Spatial Analysis of Geochemical and Geologic Information from the Tonopah 1 x 2 Quadrangle, Nevada

. U.S. GEOLOGICAL SURVEY BULLETIN 1878

Spatial Analysis of Geochemical and Geologic Information from the Tonopah 1 X2 Quadrangle, Nevada

By JOHN L. DWYER and J. THOMAS NASH

Experimental application of a geographic information system to the analysis and mapping of information used in a typical mineral resource assessment

U.S. GEOLOGICAL SURVEY BULLETIN 1878

U.S. DEPARTMENT OF THE INTERIOR

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991

For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25.425 Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Dwyer, john L. Spatial analysis of geochemical and geologic information from the

Tonopah 1 X2 quadrangle, Nevada I by John L. Dwyer and J. Thomas Nash.

p. cm.-(U.S. Geological Survey; 1878) Includes bibliographical references. 1. Mines and mineral resources-Nevada-Data processing. 2. Geographic

information systems. 3. EROS Data Center. I. Nash, j. Thomas (John Thomas), 1941- . II. Geological Survey (U.S.) Ill. Title. IV. Series. QE75.B9 no. 1878 [TN24.N3] 557.3 s-dc20 91-6650 [557.3'.09793] CIP

CONTENTS

Abstract 1 Introduction 1 Geologic setting 3 Introduction to geographic information systems 3 Preprocessing of data prior to analysis 5 Geochemical backgrounds and anomaly thresholds 8 Spatial models useful for resource assessment 8 Geochemical map methods 10

Interpolated surface (array) maps 11 Drainage basin maps 12

Spatial models of mineral deposits 12 Skarn deposit model 13 Epithermal deposit models 15

Quantification of spatial models 17 Discussion 18

Database requirements 18 Computing requirements 19 Output options 19 Cost effectiveness 19 Spatial maps for geochemistry 20 Suggestions for future research 20

References cited 20

PLATES

[Plates are in pocket]

1. Representative geochemical and geologic maps of the Tonopah 1 >

3. Representative geochemical and geologic maps of the Tonopah 1 X2 quadrangle, Nevada: Map A. Distribution of total concentrations of arsenic, antimony, molybdenum, and zinc after individual element values have been adjusted according to dominant drainage-basin lithologies. Map B. Distribution of total concentrations of arsenic, antimony, molybdenum, and zinc after individual element values have been adjusted according to dominant drainage-basin lithologies, excluding areas of exposed plutonic rocks. Map C. Areas favorable for Carlin-type gold deposits, on the basis of geological and geochemical criteria.

FIGURES

1. Map showing location of major mines and prospects in the Tonopah 1 X2 quadrangle 2

2. Map showing generalized geology of the Tonopah 1 X2 quadrangle 4 3. Schematic diagram of GIS subsystems, interfaces, and example datasets 5 4. Diagrams showing spatial representation and encoding of point, line, and

polygon features and attributes 6 5. Diagram showing hierarchical scheme for encoding sampled drainage areas 7 6. Outline of the methodology for spatial representation of geochemical

results 10 7. Maps showing stream-sediment geochemical data in Tonopah 1 X2

quadrangle 11 8. Schematic diagram of spatial information that is useful for proximity

analysis 11 9. Conceptual illustration showing the use of buffer generation to define

Paleozoic carbonates that crop out within 2 km of an exposed granitic intrusive 14

10. Geologic map showing geologic units permissive for skarn deposits in Tonopah 1 X2 quadrangle 15

TABLES

1. Summary of analytical results for selected elements 9 2. Recalculated background concentrations for three major rock types in the

Tonopah 1 X2 quadrangle 10 3. Descriptive model of copper-tungsten skarn deposits 13 4. Descriptive model of volcanic-associated epithermal deposits 16 5. Descriptive model of sediment-hosted epithermal deposits 17

IV Contents

Spatial Analysis of Geochemical and Geologic Information from the Tonopah 1 X2 Quadrangle, Nevada

By John L. Dwyer and j. Thomas Nash

Abstract

Advanced computer techniques in a geographic information system (GIS) at the EROS Data Center were used for the spatial analysis of geochemical and geologic information for the Tonopah 1 X2 quadrangle, Nevada. The GIS techniques permitted rapid mapping of geochemical trends and development of spatial models of mineralized environments for resource analysis. Information in the GIS included a simplified geologic map (38 map units), structure, chemistry of

