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Geology of Kennecott Copper Corporation's Safford copper deposit Annan Cook and R. F. Robinson, 1962, pp. 143-147 in: Mogollon Rim Region (East-Central Arizona), Weber, R. H.; Peirce, H. W.; [eds.], New Mexico Geological Society 13th Annual Fall Field Conference Guidebook, 175 p.

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GEOLOGY OF KENNECOTT COPPER CORPORATIONS SAFFORD COPPER DEPOSIT

By ANNAN COOK and R. F. ROBINSON Resident Geologist, Safford Project, Kennecott Copper Corporation.

Senior Geologist, Southwest District, Bear Creek Mining Company.

LOCATION AND HISTORY GEOLOGY Kennecotts Safford Development Project is located in The sequence of rocks exposed in the area is shown the Lone Star Mining District, thirteen miles northeast of in the tabulation below: the town of Safford, county seat of Graham County, Ari- zona. The nearest active mining property is Phelps Dodges Sedimentary and Volcanic Rocks Morenci open-pit copper mine, located nineteen airline Formation Rock Types Thickness Age Alluvium, Gila Sands, siltstones 0-2,000 Quaternary- miles to the northeast (or 60 miles by road). Recorded conglomerate conglomerates Pleistocene production from the Lone Star Mining District up to 1907 Unconformity amounted to only 110,000 lbs. of copper — mainly from Younger Vol- Basalt, tuffs 0,-1,000 Tertiary(?) the San Juan Mine. This production was mined from small conic rocks agglomerates shear zones or veins which occur in quartz-monzonite por- Unconformity to disconformity phyry, andesite, or . Intermediate Andesite, tuffs, 0-2,000 Post- flows, agglomerates plus mineral The mining district lies south of the Gila Mountains, Volcanic rocks which are covered by later unmineralized basalt, andesite, Unconformity A agglomerate and . From 1949 to 1952 Consolidated Older Andesite, flow- 2,500 plus Pre- Copper Mines Corporation and the American Metal Com- Volcanic breccia, tuffs mineral Cretaceous(?) pany, Ltd., optioned some of the existing claims, drilled rocks four holes in the outcropping mineralized , and Unconformity Cambrian ( ?) five holes through the basalt-covered area. While only Quartzite Quartzite (?) low-grade copper mineralization was encountered, some of Intrusive Rocks the mineralization was found to occur as disseminations in Formation Age the andesites and rhyolites. Basalt dikes Post-mineral

In 1955 Bear Creek Mining Company (Kennecotts ex- , quartz-latite, Older than Unconformity A rhyolite and quartz- (Intrusive into Older Volcanics and ploration subsidiary) commenced reconnaissance geologi- diorite dikes and plugs Lone Star pluton) cal mapping in the Gila Mountains. Although results of the previous drilling were not available to Bear Creek at Quartz-monzonite porphyry Older than Unconformity A (Intrusive into Older Volcanics) this time, the sheared, altered and iron-stained outcrops Lone Star pluton (quartz- Older than Unconformity A looked encouraging enough to warrant further investiga- diorite, diorite) (Intrusive into Older Volcanics) tion. The same area was therefore optioned in 1956. As will be seen from the tabulation, and also from After the area was mapped geologically — assisted by the attached geological map (Fig. 1), the oldest rocks in geophysical surveying and geochemical sampling — a dia- the area consist of the small outcrops of quartzite of pos- mond drilling program was commenced. When drilling sible Cambrian age. Next younger in age is the group of indicated the existence of a sizeable low-grade copper de- Older Volcanics, consisting mainly of andesite with some posit underlying the unmineralized later volcanics in the flows and lithic tuffs, strongly sheared by northeast fault Gila Mountains, the property was purchased by the Com- zones. Intrusive into the andesite is the Lone Star pluton, pany in 1959. consisting of diorite and quartz-diorite. Still later intru- Since this time development work has been carried sions include a quartz-monzonite porphyry plug (at the on at the property. In 1961 a development shaft was sunk San Juan Mine) and numerous northeasterly-trending dikes, to a depth of 804 feet, followed by 1,500 feet of cross- and probably three plugs of latite, quartz-latite and rhy- cutting and 1,500 feet of drifting on the 3900 level (754 olite — only one of which is exposed at the surface. feet from the collar). Both underground and surface drill- Dikes of these rock types also intersect both the quartzite ing is currently being done. and the Lone Star pluton.

