DIFFERENTIATION IN LAVAS OF THE KEWEENAWAN SERIES AND THE ORIGIN OF THE COPPER DEPOSITS OF MICHIGAN
DIFFERENTIATION IN LAVAS OF THE KEWEENAWAN SERIES AND THE ORIGIN OF THE COPPER DEPOSITS OF MICHIGAN
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KEWEENAWAN系列熔岩的分化和密歇根州铜矿的成因
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发表时间：
1951
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影响因子：
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通讯作者：
H. R. Cornwall
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文献类型：
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作者：
H. R. Cornwall
本文对密歇根铜矿区Keweenawan系列的5条熔岩流进行了研究和化学分析，特别注意：(1)熔岩流上部伟晶岩部分的化学和矿物学变化；(2)分化过程；(3)将单个流动中发现的岩石类型与构成熔岩系列的流动类型进行比较；(4)流体分异与铜矿成因的关系。导致气流分化的主要因素可能是：(1)以中等速度冷却，且主要从顶部冷却；(2)差动沉降晶体分选；(3)挥发物的迁移和局部浓度；(4)岩浆周期性对流翻转。对流可能在较薄的气流中不活跃，在较厚的气流中也可能不活跃。人们提出了不同的假设，一种假设对流翻转很重要，另一种假设并不重要。该系列中所有类型的流动都可以用绿岩流的分异产物来近似表示，表明所有的流动都可以从一种类似绿岩流的岩浆中分异，但可能比绿岩流的岩浆更具基性。在考虑铜矿床的成因方面，作者简要地回顾了文献，并提出了新的证据。矿床的形成模式或成因仍有待研究：(1)矿床为后成型，由下伏侵入岩上升的热溶液沉积而成。(2)矿床是同生和后生共同作用的结果。火山喷气孔、温泉或盐湖中铜的原始浓度被后来上升的火成岩喷发所重新加工。(3)杏仁核沉积主要是同生的，仅受后期上升溶液作用的适度改变。
Five lava flows in the Keweenawan series of the Michigan copper district have been studied and chemical analyses made, with special attention to: (1) chemical and mineralogic variations within the upper, pegmatitic portions of the flows; (2) processes of differentiation; (3) comparison of the types of rocks found within a single flow with the types of flows that make up the lava series; (4) the relation of differentiation in the flows to the origin of the copper deposits. The primary factors in the differentiation of the flows were probably: (1) cooling at a moderate rate, and predominantly from the top; (2) crystal sorting by differential settling; (3) migration and local concentration of volatiles; (4) periodic convective overturn of the magma. Convection currents probably were not active in the thinner flows and may not have been in the thicker. Alternate hypotheses have been presented, one assuming that convective overturn was important, the other that it was not. All types of flows in the series can be approximated by differentiation products of the Greenstone flow, indicating that all the flows could have been differentiated from a magma similar to, but perhaps slightly more mafic than, that of the Greenstone flow. In considering the origin of the copper deposits, the writer has briefly reviewed the literature and presented new evidence. The following modes or origin of the deposits appear still open to consideration: (1) The deposits are epigenetic, and formed by deposition from hot solutions ascending from an underlying intrusive. (2) The deposits are the result of a combination of syngenetic and epigenetic processes. Primary concentrations of copper in fumaroles, thermal springs, or playa lakes were reworked by later, ascending igneous emanations. (3) The amygdaloidal deposits are primarily syngenetic and have been only moderately modified by the action of later ascending solutions.