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首頁> 外文學(xué)位 >Orogenic gold formation and tectonic evolution of the Grass Valley gold district and temporal correlations of gold deposits in California.
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Orogenic gold formation and tectonic evolution of the Grass Valley gold district and temporal correlations of gold deposits in California.

機(jī)譯：加利福尼亞草谷金礦區(qū)的造山帶金形成和構(gòu)造演化以及加州金礦的時間相關(guān)性。

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With a total past production of 13 Moz of lode gold, the Grass Valley gold district of the Sierra Nevada foothills province is the historically most productive lode gold source in California. Despite its economic importance, an understanding of the broad processes controlling the gold formation is lacking. Two distinct vein sets are present in Grass Valley: a north-trending set (N-S veins) hosted by the Grass Valley granodiorite and an east-trending set (E-W veins) hosted within mafic-ultramafic rocks. Questions of how these relate to each other and if they are products of the same event or different events remain to be answered. Some of the previously published data are conflicting, and the timing of gold formation for the district seems inconsistent with previous interpretations of orogenic gold formation in the Cordillera of California, particularly when viewed relative to the much better studied Mother Lode belt in the southern Sierra Nevada. A geochemical and geochronological characterization of the ore-hosting granodiorite is also lacking.;The present study represents the first detailed modern study on the Grass Valley gold district. The research included a detailed microanalytical and geochronological study of the ore-hosting granodiorite and the orogenic quartz veins. It is shown the ore-hosting Grass Valley granodiorite was emplaced at 159.9 +/- 2.2 Ma (U-Pb zircon) at temperatures of nearly 800 °C and at paleodepths of approximately 3 km. It rapidly cooled to below 300 °C between 162-160 Ma (40Ar/39Ar hornblende and biotite). After crystallization, the intrusion underwent brittle fracturing concurrent with N-S vein formation. The hydrothermal fluids interacted with the granodiorite and formed monazite and xenotime as alteration products, permitting U-Pb geochronology. An age of 162 +/- 5 Ma for vein formation was determined for xenotime. This age is indistinguishable from the intrusive age, but must have occurred after the pluton was cool enough to undergo brittle fracturing. The hydrothermal monazite and xenotime have markedly different geochemical characteristics than magmatic phases. Magmatic monazite from the Grass Valley granodiorite has Th concentrations up to 11.6 wt.%, whereas the hydrothermal monazite has maximum Th concentrations of 0.2 wt.%. The REE profiles are also significantly different, including a strong negative Eu anomaly for the magmatic phases and no Eu anomaly for the hydrothermal phases. Therefore, despite this age overlap between magmatism and hydrothermal activity, they are not genetically related. This implies that the vein-hosted phases are not xenocrysts and also did not form from an evolving magmatic-hydrothermal system, but are instead formed by orogenic fluids. A second hydrothermal event formed the E-W veins at ~152 Ma, isolated in time from any regional magmatism.;In addition to the geochronological research on the ore-hosting granodiorite and the veins, a detailed paragenetic investigation was performed on the orogenic veins as they are remarkably undeformed. In contrast to typical orogenic gold deposits displaying textures indicating brittle-ductile deformation and recrystallization, those of Grass Valley only display minor brittle fracturing of quartz and pyrite. Optical microscopy and optical cathodoluminescence imaging revealed the presence of multiple generations of quartz characterized by different luminescence responses and concentrations of secondary fluid inclusion trails. Pyrite crystallized following quartz precipitation. Gold