Vol. 19, No.4 Investigations • Service • Information Winter 1989

Geology of theVulture Gold Mine

Geologic Setting

The approximately 1 million tons of oremined had an average grade of 0.35ounces per ton of gold and 0.25 ouncesper ton of silver. In spite of significantgold production, the deposit has receivedlittle geologic study until recently(Reynolds and others, 1988; White, 1988).Recent geologic mapping and laboratorystudies by the authors of this article,drilling, and deposit evaluations have ledto a much better understanding of thegeologic characteristics, age, origin, andevolution of the deposit.

New mapping in the Vulture Moun­tains was partially supported by the U.S.Geological Survey and Arizona Geologi­cal Survey Cooperative Geologic Map­ping (COGEOMAP) program. Results ofthese investigations have implications forexploration strategies in the Vulturemine area and in similar highly extendedareas elsewhere in Arizona.

Rocks in the Vulture Mountainsconsist of a variety of Proterozoicmetamorphic and igneous rocks, a Creta­ceous granite or granodiorite pluton, andlower to middle Miocene volcanic andsedimentary rocks. Large-magnitude,middle Miocene extension, common tomost of western Arizona, was accommo­dated in the Vulture Mountains bymovement on numerous listric and planarnormal faults. Normal faults and faultblocks were tilted to the east or north­east during extension. Miocene stratanow typically dip steeply or are locallyoverturned to the east or northeast andfaults dip gently to the west or south­west (Figure 1).NE

A'

The Vulture mine in the VultureMountains of west-eentral Arizona is oneof Arizona's largest historic gold mines.The mine yielded approximately 340,000ounces of gold and 260,000 ounces ofsilver. from 1863 to 1942 (White, 1988).

III Tertiary basalt lavas

Tertiary rhyolite lavas and tuffs, and sedimentary rocks

§ Cretaceous granitoid

~ Proterozoic granite and schist

.J,. Low-angle normal fault, hachures on upper plate

1. High-angle fault, bar and ball on downthrown side

SWA

o 10 20 km,---I__-----'__-----'1 -'-----_-----'1

by Jon E. Spencer, Stephen J. Reynolds,Michael J. Gmbensky, John T. Duncan

Arizona Geological Surveyand Don C. White

521 E. Willis St.Prescott, AZ 86301

CROSS SECTION Geology of the Vulture Mine

Figure 1. Simplified geologic map and cross section of the Vulture Mountains (from Gmbenskyand others, 1987; Gmbensky and Reynolds, 1988; and M.J. Grubensky, unpublished mapping).

Mineralization and alteration at theVulture mine occurred primarily withinand directly adjacent to a north-dippingquartz porphyry dike that extends east­ward from a Late Cretaceous pluton andintrudes Proterozoic crystalline rocks(Figures 2 and 3). Moderate to severealteration of the dike and wall rockshas converted feldspar and mafic miner-

3000

feeto 500 1000 m,---'_--'-_---'---,_---'---_---J'

1000

750~~~~~;;;;"'9'77'~_~_.~__~.{.~ 2000

500

meters

-----

Figure 2. Simplified geologic map of the Vulture mine area and fluid-inclusion sample locations.

als to fine-grained sericite, hematite,and clay minerals. Altered dike rockscommonly consist of quartz "eyes" in afine-grained matrix of alteration miner­als. Gold is concentrated in quartzveins and in silicified and altered rockswithin and adjacent to the dike (Figure3). Gold is present as either nativemetal or electrum and is associated withpyrite, argentiferous galena, and minoramounts of chalcopyrite and sphalerite.There is a positive correlation amongabundances of secondary silica, sulfides,and gold (White, 1988).

The Miocene volcanic rocks northeastof the Vulture mine were deposited on

Fluid-Inclusion Characteristics

Conceptual restoration of the rocksof the Vulture mine area to their pre­rotation orientation reveals the approxi­mate geometry of the ore deposit at thetime of mineralization. Mineralizatioand alteration originally occurred alonga north-northeast-trending subverticaldike that projected upward from thestructural top of a Cretaceous granitoidpluton (Figure 4A). The association ofgold with the dike (Figure 3) and grada­tion of the dike into the granitic rocksof the pluton indicate that gold mineral­ization was intimately related to Creta­ceous magmatism and dike emplacement.Later erosion and subsequent burial bylower Miocene volcanic rocks (Figure 4B)was followed by structural dismember­ment and tilting (Figure 4C) and eventu­al uncovering by late Cenozoic erosion.The Astor fault (Figure 3), which isprobably one of the youngest faults inthe area, cuts the deposit and has dis­placed its down-dip continuation by anunknown amount (White, 1988).

Fluid inclusions are bubbles of liquidand gas that are trapped inside mineralsduring mineral formation. The composi­tion of fluids in inclusions that weretrapped in mineral deposits at the timeof deposit formation reflects the compo-~sition of the aqueous fluids from which"the deposits formed. One can determinethe salinity of the inclusions by measur-ing the freeZing temperature of thetrapped fluid. The minimum temperatureof the fluid at the time it was trappedcan be determined by heating the sampleuntil the two phases (liquid and gas) inthe inclusion become one. (This is calledthe homogenization temperature.) Fluidinclusions that formed during precipita-tion of host minerals are called primary,whereas those that formed later alongfracture planes are called secondary.

Quartz veins are numerous over abroad area around the Vulture mine.Samples of veins were collected from anarea (Figure 2) that represents an origi­nally vertical cross section through theVulture mine and that includes morethan 1 kilometer of paleodepth range.Homogenization temperatures of primaryand secondary fluid inclusions vary fromapp.roximately 200°C to 320°C and calcu­lated salinities vary from approximately1 to 18 percent NaCl equivalent byweight. Homogenization temperaturesand salinities generally decrease withdecreasing paleodepth (Figure 5). Thesefluid-inclusion data reveal the tempera-~.\tures and salinities of the hydrothermal••fluids that were probably undergoingconvective circulation above the Creta­ceous intrusion and that were respon-

A"

Low-angle normal fault, dashed whereinferred, ,dotted where concealed

"\ Attitude of beds

~ Attitude of overturned beds

"\ Attitude of flow foliation

A e Fluid-inclusion sample

the Proterozoic crystalline rocks thathost the Vulture mine gold deposit(Reynolds and others, 1988). The origi­nally horizontal volcanic strata and theircrystalline substrate have been rotated70° to 90° so that bedding is nowalmost vertical. Rocks exposed in theVulture mine area, therefore, representan originally vertical cross section thathas been tilted approximately 80° to theeast by rotational normal faulting. Themap view (Figure 2) represents what wasoriginally a vertical cross-section view;what is visible in a north-south crosssection (Figure 3) was originallyhorizontal.

