Breccia-Hosted Gold on the Northern Burin Peninsula, Newfoundland

Current Research (2000) Newfoundland Department of Mines and Energy Geological Survey, Report 2000-1, pages 299-309

BRECCIA-HOSTED GOLD ON THE NORTHERN BURIN PENINSULA, NEWFOUNDLAND

J.G. Hinchey, C.F. O'Driscoll1 and D.H.C. Wilton Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5

ABSTRACT

The Lodestar prospect consists of sulphide–oxide mineralized breccias of magmatic–hydrothermal or phreatomagmatic origin. The prospect contains significant gold mineralization within polylithic breccias associated with the Powder Horn Intrusive Suite, near Goobies. The best grades obtained by exploration companies from chip-channel sampling of the mineralized breccia consist of 4.98 g/t Au and 14.8 g/t Ag over a 15.9 m width, along with one area that returned an assay of approximately 2.0 % Zn. Earlier assays returned values of 6.13 g/t Au over 4.7 m and 4.91 g/t Au over 3 m. Recent samples chan- neled by the owner of the property indicate 5.6 g/t Au over 8.5 m including 12.6 g/t Au over 1 m.

The Powder Horn Intrusive Suite is a multi-phase intrusive suite, having both pre- and post-brecciation (mineralization) phases. Pre-mineralization rocks consist of sedimentary units of the late Neoproterozoic Musgravetown Group, which are hornfelsed in areas close to the intrusion, medium-grained gabbro/diorite, and minor felsic material. Post-mineralization phases include fine-grained gabbro/diorite, felsic phases, and diabase dykes.

The main mineralized breccia, which occurs at the contact of the host sedimentary rocks and the pre-breccia gabbro/diorite, is exposed over approximately 25 m and contains gold mineralization in association with copper, arsenic, and zinc. The mineralization dominantly occurs as the matrix of the breccia, whereas unmineralized sections of the breccia contain actinolite/chlorite within the matrix. The breccia clasts consist of the host sedimentary material, pre-breccia gabbro/diorite, quartz–feldspar porphyry, diabase, and minor fine-grained granite. The sedimentary and gabbro/diorite clasts were locally derived from the wall-rock, however, the quartz–feldspar porphyry clasts have not been observed in outcrop in the area of the showing and are therefore assumed to be derived from depth. In places, these porphyry clasts contain pre-breccia mineralization, and they also appear to be finely comminuted, thereby producing a rock-flour matrix in portions of the breccia and possibly indicating transportation from depth. Many characteristics of the Lodestar prospect correspond to a magmatic- hydrothermal origin for the breccia, produced by fluid exsolution from a gold-bearing intrusive system. The presence of exotic mineralized quartz–feldspar porphyry clasts within the breccia suggests that mineralization may be related to a deep Cu–Au porphyry system. If so, the prospect may have formed in a transitional environment between a deep mineralized porphyry system and a shallow epithermal system. The Lodestar prospect is, therefore, very significant because it indicates the potential for additional porphyry-style mineralizatilon within the Powder Horn Intrusive Suite, as well as elsewhere on the Burin Penin- sula.

INTRODUCTION only about twenty years (Hussey, 1978; Taylor et al., 1979; McKenzie, 1983, 1986; O'Driscoll, 1984). During this peri- Gold mineralization has been known to occur in rocks od, numerous discoveries have been made and large, aurif- of the Avalon Belt (O'Brien et al., 1998) of the southern erous, alteration belts have been delineated (e.g., Huard and Appalachians for the past 200 years. The Avalon Belt was O'Driscoll, 1984, 1985, 1986; McKenzie, 1986; Tuach et al., the site of North America's first gold rush (Carpenter, 1972), 1988; Huard, 1990; Hayes and O'Driscoll, 1990; O'Brien et however, in the Newfoundland part of the belt, widespread al., 1996, 1998, 1999; Dubé et al., 1998). gold mineralization has been suggested and documented for

