U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Principal facts for gravity profiles collected near the Osgood Mountains and the Slumbering Hills, north-central Nevada by V. J. S. Grauch and Robert P. Kucks U. S. Geological Survey MS 964, Federal Center Denver, CO 80225-0046 Open-File Report 97-85 1997 This report is preliminary and has not been reviewed for conformity with U. S. Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U. S. Geological Survey. These data have been approved for release and publication by the Director of the USGS. Although these data have been subjected to rigorous review and are substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, they are released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. The digital data, text of this report (figures in separate files) can be downloaded via 'anonymous ftp' from a USGS system named greenwood.cr.usgs.gov (136.177.21.122). The files are located in a directory named /pub/open-file-reports/ofr-97-0085 and are described in an ASCII file named readme.txt. The data contact is: V. J. S. ("Tien") Grauch phone: 303-236-1393 email: tien@usgs.gov U. S. Geological Survey Box 25046, MS 964 Federal Center Denver, CO 80225-0046 ABSTRACT This report presents principal facts for gravity stations collected along profiles near the Osgood Mountains and Slumbering Hills, north- central Nevada. These include (1) data collected near the Osgood Mountains by U. S. Geological Survey (USGS) personnel in the years 1989, 1990, and 1993; and (2) data released to the USGS by Battle Mountain Gold (now Battle Mountain Exploration) that were collected in 1989 near the Osgood Mountains and the Slumbering Hills. The digital data, text of this report (figures in separate files) can be downloaded via 'anonymous ftp' from a USGS system named greenwood.cr.usgs.gov (136.177.21.122). The files are located in a directory named /pub/open-file-reports/ofr-97-0085 and are described in an ASCII file named readme.txt. This information is also contained below in Table 1. INTRODUCTION This report presents principal facts for gravity stations collected along profiles in the Getchell trend area, Osgood Mountains, north-central Nevada. These include (1) data collected near the Osgood Mountains by U. S. Geological Survey (USGS) personnel in the years 1989, 1990, and 1993; and (2) data released to the USGS by Battle Mountain Gold (now Battle Mountain Exploration) that were collected in 1989 near the Osgood Mountains and the Slumbering Hills. The gravity data collected by the USGS were intended to supplement studies related to the USGS Getchell trend airborne geophysics demonstration project, which was most active during the years 1988-1992. The Getchell gold trend is located on the east side of the Osgood Mountains and was originally so-named due to the north-northeast alignment of five sedimentary-hosted disseminated gold deposits (often called Carlin-type deposits): Preble, Pinson, Mag, Getchell, and Twin Creeks mines (Figure 1). The airborne demonstration project was designed to employ a diverse set of geophysical and remote- sensing techniques to test the application of integrated and comprehensive airborne geophysical surveying for mineral exploration or assessment in covered terranes. Using a diversity of data-acquisition techniques helps optimize constraints on subsurface interpretations as opposed to using only one technique. A desire for further diversity of techniques in the Getchell trend demonstration project led the USGS to acquire gravity data to complement the airborne geophysical data. Gravity methods are designed to determine variations in the Earth's gravity field that are due to differences in bulk densities of the rocks in the subsurface. In the Getchell trend area, the most significant density differences occur at basin-and-range faults that juxtapose low-density alluvium against moderate- to high-density bedrock. Thus, gravity data can provide excellent guides for detecting and defining these types of faults in this area. Prior to the initiation of the Getchell trend demonstration project, gravity data acquisition had been limited. During the project, gravity data were collected along profiles selected to match several individual flight lines in the airborne aeromagnetic/electromagnetic survey. These lines were flown specifically to span longer distances on either side of the main survey area (Grauch and Bankey, 1994). Acquiring gravity data at a spacing to give coverage comparable to the coverage of the airborne data would have been prohibitively expensive. The profiles collected for Battle Mountain Gold were part of the company's exploration program. Ground-based electrical, magnetic, and gravity data collected before the airborne surveys were flown are presented in Abrams and others (1984), Heran and Smith (1984), Heran and McCafferty (1986), and Hoover and others (1984; 1986). The gravity data reported in Abrams and others (1984) are partly superseded by the profile data presented here (discussed below). The Getchell digital airborne geophysical data were originally released through the U.S. Department of Commerce's National Geophysical Data Center