Application of GIs Technology for Resource Estimation of Offshore Placer Deposits in Nome, Alaska

Scott ~uang' , Gang hen', Scott ~aybrie?, Ky[e L. ~rennan"

ABSTRACT Data h m about 3200 drill holes in the offshore area at Nome, Alaska was digitized and processed. Most drill hole files have associated lithology, gold value, and blow count descriptions for each drill hole. The gold intensity was used to present the potential gold resource in the study area on the world wide web. A web page was created that gives background information on the project, descriptions about the Nome area, maps of the claim blocks that include all the normalized slices of gold deposits, and links to other relevant sites on the web. The site is located at httD://mineral.uafsme.alaska.edu/nome~roiect~nomehome.htm

INTRODUmION Beach and offshore resources of heavy refkctory minerals are widespread along the coast of Alaska, The resources demonstrated at Nome are, however, the richest yet found in Alaska. They have been exploited but also contain significant demonstrated resources that may be developed in the future. ~ecause-of the extent and richness of the Nome resources, they also have been studied and documented extensively.

~ o m e has been one of the most active areas of placer mining in the State of Alaska since the late 1800's. Major gold production took place there as recent as 1990. Production waned because of limited technology and because the gold offshore could not be economically retrieved. In 1994, the USGS Marine Geology Plan identified the Bering Sea, Alaska, as a region wananting a systematic investigation for its heavy mineral potential. Developing the mineral sources in this region would encompass a vast array of factors as well as the cooperation of federal and state government agencies and the mineral industry.

Given that new technology such ai Geographic Information System (GIs) is widely available, it should be applied to the development of the mineral sources in the Bering Sea, Alaska GIs can be easily used to integrate various geologic, geochemical, geophysical, topological, and other maps with digital data sets to expedite mineral resource exploration and reserve estimation. It is expected that by applying GIs technology with the available information a better understanding can be obtained of the geological, geophysical, and geochemical characteristics of near-shore mineral deposits as well as the geologic systems under which they formed a q i how they accumulated in the Bering Sea region.

The Department of Mining and Geological Engineering, School of Mineral Engineering at the University of Alaska Fairbanks received a research grant h m the Minerals Management Services, U.S. Department of the Interior in 1998. The study applies Geographic Information System (GIs) technology to a systematic investigation of the Nome area in order to determine the resource potential for offshore strategic heavy metals, specifically the gold deposits. The first phase of this project included data compilation and analysis and web site development

Assembly and Cornpilatioh of Information. One of the tasks involved the collection and compilation of information related to geologic characteristics, geochemical and geophysical signatures, borehole data, economic considerations, oceanographic factors,

1 Department of Mining and Geological Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775-5800 2 Shannon and Wilson, Inc. Fairbanks, AK 99707 . .

3 Shannon and Wilson, Inc. Anchorage, AK 995 18

submarine topography, and potential environmental impacts. Search of literatures using internet also was carried out with an emphasis on the GIs application.

Another large data source, perhaps the most important information for this study, came from a huge amount of data that was collected by Western Gold Exploration and Mining Company, Limited Partnershrp (Westgold). Westgold operated the bucketline dredge BIMA h m 1985 through 1990. The BIMA was used to dredge gold from the ocean floor at the Nome Offshore Placer Project.

Development of GIS Structure This project used a hierarchical structure with its data storage and retrieval for its geographic information system. Input activities involved the digitization of maps of geperal, geologic, bathymetric, and contoured gold values. The creation of these digital maps was an involved process because most current GIs operations still involve manual conversion of analog data to digital format. The maps had to be sorted, digitized, and edited so that they were compatible in ARC format. ARC/TNF0@ Version 7.1 for and Windows NT and Arcview were the two ARC programs used in this project.

Delivery of Web-based Results Several formats are used to give the results of this project. Research reports, papers, and student theses are the primary outlets of information. Many relevant maps illustrating potential maps have been generated. All of the data is on computers, and a major part of it is available onthe web site of the School of Mineral Engineering, University of Alaska Fairbanks.

NOME OFFSHORE GEOLOGY AND STRUCTURES Offshore sediments consist of varied lithologies, the majority of which have been mapped onshore. These lithologies include: red granite and quartz monzonite, which are common with abundant chlorite schist, marble, marble schist, limestone, graphitic schist, graphitic-siliceous calc-silicate rock, biotite schist, and rare polished round, green quartz pebbles (Howkins, p.19, 1992). Fine grain marine sediments deeply bury offshore bedrock east of Nome; to the west offshore bedrock is just below the sea bottom. Figure 1 is a map of the sediment on the ocean floor.

