GEOLOGY AND GOLD MINERALIZATION OF THEGOLD BASIN-LOST BASIN MINING DISTRICTS,MOHAVE COUNTY, ARIZONABy TED G. THEODORE, WILL N. BLAIR, and J. THOMAS NASHABSTRACTThe Gold Basin and adjacent Lost Basin mining districts are in northwestern<strong>Arizona</strong>, south <strong>of</strong> Lake Mead and justwest <strong>of</strong>the Grand WashCliffs. Gold in quartz veins was apparently first discovered in the areain the 1870's. Recorded production from the districts between 1901 and1942 includes 13,508 oz gold and 6,857 oz silver and has a value <strong>of</strong>about$359,000, <strong>of</strong> which 98 percent is credited to gold. Most recorded productionfrom lode deposits was from mines in the Gold Basin district,which is in the southern White Hills, whereas the bulk <strong>of</strong> placer productionwas along the east flank <strong>of</strong> the Lost Basin Range, about 16 kmto the northeast across Hualapai Valley. The districts have been idlesince about 1942, except for very small scale placer mining.Most known occurrences <strong>of</strong> lode gold in the districts are associatedwith widespread quartz-cored pegmatite vein systems that were probablyemplaced episodically during Early Proterozoic, Middle Proterowic,and Late Cretaceous time into Early Proterozoic metamorphic andigneous rocks. Most <strong>of</strong> the veins apparently were emplaced during theLate Cretaceous, and they were localized along both high- and low-anglestructures in the Early Proterozoic rocks. These veins are associatedspatially and possibly genetically with two-mica magmatism <strong>of</strong> presumedLate Cretaceous age. A Late Cretaceous two-mica monzogranite, whichincludes some- episyenite, crops out in an area <strong>of</strong> 4 to 5 km 2 in thesouthern part <strong>of</strong> the Gold Basin district. Some gold is found also in smallepisyenitic alteration pipes and in veins along a regionally extensive,low-angle detachment surface that has been traced for at least 30 kmalong the western flank <strong>of</strong> the White Hills and crops out conspicuouslyin their southern part.Hydrothermal micas from selected veins in the districts give K-Arages <strong>of</strong> 822, 712, 69, 68, and 65 Ma, and those from pipes give ages<strong>of</strong> 130 and 127 Ma. The oldest ages (822 and 712 Ma) may reflect resetting<strong>of</strong> veins emplaced penecontemporaneously with the 1,400-m.y.-oldgranite <strong>of</strong> Gold Butte, which crops out just to the north <strong>of</strong> Lake Mead.The ages from pipes (130 and 127 Ma) must reflect either the presence<strong>of</strong>excess radiogenic argon in the hydrothermal environment <strong>of</strong>the evolvingpipes or contamination <strong>of</strong>the dated mineral separates by Proterozoicmica and (or) feldspar. Primary white mica from the two-mica monzogranitegives a K-Ar age <strong>of</strong> 72 Ma.Most occurrences <strong>of</strong> gold in the veins and pipes probably reflect eitherremobilization <strong>of</strong> gold from gold-bearing, near-surface Proterozoicmetabasite or anatectic incorporation <strong>of</strong> gold into Late Cretaceous,two-mica magmas from very deep gold-bearing Proterozoic sources.Deposition <strong>of</strong> gold occurred in a mesothermal environment during thegalena-, chalcopyrite-, and ferroan-carbonate-bearing stages <strong>of</strong>the veins.Homogenization studies <strong>of</strong> fluid inclusions prominent in the veins andpipes yield temperatures mostly in the range 150 to 280°C. Early-stagetrapping temperatures in the pipes were probably about 330°C, andpressures in the range 50 to 70 MPa can be inferred. Fluids weremoderately saline, mostly 4 to 16 weight percentNaCI equivalent, nonboiling,and contained appreciable amounts <strong>of</strong> carbon dioxide