precipitates relatively late in the paragenetic sequence entirely independent of quartz and is found within fractures in quartz, in fractures and voids within pyrite, and intergrown with galena and mica. The time of gold mineralization is recorded in pyrite by a chemically distinct growth zone containing arsenic, and nickel and cobalt zones with pyrites found in the E-W veins hosted in mafic-ultramafic rocks. The formation of quartz due to adiabatic decompression indicates the importance of pressure fluctuations in vein formation. The correlation of elements derived from the fluid (Ag, As, and Au) and those from the host rock (Co, Ni, and Pb) indicate the importance of fluid reactions with the local host rock during mineralization.;Developing a regional scale view of gold mineralization in the Cordillera of California can help shape the understanding of how gold deposit formation relates to various stages in the late Mesozoic tectonic evolution of California. To constrain the timing of gold mineralization in the other major gold province of California, white mica was separated from samples from eight deposits. Four of these exhibited evidence for excess argon interpreted to result from intense deformation and (or) the presence of mineral inclusions. The other four samples had argon isotope age spectra that provided plateau ages of ~160-140 Ma. The maximum age corresponds to a major plate reorganization and initial gold mineralization in the Sierra Nevada. The minimum age corresponds to the initiation of the lateral offset of the Klamath Mountains westward of the Sierra Nevada and the active arc. This marks the termination of both hydrothermal activity and magmatism in the Klamath Mountains. However, orogenic gold formation within the Sierra Nevada foothills continued as it was still located on the active arc.
機(jī)譯：內(nèi)華達(dá)山脈山麓省的草谷金礦區(qū)過去的黃金總產(chǎn)量為13莫茲，是加利福尼亞州歷史上產(chǎn)量最高的金礦資源。盡管其具有經(jīng)濟(jì)重要性，但仍缺乏對控制金形成的廣泛過程的了解。草谷中有兩種不同的脈集：由草谷花崗閃長巖主持的北向脈（N-S脈）和在鐵鎂質(zhì)-超鎂鐵質(zhì)巖石中主辦的東向脈（E-W脈）。這些問題如何相互關(guān)聯(lián)以及它們是同一事件還是不同事件的產(chǎn)物，尚待回答。某些先前公布的數(shù)據(jù)相互矛盾，該地區(qū)金礦形成的時間似乎與加州科爾迪勒拉造山金礦的先前解釋不一致，特別是相對于內(nèi)華達(dá)山脈南部研究得更好的母親洛德帶而言。還缺乏對寄主花崗閃長巖的地球化學(xué)和地球年代學(xué)表征。;本研究代表了草谷金礦區(qū)的第一個詳細(xì)的現(xiàn)代研究。該研究包括對含礦花崗閃長巖和造山石英脈的詳細(xì)的微觀分析和年代學(xué)研究。結(jié)果表明，在近800°C的溫度和約3 km的古深度處，礦石儲藏處的Grass Valley花崗閃長巖放置在159.9 +/- 2.2 Ma（U-Pb鋯石）上。在162-160 Ma（40Ar / 39Ar角閃石和黑云母）之間迅速冷卻至300°C以下。結(jié)晶后，該侵入體經(jīng)歷了脆性破裂，同時形成了N-S脈。熱液與花崗閃長巖相互作用，并形成獨居石和異種時間作為蝕變產(chǎn)物，從而允許U-Pb年代學(xué)。確定了異種時間的靜脈形成年齡為162 +/- 5Ma。這個年齡與侵入年齡是無法區(qū)分的，但是一定是在巖釘冷卻到足以發(fā)生脆性破裂之后才發(fā)生的。水熱獨居石和異種時間具有與巖漿相明顯不同的地球化學(xué)特征。來自Grass Valley花崗閃長巖的巖漿獨居石的Th含量高達(dá)11.6 wt。％，而水熱獨居石的最大Th含量為0.2 wt。％。 REE分布也顯著不同，包括巖漿相的強(qiáng)烈Eu負(fù)異常和熱液相沒有Eu異常。因此，盡管巖漿作用和熱液活動之間存在這個年齡重疊，但它們在遺傳上并不相關(guān)。這意味著，這些脈管相既不是異晶相，也不是由不斷演化的巖漿熱液系統(tǒng)形成的，而是由造山流體形成的。第二次熱液事件在約152 Ma處形成了EW脈，及時與任何區(qū)域巖漿作用隔離開來;除了對賦礦花崗閃長巖和脈的地質(zhì)年代研究外，還對造山脈進(jìn)行了詳細(xì)的同生研究。沒有明顯變形。與典型的造山型金礦顯示出表明脆性韌性變形和再結(jié)晶的質(zhì)地不同，草谷的那些僅顯示出石英和黃鐵礦的較小脆性破裂。光學(xué)顯微鏡和光學(xué)陰極發(fā)光成像揭示了多代石英的存在，其特征在于不同的發(fā)光響應(yīng)和次生流體包裹體痕跡的濃度。石英沉淀后黃鐵礦結(jié)晶。金在共生序列中相對較晚地析出，完全不依賴于石英，并且在石英的裂縫中，黃鐵礦的裂縫和空隙中發(fā)現(xiàn)，并與方鉛礦和云母共生。在黃鐵礦中，通過化學(xué)上截然不同的生長區(qū)記錄了金礦化的時間，該生長區(qū)包含砷，在鐵鎂質(zhì)-超拉菲鐵礦巖體的E-W脈中發(fā)現(xiàn)了鎳和鈷以及黃鐵礦。由于絕熱減壓而形成的石英表明在靜脈形成中壓力波動的重要性。流體中的元素（Ag，As和Au）與基質(zhì)巖石中的元素（Co，Ni和Pb）之間的相關(guān)性表明了礦化過程中與局部基質(zhì)巖石進(jìn)行流體反應(yīng)的重要性。加利福尼亞州科迪勒拉的金礦化可以幫助人們理解金礦床形成與加利福尼亞中生代晚期構(gòu)造演化的各個階段之間的關(guān)系。限制加利福尼亞其他主要黃金省的金礦化時間，從八個礦床的樣品中分離出白云母。其中有四個證據(jù)表明過量氬氣被認(rèn)為是由于劇烈變形和（或）礦物包裹體的存在所致。其他四個樣品具有氬同位素年齡譜，可提供約160-140 Ma的平穩(wěn)年齡。最大年齡對應(yīng)于內(nèi)華達(dá)山脈的一次主要板塊重組和最初的金礦化。最小年齡對應(yīng)于內(nèi)華達(dá)山脈西側(cè)的克拉馬斯山脈的橫向偏移和活動弧線的開始。這標(biāo)志著克拉馬斯山脈的熱液活動和巖漿活動都終止了。但是，內(nèi)華達(dá)山脈山麓內(nèi)的造山金礦繼續(xù)形成，因為它仍位于活動弧線上。

著錄項

  • 作者

    Taylor, Ryan D.;

  • 作者單位

    Colorado School of Mines.;

  • 授予單位 Colorado School of Mines.;
  • 學(xué)科 Geology.;Geochemistry.
  • 學(xué)位 Ph.D.
  • 年度 2015
  • 頁碼 164 p.
  • 總頁數(shù) 164
  • 原文格式 PDF
  • 正文語種 eng
  • 中圖分類
  • 關(guān)鍵詞

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