Glaciation Drift h m past glaciations covers most of the area, the most extensive of which is a surface drift sheet deposited during the Nome River glaciation of the middle Pleistocene (Bundtzen et al., p. 2, 1994). The subsequent Stewart River, Salmon Lake, and Mount Osborn glacial periods were much smaller and restricted to higher elevations and mountain valleys (Howkins, p. 26, 1992).

At least twice in history, glaciers have advanced past the Nome coast. The first event occurred in the early Pleistocene, and the second event was the Illinoian Glaciation. Between the mouths of the Nome River and Rodney Creek, glaciers extended several miles seaward beyond the present coastline. These glaciers eroded mineralized bedrock and alluvial placers in the hills north of Nome. These glaciers also sheared into underlying marine sediments, leading to layering of glacial till and marine clayey silt. This glacial action led to the emergent beach deposits in the Nome area being a major source of the gold (Nelson and Hopkins, pp. 7-8, 1972).

Sediment-eontrolled Structures Eight major sediment-controlling structures are given in Figure 2. The Back Trough feature is composed of sand and gravel facies sediments with some occurrences of mud. The Central Core is the dominant structural element. It is composed mainly of diamict facies sediments and shows an asymmetrical outline. Sediment in the Fore Trough is mud-rich and has a low gold content. The Thrust Zone is an area of displaced mud caused by glacial advance over the troughs, hence the name Thrust Zone. The Transition Zone is an extensive atea of mature gravel and sand with interbedded mud. The Marine Fringe is the seaward margin of the Central Core with washed sediment composed of gravel and sand facies. East of the Central Core, the East Flank is composed of typical high-energy shallow marine deposited sediment. The West Flank is an area dominated by gravels with less distinct boundaries than the East Flank (Howkins, pp. 62-74, 1992). Seven identifiable zones are given. These include the coastal zone, the Penney River delta, the Transition Zone and Central Core high, the Yukon horst plateau, and the Thrust Zone ridge and the Norton Sound Basin (Howkins, p. 79, 1992).

ha-ke GraveueoUldera rn sand

Mud and Sandy Mud Ocean Floor

Meters

Figure 1. Lithologic map of the ocean floor off the coast of Nome (surficial geology summary digitized &om Howkins, p. 160, 1992).

Current net-longshore drift in the area is to the east This trend has also been the case in the past in reference to the submerged beach deposits, through studying pebble size, roundness and quartz content. The bottom sediment distribution near Nome is influenced by several factors, namely present longshore and offshore currents, subaerial sediment deposits h m past streams and glaciers, and effects of past wave action tkom shoreline transgressions and regressions on these subaerial sediment deposits (Nelson and Hopkins, p. 9, 1972).

Economic Geology Four types ofland-based metallic mineral resources are found in the Nome area. These resources include:

gold-polymetallicquartzcarbonate veins, stratiform, massive sulfide-barite deposits associated with felsic metavolcanic schist'and metafelsite centers, massive sulfide-iron deposits hosted in carbonate-dominated terranes of uncertain origin, and heavy mineral placer deposits.

Polymetallic Gold Quartz Veins. These veins are the source of most of the significant gold resources that is known in the Nome district. They cut pelitic and mafic schists and carbonates and are believed to be a product of regional metamorphism. The mineralization package with these veins include As-Sb-Ag-Zn- Pb-Cu-W-Au. Example deposits are Rock Creek for gold, Sophie-Gulch for gold and tungsten, the Mt. Distin area for gold and antimony, the Charley Creek area for gold-bismuth-arsenic vein deposits, and others. There are at least three types of veins that have been recognized. Chalcopyrite-sphaleritequartz- carbonate veins are one type. They are seen as boudins rolled around F1 fold axes. Saddle reef quartz- gold-polysdiide veins are a second type. The third deals with brittle vein systems that show distinct high angle crosscutting of metamorphic stratigraphy.

Major Geologic Structures

Figure 2. Sediment-controlled structures map for offshore Nome (digitized from Howkins, p. 155, 1992).