___." High-angle fault, dashed whereinferred, dotted where concealed

AI

.J-U-~ ..... ,

+

2000 3000I' , I

500 1000

1000

Aphyric rhyolite (Miocene)

Tuff and altered rhyolite (Miocene)

Basalt and andesitic flows (Miocene)

Granite (Cretaceous)

Metamorphic rocks (Early Proterozoic)

A

feet 0,

~NI

meters 0

Arizona Geology, vol. 19, no. 4, Winter 1989

------------------------------------------------~ -,~""~~.~~.".~~~,•....._._------- --------~-~

wz~

w'"~~~

>~

III~ III

500 1000 1500 2000

w Iz~

w

\j'"

I~

~~

>~

i~I,500 1000 1500 2000

Paleodepth (Meiers)

Recognition of this type of ore­deposit tilting and possible structuraldismemberment has implications for ex­ploration strategies in extended areas.Specifically, mineral exploration inhighly extended areas characterized byrotational normal faulting may be facili­tated by the knowledge that mineraldeposits may have been tilted 800 fromtheir original orientation. Such rotationprovides a natural laboratory for thestudy of mineral deposits because the

14

18,--------------T---,

16

Figure 4 (left), Evolutionary block diagramof the Vulture mine area. Although only onegeneration of normal faults is shown, rotationprobably occurred by movement on two ormore generations of normal faults and ismore complex than is shown here.

Figure 5 (below), Paleodepth versus salinity(upper diagram) and homogenization tempera­ture (lower diagram) for fluid inclusions fromquartz veins in the Vulture mine area. Paleo­depth is the distance perpendicular to theapproximately vertical disconformity at thebase of Miocene volcanic rocks in the Vulturemine fault block. The actual depth of Vulturemine rocks at the time of mineralization wasprobably 1 to several kilometers.

4

12

00

340

320

300

N '0 280

2ciE 260~

240

220

200

1800

Vulture mine gold deposit

..f:33:; Fault

1"'- Drill hole ,

--' Rocks with >0.01 oz/ton gold

Middle Miocene sedimentary rocks

LowerMiocenevolcanicrocks

Cretaceous granitic intrusion

®CRETACEOUS

Area ofgoldmineralization

Proterozoiccrystallinerocks

®EARLYMIOCENE

©MIDDLEMIOCENE

Normal--­fault

Presentlandsurface

Normalfault

Conclusion

Figure 3 (below), Geologic cross sectionthrough the Vulture mine (modified fromWhite, 1988 and unpublished data). SeeFigure 2 for location.

sible for much or all of the mineraliza­tion and alteration at the Vulture mine.Greater fluid temperatures at greaterdepths probably reflect heat from the

.. magma intrusion (now the granitoid• pluton) that lay beneath the Vulture

mine deposit. Downward-increasing fluidsalinities may reflect a downwardincrease in the proportion of aqueousfluid expelled by the magma duringcrystallization.

s

Recent geologic mapping of the Vul­ture Mountains and adjacent ranges hasestablished that the area has undergonelarge-magnitude extension as a result ofrotational normal faulting (Grubenskyand others, 1987; Stimac and others,1987; Grubensky and Reynolds, 1988; seealso Rehrig and others, 1980). Geologicmapping in the Vulture mine area indi­cates that this area has been faultedand tilted like most of the range andthat the Vulture mine gold deposit hasbeen tilted approximately 800 (Reynoldsand others, 1988). Drill-hole assay datashow that mineralization is associatedwith a dike that extends from the struc­tural top of a Cretaceous pluton (White,1988). Fluid-inclusion studies indicatethat mineralization at the Vulture mine

? deposit occurred within a larger systema of circulating aqueous fluids in whichw'temperature and salinity increased down-

ward toward a crystallizing magma body.

Arizona Geology, vol. 19, no. 4, Winter 1989 3

-------------deposits are exposed in what was origi­nally a near-vertical cross section. Thistype of extensional faulting may also cutan ore deposit' into two or more piecesand leave them in shinglelike imbricatefault blocks separated from each otherby several kilometers (e.g., Lowell, 1968).

References

Grubensky, M.J., and Reynolds, S.J., 1988, Geo­logic map of the southeastern Vulture Moun­tains, west-central Arizona: Arizona Geologi­cal Survey Open-File Report 88-9, 16 p., scale1:24,000.

Grubensky, M.J., Stimac, J.A., Reynolds, S.J., andRichard, S.M., 1987, Geologic map of thenortheastern Vulture Mountains and vicinity,central Arizona: Arizona Bureau of Geologyand Mineral Technology Open-File Report87-10, 7 p., scale 1:24,000.

Lowell, J.D., 1968, Geology of the Kalamazooorebody, San Manuel district, Arizona: Eco­nomic Geology, v. 63, p. 645-654.

Rehrig, W.A., Shafiqullah, M., and Damon, P.E.,1980, Geochronology, geology, and listric nor­mal faulting of the Vulture Mountains, Mari­copa County, Arizona, in Jenney, J.P., andStone, Claudia, eds., Studies in western Ari­zona: Arizona Geological Society Digest, v.12, p. 89-110.

Reynolds, S.J., Spencer, J.E., DeWitt, Ed, White,D.C., and Grubensky, M.J., 1988, Geologic mapof the Vulture mine area, Vulture Mountains,west-central Arizona: Arizona GeologicalSurvey Open-File Report 88-10, 4 p., scale1:24,000. A,

Stimac, J.A., Fryxell, J.E., Reynolds, S.J., Richard,..S.M., Grubensky, M.J., and Scott, E.A., 1987,Geologic map of the Wickenburg, southernBuckhorn, and northwestern HieroglyphicMountains, central Arizona: Arizona Bureauof Geology and Mineral Technology Open-FileReport 87-9, 13 p., scale 1:24,000,2 sheets.

White, Don, 1988, Geology of the Vulture mine,Arizona: American Institute of Mining, Metal­lurgical, and Petroleum Engineers, Society ofMining Engineers Preprint 88-44, 5 p.

State Geological Survey - U.S. Geological SurveyMeeting Held in Tucson

Arizona Geology, vol. 19, no. 4, Winter 1989

Figure 2 (left), Western State geologistsmeet to discuss mutual concerns. Top row,left to right: Bob Forbes (Alaska), Ed Ruppel(Montana), Jon Price (Nevada), Don Haney(Kentucky; President of the Association ofAmerican State Geologists), and Larry Fellows(Arizona). Seated, left to right: Eric Schuste_.....\(Deputy Director, Washington), Jim Davi~

(California), Earl Bennett (Idaho), Jamie Rob­ertson (Deputy Director, Ne:w Mexico), andLee Allison (Utah).

(Figure 2). USGS geologists held avariety of postmeeting functions at theirArizona Field Office.

