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Brusson area Au, NW Italy
A Baro-acoustic decrepitation study on
a well documented mine
Introduction
The Brusson area of North-western Italy hosts several gold mines
which have been worked since roman times. These have also been the
focus of intensive recent research by L.W.
Diamond and others. The
quartz veins occur in brittle fracture structures within gneiss and
marble host rocks, but gold only occurs in the quartz veins within the
gneiss host rock. The Fluids which formed the auriferous quartz veins
were of metamorphic origin during the alpine orogen and Diamond has
determined time constraints on the fluid processes suggesting that
hydrothermal activity lasted less than 2 Ma.
37 samples were collected from the Fenilia mine area for baro-acoustic
decrepitation. Because the mine adits were inaccessible, the samples
came from dump material at each of 3 adit entrances at different mine
levels as well as surface quartz outcrop from the overlying carbonate.
The mine is located in the north-western alps of Italy in Val d'Ayas.
The sample locations were recorded using a GPS. Samples
came
from adit entrances and mining dump waste on the hillside at 4
different levels.
The cross section shows the lower levels of the Fenilia vein
hosted by paragneiss. These levels are auriferous. At the uppermost
level 4, the Fenilia vein is hosted within carbonate rocks and is now
barren of gold. Past studies have tried, unsuccessfully, to explain the
lack of gold in the Fenilia quartz vein where it is hosted by
carbonate.
Brusson decrepitation results
Samples were collected from adit entrances on 4 levels of the
Brusson mine, specifically including the uppermost level which is
hosted by carbonate rocks and is considered to be barren of Au.
Each site is allocated a single sample number, at which
multiple
quartz fragments were collected over several metres radius and the
quartz is from unknown locations within the
adjacent adit. Each fragment is allocated an alphabetic suffix and
analysed
separately, with different “RUN” numbers.
Before analysis each quartz fragment was briefly described to
see
if there was a correlation between visible quartz characteristics and
decrepitation response.
The sample details are listed here.
The
locations
were
fixed
by GPS and waypoint names IT6 – IT9 are these
positions and are plotted on the topography map.
In addition, several samples from nearby areas were collected
as
“background” quartz, but there were few opportunities to collect
such background samples.
Interpretation
The complete decrepitation analyses for all these samples are
shown in the plots later
in
this page. Low temperature decrepitation peaks indicative of
high CO2
contents in the inclusion fluids are common, but there are variations
which do not relate to quartz hand sample description or presumed Au
content. In particular, the top level samples (H2053-H2056, sample
1957) are not obviously different to samples from the lower levels
which are considered to be auriferous. Most, but not all
the samples have a low Temp Peak due to CO2/CH4
rich fluids.
Some of the samples from the 2 nearby
background areas also show CO2
so it seems that CO2
content
alone does not relate closely to Au content at Brusson. Or perhaps
the CO2 anomaly halo around mineralisation is so large that
all the
quartz in
this region is within the anomaly.
At first inspection, the initial plot of all samples together,
colour coded by mine level and shown here, does not distinguish
between the lower auriferous levels and the uppermost level 4 barren
sample.
However, plotting this data using a special logarithmic y axis to
accentuate the important region from 20 to 500 counts reveals
that there are 3 distinct types of quartz. Type 1 quartz has high
levels of contained CO2. Type 2 quartz has lesser, but still
substantial, levels of CO2. Type 3 quartz lacks CO2
rich fluid inclusions. In the following plot these groups are
distinguished and a scatter plot (inset) of quartz types versus mine
level shows that there is no type-3 quartz at the uppermost mine level.
So the decrepitation data DOES distinguish a difference in
the quartz which correlates with the known lack of gold in the quartz
vein at the uppermost, carbonate hosted level 4.
To facilitate comparison between these complex decrepitation
curves, each curve was deconvolved to determine the component skewed
gaussian populations which combine to generate the overall result.
These data samples can comprise between 3 and 5 sub-populations. The
peak temperature of the fitted peaks is summarized in the following
table. The curve fitting and some of the data are shown here.
The complexity of these samples with (often) 5 component populations is
unusual and interesting. It is also unusual to be able to distinguish a
population such as Peak4 with a temperature around 560 C, which is
unusually close to the inversion-related peak at 600 C.
Component population
temperatures of each sample at Brusson
This peak temperature data is plotted below, grouped by both
the mine level of the sample and also the quartz type. In this plot,
the radius of each plotted circle represents the total number of
decrepitation counts in that decrepitation peak. Note the
absence of green "type 3 quartz" samples from the uppermost (1710m)
level.
Conclusions
The decrepitation data reveals that there is a significant
difference in the fluid inclusion populations in quartz from the
uppermost level when compared with the quartz samples from the 3 lower
levels. This observation can be used to define a model for the
formation of the Fenilia quartz vein which explains the lack of gold
where this vein is hosted by carbonate rocks.
