دانلود رایگان ترجمه مقاله تعیین متاسوماتیسم در کانسارهای طلا – نقره اپی ترمال – Geoscienceworld 2011
دانلود رایگان مقاله انگلیسی + خرید ترجمه فارسی | |
عنوان فارسی مقاله: |
تعیین متاسوماتیسم در کانسارهای طلا – نقره اپی ترمال: یک مطالعه موردی در منطقه Waitekauri، نیوزلند |
عنوان انگلیسی مقاله: |
Quantifying Metasomatism in Epithermal Au-Ag Deposits: A Case Study from the Waitekauri Area, New Zealand |
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مشخصات مقاله انگلیسی (PDF) | |
سال انتشار | ۲۰۱۱ |
تعداد صفحات مقاله انگلیسی | ۳۲ صفحه با فرمت pdf |
رشته های مرتبط با این مقاله | زمین شناسی |
گرایش های مرتبط با این مقاله | سنگ شناسی، زمین شناسی محیطی |
چاپ شده در مجله (ژورنال) | زمین شناسی اقتصادی – Economic Geology |
رفرنس | دارد ✓ |
کد محصول | F1137 |
نشریه | Geoscienceworld |
مشخصات و وضعیت ترجمه فارسی این مقاله | |
وضعیت ترجمه | انجام شده و آماده دانلود |
تعداد صفحات ترجمه تایپ شده با فرمت ورد با قابلیت ویرایش | ۱۳ صفحه با فونت ۱۴ B Nazanin |
ترجمه عناوین تصاویر و جداول | ترجمه شده است ✓ |
ترجمه متون داخل تصاویر | ترجمه نشده است ☓ |
ترجمه متون داخل جداول | ترجمه نشده است ☓ |
درج تصاویر در فایل ترجمه | درج شده است ✓ |
درج جداول در فایل ترجمه | درج شده است ✓ |
کیفیت ترجمه | کیفیت ترجمه این مقاله متوسط میباشد |
توضیحات | ترجمه این مقاله به صورت خلاصه انجام شده است. |
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چکیده |
بخشی از ترجمه |
چکیده |
بخشی از مقاله انگلیسی |
Abstract Abstract Major element geochemical exploration for epithermal deposits can extend the range of traditional pathfinder elements to a 1- to 10-km scale, and with knowledge of protolith composition, mass changes associated with hydrothermal alteration can be quantified. In the Hauraki goldfield of New Zealand, altered andesites and dacites host epithermal Au-Ag deposits and prospects. The major element compositions of equivalent unaltered rocks correlate with whole-rock Zr/TiO2, an immobile element ratio that is preserved during K metasomatism. We used this feature to estimate the initial composition and calculate a mass balance for veinless altered rocks in the Waitekauri area along a 3-km-wide section that extends from the central Waitekauri fault to the periphery of the alteration zone. The total transferred mass is equal to approximately 11 percent of rock mass in illite-dominated altered rocks, and 24 percent of rock mass in adularia-dominated altered rocks. On average mass losses exceed gains. Potassium was gained in most altered rocks, which contain illite and/or adularia as K-bearing hydrothermal minerals. Silica was gained in adularia-quartz−rich rocks close to the Waitekauri fault. Other major elements are preferentially lost (Ca, Na, Fe, Mg) or effectively immobile (Al, Ti). The greatest K and Si gains occur in adularia-rich rocks that surround Au deposits along the Waitekauri fault, whereas K gains are progressively lower and Si gains are mostly insignificant in deposits and prospects farther east where illite or interstratified illite-smectite is the dominant K-bearing mineral. In contrast, Na and Ca losses do not increase significantly from the periphery to the core of the Waitekauri area, because losses are commonly complete and therefore limited by the initial concentration. However, the K and Si gains correlate with other measures of K metasomatism including K/Sr and Rb/Sr values and molar (M) K/(K + Na + 2Ca) values, and together these parameters vector from the barren periphery to the orebodyhosting center of the Waitekauri area. In contrast to major element trends, the pathfinder elements As, Sb, and Hg define more local hydrothermal alteration cells within the larger Waitekauri area, some of which surround Au deposits. ۱٫ Introduction EPITHERMAL DEPOSITS are surrounded by altered host rocks that have exchanged elements with hydrothermal fluids. The magnitude of gains and losses is generally greatest close to epithermal veins, so a regional gradient in the magnitude of gains and losses can point to epithermal mineralization. Many studies have successfully used trace pathfinder elements, such as As, Sb, Hg, Tl and base metals, for vectoring toward mineralization over distances on the order of tens to hundreds of meters (e.g., White and Hedenquist, 1995; Carlile et al., 1998; Hedenquist et al., 2000). However, because path – finder elements are mainly confined to veins and adjacent wall rocks, major element mass changes in altered host rocks away from veins have the potential to significantly extend the range of geochemical exploration techniques up to several kilometers (Clarke and Govett, 1990; Madeisky, 1996; Sherlock, 1996; Warren et al., 2007). Nonetheless, major element mass change is difficult to quantify on concentration data alone because major element concentration must sum to 100 percent, which attenuates gains and losses, and compared to pathfinder elements, major element mass changes are small relative to the initial concentration. A mass-balance approach is required to accurately assess major element mass gain and loss, which in turn requires the composition of the protolith to be accurately known. Determining the protolith chemistry of