Two major discussion sessions wereheld at the joint meeting: (1) the Min­eral Resources Data System (MRDS), acomputerized database maintained by theUSGS, and (2) outreach activities inearth science education. A half-day fieldtrip was taken to observe detachment­fault geology and the impacts of ground­water withdrawal, subsidence, and earthAfissures in the Picacho basin (Figure 3). -..,

The 1990 meeting will be cohosted bythe USGS and Idaho Geological Surveyin Moscow, Idaho.

Figure 3 (above). AZGS geologists PhilPearthree and Steve Reynolds discuss areasof subsidence and earth fissures in the Pica­cho basin with field-trip participants.

The annual meeting of the directorsof western State geological surveys andkey U.S. Geological Survey (USGS) staffwas held in Tucson October 22-25 at theGhost Ranch Lodge. The purposes of themeeting were to improve communicationbetween staff of the State and Federalsurveys; learn about current activities,projects, and concerns (Figure 1); andexplore ways of fulfilling the respectivestatutory mandates more effectivelythrough improved coordination and coop­eration. Ten of the 13 Western Stategeological surveys were represented;approximately 20 USGS staff members,primarily from the Office of MineralResources, were also present.

Western State geologists held an all­day business meeting at the ArizonaGeological Survey (AZGS) on October 21

Figure 1. Representatives from the AZGS andUSGS discuss the Cooperative Geologic Map­ping (COGEOMAP) program. Left to right:Larry Fellows (AZGS Director and State Geol­ogist), Steve Reynolds (AZGS Research Geolo­gist), Ben Morgan (USGS Chief Geologist,Reston), and Dave Russ (USGS AssistantChief Geologist for Programs, Reston).

-The Tucson CAP Tunnel:

A Lesson in Engineering Geology

by Brad HerbertU.S. Burea,u of Reclamation

In 1973 construction began on theCentral Arizona Project (CAP) at LakeHavasu on the western Arizona border.Today, 16 years later and nearly 330miles farther, the continuous ribbon ofcanals, pipelines, tunnels, and pumpingplants is at Tucson's doorstep, withcompletion scheduled for 1991. The lastleg of the project bringing potable waterto Tucson is now under construction.This final phase involves the excavationof an 8,340-foot-Iong, 12-foot-diametertunnel through the southern TucsonMountains. The tunnel will conveytreated water from Tucson Water'streatment plant on the west side of themountains to the utility's distributionsystem on the east.

National Projects Inc. (NPI) fromBoise, Idaho constructed the $12.6­million tunnel under contract with theU.S. Bureau of Reclamation (BOR). NPIexcavated and supported the tunnel,which was completed on October 25,and is now constructing an 8-foot-

(~ inside-diameter pipeline within the exca­_ vation to carry the water under pressure

to the outlet, just west of Star PassGolf Course.

Conception and Siting

Originally, the treated water was tobe pumped through a 5-mile-Iong buriedpipeline that would run east along theTucson-Ajo Highway to Robles Pass,where it would turn north and snakethrough the mountains to its terminus.When BOR geologists began investiga­tions, however, they discovered problemswith that location. A portion of the pro­posed pipeline traversed a steep colluvialslope. Water from a large wash had al­ready eroded the toe of this slope andcaused it to fail in one area. Construct­ing the pipeline across this slope wouldhave been extremely difficult and assur­ing its stability would have been costly.

Another design problem uncoveredduring geologic investigations occurredwhere the pipeline was to pass througha small saddle within the Shorts RanchAndesite. The design called for a cutapproximately 40 feet deep through thissaddle. When an exploration core hole

~ was drilled in this area, geologists• discovered that the condition of the

rock was much worse than they expect­ed. Less than 60 percent of the corewas recovered, and what was recovered

Arizona Geology, vol. 19, no. 4, Winter 1989

displayed evidence of extensive shearingand fracturing. Constructing a safe andusable excavation would have necessitat­ed removing a hilltop on the west sideand extending the cut on the east sidenearly 100 feet up the mountain.

After these problems came to light,BOR designers examined several otherconstruction-related problems more care­fully. They realized that a less costly,more efficient, and more environmentallysound alternative was required. In 1986they discussed the idea of a combinationpipeline/tunnel. Initial cost analysesindicated that the tunnel option waseconomically feasible and geologic inves­tigations were begun.

After initial reconnaissance andreview of pertinent literature, BORgeologists began detailed geologic map­ping in the fall of 1986 at a scale of1:4,800 (1 inch equals 400 feet). Theresults of that mapping helped the geol­ogists pinpoint areas' that posed possibleproblems for tunnel excavation and sta-

Figures 1a and 1b (above and left), TheTBM began its assault on the Tucson Moun­tains on March 17 as it ground into theshotcrete-covered rock surface at the inletportal. The cylindrical TBM contains thecutter head, operating controls, motors, andhydraulic systems. It drags behind it an 80­foot-long trailing gear (the open rectangularstructure), which contains electrical panels,air and water hoses, and the first portion ofthe muck (cuttings) conveyor system.

bility. They conducted further investi­gations in these areas, including drillingsix core holes on or near the alignment:two at both the inlet and outlet portalswhere stability could be a problem andtwo along a large fault that crossed theproposed tunnel. The drill holes alongthe fault revealed a 30-foot-thick claygouge zone at tunnel level. This knowl­edge enabled BOR designers to plan foralternative support and excavationmethods in this area.

Geologic mapping also revealed thepossible existence of another, previouslyunmapped fault, which is a southwesternextension of a fault that Kinnison (1958)mapped. This fault closely paralleled aportion of the proposed tunnel align­ment, a situation that could have madeexcavation and support difficult. Near­surface seismic refraction and resistivitysurveys were conducted to confirm theexistence of the fault. Although theresults of the seismic survey were in­conclusive, the resistivity surveyrevealed the existence of conductiveground at the suspected fault, indicatingthe presence of clay or moisture. These

5

As the excavation progressed, geolo­gists prepared a descriptive log. On theT8M between the crown and sideshields, a 4-inch "window" provided aview of the rock. From studying this_window, ge610gists recorded the locations..of significant geologic features. Theyalso viewed the entire excavation for ashort time just before a ring was con­structed. Occasionally, they were allowedto examine the exposed cuts more thor­oughly (Figure 3). Much of the exposedrock was completely covered with dustand mud; rock samples were, therefore,taken regularly for a detailed descriptionof the lithology throughout the tunnel.

BOR geologists are compiling andanalyzing all geologic data collectedduring tunneling for a final report. Thisreport will include detailed descriptionsof stratigraphy, structure, and lithologiesin the tunnel, as well as how theseparameters affected the excavation.

Figure 2. View from the rear of the TBM back through the trailing gear toward the inletportal. The concrete segments in the foreground are ready to be set in place by the hydraulicerector arm (not pictured).

findings persuaded BOR designers toshift the alignment to the northwest toavoid the possible adverse effects of thefault.

