Page 29 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

Geophysical Investigations in the Lake Valley Area
By M.D. Kleinkopf, D.P. Klein, and R.A. Wise
Abstract
This study is part of a U.S. Geological Survey investigation of the geology and mineral deposits for the Bureau of Land Management designated Lake Valley Area of Critical Environ­ mental Concern. Analysis of aeromagnetic, gravity, and electri­ cal data provided information about depths of burial and configurations of faults, igneous intrusions, calderas, and rift basins in the area of the Lake Valley mining district.
The boundary of the Emory cauldron is well expressed in the aeromagnetic anomaly data by highs and linear anomalies that reflect igneous intrusions and faults in the structural wall of the ring complex of the cauldron. A corresponding 30-mGal Bouguer gravity low narrows to the southeast, reflecting a trough or graben of volcanic rocks. The proposed graben extended to the northwest and may have been the precursor and structural control for emplacement of the Emory cauldron.
Of particular interest were investigations of the distribution of geologic units and structure associated with the Lake Valley fault that forms the southwestern boundary for silver-manganese mineralization. Interpretations were based largely on geologic models developed from resistivity and gravity profiles. Compos­ ite layered-earth inversions (one-dimensional) of audiomagneto­ telluric measurements show discontinuities in the lateral distribution of resistivity units across the inferred projections of the Lake Valley fault southeast and northwest of the mining dis­ trict. Steep gradients along three Bouguer gravity profiles across the study area suggest fault throws of about 300 meters at loca­ tions inferred from geologic mapping.
Introduction
Aeromagnetic and gravity anomaly maps provided infor­ mation about the subsurface configuration and the areal distribu­ tion of geologic structure and lithology in the area of the Lake Valley mining district. Gravity modeling of three profiles across the area was used to prepare interpretive geologic sections show­ ing faults, volcanic features, rift basins, and deep structure in the lower crust and upper mantle (figs. 4–6). Although the promi­ nent aeromagnetic expressions associated with the Emory caul­ dron and the exposed igneous intrusions near Hillsboro appear to show a great deal of geologic information, it is beyond the scope of this report to make a detailed analysis of these anomalies.
Aeromagnetic and gravity anomalies result from juxtaposi­ tion of rocks of contrasting physical properties. Aeromagnetic anomaly data distinguish highly magnetic mafic rocks, such as
basalts, from weakly to moderately magnetized rocks, such as granites, hydrothermally altered rocks, and most sedimentary rocks. Aeromagnetic anomaly data generally provide more detail about shallow structure and lithology than gravity data because of the nearly continuous measurement of magnetic responses along flight traverses compared with the generally randomly distributed points of gravity observations at spacings that often exceed 3.2 km. Aeromagnetic anomaly data com­ monly exhibit definitive signatures of intrusions and faults asso­ ciated with ring structure and faulted volcanic rocks. Gravity anomaly data help identify large lithologic units and basins as well as major fault zones in the crust. For example, gravity lows may reflect granite plutons intruded into higher density terrane.
Results of earlier studies of magnetic and gravity anomaly data applied to geologic framework and mineral resource inves­ tigations that include the Lake Valley area have been reported by Adams and Keller (1994); Cordell (1983); Cordell and Grauch (1985), Kleinkopf (1997); and Schneider and Keller (1994).
Observations on Aeromagnetic and Gravity Anomalies
Aeromagnetic Anomaly Data
The aeromagnetic anomaly maps (figs. 1 and 2) were com­ piled from data purchased from the firm of Pearson, deRidder and Johnson, Inc. The survey was flown at line spacings of 0.53 km and at a mean terrane clearance of 152 m. The aeromagnetic maps in this report are at scales of 1:500,000 or smaller. Under the purchase agreement, the data cannot be published at scales larger than 1:500,000, or released in digital format. These data provide considerably more detail about the geology than the published data that were collected from surveys at higher alti­ tude and wider line spacings (Cordell, 1983).
The total-intensity aeromagnetic anomaly data (fig. 1) were reduced to pole. The reduced-to-pole map corrects anomaly locations for inclination of the Earth’s magnetic field and shifts anomaly centers over the causative sources. A residual total- intensity aeromagnetic anomaly map (fig. 2) was prepared by wave-length filtering (50 km high-pass) the reduced-to-pole magnetic data. The processing was done with software that uses fast Fourier transforms to convert aeromagnetic anomaly data in the space domain to the frequency domain (Hildenbrand, 1983). The filtering enhances definition of anomalies due to sources in the upper crust, on the assumption that anomaly wavelength is
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