1 Mineral Deposits Section

299 CURRENT RESEARCH, REPORT 2000-1

In 1998, significant gold mineralization was discovered of the Sall the Maid Intrusive Suite and diorite, gabbro and in polylithic breccias associated with the Powder Horn felsic dykes of the Powder Horn Intrusive Suite. Intrusive Suite in the Goobies area by Mr. Leroy Smith, a prospector, from Arnold's Cove Station. He staked the area, Connecting Point Group (Hayes, 1948) rocks are locat- and optioned it to Noveder Inc. Channel samples collected ed on the eastern side of the map area and are separated from by them returned values of 6.13 g/t Au over 4.7 m and 4.91 other rocks in the area by the Come-by-Chance fault (Figure g/t Au over 3 m. Noveder Inc. re-optioned the property to 2). They include green, grey and brown, regularly bedded NDT Ventures Ltd. They carried out an exploration program and laminated sandstone, siltstone and shale. In places, thick during December, 1998 that included prospecting, gridding, massive sandstone beds occur within the thinly bedded soil geochemical sampling, ground geophysical surveying, sequences. A medium- to coarse-grained green conglomer- backhoe trenching and rock sampling. The best results ate unit occurs toward the top of the sequence and was pre- obtained by NDT from chip-channel sampling in mineral- viously mapped as part of the Musgravetown Group ized breccia of the main showing returned values of 4.98 g/t (McCartney, 1967). It lies along strike of and is similar to, Au and 14.8 g/t Ag over a 15.9 m. In addition, one area rocks mapped as part of the Connecting Point Group on returned an assay of approximately 2.0% Zn. Ownership of Long Island, Placentia Bay (O'Driscoll and Muggridge, the property reverted back to Mr. Leroy Smith in 1999. He 1979; Figure 1). named the occurrence the "Lodestar Prospect" and did additional trenching and sampling in December, 1999. This The Musgravetown Group (Hayes, 1948) underlies the work has extended the main zone and added previously central part of the map area (Figure 2). It consists of green unknown zones. Recent channel samples indicate 5.6 g/t Au to grey, graded and crossbedded sandstone, pebbly sand- over 8.5 m including 12.6 g/t Au over 1 m (L. Smith, per- stone, conglomerate and grey to black thinly bedded sand- sonal communication, 2000). stone and shale. In places, detrital magnetite is common as thin lenses and wisps within the beds. These rocks have been The purpose and scope of this project by the senior altered to a dense black hornfels close to the Powder Horn author is to complete a detailed examination of the geology, Intrusive Suite. Sedimentary features can generally be rec- geochemistry, geochronology, petrography, and metallogeny ognized within the hornfelsed rocks. of the Powder Horn Intrusive Suite as part of a M.Sc. thesis at the Department of Earth Sciences, Memorial University, The Marystown Group (Strong et al., 1978) rocks are under the supervision of Dr. Derek Wilton. exposed in the northwestern part of the map area (Figure 2). They are composed of mafic and silicic volcanic flows and Previous work in this area includes regional mapping pyroclastic rocks, including crystal, lithic and crystal-lithic by Rose (1948), Anderson (1965), McCartney (1958, 1967), tuffs and agglomerates. These rocks have been isoclinally O'Driscoll (1977) and O'Driscoll and Hussey (1976/77). No folded and are overprinted by a strong penetrative north- to mapping more detailed than 1:50 000, or exploration has northeast-trending, steeply dipping foliation and have been been previously done in the area. The recent discovery pro- regionally metamorphosed to chlorite and sericite schists. vides a new target for additional exploration. These rocks were originally included in the Love Cove Group because of their similar composition and intense REGIONAL GEOLOGY deformation. It has been demonstrated in the Bonavista Bay area that the Love Cove Group underlies the Connecting The Goobies–Come By Chance area (Figures 1 and 2) Point Group conformably (O'Brien and Knight, 1988) and is divided into two structurally contrasting geological ter- has an age of 620 ± 2 Ma (O'Brien et al., 1989). However, a ranes separated by a system of faults that are an extension of recent age of the rocks on the Burin Peninsula indicates they the Paradise Sound Fault. Southeast of the fault, the area is are ca. 572 Ma (O'Brien et al., 1999) and are younger than underlain by a thick succession of late Neoproterozoic sedi- the Love Cove Group. The Marystown Group rocks are mentary rocks (Connecting Point Group) that is overlain by therefore equivalent in age to rocks of the Musgravetown the mixed volcanic and sedimentary assemblage of the Mus- Group, which unconformably overlies the Connecting Point gravetown Group. These, in turn, are overlain or faulted Group. against Lower Cambrian shales. Rocks in this belt have been openly to tightly folded about northeast-trending axes and The Random Formation (Walcott, 1900) overlies rocks do not display any schistosity. Northwest of the fault lies a of the Musgravetown Group in the north-central part of the belt of chlorite–sericite schists derived from volcanic, sedi- map area and consists of white to brown orthoquartzite, mentary and intrusive rocks and are now considered to be quartz sandstone and interbedded dark grey to green mica- part of the Marystown Group. Intrusive rocks in the area ceous siltstone and sandstone. Rocks of the Adeyton Group (Figure 2) are granite, quartz–feldspar porphyry and diorite (Jennes, 1963) overlie the Random Formation and are char-