Stratiform Massive Sulfide Deposits. This deposit type involves the informal Aurora Creek section of muscovite-feldspar metavolcanic schist, carbonate, pelitic schiit, and graphitic schist. Mineralization of a premetamorphic origin includes lenses and disseminations of sphalerite, galena, chalcopyrite, and massive barite. A premetamorphic origin for the mineralization is inferred because it and the host rock are both complexly folded. ' .

Carbonatehosted Stratabound Polymetallic Deposits. These sulfide-bearing deposits have an unknown origin. Proposed mineral deposit types include sedex, volcanogenic massive sulfide, intrusive related replacement, and hydrothermal veining.

Placer Deposits. Classic examples of beach placers are seen at Nome, Alaska, on the Seward Peninsula. These placers show most features common to gold beach placers. The Nome district is the second leading producer of placer gold in Alaska The main source for mining in the Nome area'has been placer deposits. From 1898-1993, an estimated 4,822,569 ounces has been produced &om stream, colluvial, glacial, and marine strandline placers. Roughly 70% of this placer gold comes kom the smdline deposits.

The gold very much follows the coastline immediately offshore. Concentrations can also be seen in the area of the Thrust Zone and the P e ~ e y River delta. A significant byproduct kom this placer mining is a large volume of highquality, washed tailings suitable for foundation and construction purposes (Bundtzen et al., pp. 3-4, 1994).

Five placer types have been classified by Boyle for the area near Nome. These types include: 1) Eluvial - angular, coarse gold, occurs on or in the upper part of disintegrated schists 2) Gulch, creek, stream, and river gold fine to coarse - nuggets common 3) High-bench placers - stream placers laid down in former drainage patterns 4) Gravel-plain placers - most are small and lean, scattered irregular accumulations disseminated

through the gravels 5) Beach (strandline) placers - modem, buried, and raised types (Boyle, p. 371, 1979).

The origin of the gold-rich beaches at Nome can be traced &ee steps.- The first step involves the presence of leangold gravels on the coastal plain iiom deltas. Next, coastal uplift periods occur, each new beach is subjected to wave action, and the result is the gold-rich gravels become more concentrated. Also, some gold concentration by streams occurs with their dropping of sediment loads in shallow areas along the beaches. The final progression deals with the idea that glaciation does not necessarily destroy andfor prevent the formation of placers. Glaciation can lead to concentration of gold, as is seen in Nome.

METHODOLOGY The work for this project began with the retrieval of drill hole data &om over 100 diskettes (5 % and 3 !h in) that were made by Nova Natural Resources around 1990. The data on these disks spans several years of collection. This data set is the same used by Celia Howkins in her thesis.

The data comes h m drilling done o6hore the coast of Nome over several miles. The coverage of the drill holes is divided into twelve claim blocks per WestGold database. These blocks are: 1) Coho, 2) Halibut, 3) Herring, 4) Humpy, 5) King, 6) Other A, 7) Other B, 8) Other C, 9) Pink, 10) Red, 1 1) Silver, and 12) Tomcod.

Total, there are about 3200 drill holes. The data consists of several descriptive liles for each drill hole. Most drill hole files have associated~lithology, gold value, and blow count descriptions for each depth interval of each drill hole. These file types are the principle sources of information for this project. There is also a geologic key in the data that gives a description of each geology type intercepted by the drill holes. The gold values were given in two units, oz/m3 and odyd3, along with normalized values. The normalized gold value was the relative gold concentration as compared with a reference gold concentration (i.e. 0.02405 oz/m3). Gold intensity was used in this study to further divide the range of the normalized gold values (i.e. 4 to 45) into 9 even intervals with intensity 9 having the highest gold concentration. Blow count data, telling how many blows were needed to drive each barrel through the sample length of 30 cm (1 ft), was also given to allow for hardoess determinations.

Three programs, Generic Cadd 5.0 and Geomath's Geological Database Management program, and RBASE 2 were used by Howkins in the early 90's in her ore reserve analysis of the data The files in RBASE were the primary sources of the data used in this project. Because of the age of these programs and their use of outdated file extensions, a great deal of time was spent to convert the data into *.W files. This conversion to *.m files was done because it would allow for easy use of this data with a variety of programs. The drill hole files also had to be sorted into the appropriate claim blocks in which they were located.