10 I

I

20 30 MILES I

20 30 KILOMETERS

EXPlANATION Gold-silver deposit

C Mercury deposit QUADRANGLE LOCATION

o Tungsten skarn deposit

Copper-molybdenum deposit

Silver-base metal deposit

V Antimony

Figure 1. Map showing location of major mines and prospects in the Tonopah 1 X2 quadrangle. Symbols represent six major deposit types known in the area.

experimental application of newly developed computer technology (Dwyer and others, 1987). In this report we discuss computer techniques available for spatial analysis of geologic information using integrated computer hardware and software called a geographic information system (GIS). The re~ults of GIS processing described here are but a few of the possible interpretations of the data and should not be considered final resource assessments because not all the data and expertise of the Tonopah CUSMAP geoscientists have been incorporated.

The Tonopah quadrangle has been thoroughly prospected for about 130 yr; many mines and prospects have been discovered (fig. 1), but additional, undiscovered resources are presumed to remain. From 1981 to 1986

the quadrangle was studied by a team of U.S. Geological Survey geologists, geochemists, and geophysicists. Geology of about 30 percent of the quadrangle was mapped at a scale of 1:62,500 or larger, and topical studies addressed the geology and mineral deposits in many areas of the quadrangle.

Geochemical surveys were made of mineralized rocks, and stream sediments were collected and analyzed to characterize regional geochemistry. A folio of maps that present the geologic, geochemical, and geophysical information is being published as Miscellaneous Field Studies Map MF-1877-A-J. Part of the information from this multidisciplinary study was converted to a digital format to serve as the database for the present investigation.

2 Spatial Analysis of Geochemical and Geologic Information from the Tonopah 10X2 Quadrangle, Nevada

The major focus of this investigation has been spatial analysis of geochemical information. The geochemical data have been reported elsewhere by the team of chemists who worked on the Tonopah CUSMAP project (Fairfield and others, 1985; Siems and others, 1986; Hill and others, 1986). Geochemical maps at a scale of 1 :500,000 were presented by Nash and Siems (1988). The data for stream sediments, definitions of element anomalies, and relations of geochemical trends with geology and mineral deposits have been discussed by Nash (1988) in a companion publication, which is based chiefly on manual methods of data inspection and map overlays. One objective of the present investigation i~ to compare the efficiency and accuracy of the manual and automated processing of information. A second objective is to identify factors that affect the extent to which GIS techniques can enhance the efforts of regional geochemical studies. A traditional limitation of stream-sediment geochemical surveys is the fact that individual sample analyses represent the displaced, modified, and homogenized chemical signatures of rocks and mineral deposits that are resident within a specific drainage basin. In a regional survey that involves more than a thousand sample sites, as in the Tonopah CUSMAP project, it is very time consuming by manual methods to relate repeatedly sample analyses to geology upstream for each sample site. The GIS-based capability to associate spatially drainage basins with corresponding stream-sediment geochemical data provides an accurate and expedient means for representing, analyzing, and evaluating geochemical and associated data. In order to realize more completely the analytical capabilities afforded by a GIS, an initial investment in time and effort is required to encode properly the basic datasets.

GEOLOGIC SETTING

The Tonopah quadrangle comprises about 19,300 km2 and is within the Basin and Range physiographic province. The region is characterized by generally north trending, uplifted mountain ranges separated by broad alluviated valleys. The ranges, in which there is abundant rock exposure, constitute roughly 50 percent of the area (fig. 2). Rocks range in age from latest Precambrian to Quater-nary (Albers and Stewart, 1972; Kleinhampl and Ziony, 1985; Whitebread, 1986). Pre-Tertiary sedimentary rocks are chiefly dark eugeoclinal facies in the west and miogeo-clinal carbonate facies in the east. Triassic to Cretaceous plutonic rocks are exposed in the central and western ranges. Tertiary ash-flow tuffs, flows, intrusions, and volcaniclastic rocks are present in all the ranges and comprise most of the rocks exposed in the eastern part of the quadrangle. Quaternary basalt flows are present in the southeastern part of the area. Major orogenies in the late Paleozoic and Mesozoic produced thrusts and complex