in February, 1989. Digital grids of many of these data sets are contained in Grauch and others (1993), along with interpretative grids and descriptions of the survey specifications and data files. Other publications related to the airborne geophysical demonstration project include Hoover and others (1991), who reviewed preliminary results of all the airborne surveys; Pitkin, (1991, gamma-ray data); Grauch and Bankey, (1991, aeromagnetic data); and Pierce and Hoover (1991, electromagnetic data). Maps at 1:100,000 scale and brief discussions of the aeromagnetic and electromagnetic data are presented in Grauch and Bankey (1994) and Wojniak and others (1994), respectively. Further discussions of the data are contained in Grauch and others (1991), Wojniak and Hoover (1991), and Grauch and Hoover (1993). DATA COLLECTION Gravity measurements were acquired along profiles IP (Iron Point), FMG (FirstMiss Gold), and OC (Osgood Creek) by the USGS in 1989; along profile SR (south regional) by the USGS in 1990 and 1993; and along profiles LB1, LB2, LB3, FM2, and FM3 by Mining Geophysical Surveys for Battle Mountain Gold Company in 1989. Locations of these profiles are shown in Figures 2 and 3. Data descriptions for the profiles collected by the USGS and those collected by Mining Geophysical Surveys are described separately below. USGS Profiles Data for profiles IP, FMG, and OC were acquired using LaCoste- Romberg gravity meter G-2 in 1989. Data for profile SR were collected in 1990 using LaCoste-Romberg gravity meter G-550 and in 1993 using LaCoste-Romberg gravity meter G-191. The hiatus in data collection of profile SR was caused by equipment failure and scheduling difficulties. Locations along all profiles were surveyed using a Hewlett-Packard total station. The profile lines were initially planned to match as closely as possible the location and orientation (about N58oW) of the extra-long flight lines in the airborne/electromagnetic survey (Grauch and Bankey, 1994). Reference positions along the profiles were determined by triangulation from points with known control, such as benchmarks, section corners, and surveyed drill-hole locations. The locations of individual stations were determined by turning horizontal and vertical angles from a reference point. Back-surveying the locations of stations was not attempted because of the considerable logistical difficulties involved and because the flat terrain of the area lessened the need for the accurate positioning that is usually required for terrain corrections during gravity data reduction. Distances between stations were generally 305 m (1000 feet). The gravity stations were referenced to the U.S. Department of Defense base station ACIC0474-1 at Winnemucca, Nevada (figure 4), which is part of the International Gravity Standardization Net, 1971 (International Association of Geodesy, 1974). An additional temporary base, tied to the primary base, was set for convenience of operation. Linear meter-drift was calculated by making repeat observations at the primary or temporary base. Profile FMG is generally located along a portion of the same profile as reported in Abrams and others (1984; stations 112-132). Comparison of gravity data from the two different periods of data collection at nearly coincident stations show differences as much as 0.5 mGal, which can present some difficulties in data processing and interpretation. These differences may be caused by inaccuracies in the locations of the stations collected by Abrams and others; they located these stations by inspection of 1:24,000-scale topographic maps and by distance estimates made in reference to power poles along the road (G. Abrams, oral commun., 1993). Therefore, the more recently acquired data are judged to supersede the earlier data along the extent of the FMG profile because the positions of the more recent data were measured more accurately. Company Profiles Profiles LB1, LB2, and LB3 near Lone Butte, profiles FM2 and FM3 near Iron Point (figure 2), and profiles SH1 and SH2 near the Slumbering Hills (figure 3) were collected by Mining Geophysical Company in 1989 using LaCoste-Romberg gravity meter G-849. Location maps and original records of meter readings, including station elevation and location, were obtained by the USGS from Battle Mountain Gold Company in 1990. Information on how the station positions and elevations were determined was not available. The gravity stations were referenced to the U.S. Department of Defense base station ACIC0474-1 at Winnemucca, Nevada (figure 4), which is part of the International Gravity Standardization Net, 1971 (International Association of Geodesy, 1974). An additional temporary base, tied to the primary base, was set for convenience of operation. Linear meter-drift was calculated by making repeat observations at the primary or temporary base. ELEVATION CONTROL FOR THE USGS DATA The station elevations for the USGS data were obtained by turning vertical angles from known control using the HP total station. Known control points were primarily referenced to accurately surveyed elevation points on 1:24,000-scale topographic maps. Using this technique, elevations may have