Aluminous granite as a source magma of hydrothermal ore deposits; an experimental study

Abstract

Partition ratios of zinc and lead between synthetic granitic melts and aqueous phase were determined at 800øC and under PH•o = 3.5 _ 0.25 kb. Two starting glasses in the system SiO2-A12Oa-CaO-Na20 were prepared. Their bulk chemical compositions are similar, i.e., SiO2 content of 75.65 percent in glass 1 and of 74.68 percent in glass 2. Glass 1 is prepared to be slightly alkaline with 1.89 percent normative wollastonite and glass 2 is slightly aluminous with 2.42 percent normative corundum. The partition ratio of sodium between aqueous fluid and melt, K•a, can be expressed by the following equation over the whole range of chloride concentrations in the aqueous solution m v (mole/kg), regardless of the c• normative composition of the melt: K•a = 0.468 X m v ci. However, the partition ratio of zinc, K •' for the aluminous melt is more than two orders zll, of magnitude larger than that for the alkaline melt. K•. varies with the square of the chloride concentration in the aqueous phase and is satisfied by the following two equations: K r = 0.0833 X (alkaline melt 1). z,, = 9.40 X (m•::•) '• (aluminous melt 2) and K •' •m' • Zu I, CI] A similar contrast was also observed in the partition ratios of lead between aqueous phase and melts: K]•, = 2.26 X (m•:O '• (aluminous melt 2) and K•, = 0.0474 X (m•:•) • (alkaline melt 1). The log of the partition ratio shows an excellent inversely proportional relation with the pH (at 25øC) of the aqueous solution which was equilibrated with granitic melts and quenched. A slope of -2 is obtained for alkaline melt and is reasonably explained by the equation, PbO,,,,4t + 2H•+• .... + 2C1• .... = PbCI• ....... + H,20. The slope for the aluminous melt, -V•, may be explained only by complex reactions involving silicate ligands in the melt.The results indicate that only aluminous granite is capable of producing hydrothermal fluids which would contain a significant amount of ore metals. In other words, aluminous granite can be "ore-bearing" by postmagmatic processes. This is likely to happen when a granitie magma which originally was slightly alkaline changed its composition to slightly aluminous during diapiric uplift by assimilation of pelitic sediments. If this compositional evolution happened, then granitic magma would tend to store up base metals during the early stages of its evolution and release them very efficiently in the later stages to form a hydrothermal ore solution. Introduction IN 1972, Holland published a milestone paper titled "Granites, solutions, and base metal deposits." In the paper, he proved that hydrothermal solutions in equilibrium with granitic magmas can contain (a high concentration of) ore metals. He concluded (p. 292), "the classic hypothesis of the derivation of post-magmatic hydrothermal ore deposits from granitic magma is found to be in excellent health." However, the problem of so-called ore-bearing and barren granites is left unexplained. The present study investigates experimentally the problem in terms of a slight difference in the chemistry of granitic m...

Rare Earth Elements as an Indicator to Origins of Skarn Deposits: Examples of the Kamioka Zn‐Pb and Yoshiwara‐Sannotake Cu(–Fe) Deposits in Japan

“…Thus the chlorine content of the fluid phase may be critical to the REE contents of the magmatic fluid, as is like to other major, trace, and ore elements in hydrothermal fluids (e.g., Burnham, 1967;Holland, 1972;Urabe, 1985;Webster et al, 1989). The strong influence of pressure on K Cl (wt% Cl in fluid / wt% Cl in granitic melt) is well established for granitic systems (e.g., Kilinc and Burnham, 1972;Shinohara et al, 1989;Webster, 1992a).…”

“…Therefore it is assured that various conditions in granitic system decide the chlorine content in the hydrothermal fluid released from the granitoids, and in turn the chlorine content has a significant effect on the absolute content of REEs in the fluid. However, it is generally considered that magmatic fluids responsible for base metal ore deposits had relatively high chlorine contents (Holland, 1972;Urabe, 1985). Such magmatic fluids with high chlorine contents are expected to have general features including no Ce anomaly, a positive Eu anomaly, and a high (La/Yb) cn ratio, while absolute REE abundances may differ considerably.…”

Experimental study on tin partition between granitic silicate melt and coexisting aqueous fluid

“…./ Previous investigations on trace-element distribution have shown that melt polymerization (expressed as the number of non-bridging oxygen per tetrahedral cation (NBO/T)) has a distinct effect on the partitioning behavior of trace elements, including ore metals (Urabe, 1985;Lowenstern et al, 1993;Kohn and Schofield, 1994;Tang, 2003). The increase of alkali as network-modifiers in silicate melt can lead to an increase in the concentrations of NBO atoms and a decrease in the degree of melt polymerization (Jiang et al, 2003).…”

“…Many experimental studies have indicated that the formation of ores associated with magmatic rocks depends, to a great extent, on partitioning of metal elements between melt and fluid, which depends on the composition of melt, the composition of fluid derived from the melt and the physicochemical conditions under which the partitioning occurs (Holland, 1972;Feiss, 1978;Candela and Holland, 1984;Urabe, 1985;Candela, 1989;Keppler and Wyllie, 1991;Lowenstern et al, 1991;Peiffert et al, 1994;Candela and Piccoli, 1995;Chantal et al, 1996;Webster, 1992aWebster, , 1997Bai et al, 1998;Halter et al, 2002). Some experimental studies have revealed that different melts have distinct effects on the partitioning of Cu, Pb, Zn, U between melt and coexisting aqueous fluid (Chantal et al, 1996;Bai et al, 1998;Tang, 2003).…”

Controle polygenetique sur la localisation des gisements proximaux et distaux dans le champ polymetallique-stannifere a Xinlu Guangxi Chine meridionale

“…The peraluminous property of phase III granites is favourable for the partitioning of oreforming elements into hydrothermal fluids. The experimental data by Urabe (1985) reveal that the partition ratio of Zn (Znfi u id/Zn me lt) for aluminous melt (CIPW normative corundum is 2.42 %) is more than two orders of magnitude larger than that for the alkaline melt (CIPW normative corundum is 0 %), and a similar contrast was observed for the partition ratio of Pb. From his experiments he concluded that only aluminous granite is capable of producing hydrothermal fluids which contain a significant amount of ore metals while alkaline melt tends to retain these metals instead of releasing them into the hydrothermal fluids.…”