The present research deals with the morphological, compositional and textural aspects of quartz and cassiterite crystals of the Mocambo Granite (MG) and of associated greisens bodies, belonging to the Velho Guilherme Intrusive Suite, Carajás Province, and its relation with the tin mineralization. The study was performed with the aid of scanning electron microscope (SEM), using catodoluminescence (CL) images, semiquantitative analyzes by energy dispersive spectroscopy (EDS) and by electronic microprobe (EM) analyzes. Different facies and greisenized rocks of the MG were studied and it was possible to identify five types of quartz, called Qz1, Qz2, Qz3, Qz4 and Qz5. The Qz1, considered the most earlier type and of magmatic origin, can be found in all facies, being less frequent in the greisens. It appears as
anhedral phenocrysts to luminescent sub-rounded (light gray), with varying degree of fracturing, as well as fine-to medium-grained crystals dispersed in the groundmass. Luminescent nuclei with alternating or reasorbed alternating light-dark zonations are common. Qz2 is posterior to Qz1 and not luminescent (dark gray); is present in all facies, but is rare in the greisen. It usually occurs as irregular discontinuous stains or filling fractures and shafts that section the Qz1, suggesting a process of intense replacement. The Qz3 does not show luminescence. It occurs in almost all facies filling a fracture that cuts Qz1 and Qz2. The
Qz4 is present in the most evolved and intensely altered rocks, in the greisen and in veins or interstitial cavities, usully associated with cassiterite crystals. It is represented by euhedral, medium-greined, slightly fractured crystals, with well-defined light-dark zoning and variable thickness. Qz5 occurs sectioning and forming irregular spots on Qz4, being associated generally with wolframite or wolframite + cassiterite in quartz veins. They are slightly fractured, luminescent, meduium-to-coarse greined anhedral crystals. Qz1 and Qz2 from porphyritic syenogranite to monzogranite facies show high Ti concentration (9.5 - 104 ppm)
and low Al (10 - 149 ppm). Qz1, Qz2 and Qz3 crystals from the aplitic alkali-feldspar granite facies presented slightly lower Ti contents (5 - 87 ppm) in comparison to SGMP quartz values and Al values that reach 2065 ppm. In the Qz1, Qz2 and Qz3 of the greisenized rocks, the Ti presented lower contents (0.0 and 62 ppm) and variable Al content (0 - 167 ppm). In the Qz4 crystals of the mineralized greisenized rocks in cassiterite, the Ti did not exceed 20 ppm, while Al presented strong enrichment, exceeding 3000 ppm. In the mineralized quartz veins in wolframite or wolframite + cassiterite, consisting mainly of Qz5, the Ti and Al concentrations presented generally low values, with maximum contents of 7 and 77 ppm, respectively. The cassiterite is occur as anhedral to subhedral fine-to coarse-grained crystals, anhedral to subhedral, associated to chlorite, muscovite, fengite and siderophyllite in gresenizeds rocks or commonly included in wolframite crystals in quatz veins. Shows light brown to reddish color and high colors of interference. More developed crystals show concentric zoning. Analyzes carried throug ME showed that in addition to Sn, cassiterites have lower concentrations of Fe, Ti, W, Nb and Mn traces. The concentrations of Fe, Nb, Ti and W are higher in the darker spots, while Sn shows higher purity in the lighter parts of the crystals. Cassiterites associated with Qz5 (hydrothermal) are often included in wolframite crystals or are partially substituted by it. This study showed that quartz was an excellent marker of the magmatic evolution and late alteration resulting from hydrothermal processes that operated in the Mocambo granite. It was possible to distinguish one magmatic and four hydrothermal types of quartz. The CL images indicate that the tin mineralization is present in the most evolved rocks and hydrothermally altered as in greisenized rocks and quartz veins, where the cassiterite is associated with Qz4 or Qz5 + wolframite. Qz5 suggests a possible mineralizing hydrothermal event of wolframite, subsequent to the origin of the cassiterite associated with Qz4.
|
