Geochemistry and Stable Isotope Composition of Altered Rocks at the Golden Cross Epithermal Au-Ag Deposit, New Zealand
Geochemistry and Stable Isotope Composition of Altered Rocks at the Golden Cross Epithermal Au-Ag Deposit, New Zealand
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DOI：
10.2113/gsecongeo.102.5.841
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发表时间：
2007-08
期刊：
影响因子：
5.8
通讯作者：
J. Mauk;M. Simpson
中科院分区：
文献类型：
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作者：
J. Mauk;M. Simpson
金十字金矿位于新西兰Coromandel半岛，是一个晚中新世火山岩型浅成热液型金银矿床，与石英±方解石±阿莫里亚±伊利石组合有关。含金和含银的石英脉产于蚀变强烈的流体和少量安山岩和英长岩组成的火山碎屑火山岩中。与其它与石英±方解石±阿胶石±伊利石组合相关的火山型浅成热液矿床一样，金十字矿区全岩地球化学的主要控制因素是热液蚀变对火成岩斜长石的破坏和热液流体的钾交代作用。定量质变计算表明，无论何种岩石类型（安山岩还是英安岩），热液蚀变岩在帝国脉系和脉网周围富集K、Rb、As和Sb。Na和Sr在矿脉周围富集，Fe局部富集。硅、钙、镁、钡的富集和失缺变化较大，其中硅在紧邻矿脉处富集，钙仅在局部富集。这些元素的富集和衰竭趋势与蚀变程度和蚀变矿物学有直接关系。强蚀变岩石中K和Rb的添加，Na和Sr的损失，反映斜长石被阿龙石和伊利石取代。钙的损耗也反映了斜长石的破坏，尽管钙的损失被热液方解石的形成局部抵消。远离脉体的Na和Sr越高，表明斜长石保存越好，边缘岩石蚀变越少。钾、铷、钠、锶的异常范围在脉东200 ~ 350米，比As和Sb的异常范围更广，As和Sb的异常范围在脉东150 ~ 200米。金十字的地球化学分带模式与其他浅成热液矿床的地球化学分带模式非常相似，这些浅成热液矿床具有石英±方解石±阿龙石±伊利石组合。钾、Na、Rb、Sr和Ca的富集和衰竭模式提供了对蚀变强度和类型的洞察，从而补充了传统的Au、Ag和其他探路者元素（如as和Sb）的分析，这些元素用于寻找浅成热液型金银矿床。在金十字，钾、Na、Rb、Sr和Ca的富集和枯竭模式比贵金属和其他微量元素探路者定义的晕更大，从而为勘探提供了更大的目标区域。钾交代作用尤为重要，因为帝国脉系发生在钾含量显著富集的蚀变岩中。钾交代作用可以通过原始的钾氧数据来评估，或者更有效地通过钾质量变化、(2Ca + Na + K)/Al与K/Al (wt % k2o * 100)/ppm Sr、Rb/Sr比和K数[K/(2Ca + K + Na)摩尔]来评估。金十字金矿的全岩氧同位素模式与其他浅成热液矿床的氧同位素模式相似，这些浅成热液矿床具有石英±方解石±阿莫里亚±伊利石组合。低δ 18o值定义的区域小于蚀变矿物野外填图或全岩主微量元素地球化学定义的区域。
Golden Cross is a late Miocene volcanic-hosted epithermal Au-Ag deposit associated with quartz ± calcite ± adularia ± illite assemblages located in the Coromandel peninsula, New Zealand. Gold- and Ag-bearing quartz veins occur in intensely altered flows and lesser pyroclastic volcanic rocks of andesitic and dacitic composition. The primary control of whole-rock geochemistry at Golden Cross, as at other volcanic-hosted epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages, is the destruction of igneous plagioclase by hydrothermal alteration and K metasomatism from hydrothermal fluids. Quantitative mass change calculations indicate that, irrespective of the rock type (andesite versus dacite), hydrothermally altered rocks are enriched in K, Rb, As, and Sb around the Empire vein system and stockwork. In contrast, Na and Sr are strongly depleted around the veins, with Fe locally depleted. Silicon, Ca, Mg, and Ba are variably enriched and depleted, with Si enriched immediately adjacent to the veins and Ca only locally enriched. The enrichment and depletion trends for these elements are directly related to the degree of alteration and the alteration mineralogy. Intensely altered rocks show addition of K and Rb, coupled with loss of Na and Sr, which reflect the replacement of plagioclase by adularia and/or illite. Calcium depletion also reflects the destruction of plagioclase, although Ca loss is locally offset by the formation of hydrothermal calcite. Progressively higher Na and Sr away from the veins reflect greater preservation of plagioclase and the transition into less altered rocks on the margins. Potassium, Rb, Na, and Sr all define anomalies that extend 200 to >350 m east of the veins and are more widespread than As and Sb, which are anomalous within 150 to 200 m of the veins. The geochemical zonation patterns identified at Golden Cross are very similar to those at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns provide insight into the intensity and type of alteration, thereby complementing traditional analyses of Au, Ag, and other pathfinder elements, such as As and Sb, that are used to explore for epithermal Au-Ag deposits. Potassium, Na, Rb, Sr, and Ca enrichment and depletion patterns at Golden Cross are larger than halos defined by precious metals and other trace element pathfinders, thereby providing a larger target area for exploration. Potassium metasomatism is particularly important because the Empire vein system occurs in altered rocks that are significantly enriched in K. Potassium metasomatism can be evaluated by raw K 2 O data or, more effectively, by K mass changes, molar element ratios of (2Ca + Na + K)/Al versus K/Al (wt % K 2 O * 100)/ppm Sr, Rb/Sr ratios, and K numbers [K/(2Ca + K + Na) molar]. Whole-rock oxygen isotope patterns at Golden Cross are similar to those described at other epithermal deposits associated with quartz ± calcite ± adularia ± illite assemblages. However, the area defined by low δ 18 O values is smaller than the area that can be defined by field mapping of alteration minerals or whole-rock major and trace element geochemistry.