Descriptions of mineral occurrences and interpretation of mineralized rock geochemical data in the Stikine geophysical survey area, southeastern Alaska by Cliff D. Taylor1 Open File Report 03-154 2003 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This report is intended for immediate release. It will be superceded by the release of this report as an integral chapter in a Professional Paper on the results of a larger body of work in the Stikine geophysical survey area. U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 1 U.S. Geological Survey, Denver, Colorado 1 DESCRIPTIONS OF MINERAL OCCURRENCES AND INTERPRETATION OF MINERALIZED ROCK GEOCHEMICAL DATA IN THE STIKINE GEOPHYSICAL SURVEY AREA, SOUTHEASTERN ALASKA By Cliff D. Taylor U.S. Geological Survey Central Mineral Resources Program Box 25046 Federal Center, MS-973 Denver, CO 80225-0046 ABSTRACT Detailed descriptions of some of the more significant mineral occurrences in the Stikine Airborne Geophysical Survey Project Area are presented based upon site-specific examinations by the U.S. Geological Survey in May of 1998. Reconnaissance geochemical data on unmineralized igneous and sedimentary host rocks, and mineralized rocks are also presented and are accompanied by a brief analysis of geochemical signatures typical of each occurrence. Consistent with the stated goal of the geophysical survey; to stimulate exploration for polymetallic massive sulfides similar to the Greens Creek deposit, the majority of the described occurrences are possible members of a belt of Late Triassic mineral deposits that are distributed along the eastern edge of the Alexander terrane in southeastern Alaska. Many of the described occurrences in the Duncan Canal-Zarembo Island area share similarities to the Greens Creek deposit. When considered as a whole, the geology, mineralogy, and geochemistry of these occurrences help to define a transitional portion of the Late Triassic mineral belt where changes in shallow to deeper water stratigraphy and arc-like to rift-related igneous rocks are accompanied by concomitant changes in the size, morphology, and metal endowments of the mineral occurrences. As a result, Late Triassic mineral occurrences in the area appear as: 1) small, discontinuous, structurally controlled stockwork veins in mafic volcanic rocks, 2) small, irregular replacements and stratabound horizons of diagenetic semi-massive sulfides in dolostones and calcareous shales, and as 3) larger, recognizably stratiform accumulations of baritic, semi-massive to massive sulfides at and near the contact between mafic volcanic rocks and overlying sedimentary rocks. Empirical exploration guidelines for Greens Creek-like polymetallic massive sulfide deposits in southeastern Alaska include: 1) a Late Triassic volcano-sedimentary host sequence exhibiting evidence of succession from tectonic activity to quiescence (such as conglomeratic and/or mafic volcaniclastics or flows overlain by platform carbonate or shale sequences), 2) presence and proximity to Late Triassic mafic- ultramafic intrusions, 3) presence of quartz-carbonate-fuchsite altered ultramafic bodies, 4) pyritic, graphitic shales, 5) presence of barite and/or iron-manganese-rich carbonates, 6) low-iron sphalerite and antimony-rich sulfosalt minerals, 7) a geochemical signature including Fe-Zn-Pb-Cu-Ag-Au-Sb-Hg-As-Cd-Ba-Mn-Mo-Tl and the ultramafic-related 2 suite of elements Ni-Cr-Co, and 8) a geophysical signature characterized by the coincidence of a sharp resistivity contrast with evidence for buried intrusive rocks. Critical factors for the development of larger, economic orebodies are significant thickness of pyritic, graphitic shale indicating that a locally reducing sedimentary setting was established and that accumulation of an insulating shale blank occurred, and proximity to Late Triassic aged hypabyssal mafic-ultramafic intrusive rocks. INTRODUCTION This report presents descriptions of twenty-five mineral occurrences and preliminary interpretations of their associated anomalous metal geochemical signatures. The descriptions consist of four sections detailing the location, host rocks, morphology and mineralogy, and geochemistry of each occurrence. The first three sections of each description are based on the author’s observations at the occurrences during fieldwork in May 1998 and, for a subset of the occurrences, during the summers of 1992 and 1993. Where available, previous descriptions and interpretations are incorporated and discussed in light of the new information generated during the present survey. Also presented are tables containing the unedited results of ICP-40 (inductively coupled plasma