errors generally within 2 feet (60 cm) from the known control points depending on climatic conditions. However, errors in the estimation of terrain corrections give rise to the greatest uncertainty in Bouguer anomaly values. Computer-generated terrain corrections for flat areas, as is the case for most of the stations in this report, are generally negligible. Where the stations are located within more moderate relief near the Osgood Mountains, the terrain was digitized from 1:24,000 scale topographic maps. The error in elevation from these maps is 10 feet (3 m), half the contour interval. DATA REDUCTION Computer programs existing at the USGS were used to obtain principal facts and terrain-corrected gravity values from all the profile data. A program written by M. Webring and R. Wahl (USGS, unpub. program, 1983) was used to reduce data from the IP, FMG, and OC profiles. An updated version of this program, written by M. Webring (unpub. program, 1996), was used to reduce data from the rest of the profiles. These programs reduced the gravity meter-readings to observed- gravity values by calculating and correcting for earth-tide and linear meter-drift, and computed free-air and Bouguer anomalies using the vertical gradient and curvature equations given by Cordell and others (1982) and in the computer program BOUGUER (Godson, 1988). The theoretical gravity value was calculated using the 1967 formula of the Geodetic Reference System (International Association of Geodesy, 1971). Outer-zone and some inner-zone terrain corrections were computed using a program that corrects for the gravity effects of terrain from a radius of 0.865 km to a radius 166.7 km away from each station using the method of Plouff (1977). These computed terrain corrections use mean-elevation data digitized on a 15-second grid for corrections from 0.865 to 5 km, 1-minute terrain data for corrections from 5 to 21 km, and 3-minute terrain data for corrections from 21 to 166.7 km. A density of 2.67 g/cm3 was used to calculate terrain corrections, giving the corrections and gravity anomaly values listed in Appendix A. For stations along profiles IP, FMG, and OC near steep topography, inner-zone (0 to 0.865 km) corrections were calculated using the method of M. Webring (USGS, unpublished, 1984). This method incorporates the minimum-curvature gridding algorithm of Briggs (1974) to define the topographic surface close to the station using hand-digitized data and calculates the gravity effects of small cylindrical sections of the Hammer zones using the method of Olivier and Simard (1981). DIGITAL FILE DESCRIPTIONS The digital data and text of this report (figures in separate files) can be downloaded via 'anonymous ftp' from a USGS system named greenwood.cr.usgs.gov (136.177.21.122). The files are located in a directory named /pub/open-file-reports/ofr-97-0085 and are described in an ASCII file named readme.txt. This information is also contained below in Table 1. TABLE 1. FILE DESCRIPTIONS FILE NAME FILE TYPE DESCRIPTION ----data files----- profiles.lis ascii text Listing of principal facts in 80-column format with labeled columns profiles.pos UNIX binary, Station identifier, longitude (degrees), sequential post file latitude (degrees), and 6 channels of (see below for principal facts (see below) for all description of format) profile data. profiles.asc ascii post file (see Same as profiles.bin but in ascii below for description FORTRAN format (a8, 8g16.8). of file format) -----figures----- fig1.eps fig2.eps encapsulated Figures 1, 2, 3, and 4 in encapsulated fig3.eps postscript postscript, no preview fig4.eps fig1.tif fig2.tif tagged image file Figures 1, 2, 3, and 4 in ".tif" fig3.tif format bitmap format fig4.tif fig1.prn fig2.prn postscript print Figures 1, 2, 3, and 4 in a format fig3.prn files to print on 300 dpi postscript fig4.prn printers ----text-------- readme.txt ascii text This file: Text describing digital files. text.txt ascii text A file containing the text of open-file report 97-85, without figures or Appendix A. text.prn postscript print A print file of the text of file open-file report 97-85, without figures, for 300 dpi postscript printers. POST FILE FORMAT USGS post file format consists of an eight-character station identifier, followed by eight real numbers, called channels. The channels for the post files associated with this report are described below. CHANNEL DESCRIPTION 1 Longitude in decimal degrees (west longitudes are negative) 2 Latitude in decimal degrees 3 Station altitude in meters above sea level 4 Observed gravity in mGal with a constant value of 980,000 mGal removed 5 Inner-zone terrain correction (zones A-F), in mGal 6 Outer-zone terrain correction (zones G-X), in mGal 7 Free-air gravity anomaly, in mGal 8 Complete (terrain-corrected) Bouguer anomaly, reduced using a density of 2.67 g/cc, in mGal ACKNOWLEDGMENTS We are grateful to Wayne Wojniak, who helped collect field data; to Mike Webring, for helpful suggestions on terrain corrections and references for gravity reduction equations; and to Battle Mountain Exploration, who graciously donated their gravity data. REFERENCES CITED Abrams, G. A., Moss, C. K., and