n Alas - Alaska Division of Geological & Geophysical Surveys - State ...
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n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

n Alas - Alaska Division of Geological & Geophysical Surveys - State ... n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

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of relative Lith01oglc abundances withh mOr8.ines and conventional aitphoto interpretation of moraihal 0rigin, mejor bedrock units were identified, and their &istribution delineated to produce a generalized Lithologic map. Morainal debris represented regional and contact(?)-met8 morphic mks and hypabyssal to plutonic igneous rocks; no sedimentary rock were observed. Structural interpretations were based primfully on observation and analysis of planar features (joints, small faults, veins, and dikes) and minor folds visible in individual constituents of the boulder- and, to some extent, the cobbleaize fractions of the morainal material. Fold styles, relative abundance of planar features, type of igneous contacts, and crosscut tIng relations were combined with other observations (Lithologic differentiation, vislble mineralization, and air~hoto data) to delineate the stvle and orientation of mGor struct&es and stratigrapic relations. On the basis of an integration of these observations, the catchment area is composed of well-foliated isoclinally-folded east-west-trending schist and phyLlite with minor interlayered quartzite and marble, discordantly intruded by one granitic and one gabbroic tgneous body. The evfduation of mineral potential lnvolved two distinct but complementary Unes of investigation: (1) identification of metallic rninerab in the pebble-, cobble-, and boulder-size fractions; and (2) a 31element se miquantitative spectrographic analysis of the nonmagnetic heavymineral separate from the sand-size fraction of each sample. Pyrite was the dominant metallic mineral identified, both dissemlnated and as thin (0.5-1.0 rnm thlak) veinlets. Much of this pyrite is directly related to metamorphism of the host rwk but is not related to potential ore-bearing systems. Sjgnificant amounts of copper minerallzation, in the form of chalcopyrite, bornite, azurite, and malackite, were found In the coarser fractions from several moraines. ?his mineralization was observed as flnely disseminated chalcopyrite within the rnetasedlmentary rocks or, more commonly, as vein fillings, aommonly associated with quartz and pyrite. A boulder of massive sulfide (chalcopyrite, arsenopyrite, bornite, and other sulfide minerals), measuring 15 by 20 cm and wstghing 18 kg, that was recovered from the eastern tributary indicates that concentrations of massive sulfides exlst in the catchment wea. Tungsten and molybdenum mineralization (schelite and powellite) was identified during microscopic and ultraviolet examination of the nonmagnetic heavymineral separates (R. B. Tripp, written commun., 1981). Evaluation of the semiquantffatfve-spechgraphic-analysis data was based on the selection of 11 metallic elements (Ag, As, Cr, Cu, Mo, Ni, Pb, Sb, Sn, W, Zn) representative of ore-mineral assemblages. Two hktograms were prepared for each element: (1) a frequency histogram showtng the concentration (in parts per million) fmrn the lowest to highest vduer fg. 60&, and (2) a plot of anomalous occurrence versus identified source moraine on the glacier (fig. 60g). The concentration histogram was utiLized to semiquan titstively deheate anomdous concentration values, defined as those values lying above an abrupt break in slope of a curve fitted to each set of data. The anomalous occurrence-versussource m oralne his- tcgram (fig. 60% diagrams the source-are8 @atd mtion d mom&us vdues. Four major tYPS of anomalies were identilied: (1) a mdtielement (Cut Agi Pb, Zn, Sb, Ni) anomaly, (2) a tungsten-molybdenum anOmalv. (3) a silver-lead anomaly, and (4) twf, spa- tUy di&ete chromium anomaliei(figs. 60% 61). Two exploration targets were outlined through integration of visibleraineralization observations and geochemical analyses. The first target is the multi- element anomaly area (1, f&s. 60% 811, in which the copper mineralization is attributed to a hydrothermal- vein system genetically related to a monzonite-grano- diorite complex. On the bask of composition, texture, and the apparent absence of widespread hydrothermal- alteration effects in the plutonic rocks, the likelihood of a welldeveloped porphyry system is small. The second target, the tungsten-molybdenum-anomalous area (2. fim. 608. 611. is interoreted to be derived from a' s& dG;csit 'spatially ' associated with the gabbmic intrusive corn lex. The chromium-anomalous area (3, figs. 605 BJ is interpreted to reflect the relatively high chromium content of a gabbroic Intru- sive body. The silver-lead-anomdous area (4, figs. 603 6 1) is based on values from only one moraine and is, therefore, ass'qed a low exploration-target priority. REFERENCE CI'TED Eveman, E. B., Stephens, G. C., Curtin, G. C., and Tripp, R. B., 1982, Gemhernial exploration using englacid debris, 5 Coonrad, W. L., ed., The United States Oeological Survey in Alaska: Accomplishments during lg8O: U.S. Geological Survey Circuler 844, p. 108-109. Placers and plecer mhhg in the HeaZg quadmgle, southern Ah&fi Gold was originally discovered on VMez Creek In 1903, and soon afterward the buried "Tammany ChannelN was determined to contain rich concentra- tions of phcer gold. Approximately 27,000 troy oz of gold has been produced from the channel (Smith, 1981); however, a substantial part of the buried channel is still unmined. The ChanneL is very steep sided, similar to the present Valdez Creek. Where mined, it appears as a gage, 45 to 60 m deep and at least 90 m across at the top. The upper part of the channel and the w- rounding bedrock bench Is mantled with a blanket of glacial drift, as much as 23 m thick. As much as 15 m of the buried alluvial channel gravel may be gold bearing. Smith (1970) conducted a seismic*fraction survey to locate additionel buried incised channels in the Valdez Creek area. He identified a deposit, de- scribed as the DyeU bench gravels, containing more than 27 million rn of aurifecous gravel. A buried gold-bearing gravel-filled channel in the Valdez Oeek drainage ic currently being mined on a moderately large scale by the Dmall NUning Co. A large washing plant that was assembled in summer 1980 has subsequently been put into operation process- ing the gold-bearing gavel in the buried nTammany Channel