and, inplaces, fluorine. Such fluids associated with the deposition <strong>of</strong> gold inthese districts largely bridge the fluid-composition interval between fluidsassociated with other epithermal precious-metal and porphyry copperdeposits.Approximately 350 compositional analyses <strong>of</strong> samples <strong>of</strong> native goldfrom 20 mines in the Gold Basin district and from 48 veins in the LostBasin district show silver contents from 6 to approximately 50 weightpercent, and copper contents from 0.01 to 0.5 weight percent. Metalzonation and a possible relation to a porphyry copper system at depthcan be inferred from some <strong>of</strong>these chemical data. Differences betweenthe composition <strong>of</strong> placer gold from 24 occurrences in the Lost Basindistrict and that <strong>of</strong>gold in nearby lode sources suggest that other sourcescontributed gold to the placers or that locally derived grains were enrichedby oxidation and weathering <strong>of</strong> the lodes.INTRODUCTIONThe Gold Basin-Lost Basin mining districts <strong>of</strong> northwestern<strong>Arizona</strong> are in Mohave County, 120 km southeast<strong>of</strong> Las Vegas, Nev., and about 95 km north <strong>of</strong> Kingman,Ariz. (fig. 1). These districts comprise primarily goldbearingvein deposits containing minor byproduct lead,silver, and copper, and placer gold deposits. Gold was firstdiscovered there in the 1870's. The districts lie adjacentto each other south <strong>of</strong> Lake Mead and west <strong>of</strong> the GrandWash Cliffs, which mark the boundary <strong>of</strong> the ColoradoPlateau. The Gold Basin district is mostly in the southernWhite Hills, and the Lost Basin district is to the east,across Hualapai Wash; they are mostly in the GarnetMountain 15-minute quadrangle (fig. 2). In this report, wefollow a broadly defined, strictly geographic assembly <strong>of</strong>mineral deposits and occurrences into mining districts.Our Gold Basin mining district includes the Gold Basin,Cyclopic, and Gold Hill mineral districts <strong>of</strong> Welty andothers (1985), and our Lost Basin mining district includesthe Lost Basin and Garnet Mountain mineral districts <strong>of</strong>Welty and others (1985). In their classification, Welty andothers (1985) grouped known metallic mineral occurrencesand deposits according to metallogenic criteria. Thisreport summarizes field and laboratory investigations inwhich a remarkable suite <strong>of</strong> gold-bearing samples werecollected from wide-ranging localities in these districts.Included are discussions <strong>of</strong> the environment(s) and age1
2 GEOLOGY AND GOLD MINERALIZATION OF THE GOLD BASIN-LOST BASIN MINING DISTRICTS, ARIZONA<strong>of</strong> gold deposition based on geochemical studies, fluidinclusionstudies, and K-Ar isotopic dating, all supplementedby observations with the scanning electronmicroscope (SEM). For our investigations, lode and placersamples containing visible gold were collected from morethan 30 localities.ACKNOWLEDGMENTSThe preceding geologic investigations <strong>of</strong> P.M. Blacet(1975) <strong>of</strong>the U.S. Geological Survey in the Garnet Mountainquadrangle during the late 1960's and early 1970'sand <strong>of</strong> E.J. Krish (1974) and A.J. Deaderick (1980) pro-120 0 114 0EXPLANATION40 035 0oI:", c~~~ ~/I~.., MOUNTAINVA/ ~I,~" MINING'\'j0., ~TRICT LAS ~\>'"I-'~ \ VEGAS,,0/ (j "1.-''''~I'''- I ISTUDY\-, ,,~ M )AREA \ SOUTH VIRGINOJAVE \ MOUNTAINSDESERT 0 \I, Kingman ,,IIThrust fault- Sawteeth onupper plateBatholithic rocks/---//-- -- --- -f// III;1 III,." z.. Boundary <strong>of</strong> Colorado/// Plateaus province//6\ ELDORAD,O "~ MOUNTAINS ",~ ~ "'" ,f MESQUITE MINING DISTRICT"'"100 200Ir~~ /~\
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