
plagioclase phenocrysts (“‘20%), up to 2 mm in length, and
possibly sparse quartz phenocrysts. This unit is very similar to
texture to Intrusion 2, but is differentiated because of the
intense clay alteration, which does not affect Intrusion 2.
Intrusion 2
Intrusion 2 occurs as two separate semi-circular stocks, both
approximately 150 m to 200 m across. One occurs in the center of
the pit, cutting the core of Intrusion 1, and the second occurs on
the northwestern margin of Intrusion 1. Both intrusions are in
contact with Intrusion 3, and there are large fragments of schist
along the margin of the northwestern stock. The stocks carry
economic gold grades. The rock is a fine-medium grained
porphyry. It is comprised of abundant (20-30%) plagioclase
phenocrysts up to 2 mm in length in a dominantly K-feldspar
groundmass. No quartz phenocrysts were observed. This unit can be
weakly-moderately magnetic. Compositionally this unit would lie in
the syenite — monzonite fields.
Intrusion 3
Intrusion 3 is the youngest intrusive body at Kisladag. It forms
a semi-circular stock near the center of Intrusion 1, west of the
central Intrusion 2 stock, and extends into an elongate, steeply
dipping to vertical dyke-like body to the west, extending beyond
the limits of the proposed pit. It has been traced at least 450 m
west of the pit. The contacts of this unit with other rocks are
generally well preserved, and the drop in gold grade is abrupt at
the contacts. It is a fine grained porphyritic unit, comprised of
20% to 30% plagioclase phenocrysts, up to 4 mm in length, sparse
quartz and biotite phenocrysts (both <5%), and amphibole
phenocrysts (5-10%). The amphibole phenocrysts have been
preferentially altered to secondary mafic minerals, but their
prismatic shapes are preserved. This intrusion is typically
magnetic, due to the presence of very fine grained disseminated
magnetite in the groundmass. The magnetite is probably a primary
component of this unit. Its bulk composition places it in the
quartz-syenite - quartz-monzonite fields.
Mineralization
The Kisladag deposit consists of porphyry—style gold
mineralization centered on a series of overlapping sub-volcanic
and extending outward into the surrounding volcanic and
volcanoclastic rocks. Gold mineralization with traces of
molybdenum, zinc, lead and copper encircles the late barren stock
(lnt—3). Higher—grade gold mineralization (> 1 g/t Au) is
associated with Intrusive 1 and forms a horseshoe shaped zone
around the northern, southern and eastern sides of Intrusive 3.
The higher gold grades are associated with multiphase quartz
sulphide stockwork and pervasive silicification. The mineralized
zones dip outward in a bell-shaped body; sub parallel to the
contact of the stock. Gold is associated with at least three
phases of partially overlapping stockwork veining and brecciation.
These include:
- Intense quartz—tourmaline stockwork veining
- Hydrothermal breccias (± gold)
- Multiple phases of quartz~pyrite veining with
local silica flooding with gold
- Late sulphide rich quartz veining with traces
of molybdenum, sphalerite, galena and tetrahedrite (±
gold).
In general, the amount of stockwork veining decreases with depth,
especially below 650 masl elevation. Higher—grade mineralization
(above 2 ppm Au) has been traced from surface to depths greater
than 250 m below surface. Lower—grade mineralization, grading
between 0.5 and 1.0 ppm Au has been traced to the deepest levels
drilled on the property approximately 600 m to 700 m below
surface.
Tourmaline is locally abundant in every phase of mineralization
except the late vuggy silica. it typically occurs as very fine,
anhedral grains disseminated in the host rock and silica gangue.
Pyrite is the dominant sulphide mineral present with visual
estimates averaging around 4% in the primary ore zone. Locally as
much as 15% pyrite can be present. Other sulphide minerals
identified in microscopic studies include chalcopyrite,
sphalerite, tetrahedrite, galena and molybdenite. Oxidation tends
to be deeper on the uphill (southern) side of the deposit (from 30
m to 80 m deep) as compared to the downhill (northern) side of the
deposit, where oxidation is limited to between 20 m and 50 m below
surface. There is also a broad east-west trend, with slightly
deeper oxidation on the east side (50 m to 100 m) versus the west
side of the deposit where oxidation ranges from 30 m to 60 m deep.
