Sericitic and Advanced Argillic Mineral Assemblages and Their Relationship to Copper Mineralization, Resolution Porphyry Cu-(Mo) Deposit, Superior District, Pinal County, Arizona by Alexander Raine Winant A Prepublication Manuscript Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 2010 1 STATEMENT BY THE AUTHOR This manuscript, prepared for publication in Economic Geology, has been submitted in partial fulfillment of requirements for the Master of Science degree at The University of Arizona and is deposited in the Antevs Reading Room to be made available to borrowers, as are copies of regular theses and dissertations. Brief quotations from this manuscript are allowable without special permission, provided that accurate acknowledgment of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the Department of Geosciences when the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. __________________________________________ _____________ (author’s signature) (date) APPROVAL BY RESEARCH COMMITTEE As members of the Research Committee, we recommend that this prepublication manuscript be accepted as fulfilling the research requirement for the degree of Master of Science. Dr. Eric Seedorff__________________________ _____________ Major Advisor (type name) (signature) (date) Dr. Mark D. Barton____________________________ _____________ (type name) (signature) (date) Dr. Frank K. Mazdab _________________________ _____________ (type name) (signature) (date) 2 Sericitic and Advanced Argillic Mineral Assemblages and Their Relationship to Copper Mineralization, Resolution Porphyry Cu-(Mo) Deposit, Superior District, Pinal County, Arizona Alexander R. Winant and Eric Seedorff Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona 1040 East Fourth Street, Tucson, Arizona 85721-0077 Hamish R. Martin Resolution Copper Company, 47206 N. Magma Shaft #9 Road, Superior, Arizona 85273 Frank K. Mazdab and Mark D. Barton Lowell Institute for Mineral Resources, Department of Geosciences, University of Arizona 1040 East Fourth Street, Tucson, Arizona 85721-0077 3 Abstract The Resolution deposit is a giant, deep, high-grade deposit in the Laramide porphyry copper province of Arizona that is currently being developed. This study focuses on the features at Resolution that formed from acidic hydrothermal fluids (including sericitic and advanced argillic alteration types) that are well developed in the upper part of the system. The distribution of alteration-mineralization features are illustrated along two, roughly perpendicular fences of drill holes that were logged with concurrent mineral identifications made with a PIMA™ infrared spectrometer and ultraviolet light and supplemented with subsequent reflected and transmitted light petrographic observations. Hydrothermal minerals formed during intense hydrolytic alteration at Resolution commonly are related to multiple superimposed, crosscutting events. Though showing some degree of stratigraphic control, particularly at deep levels, the distribution of hydrothermal minerals and mineral assemblages shows only weak degrees of structural control at the deposit scale. The intermediate sulfidation opaque assemblages containing chalcopyrite characterize the many hydrothermal mineral assemblages that formed potassic alteration of igneous rocks, skarn, and calc-silicate hornfels, which are best developed outside the region of this study. Earlier sericitically altered rocks contain pyrite ± chalcopyrite, but later sericitic and advanced argillic assemblages contain higher sulfidation state opaque assemblages, such as pyrite + bornite ± chalcocite with kaolinite, dickite, and topaz, with lesser alunite, pyrophyllite, and zunyite. 4 Veins with assemblages characteristic of advanced argillic alteration consistently offset veins associated with sericitic alteration. Most of the advanced argillic assemblages at Resolution formed at relatively low temperatures, stable with kaolinite and dickite. Resolution contains fairly high levels of fluorine. The most important fluorine-bearing minerals are biotite (~3-4 wt% F), topaz (~11-12 wt% F), fluorite (~49 wt% F), and sericite (~1 wt% F), although other fluorine-bearing phases also are locally present (e.g., zunyite, 6-7 wt% F). Topaz formed at Resolution during advanced argillic alteration and the mineral has a relatively fluorine-poor composition (XF-tpz ~0.6), as is topaz from other base-metal lode deposits such as Butte, in contrast to topaz in those porphyry deposits in which a more fluorine-rich topaz occurs in sericitic and potassic assemblages. Resolution is a relatively arsenic-poor