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 section

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.

All brusson results

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.

Brusson quartz types

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
Run	Sample Number	Peak1	Peak2	Peak3	Peak4	Peak5
h2036	1954A	_	438	492	556	598
h2037	1954B	272	396	458	568	598
h2038	1954C	242	418	460	574	598	Lowest adit level
h2039	1954D	296	462	_	572	598	Location IT6
h2040	1954E	282	418	_	556	598
h2041	1954F	_	398	496	556	594
h2042	1954G	286	440	500	558	594

h2043	1955A	_	460	508	_	596
h2044	1955B	272	460	_	564	598
h2045	1955C	304	478	_	566	598	Mid-level adit
h2046	1955D	262	446	530	_	598	Location IT7
h2048	1955E	260	426	466	556	594
h2049	1955F	_	460	_	_	592

h2050	1956A	284	464	_	_	596
h2051	1956B	_	454	_	_	592	Upper adit level
h2052	1956C	284	460	_	_	596	Location IT8

h2053	1957A	260	470	_	556	598
h2053	1957A	260	420	484	558	598	Carbonate hosted zone
h2054	1957B	262	336	430	_	600	Location IT9
h2055	1957C	278	446	_	_	598
h2056	1957D	258	424	494	570	596

Details of the curve fitting procedure and some of the data are here

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.

Brusson decrep summary data

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.

Brusson model section

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.

Complete Brusson decrepitation data plots

Brusson, Lowest level adit entrance ~1560m altitude

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.

H2043-2049 Sample# 1955A-F 0.5gm -420+200u
Brusson next level up, 1660m alt. (1648 level) GPS location IT7

Brusson upper level adit entrance

Low temperature CO₂ caused decrepitation is still present at the upper adit entrance.

H2050-2052 Sample# 1956A-C 0.5g -420+200u
Brusson mine, 2 levels up (poor fix 1660m) GPS location IT8

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