MINERALOGY of LENGENBACH
The Binn valley lies almost entirely
in the Monte Leone region. The central part of this area is composed by gneiss
and serpentinites, with a band of Triassic dolomites within which is located
the Lengenbach mineralization. This sulfosalt mineralization appears only in
the dolomite outcrops those are scattered throughout the Binn valley.
The Lengenbach quarry is undoubtedly the most important of these outcrops, but
not the only one; other places of interest are Turtschi, Mässerbach and
Reckiebach. The dolomite shows its white color and it seems look like sugar,
so called saccharoid dolomites, in addition its texture reminds to marble.
The Sulfosalt Origin
Sulfosalts is characterized by the following general formula: Mx
(As, Sb, Bi)y Sz
M represents a metal, which in the case of Lengenbach,
in descending order of importance, are: Pb, Cu, Ag, Ti, Fe, Zn, Sn and Hg.
The three other formula elements are As, Sb, Bi.
First one, arsenic, is the most common in Lengenbach; the Sb is
rare and Bi has
not been found so far in this quarry.
As we left away Lengenbach quarry, Sb are gaining importance
(for example geocronite, boulangerite and bournonite). By contrast, only it has
been found one place in the whole Binntal region with sulfosalts with Bi,
it is the Turtschi outcrop, near the small town of Giessen, where was collected
the giessenite, a Bi sulfosalt, and described by Graeser in 1963.
of the Lengenbach sulfosalts occurred in two stages, as follows:
1) In the Dolomites there was an original primary
mineral association consisting mainly on galena (PbS), sphalerite (ZnS), pyrite
(FeS2) and barite (BaSO4),
among others. This type of sedimentation is comparable to that found in other
Alpine dolomite rock areas.
2) Towards the end of the Alpine metamorphism dolomite
was altered by hydrothermal solutions with high contents of As, Cu, Ag, Tl and
Sb, among others. This solution reacted with the aforementioned primary minerals
thus forming these wide range of sulfosalts, according to the following general
equation: x MS + AsySz ----> MxAsySz.
The first of all the minerals those were formed, they were the poorest in arsenic
(As) (the first was jordanite) and as the dolomite was increasing
its As percentage (by the action of the hydrothermal solution ) were formed the
most As mineral-rich series. This whole process was going on until the minerals
came to the saturation of arsenic; then began to crystallize sulfides of arsenic,
like realgar (AsS) and orpiment ( As2S3).
This explains the fact that all minerals with a similar arsenic content are collected
in the same mine area. Similarly, in a given sample you will never find together
realgar and galena, because a Pb and As sulfosalt should be formed by reaction
of the two components.
Since the content of arsenic (As) in the hydrothermal solution was much higher
than antimony (Sb), the solution became oversaturated in arsenic much earlier
than in antimony. This may be why when we left away LGB quarry there are only
arsenic low content minerals because part of this element has been replaced by
The Hydrotermal Solution
The origin of the hydrothermal solution could be associated to an old hercinian
deposit in the gneiss, in the southern part of Binntal. In this region were
found important ores of Cu and As. The primary minerals were mainly chalcopyrite
and tennantite. Due to the intense Alpine metamorphism of this deposit, As and
Cu minerals were strongly altered; all its elements were dissolved and their
constituents (Cu, As, Sb, Fe, S) were dragged by the hydrothermal solution. Primarly
this solution was directed towards the north, where it found the dolomite. Then
the chemical reaction started leading to the formation of the sulfosalts.
In one hand, it must be noted that there are major differences between the classic
geochemical area and the today working level, discovered in 1985 and operating
since 1988. First, in the traditional outcrop for sulfosalts samples are showing
individual crystals and in one sample all the crystals have a similar chemical
composition. Accordingly, it was not difficult to separate homogeneous material
for a correct identification by X-ray diffraction. By contrast, in the new working
level completely different chemical composition minerals can be found intimately
mixed making it difficult to obtain material for an X-ray diffraction. We must
use SEM-EDS methods. On the other hand, in the new level work we call your attention
in the presence of mercury and tin minerals, two metals ever found in the classic
Finally it is worth commenting on a study done in order to determine the age
of sulfosalts of Lengenbach (Purdy and Stalder, 1973). It is determined the age
of an adularia from Lengenbach by the method of the potassium-argon and the result
was 11.000.000 years. As the adularia was crystallized before sulfosalts,
this age of eleven million years is the maximum age for Lengenbach sulfosalts.