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▪ ++++ 4-.4- : +4-+ + 4- ++44+ 4+ ++++ 4,,, ÷ 4 44, 4 4-÷ ++ ++++ ...-4- +++ + + + + +++ + + 4- 4. 4- + ++ + + 44.4.4" . 4. ++ .. + .4 + + ++.4-4:4:4:++÷++ i. F AULT O -44:+: ,. 4. + t+ -:"++,,-.-:-, -7 -.--+

LEGEND

QUATERNARY I I ALLUVIUM TERTIARY DE BASALT ( YOUNGER VOLCANICS)

1 /,1 ANDESITE, TUFF, AGGLOMERATE,FLOWS. ( INTERMED. VOLCAN.) QUARTZ LATITE, LATITE, RHYOLITE TERTIARY OR LITHIC TUFF CRETACEOUS rn1 QUARTZ DIORITE, DIORITE CRETACEOUS (?) ANDESITE (OLDER VOLCANICS) CAMBRIAN (?) QUARTZITE SCALE IN FEET ==1 FAULTS, SHEARS O 2000 4000 • DRILL HOLE

Fig. I- GENERALIZED GEOLOGICAL MAP, SAFFORD PROJECT AREA ,. G S 34° 30 W N 34° 30 E U

G-12 G II

G -13 SD-7 .-r •

....., .r...., ,.: ..,,,.." ,r --h: n .....,,,, ," r, ,,, --, .r,r, ,., N. ,,, • ,,,,.,,,,, LITTLE LONE .... N. --. ...... ,,,,,, ,r , rr,/,, r,,, ,,,,,,,,,/, • LEACHED r CAPPING - ■d,/, /// // /// ////// ,/,/,//,///////////// G-38 2 rrr / --•• .4,,..,_,,X, / //////////,//1///////,/// STAR SHAFT -" .... s• ■,....1.4.,. //////////////,/////////// 77 7 \ ,., - -,,,,,,,,,,,,,,,,,,,,,, (..._ . -„- ,,‘ ,,-,.:../- . . , -- --..., a4 ) . -..,..,,• ,----• -....: -- -....-- ), ( . , = , ---_...... - ..-7•..”- 3 1 7 ,, , -... .■,\77 ELEV.400000 „ " ....,4.,

= ,

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— . -- _ 2472 , 21 7 2132

LEGEND

I I QUATERNARY ALLUVIUM SECT I ON G - 01 mil BASALT (11 INTERBEDDED TUFFS) F-1 INTERMEDIATE VOLCANICS

V....1 QUARTZ LATITE-LATITE- RHY. o soo loon X SCALE IN FEE 1 .:::1 CRYSTAL LITHIC TUFF z o ,;ILd ANDESITE 7 Fig. 2 V) X N 55°30W S55° 30 E AX

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.. ...," 4 C- 2 171 • , F.,,, I / ..-- ... ....

-.820 1908_. 17

/1195 ...---- . / _ - ... / -- ELEV. 3000 222e ♦ • / .. / r ■ / ♦ / 186e c..-- --) ■ , " ". / / .,.. -.> 2296 ... "-. / ____ •-• OUTLINE OF MINERALIZED ZONE ( LEGEND \ — ----- II

QUATERNARY ALLUVIUM SECT ION X- XI min BASALT (B INTERBEDDED TUFFS)

1%%%:, 1 INTERMEDIATE VOLCANICS 2991

1 I , NI QUARTZ LATITE - LATITE- RHY. 0 50 100 (.., SCALE IN FEET MI CRYSTAL LITHIC TUFF z o Fil ANDESITE 2 Fig.3 to NEW MEXICO GEOLOGICAL SOCIETY 4‘ THIRTEENTH FIELD CONFERENCE 147