BOR geologists analyzed and releasedthe geologic data collected during theyear-long study (U.S. Bureau of Recla­mation, 1988). A copy of this report isavailable in the Arizona GeologicalSurvey library.

Construction

Excavation of the tunnel properbegan on March 17, 1989. NPI chose toadvance the tunnel with a tunnel boringmachine (T8M) rather than use a tradi­tional drill-and-shoot (blast) method(Figures 1a and 1b). The T8M is de­signed with a slightly concave cuttinghead that turns up to 12 revolutions per

Figure 3. Folded sedimentary beds of theCretaceous Amole Group seen between theconcrete segments (left) and TBM shield(right). The top, or crown segment, is tightlysecured in place with laminated wood blocks(upper left).

minute and contains 24 separate rollercutters. Large "gripper" plates pushagainst the sides of the tunnel to stabil­ize the machine while the cutting headis thrust forward at pressures of up to4,000 pounds per square inch. Differenttypes of cutters are typically needed forhard and soft rock. This T8M, however,was designed to run equally well withthe same cutters in both rocks. Thisbecomes advantageous when mixed facesare anticipated in a tunnel. A mixed faceis encountered when two or more rocktypes are exposed at the face, or leadingedge, of the tunnel. A T8M will gener­ally deflect toward the softer rock,causing the tunnel to vary from properline and grade. Mixed faces were ex­pected in the Tucson tunnel throughoutthe megabreccia unit of the Cat Moun­tain Rhyolite sequence (Tucson MountainChaos). This TBM design was also ad­vantageous when thick fault-gouge zoneswere encountered because the time­consuming process of changing cutterswas eliminated.

Precast concrete ring segments,rather than more traditional steel setsand rock bolts, supported the tunnelwalls and crown. Each ring was com­posed of four individual segments. Thesegments were 5 inches thick and 4feet wide and were installed directlybehind the T8M with a specially de­signed hydraulic erector arm (Figure 2).This system provided immediate and con­tinuous support in all rock types. Unfor­tunately, this system also covered up therock as fast as it was' exposed, makinggeologic mapping difficult at best.

References

Kinnison, J.E., 1958, Geology and ore deposits ofthe southern section of t.he Amole miningdistrict, Tucson Mountains, Pima County,Arizona: Tucson, University of Arizona, un­published M.S. thesis, 126 p.

U.S. Bureau of Reclamation, Arizona ProjectsOffice, 1988, Geologic design data report forTucson tunnel, Central Arizona Project: 212 p.,scale 1:600, 1:2,400, and 1:4,800,5 sheets.

'11

JI

Arizona Geology, vol. 19, no. 4, Winter

-

The October 17, 1989 Lorna Prieta(San Francisco) Earthquake

by Terry C. WallaceUniversity of Arizona

and Philip A. PearthreeArizona Geological Survey

Figure 1. Map showing the location of theOctober 17, 1989 Lama Prieta earthquake epi­

~.....('.enter and aftershock zone. The traces of the'W'an Andreas, Calaveras, and Hayward faults

. in the Bay area are also shown. Any ofthese faults could be sources of earthquakesthat would be devastating to the Bay area.

larger than magnitude 5.0 since 1906.This section of the fault slipped during1906, but the amount of slip was muchsmaller than that observed on the faultnorth of San Francisco. Based on theslip deficit during the 1906 earthquake,the USGS in 1988 identified this sectionof the fault as a likely candidate for amagnitude 7.0 earthquake during the fol­lowing 5 years. The Lorna Prieta earth­quake has probably relieved the strainaccumulation on a 40-km section of theSan Andreas fault, but it has notdecreased the chances for another mag­nitude 7.0 or larger event in the Bayarea on the Calaveras or Hayward faults.Furthermore, a section of the SanAndreas fault between San FrancIsco andPortala Valley (-30 km long) could rup­ture in a magnitude 6.5 to 7.0 event.The probability of a repeat of the greatSan Francisco earthquake of 1906 re­mains very low, however, and is not ex­pected for 30 to 70 years.

The Lorna Prieta earthquake was re­corded at the Tucson seismic observatory(TUC). The seismic waves from theearthquake were so large that all theinstruments were driven off scale forapproximately 20 minutes. Ground shak­ing was recorded for nearly 3 1/2 hoursat TUe. Based on the time it took forthe ground shaking to return to normalbackground level, Tucson seismologistsestimated that the magnitude of theearthquake was 6.8.

Reference

U.S. Geological Survey, Working Group on Cali­fornia Earthquake Probabilities, 1988, Probabili­ties of large earthquakes occurring in Californiaon the San Andreas fault: Open-File Report88-398, 62 p.

Figure 2. Block diagram portraying the rela­tive motion of the Pacific and North Ameri­can plates during the Loma Prieta earthquake.The Pacific plate moved right and up relativeto the North American plate (right-lateraloblique slip) on a 700 west-dipping fault.

North American plate (right-lateralstrike-slip). The Lorna Prieta event wasa mixture of right-lateral strike-slip andthrusting motion, during which thePacific plate moved up and over theNorth American plate (Figure 2). Thefault plane dipped about 700 to thewest. Third, very little surface faultingassociated with this earthquake has beendetected, even though the aftershocksnearly reached the surface. Typicalstrike-slip earthquakes of this magnitudehave had surface displacements of 1 to 3meters.

The Lorna Prieta earthquake occurredalong a section of the San Andreasfault that had undergone no earthquakes

5pKM

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~(z, I~/<t /. l:r: 1-

~\san Jose\ ('1({) , "-

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1989EPICENTER

AFTERSHOCKZONE

At 5:04 p.m. on October 17, 1989, amagnitude 7.1 earthquake occurred alonga section of the San Andreas fault inthe Santa Cruz Mountains south of SanFrancisco. The earthquake, the largestto occur in the Bay area since 1906,caused more than $4 billion in damageand 66 deaths. During the 4 days thatfollowed the earthquake, 19 aftershocksoccurred with magnitudes larger than 4.0

The U.S. Geological Survey (USGS)designated this earthquake the LamaPrieta earthquake. Aftershocks indicatedthat a 40-kilometer(km)-10ng area on ornear the San Andreas fault rupturedduring the earthquake (Figure 1). Theevent was very unusual for several rea­sons. First, the focus was at a depth of17 km. Most of the previous SanAndreas earthquakes had foci shallowerthan 12 km. Second, faulting during theLorna Prieta earthquake was not purelystrike-slip. Typical San Andreas earth-

~uakes have been the result of horizon­~l slip on a vertical fault; the Pacific

plate moved northwest relative to the

Arizona Geology, vol. 19, no. 4, Winter 1989 7

The Most Significant Earthquakes in u.S. History

Before the Loma Prieta (San Francis­co) earthquake occurred on October 17,1989, the U.S. Geological Survey (USGS)compiled a list of the 15 most signifi­cant earthquakes in the history of theUnited States. Selection was based on acombination of magnitude, damage, andcasualties. The magnitude 7.1 LomaPrieta earthquake, which caused anestimated $4 billion in damage and 66deaths, would undoubtedly have beenincluded in this list.

