A long-lived hydrothermal system in Bushveld granites at the Zaaiplaats tin mine; lead isotope evidence
A long-lived hydrothermal system in Bushveld granites at the Zaaiplaats tin mine; lead isotope evidence
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DOI：
10.2113/gsecongeo.88.1.27
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发表时间：
1993-02
期刊：
影响因子：
5.8
通讯作者：
N. McNaughton;P. Pollard;D. Groves;R. Taylor
中科院分区：
文献类型：
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作者：
N. McNaughton;P. Pollard;D. Groves;R. Taylor
Zaaiplaats 的锡矿化位于 Lebowa 花岗岩组的碱长石花岗岩中，该花岗岩侵入 Rashoop 花岗岩组。从上接触面向下，碱长石花岗岩包括边缘伟晶岩、细粒Lease花岗岩和粗粒Bobbejaankop花岗岩。主要矿化类型包括 Lease 花岗岩中的近水平、板状、低品位矿体、Bobbejaankop 花岗岩中的浸染性锡石矿化下部带以及两种花岗岩中的浅入式和分支管矿体。流体包裹体数据显示，岩浆热液系统从 >600 摄氏度到 200 摄氏度不断演化，在 400 摄氏度以上有锡石和白钨矿沉淀。矿物成分和氧同位素分馏表明，在温度通常在 200 摄氏度到 300 摄氏度之间的普遍蚀变过程中，花岗岩内的矿物质重新平衡。这些和其他物质的侵位年龄 布什维尔德花岗岩大约是。 2050 Ma，但 Zaaiplaats 不同矿化花岗岩体的 Pb-Pb 全岩等时线年龄显示出相对有限的范围，从 Rashoop 花岗岩的 961 + 或 - 129 Ma 到 Lease 花岗岩的 1187 + 或 - 51 Ma。因此，分析的样本套件一致地记录了大约 1 年的事件。花岗岩就位后。对它们的 Pb 同位素演化进行建模表明，每个系列在 2050 年到大约 2050 年之间独立演化。 1100 Ma，表明热液循环基本上限制在每个花岗岩体内和/或流体/岩石比率较低，并且从 2050 年到约 1100 Ma，每组内的总体 238 U/ 204 Pb对于分异花岗岩来说，1100 Ma 的温度并没有异常高或低，这表明在此期间每个套件中 U 和 Pb 的行为基本上是封闭系统。约。导致 Zaaiplaats 的 U-Pb 系统关闭的 1100 Ma 事件被解释为反映了与长期热液系统的抬升和冷却以及最新阶段矿物（例如方铅矿）在 200 至 300 摄氏度的沉淀相关的阻塞温度年龄。将 Zaaiplaats 的热液系统维持在 200 摄氏度以上所需的高稳态温度约 1100 Ma。 1 岁是由放射性衰变提供的。根据目前的 U、Th 和 K 2 O 浓度计算出的当前产热特性范围从 Rashoop Granophyre 的 4.2 mu W/m 3 到 Lease Granite 的 11.5 mu W/m 3 。在考虑了最近 U 和 Th 的急剧损失后，推断矿化 Bobbejaankop 花岗岩的预风化产热为 30.3 mu W/m 3 。低水平的热液压裂与稳定同位素和铅同位素证据相结合，有效排除了外部流体对流冷却作为 Zaaiplaats 的重要热损失机制。花岗岩内的流体路径主要由晶界、微裂缝和孔洞控制，这使得内部循环和升高的固相线温度能够在约 4 公里的安置后覆盖层的热覆盖层下无限期地维持。地形的抬升和侵蚀导致铅交换阻断温度降低约 100%。 1100马。
Tin mineralization at Zaaiplaats is hosted in alkali feldspar granites of the Lebowa Granite Suite which intrude the Rashoop Granophyre Suite. From the upper contact downward, the alkali feldspar granites include the marginal pegmatite, fine-grained Lease Granite and the coarse-grained Bobbejaankop Granite. The major mineralization styles consist of subhorizontal, tabular, low-grade orebodies in the Lease Granite, a lower zone of disseminated cassiterite mineralization in the Bobbejaankop Granite and shallowly plunging and branching pipe orebodies in both granites. Fluid inclusion data show a continuous evolution of the magmatic hydrothermal system from >600 degrees to 200 degrees C, with cassiterite and scheelite precipitation above 400 degrees C. Mineral compositions and oxygen isotope fractionations indicate reequilibration of minerals within the granites during pervasive alteration at temperatures generally between 200 degrees and 300 degrees C.The emplacement age of these and other Bushveld granites is ca. 2050 Ma, yet Pb-Pb whole-rock isochron ages for the different, variably mineralized granite bodies at Zaaiplaats show a relatively restricted range from 961 + or - 129 Ma for the Rashoop Granophyre to 1187 + or - 51 Ma for the Lease Granite. Thus the analyzed sample suites consistently record an event approximately 1 b.y. after the emplacement of the granites. Modeling their Pb isotope evolution indicates that each suite evolved independently between 2050 and ca. 1100 Ma, suggesting that hydrothermal fluid circulation was essentially confined within each granite body and/or fluid/rock ratios were low, and that the overall 238 U/ 204 Pb within each suite from 2050 to ca. 1100 Ma was not unusually high or low for fractionated granites, suggesting largely closed-system behavior of U and Pb within each suite during this interval. The ca. 1100 Ma event which caused closure of the U-Pb systems at Zaaiplaats is interpreted to reflect a blocking temperature age related to uplift and cooling of a long-lived hydrothermal system and precipitation of the latest stage minerals (e.g., galena) at 200 degrees to 300 degrees C.The high steady-state temperature required to maintain the hydrothermal system at Zaaiplaats at temperatures above 200 degrees C for ca. 1 b.y. was provided by radioactive decay. Current heat production characteristics calculated from present-day U, Th, and K 2 O concentrations range from 4.2 mu W/m 3 for the Rashoop Granophyre to 11.5 mu W/m 3 for the Lease Granite. Preweathering heat production of 30.3 mu W/m 3 is inferred for the mineralized Bobbejaankop Granite after accounting for dramatic Recent losses of U and Th. The low level of hydrothermal fracturing combined with stable and Pb isotope evidence effectively rules out convective cooling by external fluids as a significant heat loss mechanism at Zaaiplaats. Fluid pathways within the granites were controlled dominantly by grain boundaries, microfractures, and vugs, which allowed internal circulation and elevated subsolidus temperatures to be maintained indefinitely under a thermal blanket of an estimated 4 km of postemplacement cover. Uplift and erosion of the terrain caused cooling through the Pb exchange blocking temperatures at ca. 1100 Ma.