Schutter, T. A., 1984, Principal facts for gravity stations in the Osgood Mountains, Humbolt County, Nevada: U. S. Geological Survey Open-File Report 84-835, 11 p. Briggs, I. C., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, no. 1, p. 39-48. Cordell, Lindrith, Keller, G. R., and Hildenbrand, T. G., 1982, Bouguer gravity map of the Rio Grande Rift, Colorado, New Mexico, and Texas: U. S. Geological Survey Geophysical Investigations Series Map GP-949, scale 1:1,000,000. Defense Mapping Agency, 1974, World Relative Gravity Reference Network, North America, Part 2: St. Louis, Missouri, Aerospace Center, DMAAC Reference Publication 25, with supplement updating gravity values to the International Gravity Standardization Net 1971, 1635 p. Godson, R. H., 1988, BOUGUER: gravity-terrain-correction program for micro-computers, version 1.0: U. S. Geological Survey Open-File Report 88-644B, Grauch, V.J.S., and Bankey, Viki, 1991, Preliminary results of aeromagnetic studies of the Getchell disseminated gold deposit trend, Osgood Mountains, north-central Nevada, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and Ore Deposits of the Great Basin, Symposium Proceedings: Geological Society of Nevada, Reno, NV, p. 781-792. Grauch, V.J.S., and Bankey, Viki, 1994, Aeromagnetic and related maps of the Getchell gold trend area, Osgood Mountains, north-central Nevada: U.S. Geological Survey Geophysical Map GP-1003-B, scale 1:100,000. Grauch, V.J.S., and Hoover, D.B., 1993, Locating buried conductive material along the Getchell trend, Osgood Mountains, Nevada: Implications for gold exploration and the carbon-gold association(?): U.S. Geological Survey Bulletin 2039, p. 237-244. Grauch, V.J.S., Hoover, D.B., and Wojniak, W.S., 1991, Subsurface structure and lithology near the Getchell gold trend, Osgood Mountains, Nevada-geophysical insights: U.S. Geological Survey Circular 1062, p. 34. Grauch, V.J.S., Phillips, J. D., Hoover, D. B., Pitkin, J. A., Livo, K. E., and McCafferty, Anne, 1993, Materials provided at the workshop "Geophysical Map Interpretation on the PC", convened April 21-22, 1993: U. S. Geological Survey Open-File Report 93-560, 150 p., 14 diskettes. Heran, W.D., and McCafferty, Anne, 1986, Geophysical surveys in the vicinity of the Pinson and Getchell mines, Humboldt County, Nevada: U.S. Geological Survey Open-File Report 86-432, 40 p. Heran, W.D., and Smith, B.D., 1984, Geophysical surveys at the Getchell and Preble disseminated gold deposits, Humboldt County, Nevada: U.S. Geological Survey Open-File Report 84-795, 78 p. Hoover, D.B., Grauch, V.J.S., Pitkin, J.A., Krohn, M.D., and Pierce, H.A., 1991, An integrated airborne geophysical study along the Getchell trend of gold deposits, north-central Nevada, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and Ore Deposits of the Great Basin, Symposium Proceedings: Geological Society of Nevada, Reno, NV, p. 739-758. Hoover, D.B., Pierce, H.A., and Abrams, G.A., 1984, Telluric traverse data release for the Getchell and Preble disseminated gold deposits, Humboldt County, Nevada: U.S. Geological Survey Open-File Report 84-846, 16 p. Hoover, D.B., Pierce, H.A., and Merkel, D.C., 1986, Telluric traverse and self potential data release in the vicinity of the Pinson mine, Humboldt County, Nevada: U.S. Geological Survey Open-File report 86-341, 26 p. International Association of Geodesy, 1971, Geodetic Reference System, 1967: International Association of Geodesy Special Publication No. 3, 116 p. International Association of Geodesy, 1974, The International Gravity Standardization net 1971: International Association of Geodesy Special Publication No. 4, 194 p. Olivier, R. J., and R. G. Simard, 1981, Improvement of the conic prism model for terrain correction in rugged topography: Geophysics, v. 46, no. 7, p. 1054-1056. Pierce, H.A., and Hoover, D.B., 1991, Airborne electromagnetic applications - mapping structure and electrical boundaries beneath cover along the Getchell trend, Nevada, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and Ore Deposits of the great Basin, Symposium Proceedings: Geological Society of Nevada, Reno, NV, p. 771-780. Pitkin, J.A., 1991, Radioelement data of the Getchell trend, Humboldt County, Nevada - Geological discussion and possible significance for gold exploration, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and Ore Deposits of the Great Basin, Symposium Proceedings: Geological Society of Nevada, Reno, NV, p. 759-770. Plouff, Donald, 1977, Preliminary documentation for a FORTRAN program to compute gravity terrain corrections based on topography digitized on a geographic grid: U. S. Geological Survey Open-File Report 77-535, 43 p. Wojniak, W.S., and Hoover, D.B., 1991, The Getchell gold trend, northwestern Nevada-geologic structure delineated by further processing of electromagnetic data collected during a helicopter survey: U.S. Geological Survey Circular 1062, p. 77. Wojniak, W.S., Hoover, D.B., and Grauch, V.J.S., 1994, Electromagnetic survey maps showing apparent resistivities of the Getchell gold trend, Osgood Mountains, north-central Nevada: U.S. Geological Survey Geophysical Map GP-1003-A, 3 plates, scale 1:100,000.