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.

The formation 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 antimony.

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 area.

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.