Shearing and fracturing along the northeast shear on which rock type is present . Sericitization is restricted to zones continued after the later intrusions. Following this, rhyolites, and quartz-latites, whereas biotitization is the area was hydrothermally altered and mineralized — mainly limited to the andesites. These two zones, occupy- principally with iron and copper-bearing sulfides. In time ing an area of approximately 7,000 by 7,000 feet, conse- the deposit probably became enriched by the formation of quently may be regarded as being equal high intensity a chalcocite blanket. Subsequently it was covered by the zones of potash introduction. so-called Intermediate Volcanics (andesite, tuffs and Where exposed, the zone of chloritization forms a flows) and the Younger Volcanics (basalt, tuffs and ag- partial halo 1,000-2,000 feet in width around the zones glomerates). Northwest block-faulting then took place, re- of sericitization and biotitization. Within this zone — sulting in the uplift and tilting of the Gila Mountains to which grades into the zone of propylitic alteration and the east and further oxidation of the presumably previous- unaltered rock — strong sericitization occurs locally along ly-enriched ore deposit. mineralized veins, shear zones, or dikes. STRUCTURE Supergene alteration has resulted in the development The most prominent structural element exposed is the of kaolinite, quartz, limonite, some sericite, and local alu- strong and dominant northeast trend of shearing in the nite and gypsum. While evidence of supergene effects is Older Volcanics. This has also been borne out by under- found at depth in most drill holes, the supergene minerals ground development. The early age of this shearing is usually can be distinguished from the hypogene alteration suggested by the probability that it guided the emplace- products. ment of the Lone Star and San Juan plutons, as well as the Alteration within the San Juan quartz-monzonite por- quartz-latite, latite and rhyolite dikes. It also guided and phyry intrusive has a roughly concentric configuration, and localized alteration and mineralization. Movement on consists of a central silicified, sericitized zone 2,500 feet some of the faults was late enough to displace the Tertiary in diameter. Adjoining andesites are biotitized in a halo basalts — e., the Trojan fault, on which there is 170 feet for an additional 1,500 - 2,000 feet, followed by an in- of throw. complete halo of chloritization approximately 1,000 feet While the northeast shearing is most intense in the wide. area straddling the Lone Star Mine (approximately 1,500 As previously mentioned, the Safford Project copper feet on each side), it also persists less intensely for 5,000 deposit occurs in the first two zones of alteration. How- feet south of the Trojan fault. To the northwest it is also ever, a pyritized zone some 20,000 feet in length and less intense inside the Lone Star pluton. However, on the 4,000 - 9,000 feet in width is associated with the ore, northwest flank of this intrusive it increases in intensity overlapping into the chloritized zone. A similar zone (as in the At Ease fault zone) but falls of again towards 3,000 - 6,000 feet in extent is present around the San the San Juan quartz-monzonite porphyry. Juan intrusive. Small amounts of gold also occur in quartz While the northwest trend of the Gila Mountains is veins situated 1,000 feet south of the orebody. Some ga- considered to have been created by block faulting, the lena was found near Corncob Hill — 4,000 feet south- Butte fault is the only major fault found to be parallel with west of the center of the mineralized zone. the range. Based on the difference in elevation of the As stated above, the orebody occurs in the Older Vol- base of the basalt on each side of this fault, the displace- conics, which were later covered by unmineralized Inter- ment here may be as much as 2,000 feet. mediate and Younger Volcanics. Below the unmineralized No extensive breccias occur within the main mineral- volcanics is a leached zone 20 - 900 feet thick. Hypogene ized section, but a tectonic breccia (500 feet by 100 feet) mineralization consisted predominantly of pyrite and chal- does occur northwest of the San Juan quartz-monzonite. copyrite occurring in vein lets or disseminations. Very minor A few structural breccias and breccia pipes (1-6 feet in quantities of bornite, molybdenite, sphalerite, galena, mag- width), and also small pebble dikes, occur in the vicinity netite, and specularite also occur. of the Lone Star pluton. Around the San Juan intrusive During later supergene enrichment, chalcocite and some of the breccias are mineralized with chalcopyrite — covellite were formed by replacement of chalcopyrite and making them pre-sulfide mineralization in age. pyrite. Because of the later uplift of the Gila Mountains, From surface and underground mapping, as well as oxidation to great depths occurred and produced the underground drilling on the Safford Project, there appear to mixed oxide-sulfide orebody being developed today. The be three main plugs of rhyolite (with attendant dikes) most characteristic and abundant copper mineral in the de- intrusive into the andesite. The rhyolite also intruded a posit is chrysocolla, with important amounts of brochantite. lithic tuff of similar composition. This tuff may have origi- Lesser quantities of cuprite, pseudo-malachite, native cop- nated earlier from the same vent or vents up which the per, antlerite, turquoise, and malachite are also present. rhyolite plugs were intruded later. The ore deposit occurs ACKNOWLEDGMENTS in the rhyolite plugs and adjoining areas of andesite, Much of the data described above are based on geo- whereas the lithic tuff is only slightly mineralized. logical mapping and petrological studies by Bear Creek ALTERATION AND MINERALIZATION geologists, particularly R. F. Robinson, Annan Cook, D. Four hypogene alteration zones have been recognized Maltzahn, W. Kurtz, A. Rose, A. Banerjee, L. Bryner, D. on the property from surface exposures, diamond drilling Huckins, D . Lobiondo, K. Martin and C. Hall and underground mapping, namely: Additional valuable assistance in mineral and rock (1) Zone of strong sericitization and silicification. identification was given by Dr. Colin Hutton and Dr. Robert (2) Zone of strong biotitization. Compton of Stanford University, and Dr. R. L. DuBois of the (3) Zone of chloritization. University of Arizona. (4) Zone of propylitic alteration or unaltered rocks. Thanks are due to Kennecott Copper Corporation for The orebody occurs in zones (1) and (2), depending permission to publish this paper. 0