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302 J.G. HINCHEY, C.F. O’DRISCOLL AND D.H.C. WILTON acterized by red, purple and green shale and slate containing has been intensely hornfelsed, (2) a medium- to coarse- many nodules and nodular beds of pink and grey algal lime- grained, equigranular, black and white gabbro/diorite, and stone. The limestones contain fossils consisting mainly of (3) one small felsic dyke, which is presumed to be pre-min- the shelly fauna hyolithids. Rocks of the Adeyton Group are eralization because it is brecciated in part of the showing exposed as faulted blocks along the Come By Chance fault where the breccia is unmineralized. The post-mineralization and along splays of it and the Paradise Sound fault. Minor rocks consist of (1) a fine-grained, dark-green, equigranular outcrops of black shale are exposed in the town of Goobies diorite to gabbro having a pilotaxitic texture, (2) pink to and may be a remnant of middle Cambrian rocks (McCart- orange quartz–feldspar porphyry dykes and fine-grained ney, 1958). granite dykes, and (3) diabase dykes.

The Sall the Maid Granite outcrops on the western edge PRE-BRECCIA UNITS of the map area and extends southwestward along the trace of the Paradise Sound fault as discontinuous intrusive bod- As previously mentioned, the sedimentary rocks (Mus- ies to the North Harbour area. It is composed of beige to gravetown Group; Unit 3, Figure 2) in the area of the intru- pink and orange, fine-grained granite, quartz ± feldspar por- sion have been intensely hornfelsed and altered. In places, phyry and green to grey diorite. It intrudes the late Neopro- however, the hornfelsed sediments still exhibit small lami- terozoic and Lower Cambrian rocks in the North Harbour nations and bands of dark and light material, which are pre- area and is presumed to be Devonian. sumably remnants of a primary depositional layering. These sediments are also commonly magnetic, indicating the pres- POWDER HORN INTRUSIVE SUITE ence of either primary detrital magnetite or the introduction of magnetite into the sediments by the intrusive rocks. In The Powder Horn Intrusive Suite is a multi-phase intru- areas close to the intrusion, the sedimentary rocks are total- sion, dominated by diorite/gabbro and lesser amounts of fel- ly hornfelsed, and appear as a very fine-grained black rock, sic and diabase material. The intrusion is exposed as an making up a thermal aureole to the intrusion. Due to the elliptical body located at the intersection of the Isthmus of grain size and the extent of the hornfelsing, identification of Avalon and the northern part of the Burin Peninsula (Figure metamorphic minerals in hand sample is difficult. Away 1) near the town of Goobies. This study was initiated to from the intrusive phases, the sedimentary rocks consist of investigate the origin of recently discovered gold mineral- thinly bedded to massive, black, grey and green sandstones, ization associated with polylithic breccias at the Lodestar pebbly sandstones, siltstones and shales, of the late Neopro- prospect. The prospect occurs in the north-central part of the terozoic Musgravetown Group. The sedimentary rocks are intrusion and is located approximately 2.5 km to the south- both overlain by and fault bounded against a sequence of west of Goobies and approximately 1.5 to 2.0 km along a quartzites and sandstones and a group of red and green woods road off the Burin Peninsula Highway (Route 210, shales containing beds and nodules of pink and grey lime- Figure 2). stone that correspond to the Cambrian Random Formation and Adeyton Group (Units 4 and 5, Figure 2). Contact relationships between the various phases of the intrusion and country rock are well exposed, and allow for The other major pre-mineralization unit consists of the the relationships between the various phases and mineral- medium- to coarse-grained, equigranular, black and white ization to be determined. Due to the lack of subsurface data, gabbro/diorite phase of the Powder Horn Intrusive Suite however, it is difficult to interpret the nature and extent of (Unit 6, Figure 2). There is some compositional variation the mineralized breccia at depth. Preliminary interpretations within this intrusive rock that gives rise to two main subdi- suggest that the mineralized breccias have a phreatomag- visions (not separated on Figure 2). The first subdivision matic or magmatic–hydrothermal origin. contains plagioclase, pyroxene, amphibole, biotite and rare quartz, whereas the second subdivision consists of only pla- Work carried out during the 1999 field season identified gioclase, pyroxene, and biotite. These rocks are slightly five significant rock units, each with internal variations; magnetic in places and are therefore assumed to contain these phases being defined as either pre- or post-brecciation accessory magnetite. A gradational transition between each (mineralization). The existence of pre- and post-breccia subdivision is observed in this intrusive phase. phases within the intrusion is important in terms of the tim- ing and mode of emplacement of the mineralization (see One small felsic dyke is brecciated and, therefore, pre- below). dates the breccia and mineralization. This is important in terms of classifying the mechanism of brecciation and is The pre-mineralization rocks consist of (1) a sedimen- dealt with below. tary unit, which is assumed to host the intrusion and which

