It is widely accepted that the complexation of Sn with Cl in magmatic-hydrothermal fluids is critical for the formation of endogenic Sn deposits. However, Sn abundance as high as 0.4 wt% has been found in some chlorine-poor but silicate-rich fluid inclusions associated with a Sn-mineralized pegmatite. To examine the role of silicate solutes in Sn mineralization, we measured the solubility of cassiterite in alkaline silicate-rich fluids by direct observation of single crystal cassiterite dissolution in a hydrothermal diamond anvil cell at 540–900 °C and 5.5–13.6 kbar. Experimental results indicated that a significant amount of cassiterite could be dissolved in both NaSiO–HO and NaAlSiO–HO fluids, and SnO solubility was positively correlated with the concentration of silicate solutes, the Na/(Si + Al) ratio of the fluid and temperature. In NaSiO–HO fluids at ∼680 °C with 24 to 45 wt% silicate solute, SnO solubility increased from 0.65 to1.67 wt%. In NaAlSiO–HO fluids at ∼715 °C with 21 to 52 wt% silicate solute, the solubility increased from 0.49 to 1.19 wt%. These values are comparable to cassiterite solubilities in HCl solutions and LiCO solutions, and are much higher than those in NaCl solutions, 0.1 mol/kg NaOH solution, and 14 wt% SiO solution. Raman spectra collected on fluids demonstrated the presence of silicate monomers and oligomers, and the complexation of Sn with these Si-(Al)-Na clusters or with (OH) could explain the observed high SnO solubility. Comparison with SnO solubility in peraluminous melt suggests that Sn can be significantly enriched in alkaline silicate-rich fluids. Modeling of progressive fluid-melt unmixing successfully reproduces the observed high Sn abundance in silicate-rich fluid inclusions. Alkaline silicate-rich fluids can therefore serve as an effective agent, alternative to chlorine-rich fluids, for Sn mobilization and mineralization, especially in pegmatite systems.

中文翻译：

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基于锡石溶解度实验的碱性富硅酸盐流体在锡矿化中的作用

人们普遍认为，岩浆热液中锡与氯的络合对于内生锡矿床的形成至关重要。然而，在一些与锡矿化伟晶岩相关的贫氯但富含硅酸盐的流体包裹体中，发现锡丰度高达 0.4 wt%。为了研究硅酸盐溶质在锡矿化中的作用，我们通过直接观察 540–900 °C 和 5.5–13.6 kbar 的热液金刚石砧池中单晶锡石的溶解情况，测量了锡石在富含硅酸盐的碱性流体中的溶解度。实验结果表明，NaSiO-H2O和NaAlSiO-H2O流体中均能溶解大量锡石，且SnO溶解度与硅酸盐溶质浓度、流体的Na/(Si+Al)比和温度呈正相关。 。在约 680 °C 且硅酸盐溶质含量为 24 至 45 wt% 的 NaSiO-H2O 流体中，SnO 溶解度从 0.65 wt% 增加至 1.67 wt%。在约 715 °C 且硅酸盐溶质含量为 21 至 52 wt% 的 NaAlSiO-H2O 流体中，溶解度从 0.49 wt% 增加至 1.19 wt%。这些值与锡石在 HCl 溶液和 LiCO 溶液中的溶解度相当，并且远高于 NaCl 溶液、0.1 mol/kg NaOH 溶液和 14 wt% SiO2 溶液中的溶解度。在流体上收集的拉曼光谱表明存在硅酸盐单体和低聚物，并且 Sn 与这些 Si-(Al)-Na 簇或与 (OH) 的络合可以解释观察到的高 SnO 溶解度。与过铝熔体中 SnO 溶解度的比较表明，Sn 在富含碱性硅酸盐的流体中可以显着富集。渐进式流体熔体分解模型成功地再现了在富含硅酸盐的流体包裹体中观察到的高锡丰度。因此，富含碱性硅酸盐的流体可以作为富氯流体的有效替代剂，用于锡的动员和矿化，特别是在伟晶岩系统中。