Rockworks99 The computer program Rockworks99 was used extensively with this project. This straightfornard Windows program uses a spreadsheet format data window to read data such as stmtigraPhic~formation elevations, X, Y, and Z data, lineations, etc. Tab-delimited ASCII text files is the format for the data files. Template files are used to set up the data sheets with the appropriate headings and column types as well as colors, symbols, and patterns. The basic features of Rockworks99 include mapping, gridding, solid modeling, stratigraphic (and lithologic) analysis, volumetric tools, hydrology, utilities, directional statistics tools, general statistics and diagrams, VRML tools, and other things such as X and Y coordinate conversions (RockWare, pp. 1-4, 1999).

A data file for each block was made in Rockworks99 with the creation of the necessary files. This feat was done by importing the text files into RockWorks99 and then saving the data file with the Rockworks99 *.atd extension. A template lile was also created in the necessary format with the appropriate h e a d i i , and this template was used for each of the fish blocks. An example of a data sheet used in Rockworks99 is given below in Figure 3.

This spreadsheet is a portion of the data from one of the claim blocks. Listed fmt are the drill hole identification number and the Easting and Northing coordinates. The Symbol column gives the symbol for each drill hole as it would be plotted on a plan view map. The gold value for each interval of the drill holes is given in the Gold Index column. The Collar column gives the depth in feet of each collar below sea level. The different lithologic layers intercepted at each drill hole interval are given in the Lithology column. Each lithology type may be assigned a pattern already defined in Rockworks99 or one created by the user. The data in the Blow Count column gives the number of blows needed to drive the length of the sampler for each interval of the drill holes.

With the completion of a claim block data file in RockWorks99, a variety of features of the program were utilized. Three-dimensional models, strip logs, fence diagrams, and other tools were used Three- dimensional solid models were created in Rockworks99 with both normalized and actual gold values for each block Solid models were done to represent the variable gold distribution " G using the X (Easting), Y (Northing), and Z (elevation) of each drill hole in a block (RockWare, p. 72, 1999). The X and Y coordinates are given in the main data sheet for each block, whereas the Z coordinates (downhole depths)

I I '. # . . I . . I I:- ** - 101 x i VW MWI Ekl S-dd Stlat YoLms Wid S - S rpontr IIli VRML W h W

l * / + l ~ ~ l ~ l e l ~ l ~ l . e g as L i .lilelSm 1vl~Ixl:r r B Q ~ B .

858166Jn31172742 - Otherhole 859 1664477 11732l7125 - Otherhde 860 1663706 1173116875 . Otherhde 861 1661347 1172293625 - Otherhole 862186207111727745 - Otherhole bd 6 4 Otherkhlcqv\862~ O t h e r b \ B 6 2 b ~

Figure 3. Example data sheet fiom RockWorks99.

are retrieved fkom linked data files. Each solid model is graphically displayed with each three-dimensional cell colored with its appropriate "G" value

Strip logs are illustrations of several possible types of information for individual drill holes. Strip logs in Rockworks99 can show stratigraphic patterns, lithology patterns, histograms, and curve traces representing point data that is linked to the data sheet. The patterns and colors for the different lithologies were manually created for this project.

ARC/INFO@ Several types of spatial data can be used in ARc/INFon. These types include coverages, grids, tins, images, tables, Spatial Database ~ n ~ i n e * (SDE*) layers, and shapefiles. Many geoprocessing tasks are possible with ARC/INFO@. Among these tasks are data automation and integration, topological editing, advanced analysis, advanced visualization, advanced cartography, and spatial data management (Environmental Systems Research Institute, Inc., pp. 2-3, 1997).

ARC/INFO@ has several important applications for this project. The most obvious use of the program is its ability to generate maps. Bathymetric maps for the offshore area of Nome that were made with ARC/INFO@ because these maps are complex and require significant computer processing time. ARCIINFO@ was also used to interpolate slices fiom the RockWorks99 block models. These models were sliced into roughly three-meter depth intervals to be used' in the gold resource calculations. These thickness intervals were based on the original drill hole data.

ArcView ArcView is a less complex windows driven version of ARc/INFo@ that was also used for several applications for this project. Arcview uses presets that limit the applications possible when working with maps. Arcview does, however, have a macro language that enables a user to perform basically the same operations as ARC/INFO". Also, surface interpolation can be done with Arcview, but the process is slower than when it is done in ARc/INFo8.

