Porphyry Copper Deposits of the American Cordillera Frances Wahl Pierce and John G. Bolm, Editors Arizona Geological Society Digest 20 1995 The Helvetia Area Porphyry Systems, Pima County, Arizona S. A. ANZALONE ASARCO Incorporated, Tu cson, Arizona ABSTRACT INTRODUCTION The Helvetia area porphyry copper deposits occur within The Helvetia area copper deposits occur within a large an extensive Laramide porphyry system located in the Santa Laramide porphyry system located in the Santa Rita Mountains, Rita Mountains, Pima County, Arizona. The entire system Pima County, Arizona. This porphyry copper system consists of consists of four separate centers of copper mineralization: four separate areas of copper mineralization - Rosemont, Peach­ Rosemont, Peach-Elgin, Broadtop Butte, and Copper Wo rld. Elgin, Broadtop Butte, and Copper World - lying within a Mineralization and alteration are primarily of the contact broad alteration zone in the northern segment of the Santa Rita pyrometasomatic type and hydrothermal alteration and zon­ Mountains. Mineralization and alteration are primarily contact ing of sulfide mineral assemblages resemble those observed at pyrometasomatic, and zoning of hydrothermal alteration and sul­ the Twin Buttes Copper Mine located approximately 30 kilo­ fide mineral assemblages is similar to those observed at the meters west of Helvetia. The stratigraphic sequence, ranging Twin Buttes and Mission Copper Mines located approximately from Cambrian Bolsa Formation to Permian Rain Va lley For­ 30 kilometers west of Helvetia. ASARCO acquired the copper mation, correlates well with sections developed in the Tw in deposits within the Helvetia area porphyry systems in 1988 and Buttes and Mission Mine areas. The Paleozoic section in the has continued the exploration and development effort since then. deposit area totals approximately 1,800 meters. A considerable amount of excellent geological work has Two Te rtiary intrusives penetrate the Paleozoic-Meso­ been completed on the Helvetia area porphyry systems but little zoic stratigraphy in the region. One, a quartz-monzonite information has been published on developments since the mid- stock, occurs primarily in the western portion of the proj ect 1950s. The geology of the copper deposits as developed by area. The other, a hydrothermally altered quartz-Iatite por­ numerous geologists over the past 75 years and the large, bulk­ phyry, is mineralized locally and the copper mineralization tonnage low-grade copper deposits outlined in recent years will in adjacent exoskarn is considered to be genetically related be briefly reviewed in this paper. to it. Limestones in contact with the porphyry have been lo­ cally metasomatically altered to lime-silicate skarns, princi­ LOCATION pally tactite. The tactite is, for the most part, grossularite or andradite garnet with varying amounts of diopside, tremo­ The Helvetia area porphyry systems are located approxi­ lite, wollastonite, quartz, and vesuvianite. Occasional Te r­ mately 50 kilometers southeast of Tucson, Arizona, in the north­ tiary lamprophyric dikes intrude the area and the immediate ern Santa Rita Mountains (fig. 1). They lie within the Basin and region is underlain by Precambrian granodiorite. Range Physiographic Province at elevations ranging from 1,402 Ore mineralization was introduced during the later to 1,890 meters. stages of pyrometasomatic hydrothermal alteration. Pri­ mary sulfide minerals include chalcopyrite, bornite, and py­ EXPLORATION AND MINING HISTORY rite with chalcopyrite and pyrite predominating. These occur principally as veinlets, coarse disseminations, blebs, Copper mineralization may have been discovered in the and clots within irregular lenticular zones lying in and gen­ Helvetia District prior to the Civil War, but no records are avail­ erally parallel to the lime-silicate areas. A limited tonnage of able for these early discoveries. The district has had a relatively secondary chalcocite and covellite occurs irregularly small production of copper ore, principally from underground throughout the deposit. In the oxidized zone, a substantial mines. In the late I 880s copper ore from the district was treated tonnage of copper occurs as chrysocolla, azurite, malachite, at the Columbia Smelter located on the west side of the Santa cuprite, and chalcanthite. Minor amounts of silver, molyb­ Rita Mountains, and the Rosemont Smelter located on the east denite, gold, sphalerite, and scheelite occur throughout the flank of the same range near Rosemont Camp (Creasey and various mineral centers. Quick, 1955). In 1903, the Helvetia Copper Company began op­ Post-ore faulting, principally high-angle normal and eration and continued until 19I I. Copper was produced almost thrust faulting, terminates all four mineral centers. The continuously from 19 I 5 to 1951. In the 1940s, some dissemi­ Peach-Elgin mineral center lies within the Helvetia Klippe nated copper mineralization in skams was mined from small and is considered to be the offset upper segment of the Cop­ open pits located in the Elgin area. Total production from the per World mineral center. Helvetia District through 1950 totaled 227,300 tons of ore con­ Approximately 570 holes have been drilled in the area taining 17,290,000 pounds of copper, 1,097,980 pounds of zinc, by various companies: Lewisohn Copper Corp., Banner and 180,760 ounces of silver (Schrader, 1915; Creasey and Mining Company, Anaconda, Anamax, and ASARCO. The Quick, 1955). deposit is amenable to open pit mining and the metallurgical After 1950, activities consisted mainly of exploration and recoveries should be similar to those achieved at the Mission development drilling. The Lewisohn Copper Company con­ and Twin Buttes Mines. ducted a drilling program in the Peach-Elgin area in 1955 and in 436 440 s. A. ANZALONE c' c GRANODIORITE (PRECAMBRIAN) o Figure Cross section through Copper World area (after Schrader, 19I5, and Drewes, 1972). Location ofcross section shown on figure 2. S. FORMA THCK SECTION DESCRIPTION noN ARKOSIC SAN>STot£ WLLOW AN> CANYON SILTSTONE. AN>ES/TE, � FORMATION 870m VOLCANIC AND CI£RT § COBBLE CONGLOMERATE 000( I-----+------!i..:::: t:i a: u QUARTZ, FINE-GRAINED, SCHERRER DOLOMITE, MINOR SIL TSTOt£ FORMATlON 220m AT BASE �z 000( Lt.fESTONE, MARL, SILTSTONE, EPITAPH a: DOLOMITE, LOCAL GYPSUM UJ FORMATION a.. 305m AN> QUARTZITE COLINA LS UMESTONE. MEDIUM-THlCK-BEDDED 105m EARP SILTSTONE, SHALE, SOME FORMATION 245m SANDSTONE AND UMESTONE HOROUIUA L1MESTOt£, THN-MASSIVE-BEDDED, � SL TSTOt£, t.tNOR SHALE AN> ut.£STONE 245m � CONGLOt.£RATE AT BASE ESCABROSA UMESTONE. THICK-MASSlVE- � Lt.£STONE 170m BEDDED. LOCAL Ct£RT S 120m SILTSTOt£, LJt.£STONE, 225- AND QUARTZITE,SHALE, Y 275m tITERBEDOEDTIft.. Figure 6. Stratigraphic column of the Helvetia area porphyry systems (after McNew, 1981). .