1 "~_liO-/ 200 1000 B roo C E % A B a b Mo W I-+ 0 NI 0 Cr E Uz:;\ ,#A CS 0 0 P I AQ r, ~b Zn Sn I+ 20 ;lo n - -dd,l E Q 0 0 CU Pb s * ,ll,,aO, l;: 1 , - . --- Moraine 30292827 26 25242322212019 1817161S 14 131211.511 10987654321 LMR CR LR LA LR Pigwe 60.-Semiquantitative-s~~t~0graphic-analyss data for selected metallic elements. A Frequency distribution of concentrations in parts per million (pprn) and Limits of anomalous concentretiond-ws). B Spat34 relation of anomalous conoentratlons to moraine source areas and exploration-target ares (box4 adjacent to the Trident Glacier, Alaska. See text for discussion. 96 I I

1<br />

"~_liO-/ 200 1000<br />

B roo C E<br />

%<br />

A<br />

B<br />

a b<br />

Mo W I-+<br />

0<br />

NI<br />

0<br />

Cr<br />

E<br />

Uz:;\ ,#A<br />

CS<br />

0 0<br />

P I<br />

AQ r, ~b Zn<br />

Sn I+<br />

20<br />

;lo n<br />

- -dd,l<br />

E Q<br />

0<br />

0<br />

CU Pb s *<br />

,ll,,aO, l;: 1 ,<br />

- . ---<br />

Moraine<br />

30292827 26 25242322212019 1817161S 14 131211.511 10987654321<br />

LMR CR LR LA LR<br />

Pigwe 60.-Semiquantitative-s~~t~0graphic-analyss data for selected metallic elements. A Frequency<br />

distribution <strong>of</strong> concentrations in parts per million (pprn) and Limits <strong>of</strong> anomalous concentretiond-ws). B<br />

Spat34 relation <strong>of</strong> anomalous conoentratlons to moraine source areas and exploration-target ares (box4<br />

adjacent to the Trident Glacier, <strong>Alas</strong>ka. See text for discussion.<br />

96<br />

I<br />

I

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