ArcView was used with this project for two main purposes. One is to create display maps. Graphical maps suitable for presentation can be easily created. The second purpose was to convert X,Y, and Z data into a comparableformat for use with ARC/INFO@. The commands for this process are already established in ArcView. The X,Y, and Z data comes fiom the three-dimensional block models generated by RockWorks99.

The Web Page One goal of this project is to present its results to the public through reports, student papers, lectures, and on the World Wide Web. A web page has been dreated that gives background information on the project, descriptions about the Nome area, maps of the claim blocks that include all the normalized slices of gold

deposits, and links to other relevant sites on the web. Eventually, the web page will include several interactive features. A person with the authority to use the actual, non-normalized information will be able to select online any order of claim blocks and then see the relevant statistics (average gold grade, total gold, standard deviation, etc.) for the blocks selected. The site is located at h~://mined.uafsme.alaska.edu/nome~roiect~nomehome.htm.

RESOURCE EVALUATION A significant part of this project was the resource estimation. The programs RockWorks99, Arcview, ARCIINFO~, and EXCEL were used for this task. First, three-dimensional block models were created for each block sing actual and then normalized gold values. The two types were created by using different columns in the gold files that are linked with each Rockworks99 spreadsheet. The data was already in this format when it was given to the project. Normalized models were created because even though it is a major goal of this project to display information on the Web, this data is privately owned. Access to the actual data on the project's web page is possible, however, with authorization h m the owner.

Once all the block models with the actual gold values were ,created, they were then exported in ASCII format for use h Arcview. The format of the slices was manually developed beforehand using the minimum and maximum sampling depths for each block. Using the first sampling depth, the next depth closest to 1.5 m (5 ft) was chosen, and then the next for a slice roughly 3 m (10 ft) thick. Once all the slices were made, a shape file was created in ArcView that included the drill hole locations for each fish block. These shape files were then used in ARC/INFO" where they were converted into point cover files and then later the rasterized grid files used with each slice.

ARcIINFo" was used to observe and record data such as the minimum and maximum gold values (ozlm3) for each slice of each block. ARcIINFo" was also used to calculate'the sum of the cells and &~d&d deviation for each slice of a block, and then the sum of cells was totaled. Each cell represents a roughly 1 m (3.3 ft) square pixel. The depth of the pixels varies, given the manual method of determining the slices discussed earlier. Each depth is a little over one meter, though. Microsoft Excel was then used to calculate the mean gold value for each slice'and to present the data table for the resource calculations for each block and the entire whole. This information is given Table 1. Further work with the slices was done with each slice being developed into a map using ArcView and then exported as a *.jpg file for informational use.

RockWorks99 - 3D Modeling Creating a three-dimensional model in Rockworks99 is a straightforward process that is accomplished strictly with windows driven commands. The f M step is to simply click on the icon: This icon brings up a window that allows the user to format the model to be created. Figure 10 gives the data input screen for the model. The spreadsheet data fiom each claim block was used with this step. The Data Type option set what column in the linked gold tiles would be used for the model generation. It was herewhere normalized or actual gold models are chosen.

The next step is the selection of modeling method. Closest Point, &verse Distance (All Points), or Inverse Distance (Octant Searching) are the modeling methods available in RockWorks99. The Closest Point- method simply uses the point closest when interpolating voxel values. The Inverse Distance (All Points) method uses all the points, and Inverse Distance (Octant Searching) includes only the closest point within each octant when interpolating voxel values. The Inverse Distance (Octant Searching) method was chosen for this project given that its accuracy was found to differ little with the Inverse Distance (All Points) method with the data for this project The third step involves selecting the model dimensions. The X- and Y-axis density and the Z-axis density are set in this window. The presets fine, medium, or coarse are available, or the densities can be set manually. The models generated for this project all were set at SO%, or medium density for both density columns. A file name and saving location are then created, and the diagram options are set. The size of the cutaway, legend, and annotation are set in this window, or the default settings can be used. The contouring options, viewer position, and exaggeration are also shown in this window.