The Fenilia auriferous quartz vein at Brusson was formed from
auriferous fluids coming from the basement, in which the Au is
transported as a HS- complex, as shown by L.W. Diamond. This was not, however, a
continuous steady flow and the flow was episodic.
During periods of high fluid upflow
rates, the effect of the weak fluid inflow from the host rock
units was minimal. The vein-flow fluid temperature remained high and
there was little chemical change so the gold present remained in
solution and passed through the section without being deposited while
type 1 and type 2 quartz was deposited.
During periods of low fluid upflow
rate, the inflow of fluids from the paragneiss host rock unit
mixed with the basement sourced fluid and lowered the temperature of
the vein-flow fluid. This resulted in phase separation of the CO2
rich fluid into an almost pure CO2 phase and an aqueous
dominated phase, with gold partitioning almost completely into the
aqueous phase. The gold enriched aqueous fluid gave rise to auriferous
quartz deposition and type 3 quartz containing few or no CO2
rich fluid inclusions. At the upper level 4, the stratigraphic inflow
fluid from the marble and serpentinite has a high pH, possibly greater
than 10. Mixing of this fluid with that in the vein increases the pH,
which increases the solubility of the Au(HS)-2
complex and also increases the
solubility of silica, despite any temperature decrease. Consequently the
quartz at this upper level lacks gold and also lacks type 3 quartz
deposition. It
is this change in pH caused by mixing with high pH stratigraphic inflow
fluids that stops the deposition of gold in the vein at this location.
Diamond also noted that the rock alteration
halo adjacent to the vein is frequently very thin, only a few cm thick.
This is inconsistent with his assumption of wall rock reaction being
the cause of depositional control. However, the model proposed here
with gold and quartz deposition being controlled by fluid mixing with
fluids ingressing from the host strata, is entirely consistent with the
observed minimal wall rock alteration halo adjacent to the vein.
The fluid system at Brusson was CO2 rich and the
fluid
inclusions show many differing populations which undoubtedly reflect a
complex fluid history of multiple events which contribute to the
complex zonation of the quartz. Despite this complexity of quartz
deposition, the decrepitation data provides key information to
understand just why the gold within the vein is controlled by the type
of host rock.
Most samples from the lower adit entrance show intense low
temperature decrepitation due to abundant CO2 rich fluid
inclusions. But the quartz is zoned and some samples lack CO2
rich inclusions.
H2036 - 2042 Sample#
1954A-G 0.5gm -420+200u
Brusson lowest adit entrance and into
adit. GPS location IT6
Brusson mid-level adit entrance - 1648 level.
Some but not all samples have low temperature CO2
caused decrepitation peaks. Note the complexity of the many inclusion
populations between 400 and 560 C. This quartz is the product of
numerous different fluid events and it is incorrect to think of it as a
single rock even though it is a single mineral phase.
Brusson Carbonate hosted quartz at ~ 1740 m altitude,
non Au mineralised.
Although outside the economically mineralized Au zone, low
temperature CO2 caused decrepitation is still prominent.
This is still quite close to the ore zone and perhaps the CO2
is part of a fairly large halo around the ore zone.
H2053-2056 Sample# 1957A-D
0.5g -420+200u
Brusson mine, carbonate hosted upper
zone, GPS location IT9 1740m RL, No Au here
Background samples
These regional samples tend to show an absence of the low
temperature peak indicating a general, but not complete, absence of CO2
in unmineralised samples.
H2032 Sample# 1952
0.5gm -420+200u coarse very
milky white qtz
En Route to brusson, italy IT4 fuchsite qtz magnesite quarry altd
serpentinite
H2033- 2035 Sample# 1953A,B,C
0.5gm -420+200u coarse
milky white qtz
Mae village, river section, near
Brusson IT5
References
Mesothermal gold lodes in the
North-western alps: A review of genetic constraints from radiogenic
isotopes.
Thomas Pettke, Larryn W Diamond and Jan D Kramers
Eur. J. Mineral., 2000, v12, 213-230.
www.geo.unibe.ch/diamond/.../Pettke,Diamond,Kramers_2000.pdf
Fluid inclusion evidence for P-V-T-X
evolution of hydrothermal solutions in late alpine gold-quartz veins at
Brusson, Val d'ayas, northwest Italian alps.
Larryn W. Diamond
Am. J. Science, v290, Oct 1990, 912-958
www.geo.unibe.ch/diamond/publications/Diamond_1990.pdf
Oligocene gold quartz veins
at Brusson, NW Alps; Sr isotopes trace the source of ore-bearing fluid
to ore a 10-km depth
Thomas Pettke and Larryn W. Diamond
Economic Geology, Jul 1997; 92: 389 - 406.
Solubility of gold in NaCl and H2S
bearing aqueous solutions at 250 - 350 C.
Ken-Ichiro Hayashi and Hiroshi Ohmoto
Geochimica et Cosmochimica Acta, August 1991 V55 #8,
2111-2126
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