atomic emission spectroscopy-40 elements), ICP-10, Au (atomic absorption spectrography after fire assay), Hg (cold vapor), and CO2 (by difference) on rock samples collected within the boundaries of the present study area. Table 1 contains the results of analyses of non-mineralized igneous country rocks, Table 2 contains the results of analyses of non-mineralized black shales primarily collected in close proximity to mineralized occurrences, and Table 3 contains the results of analyses of mineralized rocks from the twenty two occurrences described. Only the preliminary interpretations of data in Table 3 are discussed in the geochemistry section of each occurrence description. Due to the small number of mineralized samples analyzed (n=78), the presence of multiple deposit types represented by the data, and the small number of analyses of samples from each occurrence, rigorous statistical evaluation of the data was not warranted. For each occurrence described, therefore, the definition of a characteristic geochemical signature is largely subjective, based upon the author’s experience (Taylor and others, 1992, 1994, 1995a and b, 1999a and b, 2000a and b; Taylor, 1997). The fourth section of each occurrence description contains brief descriptions of the mineralized rock samples analyzed, and the resulting characteristic geochemical signature. The elements present in highest abundance are listed first, followed by less elevated but important elements (with regard to geochemical signature) in parentheses. Interpretive reports on the geochemical characteristics of the pre- Tertiary igneous rocks (Taylor and others, in prep) and the Tertiary-Quaternary igneous rocks (Karl and others, in prep) will incorporate the results of high quality XRF and ICP- MS geochemical analyses. Within each table, data for rocks collected in 1992, 1993, and 1998 are identified by their prefix. In all cases, the samples collected are representative of the rock type or mineralization present at the site. A small portion of each sample was retained for reference and the remainder, approximately 0.5 to 1.0 kilograms, were crushed and ground for geochemical analysis. Samples collected in 1992 and 1993 were analyzed in-house at the geochemical laboratories of the USGS in Denver, Colorado. 3 Samples collected in 1998 were analyzed by contract with XRAL Laboratories in Don Mills, Ontario, Canada. Although the analytical techniques are the same, accuracy, precision, and detection limits vary between the two generations of analyses. No attempt has been made to conduct an exhaustive analysis of the statistical quality of either data set. Both data sets have passed through the quality control and quality assurance procedures in place at the USGS. For a more complete discussion of analytical methods, data statistics, and probable limits to the data sets presented, see Smith (this report) and references therein. PREVIOUS WORK The earliest reports on the mineral occurrences and mining activity in the Stikine geophysical study area are those of Wright and Wright (1908), Burchard (1914), Chapin (1918), Buddington (1923), and Buddington and Chapin (1929) all of which present the results of the initial geologic and mineral resource studies of the USGS in southeastern Alaska during the first quarter of the twentieth century. Reports produced during the middle third of the century are from a variety of sources and primarily describe specific occurrences in the Stikine geophysical survey area such as the Northern Copper, Taylor Creek, and Castle Island Barite occurrences (Williams and Decker, 1932; Kerns, 1950; Twenhofel and others, 1951; Race, 1963). In the last thirty years a series of compilations listing the known mineral occurrences within various geographically defined study areas has gradually increased the existing knowledge of mineral occurrences in the area (Berg and Cobb, 1967; Cobb, 1972; 1978; Karl and others, 1980; Grybeck and others, 1984; Brew and others, 1991; Newberry and others, 1997). The most comprehensive prior study of the area was conducted during the late 1970’s and early 1980’s by the USGS during the Petersburg (and portions of adjoining quadrangles) Alaska Mineral Resources Assessment Program (AMRAP). In addition to the above compilations, the first faunally dated late Triassic mineral occurrence in Alaska was discovered in Duncan Canal which led to the recognition of a group of similar, probably late Triassic mineral occurrences in the Duncan Canal-Zarembo Island area (Berg and Grybeck, 1980). These authors identified the group of deposits as probable volcanogenic massive sulfide and related barite deposits, and Berg (1981) extended