New gold occurrence in the Swiss Alps.
The Disentis region contains two remnants of the Variscan (Hercynian) orogeny within the younger Alpine orogeny: the Aar massif to the north and the Gotthard massif to the south. The first one is limited to the south by the Rhone-Rhine WSW-ENE axis, a major feature of the Swiss Alps. The Tavetsch crystalline massif and the sedimentary Garvera zone stretch between these two major Alpine basement massifs. The first one has been recognized as a separate tectonic unit and hosts the Disentis gold mineralization. The Medel gold mineralization is hosted in the crystalline Gotthard massif, in which most of the property is situated.
The first investigations of the Plattas Valley gold-showing, between 1987 and 1990, identified a sericite-quartzmuscovite schist that forms an altered-and-mineralized zone up to 250 m wide. During the summer of 1991, Micham carried out an exploration program including mapping and drilling to test the lateral and vertical extent of the Plattas showing. Outcrops allowed the zone of alteration-and-mineralization to be traced over 5 km. The mineralization consists of abundant sulphides--such as arsenopyrite, pyrite, and pyrrhotite within these schists. Grab- and channel-sample assays ranged from 0.02-0.23 oz/mt gold. Two diamond drill holes totalling 400 m were completed from the same set-up, at minus-45 degrees and minus-85 degrees, in order to test the zone. The first one intersected 0.04 oz/mt Au over a 75 m with a segment of 37 m with 0.05 oz/ mt. The second one intersected 0.02 oz/mt over 92 m. Some 0.04 oz/mt Au was intersected over 26 m within that second zone.
Continuous, uniform gold values were encountered in both drill holes, in a highly altered and mineralized sericite schist--over a true width of 70 m dipping 45-50 degrees south. Prospecting and mapping of outcrops traced the altered and mineralized zone laterally for over 5 km, Grab samples picked from outcrops in that zone consistently gave anomalous gold values (0.02-0.08 oz/mt Au) over significant widths (5-20 m).
Twelve thin sections, made from cores taken at various depths from both holes, were studied. The rock is a sericite schist whose mineralogical composition does not vary much from one sample to the other. The usual paragenesis is: sericite, quartz, and carbonate as principal minerals; feldspar, biotite, chlorite, zircon, and tourmaline as secondary ones. The three main minerals are found in all the sections; they from a mineralogical banding with alternation layers of scaly sericite and isogranular quartz and car n for the secondary minerals on y zircon and feldspar appear in almost all the sections. Two different biotite zones were defined, the first one from a few meters to approximately 35 m deep and the second one from approximately 120-180 m deep. These zones do not mean biotite is the main mica, it is also secondary to sericite.
Sulphide content of these schists does not seem to follow any geometric pattern, it ranges from less than 0.5% to 10% of total rock. Sulphides are mostly found in the sericite layers but are sometimes found disseminated throughout the whole rock. When biotite is present the sulphides seem to associate with it. Sulphide size is very inconsistent too, they can be smaller than 0.5 microns and some quite large specimens (up to a few millimeters) were observed as well. Texture of this rock is a result of the combination of scaly sericite layers and isogranular quartz and carbonate layers. These layers usually from a mineralogical banding which can be very irregular and in a few cases is absent.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||Exploration Roundup|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Jun 1, 1992|
|Previous Article:||Protecting rights.|
|Next Article:||New helmsman charting fresh course for Cyprus.|