Doade Pesqueira Property: Geology
Pegmatites on the Property
On the picture you see our geologists Tony Spat and Rolf Burkhardt inspecting one of many, Tantalum, Tin, Lithium & Cesium bearing pegmatites (about 10 meters wide) on the Doade-Presqueira property .
There are approximately forty pegmatite dykes exposed at surface.
The geochemical and mineralogical characteristic of the pegmatites, the internal structure and condition of emplacement classifies them as LCT (Lithium, Cesium Tantalum) rare element pegmatites.
Pegmatites occur as sharply bounded homogeneous to zoned bodies within metamorphic host rocks. Hard rock lithium deposits are in close relationship with granitic pegmatite dykes rich in spodumene, petalite and possible lepidolite. Within a pegmatite field, the enrichment of the different rare metals in the pegmatite dykes is a function of their distance from the cogenetic intrusion.
Scout drilling in 2003 in the southern portion of the property intersected pegmatite dykes situated furthest from the cogenetic source. Rare element pegmatites may host several economic commodities such as tantalum (Ta-oxide minerals), tin (cassiterite), lithium (spodumene and petalite), rubidium (lepidolite and K-Feldspar) and cesium (pollucite) collectively known as rare elements, and ceramic grade kaolin rich in feldspar, mica and quartz. Dyke attitude and shape is variable ranging from tabular (Presqueira region) to moderately dipping at 30 to 50 degree westerly toward the source granite in the southern portion. A vertical profile of the various dykes in the Presqueira area along traverse A-B extending southwesterly from SO-03-10 to PRE-05-06 is shown in Picture 4.
Mineralogy
The pegmatites are leucocratic and albite is the main mineral followed by quartz, potassium feldspar and muscovite. Petalite, spodumene and most of the accessory minerals are difficult to recognize macroscopically because of their white colour and fine grain size. Cassiterite, columbite-tantalite and beryl are more easily identified by their larger grain size and dark colour. Highest grades of tin and tantalum can be correlated in the same position within the dykes whereas the higher lithium grades (petalite, spodumene) may occur in the pegmatite dykes and/or as lepidolite? in the hanging wall/footwall schists near the contact. Cesium (pollucite?) grades are reported in the host mica schist near the pegmatite-schist contact. Additional analysis for rubidium coincided in place and grade with lithium with values reaching 2,200 ppm/1m (Spat, Burkhardt, 2004). Some of the minerals believed present on Alberta 1 are show in the figure aside.
Geological Setting of the Property
|
|
The property lies in part of a much larger structural unit of the Iberian Massive during the Hercynian Orogeny. The lower and oldest portion of the Massive hosts a wide asymmetrical syncline (Forcarei Synform) with a NNW-SSE striking fold axis (see map aside). This synform sequence is up to 7 km thick and is essentially of metasedimentary lithology identified as the Parano Group which hosts the rare mineral pegmatites on our property.
The intrusion of the fertile S type Cerdedo two-mica granite along the western boundary and the development of rare element pegmatite dykes into the basal units of the Forcarei Synform occurred in the third phase of the Hercynian Orogeny (Helsen, 2002). The Beariz Granite is situated along the lower third of the concession (Picture 1) outside the eastern boundary possibly hosting historic gold and tungsten mineralization and staked in part by I.P. No. 4790 (Jonas).
|
|
The rare element pegmatites are restricted to a belt about 200m to 300m wide and extend the length of the property (14 km) in a NNW-SSE direction conformable to the regional schistosity.
There are approximately forty pegmatite dykes exposed at surface in a belt 200 to 300m wide and 14 km long in the host Parano metasedimentary schist.
The rare element pegmatites occur as 'en echelon' dykes up to 11m thick and a strike length of up to 500 m. The exception is the sill-form pegmatite in the Presqueira area that dips gently to the southwest and is up to 13 m thick (Picture 2, 3, 4)
|
|
Deposit Type
The LTC rare element pegmatites are derived from a fertile S-type, peraluminous (Al rich) granite produced by partial melting of preexisting sedimentary source rocks (Parano Sediments). The granitic melt may crystallize into several different granitic units including the two-mica granite (Cerdedo Granite) located along the Alberta 1 western boundary.
The residual melt enriched in incompatible elements (Rubidium, Cesium, Niobium, Tantalum, and Tin) and volatiles (water, lithium, fluorine, beryllium and phosphates) from such a pluton can then migrate into the host rock and crystallize as pegmatite dykes.
|
|
The lithium, cesium, tantalum (LCT) type pegmatites are almost exclusively intruded into a compressive environment during collision of continental plates and mountain building.The degree of fractionation and rare-element content of the pegmatite dykes increases with increasing distance from the fertile parent granite. (picture 5)
The abundance of rare element lithophile elements in pegmatites is the result of classic crystallization selection of compatible versus incompatible trace elements in the magma melt and not the result of anomalously enriched source rocks from which the melt was formed as a result of eutectic melting.
|
|
The crystallization of granitic magma contains several percent dissolved water and as the magma cools, feldspar is the first mineral to crystallize leaving an increasing water rich magma containing rare elements such as lithium, beryllium and niobium which because of large ionic radius do not readily substitute in the main granitic minerals When this fraction is injected into the country rock a pegmatite dyke is formed. The pegmatite dykes situated furthest from the fertile granite source crystallize at a greater depth and tend to contain higher values in tin, niobium, tantalum, beryllium and lithium minerals.
These gross characteristics are readily apparent on the Alberta 1 concession with strong albitization of the dykes and increased fractionation from west to east away from the granite/host mica schist contact and a north to south increase in lithium, tantalum and tin mineralization due to greater distance from the granite/schist contact.
Pegmatites Definition
|
|
Pegmatites are an exceptionally coarse- to gigantic-grained igneous rocks. They occur as tabular dykes, sill, lenses or veins near the margins of plutons. They tend to have zoned or layered structures, extremely variable texture of mineral aggregates and some of them are enriched in rare elements. The shape and size of pegmatites vary greatly from linear, tabular bodies with straight edges to bulbous and irregular masses to turnip-shaped bodies. Pegmatites may be several meters long and less than 1 meter thick or as much as 3000 meters long to 700 meters wide.
|
|
Pegmatites are almost as old as the earth's crust. Pegmatites of Precambrian age (2.8 to 1.0 billion years) are the most abundant and widespread. These are generally found in the stable shields of Canada, Greenland, Europe, Russia and similar geologic environments. In contrast, some of the youngest pegmatites (roughly 20 to 5 million years) are found in the Himalaya mountains of Pakistan and Nepal.
Granitic pegmatites are important sources of rare-metals, such as, tantalum, tin, lithium, rubidium, cesium, beryllium, niobium and gallium. When present in economic quantities, these rare-metals may be extracted for use in a wide range of technological applications, such as electronic capacitors, jet engines and prosthetic devices (tantalum); ceramics, lithium-batteries, pharmaceutical products, lubricants, smelting of aluminum ore (lithium). Although alternative geologic sources are available, pegmatites remain a primary source of these rare-metals.(Picture 6,7)