Abstract

Panasqueira is a large quartz-dominated, W-Cu(Ag)-Sn-rich vein deposit in northeastern Portugal. It is associated with a postkinematic greisened S-type granite and is largely hosted by pelitic schists, previously regionally metamorphosed to greenschist facies grade (P fluid < 2 kbar, T < 400 degrees C).The main-stage Panasqueira ore-forming fluids had temperatures of 230 degrees to 360 degrees C, pressures of generally less than 500 bars, and 1 to 2 m NaCl (Kelly and Rye, 1979). A typical composition of the ore-forming fluid at 300 degrees C and 1 m NaCl is estimated to have been 0.2 m Sigma K, 0.0005 m Sigma Fe, 0.02 m Sigma Ca, 0.02 m Sigma Mg, pH = 5 to 6, log f (sub o 2 ) = -34 to -35, log f (sub s 2 ) = -12.5, log Sigma S = -2.5. Such a fluid is essentially in equilibrium with the granitic assemblage, quartz + K feldspar + albite + muscovite at around 400 degrees C. Readily leachable sulfides and wolframite within the subjacent granitic bodies are therefore possible major sources of Sn, As, Cu, Zn, and W in the ore veins.Calculated solubilities of wolframite and cassiterite in the described typical ore-forming fluid suggest W and Sn concentrations of around 0.2 and 5 ppm, respectively. Combined with estimates of the total mass of hydrothermally introduced W and Sn in and around the deposit, these calculations suggest a total ore-forming fluid flux of at least several hundred km 3 . Such large fluid fluxes would be difficult to achieve through the single-pass release of magmatic or metamorphic fluids. A model of ore formation involving a multiple-pass fluid flow system is therefore proposed.

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