Earthquakes are measured in two basicways: magnitude and intensity. Magni­tude is an instrumental measure of theamount of energy released by an earth­quake, as indicated by ground motion.It is determined from the logarithm ofthe amplitude of earthquake waves re­corded by seismographs. The Richtermagnitude scale, expressed in wholenumbers and decimal fractions, theoreti­cally has no upper limit; however, thelargest earthquakes ever recorded hadmagnitudes of less than 10. The ModifiedMercalli Scale (MMS) of intensity usesRoman numerals and is based on humanjudgment of the amount of damage andeffects caused by earthquakes. It rangesfrom I (not felt) to XII (almost totaldestruction of manmade structures).

The 15 most significant earthquakesin U.S. history, listed in chronologicalorder, are as follows:(1) Cape Ann, Mass., Nov. 18, 1755­

Estimated magnitude, 6.0; maximumMMS intensity, VIII. This earthquakewas centered in the Atlantic Ocean,200 miles east of Cape Ann. It wasfelt over 400,000 square miles, fromNova Scotia south to Chesapeake Bayand from Lake George, N.Y. east intothe Atlantic. Damage was heaviest onCape Ann and in Boston, with about100 chimneys destroyed.

(2) New Madrid, Mo., 1811-12--Estimatedmagnitudes, 8.4 to 8.7; maximum MMSintensity, XI. In the most violentseries of earthquakes in U.S. history,three earthquakes (counted here asone) hit the New Madrid seismic zonein southeastern Missouri and north­eastern Arkansas on Dec. 16, 1811, andJan. 23 and Feb. 7, 1812. Damage andcasualties were not great because thearea was sparsely populated. Theearthquakes, however, were felt overthe entire United States east of theMississippi River and probably far tothe west and caused extensive changesin the land surface.

(3) Virgin Islands, Nov. 18, 1867--Esti­mated magnitude, 7.5; maximum MMSintensity, VIII. This earthquake wasfelt from the Dominican Republic to

8

the Leeward Islands. Property damage,which occurred in the Virgin Islandsand Puerto Rico, was partly caused by20-foot sea waves triggered by theearthquake.

(4) Charleston, S.c., Aug. 31, 1886­Estimated magnitude, 6.6; maximumMMS intensity, X. This earthquakekilled 60 persons. Most buildings inthe Charleston area were damaged ordestroyed; losses totalled $20 million.It was felt in New York City; Boston;Milwaukee; Havana, Cuba; and Ontar­io, Canada.

(5) Charleston, Mo., Oct. 31, 1895-Esti­mated magnitude, 6.2; maximum MMSintensity, IX. This earthquake occurrednear the junction of the Mississippiand Ohio Rivers and was the strongestshock in the New Madrid seismic zonesince the earthquakes in 1811-12. Itwas felt over 1 million square miles in23 states and Canada, caused consider­able damage, and created a four-acrelake near Charleston.

(6) San Francisco, Calif., April 18, 1906­Estimated magnitude, 8.3; maximumMMS intensity, XI. Although knownas the San Francisco earthquake, the1906 shock actually ruptured the SanAndreas fault along a 270-mile seg­ment from San Benito County north toHumboldt County. Fault slip was up to21 feet in Marin County. Damage wasestimated at more than $24 million,from both the earthquake and firesthat followed in San Francisco. Morethan 700 persons died.

(7) Mona Passage, Puerto Rico, Oct. 11,1918--Estimated magnitude, 7.5; maxi­mum MMS intensity, IX. This earth­quake was one of the most violentrecorded on Puerto Rico and was fol­lowed by a tsunami that drownedmany persons. The death toll was 116;damage was estimated at $4 million.

(8) Long Beach, Calif., March 10, 1933­Estimated magnitude, 6.2; maximumMMS intensity, VIII. This earthquakewas one of the most destructive in theUnited States because it was in aheavily settled area with many poorlyconstructed buildings, includingschools. About 115 persons were killedand hundreds more were injured.Damage was estimated at $40 million.The earthquake led to stricter con­struction codes in California to miti­gate earthquake damage.

(9) Olympia, Wash., April 13, 1949--Esti­mated magnitude, 7.1; maximum MMSintensity, VIII. This earthquake causedheavy damage in Washington and Ore­gon, killed eight persons, and injuredmany others. The earthquake was felt

eastward to western Montana an.,south to Cape Blanco, Oregon. .!

(10) Hebgen Lake, Mont., Aug. 17,1959-Estimated magnitude, 7.3; max­imum MMS intensity, X. This was thestrongest recorded earthquake in Mon­tana. It was felt over 600,000 squaremiles, from Seattle to Banff, Canadaand from Dickinson, N. Dak. to Provo,Utah. It caused massive waves on Heb­gen Lake that did not subside for 12hours. It also caused a massive land­slide that blocked the Madison Rivercanyon, creating a large lake. At least28 persons were killed. Damage wasextensive to summer homes and high­ways in the region.

(11) Prince William Sound, Alaska,March 27, 1964--Estimated magnitude,8.4; maximum MMS intensity, X. ThisGood Friday earthquake is the secondstrongest in the world during the 20thcentury. It was topped by an 8.6­magnitude earthquake in Chile in 1960.The Alaska earthquake triggered ex­tensive landslides and generated tsu­namis. It caused an estimated $311million in damage in Anchorage andsouth-eentral Alaska and killed 131persons.

(12) Seattle, Wash., April 29, 1965--EstlA.I,mated magnitude, 6.5; maximumMM~intensity, VIII. This earthquake wasfelt over 130,00 square miles of Wash­ington, Oregon, Idaho, Montana, andBritish Columbia. Seven persons died;damage was estimated at $12.5 million.

(13) San Fernando, Calif., Feb. 9, 1971-­Estimated magnitude, 6.6; maximumMMS intensity, XI. This earthquakekilled 65 persons, injured many others,and caused $1 billion in damage in theLos Angeles area. As a result of thisearthquake and the Alaskan tremor in1964, the Federal government greatlyexpanded its earthquake research andreevaluated seismic design for hospi­tals, emergency clinics, and other crit­ical facilities.

(14) Coalinga, Calif., May 2, 1983--Esti­mated magnitude, 6.7; maximum MMSintensity, VIII. This earthquake in­jured 45 persons and caused $31 mil­lion in damage, with the worst damageoccurring in downtown Coalinga. Theearthquake was felt from Los Angelesto Sacramento and from San Franciscoto Reno, Nevada.

(15) Borah Peak, Idaho, Oct. 25, 1983-­Estimated magnitude, 7.0; maximumMMS intensity, IX. This earthquak~.•was the largest recorded in Idaho. 1_was felt over 330,000 square miles.Two children were killed and damagewas estimated at $12.5 million.