Limonite is the most abundant oxide mineral, usually occurring
along fractures in thin colloform layers and as disseminated
patches around weathered pyrite and mafic minerals.
Alteration
A potassic assemblage characterized by secondary biotite, and focused on Intrusion 1 forms the core of the system. Tourmaline is present throughout the deposit, and in all phases of intrusions, but is most common in the volcanic rocks adjacent to Intrusion 1 and forms an outer alteration shell. An advanced argillic assemblage characterized mainly by alunite occurs on the outer margins, beyond and overprinting the tourmaline, partially capping the deposit and extending to depths of up to 350 m on the eastern side of the pit. A pervasive retrograde argillic assemblage of clays +/- chlorite—montmori|lite overprints all other alteration, and can extend to depths of 600 m. Surface clay weathering and oxidation is also present, and locally extends to depths of 250 m on the outer margin of the deposit. It may in part be preferentially developed on zones of advanced argillic alteration. Disseminated pyrite is ubiquitous, and may have been introduced which each stage of hydrothermal alteration, particularly the potassic and argillic stages. There is no propylitic alteration (chlorite-calcite-epidote) in or immediately adjacent to the deposit.
The alteration mineral assemblages associated with the deposit
were formed by a complex interaction of hydrothermal and meteoric
fluids. Each intrusive event contributed a high temperature phase
dominated by magmatic fluids and vapours followed by a retrograde
cooling stage associated with meteoric water telescoping in on the
earlier alteration.
Representative E-W cross section showing the relationship between
the intrusives.
Another cross section with the pit outline and grades is on the Eldorado mines website.
The baro-acoustic decrepitation study
Samples
A suite of samples were collected from representative rock types within the mine. These were provided as piles of each different rock type in the viewing area of the mine as it was not possible to access the pit. The precise locations of each sample are unknown. Where possible multiple samples from each material were collected and analysed separately and plotted together.These were analysed by baro-acoustic decrepitation as described
elsewhere on this website.
All of the results are of very low intensity, despite the use of double the normal sample size in this study. This is consistent with a magmatic origin with a restricted hydrothermal fluid overprint and lack of quartz veining.
Intrusive 1, sulphide bearing main ore zone.
Both samples contain a distinct sharp peak at about 500 C. This is not present on any other samples. This might be an ore related response or it may also be caused by sulphides which typically give narrow decrepitation peaks. But note that the sulphide observed in intrusive 2A (below) does not give a decrepitation peak at 500 C.
Intrusive 1, oxide zone
The 3 sub-samples have only a very weak response and lack similarity. Perhaps the peak at 400 C on sample h2629 is due to residual sulphide, but it is interesting that it is at 400 C not at 500 C as in the above results!
Intrusive 2A, low grade ore
There is no significant response in these 3 samples. Perhaps the
gradual rise in sample 2214C (blue) is due to fresh feldspars. It is
interesting that although sulphides were observed in the hand
specimen during preparation, there is no peak at 400 or 500 C as in
intrusive 1. Perhaps sulphides are not the cause of those peaks!!
Intrusive 3, late stage barren core zone
These 2 results are actually a repeat of the same sample. h2619 was
analysed on 0.5 gm of sample and mathematically doubled for
comparison with h2631 which was an analysis on 1.0 gm of the
identical sample. This response looks like quartz, with some
feldspars as well and is much more intense than the other results in
this study. The peak at 500 C is much broader than in intrusive 1
above and probably due to quartz rather than sulphide.
Pyroclastic, regional host rock
This material has only a weak response around 600 C which is
probably due to fresh feldspar. This is the regional
background signature from these unmineralised rocks.