altered rocks has been a hurdle in major element-based exploration techniques, because host rocks typically show incipient to intense alteration in the vicinity of orebodies, which changes their geochemistry, and they also show variable geochemistry away from orebodies; this simply reflects the heterogeneity of all rock units in the Earth’s crust but makes it difficult to select a single unaltered host-rock geochemical composition for use in mass-balance calculations (Madeisky, 1996; Sherlock, 1996; Shikazono et al., 2002; John et al., 2003; Leavitt and Arehart, 2005; Gemmell, 2007; Mauk and Simpson, 2007; Warren et al., 2007). MacLean (1990) presented a method to estimate the composition of heterogeneous protoliths to altered rocks on a sample-by-sample basis based on the bulk-rock Zr/TiO2 ratio, which allows for massbalance calculations of individual samples without the oversimplifying assumption of a single “representative” unaltered protolith. In this paper we use a variation on MacLean’s (1990) approach to assess mass changes in altered volcanic rocks that host gold mineralization in New Zealand. Epithermal Au-Ag deposits in the Hauraki goldfield, New Zealand, are predominantly hosted in calc-alkaline andesite and dacite of the Coromandel Group. The major element composition of unaltered Coromandel Group rocks correlates with SiO2 concentration, and the SiO2 concentration in turn correlates strongly with the ratio Zr/TiO2. The value of Zr/TiO2 has been used to characterize protoliths in a range of settings, including altered volcanic rocks in submarine- exhalative (Finlow-Bates and Stumpfl, 1981), altered plutonic rocks in a high-sulfidation Cu-Au deposit (Chambefort et al., 2007), metamorphosed sedimentary rocks (Hickmott and Spear, 1992), and igneous rock type in weathered rocks (Hallberg, 1984; Murphy and Stanley, 2007). Zirconium, Ti, and the other high field strength elements (HFSE), which have ionic potential values (ionic charge/radius (Å); Cartledge, 1928) between 3 and 12, are commonly found to be the most immobile elements during open-system alteration in many settings. The HFSE readily hydrolyze in solution and form hydroxides and oxides that are sparingly soluble (Baalen, 1993). Although many studies have found some mobility even of the HFSE during hydrothermal alteration, particularly at very high temperatures or at pH <2.5 (Jiang et al., 2005, and references therein), the HFSE generally represent the best approximation to an immobile element suite. Based on a reference data set of 98 fresh, unaltered Coromandel Group rocks, we establish linear regression equations that relate major element concentrations to Zr/TiO2. The protolith composition of individual altered Coromandel Group rocks can be estimated based on Zr/TiO2, because this ratio is relatively unaffected by metasomatic gains and losses of mobile elements. A mass balance can then be calculated for each rock sample. We test our approach on a suite of altered volcanic rocks that have been drilled along a 3-km-wide section, to a depth of several hundred meters; the suite includes rocks that vary from non- to intensely metasomatized. Large sample sets of this type are rare, and the current data provide an outstanding opportunity to critically assess the potential of major element-based whole-rock geochemical exploration. The Hauraki Goldfield and Hauraki Volcanic Region The Hauraki volcanic region was the main locus of subduction-related volcanism in northern New Zealand between 18 and 1.95 Ma and records the eruption of basalts, andesites, dacites and rhyolites (Skinner, 1986; Adams et al., 1994; Briggs et al., 2005). The volcanic succession erupted onto Mesozoic Waipapa terrane basement that is exposed in the northern Coromandel peninsula but progressively downfaulted to the south, where the Neogene volcanic succession completely covers the basement rocks. Regional fault sets strike north-northeast and northwest and have similar orientations to epithermal veins (Spörli et al., 2006). Contained within the Hauraki volcanic region is the Hauraki goldfield, a 100- by 40-km metallogenic province that contains approximately 50 epithermal adularia-sericite Au-Ag deposits and several porphyry Cu-Au-Mo occurrences. Although epithermal deposits occur in andesites, rhyolites, and basement rocks, over 97 percent of gold has been recovered from deposits hosted by altered andesites of the Coromandel Group (Christie et al., 2007). The Coromandel Group erupted between 18 and 2.5 Ma (Stipp, 1968; Adams et al., 1994) and comprises porphyritic basaltic andesites, andesites, and dacites with a phenocryst association of plagioclase ± augite ± hypersthene with minor magnetite, with olivine in some basaltic andesites, and commonly including hornblende (Skinner, 1986). Analogous to modern counterparts in the central North Island (Price et al., 2005), Coromandel Group rocks are mixtures of a relatively silicic (dacitic to rhyolitic) melt, represented by the groundmass, and a gabbroic crystal cargo, which commonly occurs as complexly zoned crystals, angular crystal fragments, and glomerocrysts.. |