Arizona Geology, vol. 19, no. 4, Winter

1f,ff

--------------------------------------

New AZGS Publications

•The following publications may be

urchased from the Arizona GeologicalSurvey (AZGS), 845 N. Park Ave., #100,Tucson, AZ 85719. For price informa­tion on these and other publications,call the AZGS office at (602) 882-4795.

The Contributed Report series wascreated in January 1989 for reportswritten by non-AZGS geologists that areconsidered to be significant additions tothe geologic literature on Arizona. Thisseries title describes more accurately thesource and status of these publications.Reports of this nature donated before1989 were placed in the AZGS Open-FileReport series. This latter series is nowdevoted to reports written by AZGSpersonnel. Many contributed reports areobscure and would not be readily avail­able to the public if they were notplaced in this series.

Scarborough, Robert, and Meader, Nor­man, 1981 [1989J, Geologic map of thenorthern Plomosa Mountains, Yuma [LaPazJ County, Arizona: Contributed MapCM-89-D, scale 1:24,000.

The northern Plomosa Mountains con­sist of a large fault block of dominantlycrystalline rock that has been tilted to

Ahe south and is bounded on the east~nd north by a low-angle normal fault

known as the Plomosa detachment fault.The hanging wall of the Plomosa faultcomprises a variety of crystallinemetamorphosed and multiply defo1'lllE!dPaleozoic and Mesozoic rocks, andcene volcanic andNumerous Tertiary mineral depo:sitsassociated with shearnorthern part of the range.the only geologic map ofnorthern Plomosa Mountains.

Maynard, S.R., 1989,cross sections of theof the New River Ml)ullltai:ns, Anlzolrlti:Contributed Map1:12,000,2 sheets.

This detailed geologicsouthern end of the Moorezone, a complex zone ofdeformation that separatesProterozoic rocks andportant tectonic feature.mentary and volcanic rockspresent in the range.

Chenoweth, W.L., 1989, Theprogram of the U.S. Atomic

•..Commission in Arizona: COlntributj~d

eport CR-89-A, 4 p.The exploration activities

Atomic Energy Commissioning the 1950's are well docUlne:nted

Arizona Geology, vol. 19, no. 4, Winter

AEC reports. The program of construct­ing and improving access roads to ex­ploration and mining areas, however, isless known. From 1951 to 1958, some 90projects affecting 1,253 miles of roadwere undertaken in Arizona, Colorado,New Mexico, South Dakota, Utah, andWyoming. These projects cost $17 mil­lion, $14 million of which the AECprovided. Seven projects, all totallyfunded by the AEC, were conducted inArizona: two in Gila County and fiveon the Navajo Indian Reservation inApache County. This report smnmarizesthose projects.

Chenoweth, W.L., 1989, The Carrizo"gold" mine: Contributed Report CR-89­B,26p.

Herbert E. Gregory, in his classic1917 report on the Navajo Indian Reser­vation, mentions that silver and goldwere discovered in the Carrizo Moun­tains. Gregory did not locate thisdeposit, nor is it referenced elsewherein the literature. Some old mine work­ings, once thought to be related touranium-vanadimn prospecting, are be­lieved to be the so-called Carrizo "gold"mine. Their history and geologic settingare smnmarized in this report.

Chenoweth, W.L., 1989, The geology andvr,odtlct,ion history of uranium deposits in

Salt Wash Member of the Morrisonnear Rough Rock, Apache

Contributed Report

Member of the Juras­Formation contains signifi­

ur,miU1rt-vi:l.ll<tdium deposits in thein northeastern

North of there, smallermined near the Car­

During the uraniumsome uranium was

Salt Wash near1r<:l.ding Post. This report

gel)lbgic setting and produc-ti()Jrtllis.tl?ty'ot .th'ese deposits.

C.S., 1989,un.derxr'ound workings, Mon­

£u"..,,,,;; County, Ari­Chenoweth:

LoL"-O'-U. 35 p.the two pri­

with the U.S.(iolmIl1lissilon, mapped the

Workilngs of the Mon­I.lt,lIli.utlrt-\rallladlium mine on

Thebecause the

later de­UU.HU.l~. In addition

to the maps, this report includes infor­mation on the geologic setting and pro­duction history of the mine.

Reynolds, S.J., Spencer, J.E., Laubach,S.E., Cunningham, Dickson, and Richard,S.M., 1989, Geologic map, geologic evolu­tion, and mineral deposits of the GraniteWash Mountains, west-central Arizona.'Open-File Report 89-4, 46 p., scale1:24,000.

The Granite Wash Mountains in west­central Arizona are part of the Mariafold-and-thrust belt, a belt of largefolds and major thrust faults that trendseast-west through west-central Arizonaand southeastern California. In theGranite Wash Mountains, late Mesozoicdeformation related to the Maria beltaffected a diverse suite of rocks, includ­ing Proterozoic crystalline rocks, Paleo­zoic carbonate and quartzose clasticrocks, and Mesozoic sedimentary, vol­canic, plutonic, and hypabyssal rocks.This deformation was mostly deep seatedand produced an assortment of folds,cleavages, and ductile and brittle shearzones. Several discrete episodes ofdeformation occurred, resulting in re­folded folds, folded and refolded thrustfaults, and complex repetition, attenua­tion, and truncation of stratigraphicsequences. Deformation and metamor­phism were followed by emplacement oftwo Late Cretaceous intrusions andnumerous dikes.

Mineralization includes gold depositsassociated with quartz veins, shearzones, and silicification; tungsten depos­its associated with quartz veins andshear zones; and quartz-kyanite depositssimilar to those in the southern Appala­chian Mountains that are associated withlarge gold deposits.

The geology of the Granite WashMountains was mapped between 1982and 1988 as part of the U.S. GeologicalSurvey/ AZGS Cooperative GeologicMapping (COGEOMAP) program. Thisreport, which includes a 1:24,OOO-scalemap, describes major findings aboutstratigraphy, structure, metamorphism,and mineral deposits in the area.

Welty, J.W., Reynolds, S.J., Spencer,J.E., Horstman, K.C., and Trapp, R.A.,1989, List of selected references on thegeology and mineral resources of Arizo­na: Open-File Report 89-5,162 p.

This report, which includes more than4,500 references, is the first step towardan inclusive bibliography on the geologyand mineral resources of Arizona. It wascompiled from bibliographies that werepreviously published by or are currently

9

STAFF NOTES

in-progress at the AZGS. Bibliographicentries are listed alphabetically byauthor. The list is most useful for find­ing a reference for which the author isknown, but not the date or journal.

McGaroin, T.G., 1989, Publications of theArizona Bureau of Mines (1915-77) andthe Arizona Bureau of Geology and Min­eral Technology (1977-88): Open-FileReport 89-6,12 p.