DISCUSSION The results of the resource estimation for all the claim blocks are given in Table 1. Silver and Humpy blocks have the highest amount of gold, 496,986 oz and 676,082 oz respectively. Other A and Other B

Table 1. Resource calculations for each fish block and entire claim blocks

blocks contain the fewest ounces at 109,450 oz and 5,801 oz. Humpy, at 0.039 odm3 (0.03 odyd3), and Coho, at 0.029 odm3 (0.022 odyd3), have the highest grades. The blocks with the lowest grades were Other B, 0.008 odm3 (0.006 ozlyd3), and Other C, 0.004 oz/m3 (0.003 odyd3). The total amount of gold resources for the entire claim blocks is 3,379,373 ounces with an average grade of 0.014 odm3 (0.01 1 odyd3). Tomcod, Red, and Halibut blocks encompass the most volume, Tomcod being the highest. The Other blocks have the lowest volumes, and Pink block is the next lowest. The Other blocks have the least volume is to be expected given that they cover scattered areas that were not originally blocks. With the Halibut and Silver blocks, the highest concentrations of gold values are near the surface (the first 1.5 m of depth) for both blocks. The same is basically true for Humpy, Red and Tomcod Blocks. The 3,379,373 ounces of gold was an estimate of the total resources available in the claim blocks. No cutoff grade was assumed in this project given that no mining method, gold recovery process, gold price, or other design data was associated with this project. Without this data no beneficial cutoff grade could be reasonably determined. nor could the reserve estimation.

Coho Block The gold ounces peak within the interval of 7.0 to 7.9 m (23 to 26 A) below sea level at over 44,000 ounces. The ounces drop gradually below 7.9 m (26 A), and above 7.0 m (23 A) there is little coverage and thus fewer ounces. Coho block extends the M e s t of the blocks out into the Norton Sound. Coho lies primarily in the Thrust Zone structure, and this is a ridge composed of displaced mud caused by glacial advance over the Fore Trough.

Most of the gold present is in the western half of the block, as seen with the slice maps f?om Arcview. There is also a zone that is visible fkom 6.0 to 10.7 m (19.7 to 35 A) in the eastern half, but its values are not as high as the ones to the west. There is a NW-SE trending fault that cuts across the middle of the

block that may have something to do with the gold primarily being to the west. No real variation in depth is seen with the bathymetric map.

Halibut Block Halibut is one of the larger claim blocks in terms of total gold ounces and total volume. The ounces sharply increase with the amount of drill hole coverage h m 17,000 oz at 4.3 m (14 ft) to 86,000 oz at 5.3 m (17.5 ft). The ounces stay in the 80,000 oz range where they jump to 124,000 oz at the last interval of 9.1 m (30 ft). Halibut lies mostly in the Central Core and extends into the Marine Fringe. The Central is composed mainly of diamict facies sediments. The Marine Fringe is the seaward margin of the Central , Core with washed sediment composed of gravel and sand facies. For all the claimblocks the gravel and diamict facies seem to host the highest areas of gold, and so it is not unusual that Halibut contains one of the largest amounts of gold ounces.

The 3D solid model shows that the gold in Halibut lies in the center of the block corresponding to bathymetric lows. AIJ the high concentration areas consist of relatively small areas that seem randomly scattered. These areas appear to be fault-controlled.

Herring Block The distribution of gold is mostly uniform throughout the slices. A peak value of 30,000 ounces is found at 7.9 m (26 ft) below sea level, and the rest of the slices have ounces in the 20,000's with three intervals down to 18,000 o z The 0.6-m (2-foot) slice, because it has little coverage, shows only 890 ounces. Herring falls mainly in the eastern side of the Central Core, directly behind the Halibut block. An incised river channel cuts across the Central Core in this area h m -2 1.9 m (-72 ft) and above. The western extent of glacial sedimentation is seen to the west of this channel with a cut of diamict sediment that is seen up to -18.0 m (-59 ft) (Howkins, p. 65, 1992).

Herring block lies entirely on the Yukon Horst. No faults cut across this area. Looking at the bathymetry map, the northern half of Herring is quite shallow; it only goes to 2.4 m (8 ft) of depth. The southern half drops to 6.4 m (2 1 ft). There is a distinct zone of high values just left of center of the block at the interval of 3.8 to 5.9 m (12.48 to 19.33 ft). Another exists to the right of the center of the block firom a 6.9 to 9.0 m (22.8 to 29.6 ft).

