The source of the mineralizing fluid in Archean greenstone-hosted orogenic gold deposits is widely debated, with the available geochemical and isotope data interpreted as reflecting the involvement of metamorphogenic fluid produced by devolatilization of greenstone belt metaigneous/metasedimentary rocks or granite-derived magmatic-hydrothermal fluids or both. Orogenic gold deposits form in complex geologic environment involving multiple fluid sources, reflected in the large variation in the B-isotope composition of tourmaline (δ¹¹B = −24.8‰ to +19.8‰). The δ¹¹B distribution of tourmaline from world-wide orogenic gold deposits define two peaks, one at ca. −15‰, and the other between −5 and 0‰.

In this study, we modelled the release of boron and associated B-isotope fractionation during prograde metamorphic dehydration of representative mafic and pelitic greenstone rocks using a mass balance approach and compared the results with the B-isotope composition of tourmalines measured globally from orogenic gold deposits. The results indicate that the boron content of greenstone belt metabasalts decreases from ∼26 ppm to ∼2.7 ppm by 530 °C while those in metapelites decrease from ∼60 ppm at 300 °C to ∼40 ppm by the terminal chlorite breakdown temperature of about 570 °C. In both mafic and pelitic assemblages, terminal chlorite breakdown reactions occurring during the greenschist to amphibolite facies transition (530–570 °C) release large amounts of ¹⁰B-rich fluids, which rapidly lower the δ¹¹B of the metamorphic fluid. The δ¹¹B of the fluids released from metabasalts and metapelites at these conditions are estimated to be about −14.2 to −10.8‰ and −5.7 to 0.0‰, respectively. Tourmalines precipitating from such metabasalt-derived fluid are expected to have δ¹¹B between −18.0‰ and −12.4‰ while those crystallizing from metapelite-derived fluid will have δ¹¹B between −9.5‰ and −1.6‰ for the temperature range of ore formation (300–550 °C) in orogenic gold deposits. Our calculated range of δ¹¹B for the metabasalt- and metapelite-derived metamorphic fluids matches closely with the two peaks in the δ¹¹B distribution of tourmaline from orogenic gold deposits. The δ¹¹B of metabasalt-derived fluid is highly variable (+2‰ to −15‰ between 450 °C and 610 °C) and strongly dependent on the metamorphic grade attained by the greenstone belt lithologies. The majority of measured tourmaline δ¹¹B values from orogenic gold deposits of all ages can potentially be explained by different peak metamorphic temperatures with a single fluid source from metabasalts, or by mixing between basalt-derived and pelite-derived metamorphic fluids, or both. Prior characterization of the metamorphic grade of the rocks and the temperature of mineralization are desirable before using the B-isotope composition of tourmaline for evaluating fluid source. The δ¹¹B (−14.0‰ to −3.3‰) of tourmalines measured by us from the Hutti and Kolar gold deposits as a case study can be explained by our modelled results with the isotopic variations attributed to mixing between metapelite- and metabasalt-derived fluids. Low Li concentrations in the tourmalines of Hutti and Kolar (avg. 21.7 and 28.0 ppm in Hutti and Kolar, respectively) further support a metamorphic origin for the hydrothermal fluid. The relatively higher V contents of the Hutti tourmalines (avg. 962 ppm) vis-à-vis the Kolar tourmalines (avg. 615 ppm) is suggestive of greater pelitic input in Hutti.

太古宙绿岩造山型金矿床中矿化流体的来源受到了广泛的争论，可用的地球化学和同位素数据被解释为反映了绿岩带变质/变质岩或花岗岩衍生的岩浆热液的脱挥发作用所产生的变质流体的参与。流体或两者兼而有之。造山金矿床形成复杂的地质环境涉及多个流体源，反映在电气石的B-同位素组合物中的大的变化（δ ¹¹ B = -24.8‰至+ 19.8‰）。的δ ¹¹从世界范围内的造山金矿电气石乙分布定义了两个峰，一个在约 -15‰，另一个介于-5和0‰之间。

在这项研究中，我们使用质量平衡方法对代表性的镁铁质和粉质绿岩岩石的正变质脱水过程中硼的释放和相关的B同位素分馏进行了建模，并将结果与​​从造山金矿床中全局测量的电气石的B同位素组成进行了比较。 。结果表明，绿岩带偏玄武岩中的硼含量在530°C时从〜26 ppm降低至〜2.7 ppm，而在亚氯酸盐的最终分解温度约为570时，变质岩中的硼含量从300°C的〜60 ppm降低至〜40 ppm。 ℃。在这两种基性和泥质组合，所述绿到闪岩相转变（530-570℃）期间发生的终端绿泥石击穿反应释放的大量¹⁰富B液，从而迅速降低δ ¹¹B的变质液。的δ ¹¹在这些条件下从玄武岩和释放泥质流体的B被估计为约-14.2至-10.8‰和0.0‰，分别-5.7。电气石从沉淀这样变玄武岩衍生流体预期具有δ ¹¹之间乙-18.0‰和-12.4‰，同时从泥质衍生流体那些结晶将具有δ ¹¹之间-9.5‰和-1.6‰B中的温度范围内的矿石造山金矿床中的岩层（300–550°C）。我们的计算δ的范围¹¹为metabasalt-和泥质衍生变质流体乙与在δ两个峰紧密匹配¹¹从造山金矿电气石乙分布。该δ ¹¹偏玄武岩衍生的流体的B高度可变（在450°C至610°C之间为+ 2‰至-15‰），并且强烈依赖于绿岩带岩性获得的变质等级。大多数测量电气石的δ ^11个B值从所有年龄的造山金矿可以潜在地通过不同的峰值变质温度与单一流体源从玄武岩，或由玄武岩衍生和泥质衍生的变质流体，或这两者之间的混合说明。在使用电气石的B同位素组合物评估流体源之前，需要先对岩石的变质坡度和矿化温度进行表征。该δ ¹¹由我们从Hutti和Kolar金矿中测得的电气石的B（-14.0‰至-3.3‰）可作为案例研究，通过我们的模拟结果来解释，同位素变化归因于变质岩和变玄武岩衍生的流体之间的混合。Li在Hutti和Kolar的电气石中的低Li浓度（分别在Hutti和Kolar中分别为21.7和28.0 ppm）进一步支持了热液的变质​​成因。相对于Kolar电气石（平均615 ppm），Hutti电气石的V含量相对较高（平均962 ppm），这暗示着Hutti的胶体输入量更大。