303 CURRENT RESEARCH, REPORT 2000-1

POST-BRECCIA UNITS

Post-mineralization, fine-grained, dark green, diorite to gabbro (Unit 7, Figure 2) varies throughout the intrusion. In places, this unit is simply equigranular with a pilotaxitic texture and no phenocrysts, whereas in other areas it contains phenocrysts of plagioclase and/or amphibole; some plagioclase phenocrysts being in the range of 1 to 10 cm in length. This phase of the intrusion is also slightly magnetic in places (i.e., contains some accessory magnetite). Primary sulphides (dominantly pyrite) are also disseminated throughout this phase.

The felsic dykes (Unit 8, Figure 2) associated with the intrusion range from a very fine grained aplite, to a feldspar porphyry, to a quartz–feldspar porphyry. These felsic phases are assumed to represent high-level intrusions. Plate 1. Main mineralized zone of the Lodestar prospect. CONTACT RELATIONSHIPS OF THE MINERALIZATION INTRUSIVE PHASES Contact relationships observed at the Lodestar prospect All phases of the Powder Horn Intrusive Suite intrude, (Plate 1) indicate that the breccia was emplaced in the vicin- and thermally metamorphose the surrounding sedimentary ity of the contact region between the early gabbro/diorite rocks, which are dominated by the late Neoproterozoic and the host sedimentary rocks. Both of these units were at Musgravetown Group. This relationship is evident in areas least locally sampled by the breccia as indicated by the pres- where relict primary layering or banding in the sediments is ence of clasts of these rocks within the breccia. The miner- crosscut or cut-off by phases of the Powder Horn Intrusive alized breccia has been intruded by the fine-grained diorite Suite. (Unit 7), quartz–feldspar porphyries (Unit 8), aplite dykes In some areas, small inclusions or xenoliths of sedi- (Unit 8), and diabase dykes. The fine-grained diorite has mentary rocks, commonly magnetic, are present within the well-defined chilled margins against the breccia and is also intrusive phases, thereby indicating intrusive contact rela- observed to cut off the mineralized breccia. tionships. These sedimentary xenoliths are especially common in the pre-mineralization (early) gabbro/diorite. Within the map area, the only other indication of sul- Schlieren textures are common at the contact between the phide mineralization is located at the western contact of the sediments and the early gabbro/diorite indicating possible pre-breccia gabbro/diorite and sedimentary rocks where the partial assimilation of the sedimentary rocks. presumed location of a faulted contact is marked at surface by a thick layer of rusty indurated till. In one area, the till is Intrusive contacts between the post-breccia (late) gab- cemented by gossanous iron-oxide, thereby indicating the bro and the pre-breccia (early) gabbro/diorite are generally possibility of sulphides at depth. However, it is not known if sharp and marked by chilled margins. This relationship is this mineralization is related to the Lodestar style of miner- also observed where the post-mineralization felsic dykes are alization. in contact with early and late gabbro/diorite. All phases appear to have been intruded by diabase dykes. CHARACTERISTICS OF THE MINERALIZED BRECCIAS In the southeastern part of the map area (Figure 2), a dyke of the post-breccia diorite/gabbro intrudes and ther- The mineralized breccia (Plate 2) is exposed over mally metamorphoses Cambrian shales. The contact is well approximately 20 m (Figure 3) and contains gold in associ- exposed along a brook that flows from the Powder Horn Hill ation with copper, arsenic and zinc mineralization. The min- area to the Come By Chance river. This indicates that at least eralization occurs as the matrix in parts of the breccia, this phase of the intrusion is post-Cambrian. whereas unmineralized parts of the breccia have a rock-flour