Humpy Block The gold in this block peaks at over 120,000 ounces h m 6.7 m (22 ft) to nearly 10.0 m (33 ft) below sea level (Figure 4). On either side of this interval the values drop off, but not as drastically as seen earlier. Humpy lies in the Thrust Zone, directly seaward of the center of the Central Core. The Thrust Zone. is a ridge of low relief probably formed by displaced mud h m the Fore Trough by glacial advance. Humpy has both the most ounces and highest grade of the blocks. Underneath the Thrust Zone is a horst 396 m (1300 ft) wide and 3.0 km (1.86 miles) long along strike (Howkins, p. 54, 1992). The southern edge of this horst is marked by a fault running East-West that appears to cut across the center of Humpy. Because the many gold concentrations follow the same pattern (running East-West all across the block), there is a correlation to the high areas and this fault. This pattern is readily visible at 6.7 to 11.1 m (22.04 to 36.55 ft). The remaining interval at 12.2 m ( 40,17 ft) shows a dramatic decrease in these high gold zones at 12.2 m (40.17 ft).

King Block The distribution of the gold is fairly even over the slices with the h t two slices having little coverage, with 1300 oz and 7500 oz. The peak value of 3 1,000 oz is seen at 4.1 m (13.5 ft) below sea level. King block is located on the Back Trough and Central Core. The Back Trough sediments include sand and gravel facies and some mud. King lies on the Yukon Horst in an area without any major faults.

Other Blocks These blocks were created for this project to account for the drill holes that did not fall into one of the other blocks, and so the Other blocks do not cover near as much area as the other claim blocks. The block Other A has 109,000 ounces and a peak value of 41,000 oz at 3.4 rn (1 1 ft) below sea level. There are, however, only five slices for this block. The last interval occurs at 4.5 m (14.66 ft), and so it is not possible to tell

Normalized Gold Values for the Humpy Block 22.04 feet Below Sea Level

05 0 05 1 Klbnmtem I 2 3 4 6

Above map shcms the Nonnalii Gdd Values of the Humpy Block 22.04 fee! below sea leval.

6 7 The area shown is a slice from a solid block Rlodel Unbrsity of Aklcg Fatbank

8 and the block boundary was formed by connecting -'* W M n g 9 the outer-most drill hdes. Septambmr 1,1898

I I

Figure 4. Normalized gold values for the Humpy block 6.7 m (22.04 ft) below sea level.

how the gold values trend any deeper. Other A is located farthest west mainly on the West Flank structure off the coast.

Other B block has only 5800 total ounces. Its peak value of 23 10 ounces is seen at 3.8 m (12.5 ft) below sea level. It is located directly East of Tomcod in the Western Flank just off the coast of Nome. Other C block has slightly higher total ounces (1 12,800) than Other A. The gold is distributed fairly evenly over the eight slices, and the peak value is 2 1,900 oz at 7.3 m (23.9 ft). Other c i s found off the coast and is the furthest east block A normal fault trending NE-SW runs roughly through Other C.

Pink Block The peak value of 26,600 ounces is seen at 7.3 m (24 ft) below sea level. Pink block is located mainly in the Back Trough, just off the coast and to the west of Nome and extends into the Cenhal Core. The Anvil Creek Fault cuts across Pink leaving the northwest corner of the block in a graben structure. The other section of Pink is on the Yukon Horst. The northwest comer has a high gold value at the interval from 7.4 m to 8.3 m (24.29 to 27.35 ft) that is not present at shallower depths. This area is .an ancient zone of accumulation at. Another strong peak area occurs in the southwest region of the block from 3.7 to 7.4 m (12:07 to 24.29 ft). A circular area of high values is readily visible with three distinct zones. This area is on the eastern border of the Anvil Creek Fault.

Red Block An amount of 58,700 ounces at a depth of 5.2 m (1'7 ft) below sea level is the best interval for this block. This block and the other claim blocks west of it (Silver and Tomcod) have far more ounces on more depth intervals than the blocks along the coast to the east. Red block is directly west of Pink block, and it lies within several structures. The Back Trough, Central Core, and Marine Fringe influence Red block. Red block lies almost completely on top of the graben feature immediately west of the Anvil Creek Fault. The

southern edge of Red block intercepts the Norton Basin Boundary fault, which is an asymmetrical, concave tmugh or synform that plunges to the south. It appears as a seaward dipping scarp on seismic maps (Howkins, p. 52, 1992).

