K. A. Howard
2003
<p><span>The </span>deep<span> </span>crustal<span> rocks exposed </span>in<span> the </span>Ruby<span>-</span>East<span> </span>Humboldt<span> </span>metamorphic<span> </span>core<span> </span>complex<span>, northeastern </span>Nevada<span>, provide </span>a<span> </span>guide<span> for reconstructing </span>Eocene<span> </span>crustal<span> </span>structure<span> ∼50 km to the west near the </span>Carlin<span> trend of </span>gold<span> deposits. The </span>deep<span> </span>crustal<span> rocks, </span>in<span> the footwall of </span>a<span> west-dipping normal-sense shear system, may have underlain the Piñon and Adobe Ranges about 50 km to the west before Tertiary extension, close to or under part of the </span>Carlin<span> trend. </span>Eocene<span> lakes formed on the hanging wall of the fault system during an early phase of extension and may have been linked to </span>a<span> fluid reservoir for hydrothermal circulation. The magnitude and timing of Paleogene extension remain indistinct, but dikes and tilt axes </span>in<span> the upper </span>crust<span> indicate that spreading was </span>east<span>-west to northwest-southeast, perpendicular to </span>a<span> Paleozoic and Mesozoic orogen that the spreading overprinted. High geothermal gradients associated with </span>Eocene<span> or older </span>crustal<span> thinning may have contributed to hydrothermal circulation </span>in<span> the upper </span>crust<span>. Late </span>Eocene<span> eruptions, upper </span>crustal<span> dike intrusion, and </span>gold<span> </span>mineralization<span> approximately coincided temporally with </span>deep<span> intrusion of </span>Eocene<span> sills of granite and quartz diorite and shallower intrusion of the Harrison Pass pluton into the </span>core<span>-</span>complex<span> rocks. Stacked Mesozoic nappes of metamorphosed Paleozoic and Precambrian rocks </span>in<span> the </span>core<span> </span>complex<span> lay at least 13 to 20 km </span>deep<span> </span>in<span> </span>Eocene<span> time, on the basis of geobarometry studies. </span>In<span> the northern part of the </span>complex<span>, the presently exposed rocks had been even deeper </span>in<span> the late Mesozoic, to >30 km depths, before losing part of their cover by </span>Eocene<span> time. Nappes </span>in<span> the </span>core<span> plunge northward beneath the originally thicker Mesozoic tectonic cover </span>in<span> the north part of the </span>core<span> </span>complex<span>. Mesozoic nappes and tectonic wedging likely occupied the thickened midlevel </span>crustal<span> section between the </span>deep<span> </span>crustal<span> </span>core<span>-</span>complex<span> intrusions and nappes and the overlying upper </span>crust<span>. These structures, </span>as<span> well </span>as<span> the subsequent large-displacement Cenozoic extensional faulting and flow </span>in<span> the </span>deep<span> </span>crust<span>, would be expected to blur the expression of any regional structural roots that could correlate with mineral belts. Structural mismatch of the mineralized upper </span>crust<span> and the tectonically </span>complex<span> middle </span>crust<span> suggests that the </span>Carlin<span> trend relates not to subjacent deeply penetrating rooted structures but to favorable upper </span>crustal<span> host rocks aligned within </span>a<span> relatively coherent regional block of upper </span>crust<span>.</span></p>
application/pdf
10.2113/gsecongeo.98.2.249
en
Society of Economic Geologists
Crustal structure in the Elko-Carlin Region, Nevada, during Eocene gold mineralization: Ruby-East Humboldt metamorphic core complex as a guide to the deep crust
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