The AZGS was established on July 1,1988. Its two most recent predecessorswere the Arizona Bureau of Geology andMineral Technology and the ArizonaBureau of Mines. Many of the publica­tions of these antecedent agencies areout of print but available in the AZGSlibrary. This report lists all the pub­lications released by these agencies.

Demsey, K.A., 1989, Geologic map ofQuaternary and upper Tertiary alluviumin the Phoenix South 30' x 60' quad­rangle, Arizona: Open-File Report 89-7,scale 1:100,000.

This map shows the distribution ofalluvial deposits of different ages in thePhoenix South 30' x 60' quadrangle andprovides a basis for evaluating the Qua­ternary geologic history of the area.The map was compiled from U2 high­altitude aerial photographs (scale1:129,000), natural-eolor aerial photo­graphs (scale 1:24,000), and field studies.The project was partially funded by theCOGEOMAP program.

Welty, J.W., Reynolds, S.J., and Spencer,J.E., 1989, AZMIN, a digital databasecompilation for Arizona's metallic miner­al districts: Open-File Report 89-8, 42p., high- or low-density flopPY disks.

AZMIN is the result of a 10-yeareffort to determine a classificationmethod, compile information, and createa digital database for Arizona's metallicmineral districts. In the mineral-districtclassification used for this database,known deposits are grouped according togeologic and metallogenic criteria ratherthan the geographic associations used inthe traditional mining-district approach.

AZMIN databases and programs weredeveloped on IBM-PC-eompatible micro­computers through the use of dBase IV,a database-management program.AZMIN consists of 3 database files and10 data-manipulation programs thatallow the user to search data or displaythem in various formats. AZMIN includesmineral-district and mine locations, pro­duction data, and bibliographic informa­tion. This report, the first computerdatabase that the AZGS has offered forsale, includes 42 printed pages ofdocumentation and either high-or low­density, formatted floppy disks, whichthe user must specify upon ordering.

10

Laurette E. Colton completed asupervisory training program at the(Extended) University of Arizona andreceived a Certificate in EffectiveEmployee Supervision on November 7.She has been promoted from ClerkTypist III to Administrative SupportSupervisor I.

Thomas G. McGarvin gave an over­view of the Arizona Geological Sur­vey to the Paleontological Society ofSouthern Arizona on May 8. He pre­sented talks on Arizona's gold depos­its to members of the Desert GoldDiggers and Old Pueblo LapidaryClub on June 6 and October 12, re­spectively. He also led educators onfield trips to view Tucson-area geolo­gy on April 1 and 8 and on October7 and 14. On November 18, he led afield trip to the Tucson Mountainsfor Saguaro National Monumentdocents. McGarvin, along with LarryD. Fellows, led a field trip, "AppliedGeology of the Basin and Range," onNovember 30 as part of the Phoenixregional meeting of the NationalScience Teachers Association (NSTA).On December 2, he served as coleaderof an NSTA workshop, "MineralMysteries and Rock Riddles: Class­room Methods of Identification."

Philip A. Pearthree organized and leda seminar on flood hazards associatedwith alluvial fans for the fall meetingof the Arizona Floodplain Manage­ment Association, held in TucsonSeptember 14-15. The seminar focusedon the nature of, stream behavior on

desert piedmonts and its implicationsfor floodplain management in Arizona.

Stephen J. Reynolds organized or ledtwo field trips to the Tucson Moun­tains: on April 15 for the ArizonaGeological Society and on July 7 forthe International Geological Congress.He also gave three talks: "Fluids andDetachment Faults -- Mineralization,Metasomatism, and Structural As­pects" to the New Mexico Institute ofMining and Technology and NewMexico Bureau of Mines and MineralResources on April 20; "MesozoicEvolution of Western Arizona" togeologists from Northern ArizonaUniversity and the U.S. GeologicalSurvey on May 3; and "Fluids andFaults" to the Arizona GeologicalSociety on July 11.

Jon E. Spencer gave a talk, "CenozoicExtensional Tectonics of the Mohave­Sonora Region," to the Department ofGeology at the University of NewMexico and the New Mexico Bureauof Mines and Mineral Resources onSeptember 20 and 21, respectively.He also served on the Ph.D. researchcommittee for a University of NewMexico graduate student, whose dis­sertation focused on the geology ofthe Van Diemon mine area in theBlack Mountains of Arizona.

John W. Welty gave a briefing on thestatus of mineral-resource informationon Arizona at the Western MineralsInformation Workshop, sponsored bythe U.S. Geological Survey and heldin Sacramento, Calif. in late March.

Arizona Geology, vol. 19, no. 4, Winter 1989

rI

Cooperative Geologic Mapping in Arizona:1989 COGEOMAP Update

Not mapped or mapping inadequate

Arizona Geology, vol. 19, no. 4, Winter 1989

1984-88 COGEOMAP Projects

During the 1984-85 COGEOMAP proj­ect, AZGS geologists mapped the Bighornand Belmont Mountains at a scale of1:24,000 (AZGS Open-File Report 85-14)and prepared a report containing geolog­ic, geochemical, and fluid-inclusion dataon mineral deposits in the area (AZGSOpen-File Report 85-17). The geologicmaps depict numerous normal faults thatcut moderately tilted Tertiary volcanicrocks and their underlying basement.

During 1986 and 1987, AZGS geolo­gists completed 1:24,OOO-scale maps ofthe Hieroglyphic, Wickenburg, and north­eastern Vulture Mountains (AZGS Open­File Reports 86-10, 87-9, 87-10, and 88­1). These maps show many previouslyunrecognized faults, Proterozoic banded­iron formations, a large Cretaceousgranodioritic pluton, and a Miocenevolcanic field. Some normal faults haveseveral kilometers of displacement, whichhelped to accommodate 150 percentcrustal extension. They are commonlythe loci of middle Tertiary hydrothermalalteration and mineralization.

ern United States; (4) low-angle normal(detachment) faults that have regionaltectonic and economic significance; and(5) previously undescribed areas of alter­ation and mineralization.

1988-90 COGEOMAP Projects

The 1988-89 COGEOMAP project pro­duced a 1:24,OOO-scale geologic map ofthe southeastern Vulture Mountains(AZGS Open-File Report 88-9) and theVulture mine, one of the premier golddeposits in Arizona. Geologic studies(AZGS Open-File Report 88-10 and thisissue of Arizona Geology) documentedthat mineralization at the Vulture minerepresents a midlevel, pluton-relatedCretaceous vein that has been turned onits side by Tertiary fault-block rotation.The top of the orebody was removedpre-Miocene erosion ratherfaulting, as previous int:erlJre!tatiOIlssuggested.