304 J.G. HINCHEY, C.F. O’DRISCOLL AND D.H.C. WILTON or actinolite/chlorite matrix. The breccias at the main showing, as well as proximal breccias, contain hydrothermal (secondary) magnetite in the matrix. The breccia contains rounded to angular clasts of the host sedimentary rocks, the early diorite/gabbro, quartz porphyry, quartz–feldspar porphyry, diabase and minor fine-grained granite (Plates 3 and 4). Clast size varies from <1 cm up to ~ 50 cm, but the clasts are typically <10 cm. The sedimentary and gabbro/diorite clasts were derived from the wall rocks, which the breccia transects, and therefore are presumed to be locally derived. This is supported by the fact that some clasts are brecciated in such a way that adjacent clasts have matching margins and can thus be reassembled like jigsaw puzzle pieces. In some cases, mineralization fills in small cracks in the clasts caused by the local brecciation (Plate 5). Plate 2. Mineralized auriferous breccia at the Lodestar prospect. Visible The quartz porphyry, quartz–feldspar por- mineralization consists of pyrite, arsenopyrite, chalcopyrite and mag- phyry, and fine-grained granite clasts have not netite. been observed in outcrop in the area of the showing and therefore are presumed to represent samples derived from depth. Various evidence sup- ports this idea. Some of the quartz and quartz–feldspar clasts appear to contain pre-breccia mineralization that consists of stringers or veinlets of sulphide mineralization within the clasts (Plate 6). Also, the rock-flour matrix appears to consist of finely comminuted pieces of these clasts; comminution presumably being due to the transport process. This idea of varying sources, or varying depths of clast acquisition, will be important in the classification of the breccia type and understanding of the brecciation processes that will be dealt with in future studies. No outcrop The breccia is dominantly clast-supported Post-breccia gabbro/diorite 5 metres with varying amounts of matrix (~ 5 to 15%). The Mineralized breccia matrix consists of sulphides, chlorite/actinolite and rock flour, all of which vary in proportion Pre-breccia gabbro/diorite throughout the breccia. Where mineralized, the Sedimentary rock matrix consists of arsenopyrite, pyrite, chalcopy- Figure 3. Plan view of main mineralized zone of the Lodestar prospect. rite, magnetite, bornite, and sphalerite. The matrix mineralization also overprints and forms veinlets and actual distribution of the gold with respect to the sulphide fine disseminations within some of the breccia clasts and is minerals in the matrix of the breccia, however, awaits fur- therefore, assumed to be syn- to post-brecciation. The over- ther assay results. Within the breccia, the different sulphide printing texture is predominantly associated with the quartz minerals appear to be segregated into arsenopyrite-rich and and quartz–feldspar porphyry clasts whereby mineraliza- pyrite-rich zones; the explanation for this zonation awaits tion, in the form of arsenopyrite and/or pyrite, rims and further study. overprints the clasts (Plate 7). This indicates some chemical affinity between the mineralization and these clasts. Minor alteration is visible as concentric rings around the sedimentary clasts (Plate 3), and some chlorite and epi- The gold is assumed to be associated with the arsenopy- dote alteration is also observed in the area of the breccia, rite, pyrite, and, to a lesser extent, the chalcopyrite. The although this alteration is not very substantial. Apart from

