FIELD TRIP DAY FOUR Buena Vista Hills, Humboldt Mafic Complex, Western Nevada DAVID A. JOHNSON† AND MARK D. BARTON Center for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Introduction The best exposures of syngenetic iron oxide, copper-rich, and The purpose of this portion of the field trip is to examine five-element mineralization are impractical to visit in a one- the spatial and temporal distribution of sodium-rich hy- day trip. Their locations and some of their features will be drothermal alteration and iron oxide mineralization gener- briefly described as appropriate during the day. ated by a saline hydrothermal system driven by the Middle BEGIN TRIP: Lovelock To STOP 1 Jurassic Humboldt mafic complex. The Humboldt system is of particular interest because it represents a basaltic mag- From Lovelock, drive north on I-80. The drill roads and matic end member of intrusion-driven hydrothermal systems cuts on the high peak to the east (Gypsum Mountain) are de- and, in this case, one where the fluids are largely, perhaps en- velopment work on gypsum prospects in the Middle Jurassic tirely, externally derived brines. Outcrops of Jurassic rocks Lovelock and Muttlebury Formations. These restricted-basin within the complex record multiple, mutually crosscutting marine units are coeval with the quartz-sand dominated magmatic and hydrothermal events at different structural lev- Boyer Ranch Formation, which is exposed farther to the east. els as exposed by mid-Tertiary extension. Mapping of selected All are roughly coeval with the Humboldt mafic complex. areas across this large igneous complex allows definition of Approximately 6 mi north of Lovelock, exit I-80 and turn the relationships among the magmatic, structural, and hy- right (east) onto Coal Canyon road and continue east (Fig. 1). drothermal features. In turn, these enable an interpretation The white dumps on the left side of the road are diatoma- of the overall temporal and spatial evolution of a large intru- ceous earth for the Eagle-Picher Colado Plant. The mine sion-driven hydrothermal system (Johnson and Barton, 2000). workings in the hills on the north side of the road are gold and From geology and geochemistry, one can estimate that up- antimony occurrences in the Willard District. Bentonite clay wards of 15 billion tonnes of iron and 35 million tonnes each was produced from the workings on the south side of the of copper and zinc were moved by the hydrothermal system road. At approximately 15.4 mi (9.4 mi from Coal Canyon (Johnson, 2000). What happened to these metals and what exit) a road to the north leads to Pegasus’ Relief Canyon are the implications for other systems? These are among the mine, a Carlin-type gold deposit. Gold is recovered from the issues to consider. leach pads in the valley, north of the road. Dumps on the sky- Key goals for the visit to the Humboldt complex are as line above the leach pads are from the Rochester open pit sil- follows: ver mine. At approximately 17.8 mi (11.8 mi from Coal Canyon Exit) 1. To gain an appreciation for the scale, rock types, and hy- pull over and park on the side of the road. drothermal features associated with one of many Fe oxide- rich, sodic-alteration-dominated hydrothermal systems in the STOP 1—North End of Complex, an Overview Jurassic of the western United States; This stop provides a south-facing overview of the western 2. To examine some of the key structural, igneous, and hy- half of the Humboldt Complex. From this point, looking to drothermal features, especially their crosscutting relation- the southeast we see the low Buena Vista Hills in the fore- ships and their implications for the overall time-space devel- ground, the Carson sink to the south, and the Stillwater opment of the complex; Range in the distance (Fig. 1). The road crossed the north 3. To consider the geological and geochemical constraints end of the West Humboldt Range in Coal Canyon and the on mass transfer in this system, and the implications for min- continuation of this range is seen to the southwest. Mafic eralization; and rocks of the Humboldt mafic complex are exposed in the 4. To compare the Humboldt hydrothermal system with West Humboldt Range to the southwest, the Buena Vista the Yerington district and other areas with iron oxide-rich Hills to the south, and in the Stillwater Range to the south- styles of mineralization and associated sodic-calcic alteration. east (see regional geologic map in Johnson and Barton, 2000). The open pits and mine dumps to the south are part of the There are three stops, all in the central part of the Hum- Mineral Basin district, which extends from the eastern flank boldt mafic complex. The first provides an overview