The 1988-89 COGEC)Ml\'Presulted in the1:I,OOO,OOO-scale,Arizona (AZGS Mapof three l:lUU"uUU-SCaJlenary deposits Ot:len.-.Fil.e88-4,88-17, and

Thewhich beganconcentrated

cost-sharing program, AZGS geologistshave concentrated their mapping in thePhoenix 10 x 20 quadrangle (Figure 1)and adjacent parts of west-eentral Ari­zona. This region is geologically complexand highly mineralized, but very poorlyunderstood. The Phoenix quadrangle isalso the site of rapid urban growth andmajor construction projects, such as thePalo Verde Nuclear Generating Station,Central Arizona Project, New WaddellDam, and a hazardous and toxic wasterepository.

During the past 9 years, AZGSgeologists have completed 1:24,000-scalegeologic maps of the Belmont, Big Horn,Granite Wash, Hieroglyphic, Little Har­quahala, Maricopa, South, Vulture, andWickenburg Mountains, Aguila Ridge­Bullard Peak area, and Merritt Hills(Figure 1). Parts of the Bouse Hills andBuckskin, Harcuvar, Harquahala, andWhite Tank Mountains have also beenmapped. From this mapping, major geo­logic discoveries have been made, includ­ing (1) previously unknown Mesozoicsequences and an early Mesozoic upliftevent; (2) the Maria fold-and-thrust belt,a previously unrecognized Mesozoicthrust belt; (3) a suite of quartz-kyaniterocks similar to those associated withgold on the Piedmont of the southeast-

33o _ L1--_.......::~~~----~::..-_---------=~-----""""1-330114

0

• ~ PHOENIX QUADRANGLE 1120

Mapped ~ SCALE

li::::::t::::::::::::1 Being mapped::;;~~IO~~3;:~O~~3~:O. :::::"0"D

by Stephen J. Reynoldsand Michael J. GrubenskyArizona Geological Survey

Figure 1. Status ofgeologic mapping in the Phoenix 1° x 2° quadrangle.

A major legislated responsibility ofthe Arizona Geological Survey (AZGS) isto characterize the geologic frameworkof Arizona. To help fulfill this respon­sibility, the AZGS has placed a highpriority on geologic mapping, especiallyon producing high-quality, quadrangle­scale (1:24,000) geologic maps ofpreviously unmapped areas. Such mapsincrease the understanding of the geo­logic framework, mineral potential, andgeologic hazards of an area by helpingto define the stratigraphy, structure,geologic history, and distribution andsetting of mineralization and alteration.The quadrangle-scale maps may be usedto compile intermediate-scale (e.g.,1:100,000 or 1:250,000) maps to definethe regional distribution of mineralresources and geologic hazards and iden­tify areas where additional mapping isneeded. Maps at both scales will beused to produce a new 1:500,OOO-scale

•

eologic map of Arizona.Since 1984, the AZGS and U.S. Geo­

Dgical Survey (USGS) have participatedin the Cooperative Geologic Mapping(COGEOMAP) program. As part of this

r---- Arizona Geology --------,

Arizona Geological Survey

Director & State Geologist: Larry D. FellowsEditor: Evelyn M. VandenDolderEditorial Assistant: Nancy SchmidtIllustrators: Peter F. Corrao, Sherry F. Garner

complete the Salome 1:100,OOO-scalequadrangle map (northwestern quarter ofthe Phoenix l O x 20 quadrangle). AZGSgeologists will also start mapping thtte...geology of the Little Horn Mountains i 'the southwestern quarter of the Phoenil O x 20 quadrangle. This range is aneastward continuation of the middleTertiary Kofa volcanic field and containsseveral important areas of Tertiaryprecious-metal mineralization.

After the New River and Little HornMountains are mapped, the AZGS willhave achieved one long-term goal: toproduce a regional northeast-trendingtransect from the Kofa Mountains insouthwestern Arizona to the edge of theTransition Zone. This transect, whichincludes the Kofa, Little Horn, Big Horn,Belmont, Vulture, Wickenburg, and Hier­oglyphic Mountains and New River area,will enable AZGS and USGS geologists toaddress such fundamental issues as (1)the tectonic significance of the Basinand Range Province - Transition Zoneboundary, (2) the magnitude of crustalextension in this part of the Basin andRange Province, and (3) the regionalcontrols and timing of mineralization.

Winter 1989

Governor Rose MoffordState of Arizona:

Vol. 19, No.4

Dikes associated with the younger suitecommonly intrude along low-angle faultsand are extensively K-metasomatized. Aunique series of Sr- and U-bearinglacustrine rocks overlies the youngerrhyolitic suite in the western end of therange. Even younger, largely posttec­tonic basalts unconformably overlie tiltedrocks in several parts of the range.

The AZGS recently released a1:24,OOO-scale map of the Granite WashMountains, one of the most structurallycomplex mountain ranges in the westernUnited States. This map is accompaniedby a detailed description of rock units,structural evolution, and mineral deposits(AZGS Open-File Report 89-4). Therange consists of a stack of imbricate,ductile thrust faults that juxtaposedProterozoic and Jurassic crystalline rocksdiscordantly over an upturned section ofPaleozoic and Mesozoic supracrustalrocks. The rocks show evidence of threeepisodes of thrusting, each with a dif­ferent transport direction. The thrustsheets were later folded by two genera­tions of large folds, some of which haveamplitudes of 1 kilometer. Massivequartz-kyanite rocks were discovered infour areas; these rocks are similar tothose associated with gold on the Pied­mont of the southeastern United States.

AZGS geologists will spend the 1989­90 winter field season mapping the NewRiver area and White Tank Mountains tocomplete the Phoenix North 1:100,000­scale quadrangle map (northeastern quar­ter of the Phoenix lOx 20 quadrangle).Continued mapping in the western Har­cuvar Mountains and Bouse Hills will

scale map of the entire Vulture Moun­tains (AZGS Map 27, in press). Thispublication, based on field studies con­ducted from 1986 to 1989, represents amajor advance in the knowledge of thisimportant mountain range. The rangeconsists of Miocene volcanic rocks thatdepositionally overlie a basement ofProterozoic crystalline rocks and a largeCretaceous granitoid pluton. Miocenecrustal extension formed a series ofnorth-trending, tilted fault blocksbounded by low-angle normal faults. TheTertiary units have been steeply tiltedin much of the range; each fault blockexposes an originally vertical cross sec­tion through the Miocene volcanic field.In such cross sections, rhyolitic lavaflows can be traced downward into theiroriginal feeder zones, now representedby rhyolitic dikes. Similar cross sec­tions through the Cretaceous pluton areexposed in the tilt blocks.

Two distinct suites of Miocene rhyo­litic volcanic rocks have been identifiedin the Vulture Mountains: an older I­to 1.5-kilometer-thick sequence that pre­dates extension and a younger thinnersequence that postdates much extension.

Arizona Geological Survey845 N. Park Ave., #100Tucson, AZ 85719TEL: (602) 882-4795