Abstract Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.

中文翻译：

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通过岩浆室模拟器建模揭示同化对硫化物饱和度的复杂影响：以美国明尼苏达州德卢斯复合体为例

摘要 围岩同化作用可导致岩浆中硫化物有效饱和，形成贱金属和贵金属硫化物矿床。由于缺乏合适的热力学建模工具，关于同化如何影响岩浆中硫化物饱和 (SCSS) 硫含量的详细评估一直很少。岩浆室模拟器 (MCS) 是第一个地球化学建模软件，它解释了经历补给-同化-分次结晶的开放岩浆系统中的热力学围岩相平衡。我们使用 MCS 对岩浆系统中的 SCSS 进行建模，该岩浆系统对应于明尼苏达州德卢斯复合体的鹧鸪河侵入体的母体熔体。这种侵入体在 troctolitic 和 notic 岩石中拥有几个 Cu-Ni 沉积物，这两者都显示了相邻弗吉尼亚组黑色页岩同化的证据。我们的模拟表明，如果母熔体含水（≥ 1 wt % H2O），则主要通过分馏结晶形成，而当母熔体 H2O 贫乏（≤ 0.14 wt % H2O）时，辉长岩占主导地位。从含水母体熔体形成 norite 需要约 20-30% 的黑色页岩部分熔体的选择性同化或停止块的整体同化。在分级结晶模拟中，增加母体熔体的 H2O 含量会降低 SCSS。在含水分步结晶方案中，由于橄榄石稳定性增强，FeO 从残余熔体中耗尽，SCSS 进一步降低。在同化模拟中，岩浆子系统中的残余熔体富含 SiO2、Al2O3、K2O 和 H2O 同时耗尽 FeO、MgO、CaO 和 Na2O。这些成分变化促进了硫化物饱和——这种影响在选择性而不是在整体同化情景中更为明显。微量元素模型，用作硫同化效率的代表，表明硫在选择性同化模拟中应表现为与围岩不相容的元素（DWR (S) ≤ 1），即富含早期同化的围岩。岩石流体和/或部分熔体，以满足德卢斯综合体的天然硫同位素标准。本体同化也可能足以有效地改变硫同位素组成，但它需要在岩浆中进行大量结晶，因此被认为不太可能是硫同化的主要过程。