Geochemistry, petrologic evolution, and ore deposits of the Miocene Bodie Hills Volcanic Field, California and Nevada
Geochemistry, petrologic evolution, and ore deposits of the Miocene Bodie Hills Volcanic Field, California and Nevada
复制标题
加利福尼亚州和内华达州中新世博迪山火山田的地球化学、岩石学演化和矿床
DOI：
--
复制
发表时间：
2016
期刊：
影响因子：
--
通讯作者：
P. Vikre
中科院分区：
文献类型：
--
作者：
E. A. du Bray;D. John;B. Cousens;Leslie A. Hayden;P. Vikre
祖先瀑布岩浆弧的南段包括众多的火山田，其中横跨加州-内华达州边界、莫诺湖以北的博迪山火山田(BHVF)是最大的火山田之一，也是研究最充分的火山田之一。BHVF的幕式岩浆活动跨度约为900万年，约在15~6 Ma之间，岩浆产出最大的时期是约15.0~12.6 Ma和约9.9~8.0 Ma。在地壳中浅层储层上方，约有24个毗邻并聚的喷发中心形成了多座粗安岩层状火山和众多硅质粗安岩-流纹岩穹隆杂岩，其成分变化与观察到的斑晶相的分异相一致。BHVF岩具有高钾钙碱性成分，与厚大陆地壳下与俯冲有关的大陆边缘弧岩浆的生成一致。BHVF岩石中的放射性成因同位素比率差异很大，但表明存在一定程度的地壳来源；一些较原始单元的同位素比率与较贫化的地幔来源一致。BHVF岩石的年龄和全岩成分与同位素变化都没有很好的相关性。BHVF岩中斑晶的结构和成分与伴生的岩浆储集层以开放系统行为演化一致。许多BHVF喷发单元的广泛成分多样性特征证明了储集层补给和随后的不完全均一化。同时代喷发中心产物中显著的成分差异进一步表明，负责BHVF岩浆活动的中心是由小的、离散的、成分不同的和紧密分布的储集层构成的。BHVF火山岩中赋存石英-榴辉岩和石英-明矾石浅成热液金银矿床，矿床规模约3.4Moz。黄金和28毫兹。大量的白银已经被生产出来。火山岩和所含矿床广泛同时代，这表明伴生的岩浆是参与矿床成因的热源、流体和金属的来源。石英-榴辉石矿床的特征与在-250°C来自近中性pH流体的特征一致，而石英-明矾石系统的矿床在-250°C需要更多的酸性、氧化和富硫流体。经济上可行的贵金属矿床位于Bodie和Aurora地区的断层脉状矿床中。热水流体通过可渗透的火山碎屑矿床循环，但缺乏突出的构造管道，导致大片蚀变但未矿化的岩石。
Abstract The southern segment of the ancestral Cascades magmatic arc includes numerous volcanic fields; among these, the Bodie Hills volcanic field (BHVF), astride the California-Nevada border north of Mono Lake, is one of the largest (>700 km2) and most well studied. Episodic magmatism in the BHVF spanned about 9 million years between about 15 and 6 Ma; magmatic output was greatest between ca. 15.0 to 12.6 Ma and ca. 9.9 to 8.0 Ma. About two dozen contiguous and coalescing eruptive centers above middle- to shallow-crustal-level reservoirs generated several trachyandesite stratovolcanoes and numerous silicic trachyandesite to rhyolite flow dome complexes whose compositional variations are consistent with fractionation of observed phenocryst phases. BHVF rocks have high-potassium calc-alkaline compositions consistent with generation of subduction-related continental margin arc magmas beneath thick continental crust. Radiogenic isotope ratios in BHVF rocks vary considerably but suggest somewhat enriched, crustal sources; isotopic ratios for some of the more primitive units are consistent with more depleted, mantle sources. Neither age nor whole-rock compositions of BHVF rocks are well correlated with isotopic variations. Textures and compositions of phenocrysts in BHVF rocks are in accord with the associated magma reservoirs evolving via open-system behavior. Reservoir recharge and subsequent incomplete homogenization are evidenced by the broad compositional diversity characteristic of many BHVF eruptive units. Significant compositional diversity among the products of coeval eruptive centers further suggests that centers responsible for BHVF magmatism were underlain by small, discrete, compositionally distinct, and closely spaced reservoirs. Volcanic rocks of the BHVF host quartz-adularia and quartz-alunite epithermal gold-silver deposits, from which about 3.4 Moz. of gold and 28 Moz. of silver have been produced. The volcanic rocks and contained deposits are broadly coeval, which suggests that the associated magmas are the sources of heat, fluids, and metals involved in deposit genesis. Characteristics of the quartz-adularia deposits are consistent with derivation from near-neutral pH fluids at ?250 °C, whereas those of the quartz-alunite systems require more acidic, oxidized, and sulfur-rich fluids at temperatures <250 °C. Economically viable precious metal accumulations are in fault-hosted vein deposits in the Bodie and Aurora districts. Circulation of hydrothermal fluids through permeable pyroclastic deposits but lacking prominent structural conduits resulted in large areas of altered but unmineralized rock.