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Platinum-group elements (PGE), Cu, Ni, and Cr concentrations have been determined for 134 samples of greenstone and other metavolcanic rocks of the upper Paleozoic and Triassic Wrangellia terrane and of the Upper Jurassic and Cretaceous terrane-overlap assemblage of the Gravina belt in the western part of the Coast Mountains between the Chilkat Peninsula and northern Admiralty Island and the results are summarized here. The elemental concentrations and ratios of the Gravina belt rocks are compared with corresponding data from the Alaskan-type mafic-ultramafic intrusions to test previously published hypotheses regarding their parent magmas. Finally, we speculate about the magmatic and other processes that led to the relative concentrations of the above-listed elements in these rock units. This study is limited by the sampling and analytical methods used, by the elements selected, and by the lack of data pertinent to all of the processes that may have affected the original magmas; nevertheless it accomplishes three specific purposes: (1) it summarizes an original and comprehensive set of PGE, Cu, Ni, and Cr data for these units for the first time; (2) it compares summarized Douglas Island Volcanics data with those of the Alaskan-type mafic-ultramafic complexes, and suggests a new preliminary hypothesis regarding the parent magmas of the latter; and (3) it speculates about the magmatic and other processes that led to some regular, albeit loose, grouping of the data for the different units. This last purpose is controversial because it is especially fraught with uncertainties. The rocks analyzed from the Wrangellia terrane include greenstone of Late Triassic age on Chilkat Peninsula near Haines and the Gastineau Volcanics of late Paleozoic to Late Triassic age near Juneau. The PGE-Ni-Cr concentrations and concentration ratios for these rocks supplement previous trace-element studies and may serve to further differentiate these rocks from those of Cretaceous age. The previous trace-element studies showed that the Wrangellia tholeiitic basalt metavolcanic units and the Cretaceous-age Douglas Island Volcanics alkalic basalt units erupted in different tectonic settings and have different elemental concentrations and ratios. Our new studies, like the previous ones, are of potential value to mineral explorationists and others attempting to establish the presence or absence of Triassic metavolcanic units; this question is important because the Triassic rocks locally contain volcanogenic-massive-sulfide deposits . Rocks analyzed from the Gravina belt principally belong to the Lower Cretaceous Douglas Island Volcanics exposed near Juneau. These latter rocks previously have been proposed to be the extrusive equivalent of magmas that formed Lower Cretaceous (118-100 Ma) Alaskan-type mafic and ultramafic intrusions, but our data do not support this interpretation. Instead, our study suggests that mafic dikes and plugs emplaced into the Douglas Island Volcanics either during a late stage of Ml (-120 Ma) low-grade regional metamorphism or possibly during the same time as the M2 contact metamorphism that marks the aureoles of the Alaskan-type intrusions, are the parents of those intrusions. According to our data, the metavolcanic rocks of both the Wrangellia terrane and the Gravina assemblage are characterized by dominance of Pd over Pt as shown by mean Pd/Pt ratios of 2.30 and 1.61, respectively, for the lower and upper parts of the Wrangellia terrane, and of 1.82 for the Gravina belt. Average Pt/(Pt+Pd) ratios are about the same (0.3-0.4) for all the metavolcanic units; they are significantly lower than values (0.5-1.0) for the Pt-dominant Alaskan-type intrusions. The mean total PGE concentrations of samples of the Douglas Island Volcanics and equivalents [16 parts per billion (ppb)] and of associated dike rocks (10 ppb), however, are much lower than for Alaskan-type intrusions (generally 75-100 ppb). Dikes in the Douglas Island Volcanics are Pt dominant, with mean Pd/Pt of 0.74 and Pt/(Pt+Pd) of 0.58; these values are similar to those in some of the Alaskan-type bodies, suggesting that the late dikes may represent their parental magmas, instead of the parents of the extrusive rocks of the Douglas Island Volcanics. All of these analytical values and calculated ratios are subject to significant uncertainty. The Cu, Ni, and Cr variations are complex; overall, the average Cu concentration of the units shows a general increase with increasing Pd but no systematic variation with Pt. In volcanic rocks of the Gravina belt of the Juneau area, Cu shows positive covariation with Pd and lacks systematic covariation with Pt. In contrast, in rocks of the Wrangellia terrane, Cu has a general overall increase with both increasing Pd and Pt. Neither Ni or Cr show covariation with Pt and Pd in all units. It should be noted that, in comparison with the Gravina assemblage rocks from the Juneau area, inferences regarding the Cu, Ni, and Cr concentrations of the metavolcanic rocks of both the Wrangellia terrane and the