305 CURRENT RESEARCH, REPORT 2000-1 this, the host rocks and clasts within the breccia display very little alteration.

The mineralized breccias exposed at the Lodestar prospect are interpreted to be of either phreatomagmatic or magmatic–hydrothermal origin. According to Sillitoe (1985), the main difference between these two breccia-forming processes is that the magmatic–hydrothermal system involves breccia formation due to the release of hydrothermal fluids from a magma chamber, irre- spective of the original source of the fluids con- cerned (i.e., magmatic, meteroic, connate, etc.), whereas a phreatomagmatic system involves the formation of a breccia due to the interaction of a magma with an external source of water. Another difference is that magmatic–hydrothermal breccias typically form at deep or porphyry levels and get eroded to display pipe-like forms, although Plate 3. Breccia at the Lodestar prospect containing clasts of gabbro, these need not vent to the surface (Corbett and sedimentary rocks, diabase and quartz–feldspar porphyry. Note alter- Leach, 1998), whereas phreatomagmatic (dia- ation rims in some sedimentary clasts. treme) breccias are typically associated with high-level porphyry intrusions and therefore may vent in the form of a diatreme or maar volcano, or they may occur as dykes that exploit pre-existing structures (Corbett and Leach, 1998). Many of the characteristics present at the Lodestar prospect correspond to a magmatic–hydrothermal origin for the breccia (Sillitoe, 1985), namely: 1) the presence of finely comminuted pieces of rock, termed rock flour, which makes up part of the matrix at the showing, 2) the presence of pre- and post-breccia felsic intrusions, and 3) the presence of hydrothermal magnetite in the matrix in parts of the showing.

DISCUSSION AND CONCLUSIONS

Consistent contact relationships were observed between the various intrusive phases Plate 4. Quartz–feldspar porphyry clast in mineralized breccia at the throughout the map area. The host sedimentary Lodestar Prospect. rocks were initially intruded and hornfelsed by the pre-breccia gabbro/diorite. This was followed by brecciation and mineralization at the contact of the sedi- that the mineralization and breccia process may be related to mentary rocks and the pre-breccia gabbro/diorite and then a deep Cu–Au porphyry system and that the showing may by the other intrusive phases, including the post-breccia represent some transitional environment between a deep gabbro/diorite, the felsic intrusions, and the diabase. mineralized porphyry system and shallower epithermal environment. The high-level felsic intrusions, which intrude Based upon observations from the Lodestar showing, it the pre- and post-breccia gabbro/diorite, probably represent is postulated that the hydrothermal breccia was the product late stage intrusions from a deep parental magma chamber. of fluid exsolution from a gold-bearing intrusive system. The main process driving the brecciation process in this type The presence of the exotic mineralized quartz and of environment would be the violent exsolution of magmat- quartz–feldspar porphyry clasts within the breccia suggests ic–hydrothermal fluids from cooling porphyry stocks at

306 J.G. HINCHEY, C.F. O’DRISCOLL AND D.H.C. WILTON depth (Corbett and Leach, 1998). As the magma source cools at depth, volatiles exsolve from the silicate magma, collect and become over pressured. These over-pressured volatiles eventually escape violently causing brecciation of the over- lying rocks. The volatiles commonly exploit zones of weakness such as a fault or, in the case of the Lodestar prospect, a contact between different rocks to produce the brecciation. Once the volatiles become decompressed, they propagate upward to a lower pressure regime. This allows an increased release of fluids from the magma body at depth, which results in brecciation of the above units and mixing and milling of clasts within the breccia (Corbett and Leach, 1998).

The Lodestar showing therefore, has consid- erable significance as a possible indication of a larger, Cu–Au porphyry system at depth. This showing highlights the potential for other occur- rences along the contact of the sediments and the early gabbro/diorite in the Powder Horn Intrusive Suite, and also opens up the possibility of porphyry-style mineralization on the Burin Peninsu- la as a whole. A good place to explore for these types of porphyry deposits would be in the known high-sulphidation epithermal systems that occur along the length of the Burin Peninsula. Plate 5. Polished slab of breccia with gold-bearing sulphide matrix and ACKNOWLEDGMENTS both sedimentary and felsic intrusive clasts. Some mineralization fills small cracks caused by late fragmentation of clasts. (Field of view 12 The authors wish to acknowledge the capa- cm.) ble and cheerful assistance of Greg Cochrane and Barry Sparkes in the field. Their presence, hard work and culinary skills "shortened the road" to the end of many tasks. We also acknowledge the cooperation of Leroy Smith in providing unpublished data and advice during this work. Sean O'Brien and Benoît Dubé provided and continue to provide stimulating discussions on this deposit as well as other deposits in the Avalon Zone and around the world. We thank the personnel of the Project Management Division for logistical sup- port.

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Note: Geological Survey file numbers are included in square brackets.

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