C. H. Behre Jr.
G. F. Loughlin
1934
<p><span>The </span>ore<span> </span>deposits<span> </span>in<span> the heart of the </span>Leadville<span> </span>district<span>, </span>Colorado<span>, are divisible, as shown by Emmons, Irving, and Loughlin,2 into two main classes on the basis of mineral composition: "contact-metamorphic" silicates accompanied by iron oxides, which adjoin an obscure igneous stock, and mixed sulphides accompanied by manganosiderite and quartz gangue, which cut the "contact-metamorphic" </span>deposits<span> and form a broad zone extending for 2 1/2 miles to the west and for shorter distances </span>in<span> other directions. A detailed consideration of the mineralogy of the mixed sulphide </span>ore<span> </span>deposits<span> clearly shows that they </span>in<span> turn are divisible into groups, referred to as hotter and intermediate mesothermal </span>deposits<span>. These groups, however, because of complications </span>in<span> structural control, are not ideally arranged from the standpoint of </span>zoning<span>.Emmons, Irving, and Loughlin noted that barite was a conspicuous gangue mineral </span>in<span> certain outlying </span>deposits<span> on the west slope of the Mosquito Range. The junior author during 1928-1931 had an opportunity to study the outlying </span>deposits<span> over an area of twenty square miles, which includes mines near the heads of Iowa Gulch and Empire Gulch, and the Weston Pass </span>district<span> (Fig. 1), and found that these baritic </span>deposits<span> form a somewhat distinct outer zone </span>in<span> which the primary </span>ore<span> consists of galena and light-colored zinc blende, with small amounts of silver and practically no gold, </span>in<span> a gangue of barite, jasperoid, and crystalline quartz. There is also recognized a still more remote though poorly defined zone </span>in<span> which the same sulphides are present but jasperoid is less conspicuous and barite is practically absent, whereas dolomite is the characteristic gangue mineral. The </span>deposits<span> </span>in<span> these two outer zones are fissure-fillings and comparatively small replacement bodies. They contrast strongly with the sulphide </span>deposits<span> </span>in<span> the heart of the </span>Leadville<span> </span>district<span> and are assigned to the outer mesothermal and epithermal zones. As they have not been described before, general features of these </span>deposits<span> are given </span>in<span> some detail, followed by their relations to the </span>deposits<span> within the </span>Leadville<span> </span>district<span>. Analogies are also drawn with similar </span>deposits<span> </span>in<span> other districts.</span>In<span> the preparation of this paper the authors have been impressed with the confusion that may result from too simple a conception of </span>zoning<span>. For a proper conception of </span>Leadville<span>, the geologist must have sufficient regard for structural conditions, must make due allowance for successive stages of mineral deposition within a small block of ground or even within a single </span>ore<span> shoot, and must also allow for the reactions between minerals formed at an early stage and solutions introduced during a late stage. The effects of such complications </span>in<span> the </span>Leadville<span> </span>district<span> might give the impression that certain minerals found </span>in<span> one of the inner zones were formed at relatively high temperatures, but close study of paragenesis indicates late deposition at relatively low temperatures. Again, as already stated, there is sufficient inconsistency </span>in<span> the distribution of certain varieties of </span>deposits<span> to give a confused impression of zonal arrangement unless the influence of structure on the circulation of </span>ore<span>-forming solutions is appreciated. The field facts give a far more complex picture than might be anticipated from a simple statement of the theory of </span>zoning<span>. </span></p>
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10.2113/gsecongeo.29.3.215
en
Society of Economic Geologists
Zoning of ore deposits in and adjoining the Leadville District, Colorado
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