The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, is the largest historical gold producer of the North American Cordillera. Gold mineralization is associated with shallowly dipping north-south veins hosted by the 160 Ma Grass Valley granodiorite to the southwest of the Grass Valley fault and steeply dipping east-west veins in accreted oceanic rocks to the northeast of this major fault. Quartz veins from both vein types show well-preserved primary textural relationships. Using a combination of petrographic and microanalytical techniques, the paragenetic sequence of minerals within the veins and the compositions of ore minerals were determined to constrain the mechanisms of quartz vein formation and gold deposition. The veins are composed of early quartz that formed through cooling of hydrothermal fluids derived from a geopressured reservoir at depth. The early quartz shows growth zoning in optical cathodoluminescence and contains abundant growth bands of primary inclusions. The primary inclusion assemblages and myriads of crosscutting secondary fluid inclusions have been affected by postentrapment modification, suggesting that early quartz formation was postdated by pronounced pressure fluctuations. These pressure fluctuations, presumably involving changes from lithostatic to hydrostatic conditions, may be related to fault failure of the host structure as predicted by the fault-valve model. Fluid flow associated with pressure cycling took place along microfractures and grain boundaries resulting in extensive recrystallization of the early quartz. Deposition of pyrite, arsenopyrite, and first-generation gold from these hydrothermal fluids causing recrystallization of the early quartz occurred as a result of wall-rock sulfidation. The gold forms invisible gold in the compositionally zoned pyrite or micron-sized inclusions within pyrite growth zones. The latest growth zones in euhedral quartz crystals that formed in association with this stage of the paragenesis contain very rare primary fluid inclusions that have not been affected by postentrapment modification. The hydrothermal system transitioned entirely to hydrostatic conditions immediately after formation of the latest quartz, explaining the preservation of the primary fluid inclusions. The formation of minor quartz in open spaces was followed by the deposition of second-generation native gold and telluride minerals that are commonly associated with base metal sulfides. Ore formation at this stage of the paragenesis is attributed to the rapid decompression of hydrothermal fluids escaping from the geopressured part of the crust into the overlying hydrostatic realm. There is no fluid inclusion evidence that this pressure drop resulted in fluid immiscibility of the hydrothermal fluids. Fluid inclusion evidence suggests that the north-south veins formed at a paleodepth of ~8 km, whereas the east-west veins appear to have formed at ~10 to 11 km below surface, confirming previous inferences that the NE-dipping Grass Valley reverse fault accommodated a large displacement. The findings of the study at Grass Valley have significant implications for the model for orogenic gold deposits, as the reconstruction of the paragenetic relationships provides evidence for the occurrence of two discrete events of gold introduction that occurred at different conditions during the evolution of the hydrothermal system.

中文翻译：

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造山型金矿床的共生：加利福尼亚草谷区矿化过程的新见解

位于加利福尼亚州中部内华达山脉山麓省的草谷造山带金矿区，是北美山脉最大的历史黄金生产国。金矿化与由160 Ma草谷花岗闪长岩向南倾斜的南北脉浅埋于草谷断层的西南部，以及向东向西倾斜的深海沉积岩中的东西向该大断层的东北部相关。两种脉型的石英脉均显示保存完好的主要纹理关系。结合岩相学和显微分析技术，确定了脉内矿物的共生序列和矿石矿物的成分，以限制石英脉形成和金沉积的机制。这些脉由早期的石英组成，这些石英是通过冷却深处的地压储层中的热液而形成的。早期石英在光学阴极发光中显示出生长带，并包含丰富的原生夹杂物生长带。夹杂后修饰影响了主要的包裹体组合和大量的横切次生流体包裹体，这表明早期的石英形成由于明显的压力波动而推迟了。这些压力波动，可能涉及从岩石静压到静水压条件的变化，可能与故障阀模型所预测的主体结构的断层破坏有关。与压力循环相关的流体流沿着微裂缝和晶界发生，导致早期石英广泛地重结晶。由于围岩硫化作用，从这些热液中析出了黄铁矿，毒砂和第一代金，导致了早期石英的重结晶。金在黄铁矿生长区域内按组成区域划分的黄铁矿或微米级夹杂物中形成看不见的金。与共生的这一阶段相关联的共面石英晶体的最新生长区包含非常稀少的初级流体包裹体，这些包裹体并未受到包埋后修饰的影响。最新的石英形成后，水热系统立即完全转变为静水状态，这说明了主要流体包裹体的保存。在开放空间中形成次要石英之后，沉积了通常与贱金属硫化物有关的第二代天然金和碲化物。共生阶段此阶段的矿石形成是由于从地壳的地压部分逸出到上覆静水力区域的热液迅速减压所致。没有流体包裹体证据表明该压降导致了热液流体的不溶混性。流体包裹体证据表明，南北脉形成于古深度约8 km，而东西向脉似乎形成于地表以下约10至11 km，这证实了先前的推断，即NE浸入的草谷逆断层。容纳了大位移。