system, in strong contrast to the nearby Magma vein system. The deeper part of the ore body, where potassic alteration dominates, is nearly arsenic-free, whereas the upper part of the copper ore body is arsenic-bearing. Although enargite has been observed petrographically, arsenic occurring in solid solution in other sulfides (e.g., arsenic-bearing pyrite) may be responsible for many of the local spikes in arsenic content at Resolution. Introduction Intense hydrolytic alteration of the sericitic and advanced argillic types, though known also from other types of hydrothermal ore deposits, occurs commonly in three related types of 5 magmatic-hydrothermal ore deposits, porphyry deposits, base-metal lode deposits, and acid-sulfate or high-sulfidation epithermal deposits (Meyer and Hemley, 1967; Hemley et al., 1980; Einaudi, 1982; Arribas, 1995; Seedorff et al., 2005a). Intense hydrolytic alteration, regardless of deposit type, can be pervasive or can be confined to structures or stratigraphic units; rocks exhibiting intense hydrolytic alteration can be barren to highly mineralized. Where mineralized, high- to very-high sulfidation state opaque minerals commonly are associated with advanced argillic alteration of silicate minerals (Meyer and Hemley, 1967; Einaudi, 1982; Einaudi et al., 2003). Intense hydrolytic alteration is characteristic of shallower levels of certain porphyry systems (e.g., Red Mountain, Arizona; Resolution, Arizona; El Salvador, Chile; Central deposit, Oyu Tolgoi, Mongolia) and base-metal lode deposits (e.g., Bisbee, Arizona), though sericitic and advanced argillic alteration can persist to deep levels, as at Butte, Montana (Bryant, 1968; Meyer et al., 1968; Corn, 1975; Gustafson and Hunt, 1975; Bodnar and Beane, 1980; Hedenquist and Lowenstern, 1994; Reed and Meyer, 1999; Watanabe and Hedenquist, 2001; Manske and Paul, 2002; Khashgerel et al., 2009). For the high-sulfidation epithermal deposits, links to porphyry systems are well established in certain cases (e.g., Lepanto- Far Southeast in the Philippines) but to date are lacking in many other districts (e.g., Goldfield, Nevada, and Yanacocha, Peru) (Einaudi, 1982; Arribas et al., 1995; Harvey et al., 1999; Sillitoe and Hedenquist, 2003). Likewise, it is not necessarily clear whether fluids that formed intense hydrolytic alteration represent evolution of fluids that produced potassic alteration at earlier stages or whether they 6 represent a temporally distinct hydrothermal system (e.g., Meyer et al., 1968; Brimhall and Ghiorso, 1983). Rocks exhibiting intense hydrolytic alteration commonly represent a special challenge in identifying mineral assemblages, defined as a group of minerals that appear to be stable together at the mesoscopic scale and to have formed contemporaneously (e.g., Seedorff et al., 2005a). In many cases, the hydrothermal minerals clearly are related to multiple superimposed, crosscutting events, yet the identity of the products of each event may be difficult to determine at the hand specimen scale. Moreover, the silicate minerals commonly are light colored, fine-grained, and difficult to identify with the naked eye or hand lens and in some cases petrographically, such as distinguishing between sericite and pyrophyllite. Even where the minerals can be determined by with aid of infrared spectrometers and X-ray diffraction, the textural relationships generally are lost at the spatial scales of such determinations, i.e., the minerals identified may have formed in multiple events, so the nature of the mineral assemblage remains uncertain. For these reasons, the identification of mineral assemblages within areas of intense hydrolytic alteration commonly is avoided or not deemed possible (e.g., Khashgerel et al., 2006), thereby limiting the types of geochemical or genetic conclusions that might be drawn. This study was conducted at the Resolution deposit in Arizona. The study focuses on the upper part of the Resolution system where sericitic and advanced argillic assemblages are prevalent, building on work by Manske and Paul (2002), Ballantyne et al. (2003), Schwarz (2007), and the geologic staff at Resolution, especially on previous work by Troutman (2001) 7 and Harrison (2007) on sericitic and advanced argillic alteration. The purposes of this study are to document the distribution, abundance, and compositions of associated hydrothermal minerals, to attempt to define the mineral assemblages that constitute sericitic and advanced argillic alteration, to determine the lateral and vertical changes in abundance of sericitic and advanced argillic alteration,,