Gravina assemblage in the Ketchikan and Petersburg areas are limited by the small sample population for those areas. INTRODUCTION This report summarizes platinum-group element (PGE), Cu, Ni, and Cr concentrations in 134 samples of upper Paleozoic and Mesozoic volcanic rocks from southeastern Alaska. The PGE's analyzed are Pt, Pd, Rh, Ru, and Ir. This study is part of a long-term study of the volcanogenic history of southeastern Alaska (Brew, 1968; Ford and Brew, 1978, 1987, 1988, 1993). The complete data set is in the files of co-author A.B. Ford. The primary purpose of the report is to summarize the new data on the PGE, Cu, Ni, and Cr content of the volcanic rocks. The secondary purpose is to compare the Douglas Island Volcanics PGE data with those of the Alaskan-type complexes; this topic is of interest because of uncertainty about the parent magma of the Alaskan-type complexes and because the Douglas Island Volcanics have been suggested to be the extrusive equivalent of the Alaskan-type complexes. The third purpose is to speculate on the processes that led to the concentrations now present in the different geologic units; this purpose is similar to that of Ely and others (1998) in their study of Ontong Java Plateau basalts. The fourth purpose is to explore the possibility that PGE and other data may be used to differentiate the Wrangellia metavolcanic rocks, which locally contain volcanogenic massive sulfide deposits, from the Gravina belt metavolcanics, which generally do not. Samples are principally from (1) the Wrangellia terrane of northern southeastern Alaska; it consists of two parts: (a) an upper Paleozoic and Triassic(?) and Upper Triassic lower and upper parts of the Gastineau Volcanics of Ford and Brew (1993) and (b) the Upper Triassic metabasalt of the Chilkat Peninsula and northern Admiralty Island; and (2) the Douglas Island Volcanics of Early Cretaceous age (Lathram and others, 1965). The Wrangellia rocks were assigned to the Taku terrane by Berg and others (1978), but have since been recognized on paleontological and chemical evidence to belong to Wrangellia (Ford and Brew, 1993). The Douglas Island Volcanics are part of the Upper Jurassic and Lower Cretaceous Gravina terrane-overlap assemblage that forms a discontinuous belt extending throughout southeastern Alaska (Berg and others, 1972; Brew and Ford, 1985; Brew and Karl, 1988a, b; McClelland and others, 1991; Cohen, 1992; Cohen and Lundberg, 1994). The Gravina belt is used in this paper as defined in Brew and Ford (1985). Areas sampled for the study are shown in figure 1. Samples of mafic dikes and plugs that intrude the Douglas Island Volcanics on the Glass Peninsula of northeastern Admiralty Island and a small number of samples of metavolcanic rocks from southern parts of the Gravina belt in the Ketchikan and Petersburg areas are also included in the study. These dikes and plugs are included in the study because of the possibility that they represent the parental magma of Alaskan-type intrusions (Murray, 1972; Irvine, 1974; Loney and Himmelberg, 1992; Himmelberg and Loney, 1995). The typical augite-porphyritic volcanic rocks of the Gravina belt are spatially related (Murray, 1972; Irvine, 1974; Brew and Ford, 1985) to a linear belt of distinctive, dominantly ultramafic intrusive bodies termed "Alaskan-type" (Irvine, 1973, 1974; Himmelberg and Loney, 1995). The bodies are scattered in a well-defined belt along the west side of the Coast mountains in southeastern Alaska (Taylor and Noble, 1960; Taylor, 1967; Murray, 1972; Clark and Greenwood, 1972; Irvine, 1974; Brew and Ford, 1985; Loney and Himmelberg, 1992). Himmelberg and Loney (1995) provided a comprehensive review and synthesis of the occurrence, petrology, and tectonic setting of these ultramafic and mafic intrusions. They also provided new trace-element data for three of the bodies, but PGE's were not analyzed in their study. Most of the Alaskan-type bodies were originally mapped and briefly described by Buddington and Chapin (1929) and have been subsequently studied by the workers listed above. As summarized by Himmelberg and Loney (1995), K-Ar, Ar-Ar, and Pb-U zircon age determinations on rocks from the different Alaskan-type bodies have yielded Aptian (118 Ma, Meen and others, 1991) and Albian (110-100 Ma, Lanphere and Eberlein, 1966; 111-108 Ma, Saleeby, 1992) radiometric ages, using the time scale of Harland and others (1990). Alaskan-type complexes of other ages are also reported, such as the Salt Chuck intrusion and others of early Paleozoic age in southern southeastern Alaska (Loney and Himmelberg, 1992; Brew, 1996) and bodies of Late Triassic to Early Jurassic age in a belt in British Columbia about 350 km east and northeast of southeastern Alaska (Nixon and Hammack, 1991). A cogenetic relation between some of the Alaskan-type intrusions and Lower Cretaceous Douglas Island Volcanics of the Gra

u.s. geological survey open file report/open-file report

Summary information about platinum-group elements and other trace metals in greenstones of the Wrangellia Terrane and the Gravina Belt, southeastern Alaska