Silver Block At 5.7 m (18.65 ft) peak interval of 74,000 ounces is seen. All the depth intervals fiom about 2.1 m (7 ft) and deeper have high gold concentrations; the two intervals above 2.1 m (7 ft) have total ounces that aie an order of magnitude less because of their limited coverage. Silver block lies directly west of Red block along the coast. It is centered in the Transition Zone. Mature gravel and sand with interbedded mud dominate this area. The very bottom portion of Silver is cut near horizontal by the northernmost extent of the Basin Boundary Fault. The high gold areas in this block appear to match the area covered by the Basin Boundary Fault. This high zone is visible fiom 4.8 to 9.9 m (15.89 to 32.41 ft). There is a distinct cone of high valuesvisible in the southwest corner as well. As before with the area in the Pink block, it appears faulting is creating a zone of accumulation for gold. The bathmetry map shows a circular low in this area, which would aid in accumulation.

Tomcod Block The peak interval occurs at 4.9 m (16 ft) below sea level and has 57,700 ounces. This high area runs fiom 4.0 m (13 ft) to about 5.8 m (19 ft). Tomcod is directly west of Silver block along the coastline. It covers parts of the West Flank and Marine Fringe. This block covers the most area and in about -its center is a graben. This structure is delineated by two faults trending NE-SW and NW-SE. There is also a fault running east-west in the lower portion of the block.

The gold is found mainly across the middle of the block to the eastern edge. It does not extend to the western edge. The western fault boundary of the graben seems to be a limit& feature of this high gold area. This &end is visible fiom 3.1 to 5.8 m (10.28 to 18.89 ft). There is also a small distinct high area in the lower right leg of the block fiom 5.8 to 9.3 m (18.89 to 30.37 ft). This region corresponds to a bathymetric low, and so is most likely a zone of accumulation.

CONCLUSIONS Humpy, Coho, and Silver blocks have the most gold. Beginning on the boundary area of Pink and Red blocks there are more ounces over more intervals to the west than there are to the east. This boundary follows the Anvil Creek fault. East of this fault there is limited faulting and less -depth variability as compared to the west. The only major feature affecting the claim blocks in this area is the Yukon Horst. Humpy is by far the best block, and it lies on top of a horst feature, immediately seaward of the Central Core. This area is an accumulation zone for gold-rich sediment.

The areas with high concentrations of gold generally occur in bathymetric lows that are near faults. The bathymetric lows serve as zones of accumulation, and the normal faults aid in this accumulation by weakening the soil. The enormous variability seen with the lenses of deposition can be attributed to varying stages of fluvial and marine sediment deposition. Fluvial deposition is the more dominant of the two, and it occurs when the area of deposition is above the wave base. When an area is below the wave base, marine processes deposit sediment. The current variability is seen with a sediment profile on the ocean floor that changes considerably in only a few years. Fluvial and marine deposition, when combined with periodic glacial advance over the area, has formed scattered lenses of gold-rich sediment.

REFERENCES R. W. Boyle. 1979. The Geochemistry of Gold and its Deposits (together with a chapter on geochemical

prospecting for the element). Geological S w e y of Canada. Geological S w e y Bulletin 280. T. K. Bundtzen, R. D. Reger, G. M. Laird, C. S. Pinney, K. H. Clautice, S. A. Liss, and G. R. Cruse. 1994,

Progress Report on the Geology and Mineral Resources of the Nome Mining District. Division of Geological & Geophysical Sweys. Public-Data File 94-39.

Design Science & Engineering, 1998. CIS Applications to Alaskan Near-shore Marine Mineral Resources: Phase I, Assembly and Compilation of Information Part One. Prepared for School of Mineral Engineering, Univ. of Alaska Fairbanks.

Environmental Systems Research Institute, Inc. 1997. Getting Started with ARC/INFO@:. Introducing ARC/INFO and ~ r c ~ o o l s ~ .

C. A.. Howkins. 1992. A Model for Shallow Marine Placer Depositions: Based on the Marine Gold Placers at Nome, Alaska. University of Toronto, Master's Thesis.

J. E. Mielke. 1997. Marine Mining within the U.S. EEZ. Congressional Research Service Report 97-588 ENR Lycos 20 September 1999.

C. H. Nelson and D. M. Hopkins. 1972. Sedimentary Processes and Distribution of Particulate Gold in the Northern Bering Sea Geological Survey Professional Paper 689.

NOVA NATURAL RESOURCES CORPORATION. 1994. Nome Offshore Gold Placer Project, Nome, Alaska NOVA NATURAL RESOURCES CORPORATION, Business Plan.

RockWare, Inc. RockWorks9% Instruction Manual. 1999. RockWare, Inc., Golden, CO.