from the of the West Humboldt Range to the western flank of the Still- northwest end of the complex. The second focuses on al- water Range. The district has produced more than 4 million bitized Jurassic sediments in the northern Buena Vista Hills. tons of iron ore at an average grade greater than 50 wt per- The third, composing most of the day, is a traverse through a cent iron (Moore, 1971). All iron produced from this district, partial cross section of intensely altered and mineralized rocks which is the largest of several located in the Humboldt com- in the Buena Vista mine area in the southern Buena Vista Hills. plex, has come from an area centered on the Buena Vista Hills. The dark rocks exposed on the eastern flank of the West †Corresponding author: e-mail, [email protected] Humboldt Range are Tertiary-Quaternary basalts overlying 145 146 JOHNSON AND BARTON Relief Canyon Gypsum Mountain Mine Humboldt mafic complex Lovelock Outcrop of Jurassic rocks Major Roads Field trip stops Stop 1 Thomas mine Ford Mine West Humboldt Range Stop 2 N.Buena Chocolate Butte Vista Hills Stop 3 Buena Vista mine Carson Sink N Km Dixie Valley 01020 Stillwater Range FIG. 1. TM image of the Humboldt mafic complex showing fieldtrip stops, place names, and the outcrop of Jurassic igneous rocks. Tertiary volcanic rocks. The older rocks dip up to 60° and give Ore mine and park (24.8 mi). The open pit and mine dumps evidence of the major mid-Tertiary extension seen within the to the west are the Segerstrom-Heizer mine which exploited complex. The light-colored rocks to the northeast are altered magnetite replacement bodies and veins in scapolitized gab- Tertiary felsic volcanic rocks, which host silver-antimony- broic and volcanic rocks. Production totaled at least 1.2 Mt mercury mineralization in the Antelope Springs district. tons of iron ore. Estimated reserves are greater than 35.5 Mt Spectacular isoclinal folding in Lovelock assemblage rocks in of ore with an average grade of 30 wt percent iron (Moore, the lower plate of the Fencemaker Thrust is exposed in the 1971). Walk up to the lower ridges of Chocolate Butte. skyline. Movement on the thrust postdates emplacement of The Chocolate Butte-American Ore area is one of the more the Humboldt complex. accessible and interesting outcrops of the Jurassic sedimen- tary rocks. To the east of the American Ore mine are expo- STOP 2—Northern Buena Vista Hills; Chocolate Butte; sures of pervasively albitized Boyer Ranch Formation at the Jurassic Sedimentary Rocks, Their Alteration and base of Chocolate Butte (Fig. 2). Speed (1976) interprets the Tertiary Tilting Boyer Ranch Formation as the lateral equivalent of the car- Drive south on Coal Canyon road. Pass the road on the left bonate-evaporite rocks of the Lovelock and Muttlebury For- (east) leading to the north end of Buena Vista Valley (18.8 mations exposed to the west in the West Humboldt Range mi). At approximately 21 mi take the left-hand fork (Ander- (see regional geologic map in Johnson and Barton, 2000) son Ranch Road) and continue driving south-southeast. based upon stratigraphic location and similar lithologies in The low hills to the west are composed of scapolitized and the lower part of the formation. This is consistent with more albitized Jurassic gabbros and volcanic rocks. These hills host recent work, including our own, which indicates a transition magnetite-dominated mineralization at the Thomas, Hematite, from a restricted saline basin into volcanic-rich basins with a and SE Section 29 mines. Magnetite-rich dump material is fluvial or reworked eolian sand component. periodically shipped from the Thomas mine to the west coast Here, the Boyer Ranch Formation dips 60° E and is com- for use as concrete aggregate. The hills to the south, at one o’- posed of granular to interlocking albite and rounded to sub- clock, are the northern Buena Vista Hills. Continue driving rounded quartz grains. Variable amounts of carbonate and for approximately 6 mi more and turn east (left) onto the limonite-hematite after black sands and pyrite define original small dirt road leading to the small open pit of the American bedding. Distinctive patches of chlorite ± tourmaline(schorl) 0361-0128/98/000/000-00 $6.00 146 DAY FOUR: BUENA VISTA HILLS, W. NEVADA 147 Chocolate Butte-Northern Buena Vista Hills toward the Buena Vista mine area. Most of N this section is paved; the last few miles are a broad but rough Buena Vista Hills dirt road. At approximately 9.2 mi from the fork with the An- Anderson Ranch Road Field Stop 2 derson Ranch road there will be a turn to the right. Drive straight on the smaller dirt road. Qb Km Continue driving south approximately 1.6 mi to the Buena 0 2 Vista Mine.