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GOLD COMPOSITION ANALYSIS OF THE MOORE CREEK PLACER

DEPOSIT REVEAL LARGE (>15 WT%) HG CONTENT IN GOLD; INSIGHTS

AS TO SOURCE AND ORIGIN

 

Nicholas Van Wyck1 and Rainer Newberry2                     November 2006

 

1Full Metal Minerals, Anchorage, AK 2Department of Geology and Geophysics, University of Alaska – Fairbanks.

 

Full Metal Minerals started a modest exploration program in 2006 in the Moore Creek area. Work consisted of

stream and soil sampling, limited rock chip sampling and mapping around known prospects. In addition, a suite of

panned gold flake samples were collected from several drainages feeding the main Moore Creek placer area. These

samples were mounted into epoxy plugs and microprobed for their Au, Ag and Hg values for the purpose of

addressing whether the gold placer deposit at Moore Creek was of a local or distal origin and to help focus target

selection.

 

Moore Creek is an historic placer deposit located in the Iditarod 1:250,000 quadrangle, approximately 90 km

southwest of McGrath and 47 km northeast of Flat. Historic gold production from 1911 to 1986 was estimated at

53,990 oz Au and 12,520 oz Ag (Bundtzen and others, 2003). Pay is from gravels that lie on weakly

metamorphosed Kuskokwim Group siltstones and sandstones. Cobbles within the placer gravels are composed of

basalt, the closest exposures of which are located 2.5 km to the northwest. However detailed soil sampling and

prospecting has yet to find surface exposures of gold mineralization associated with these volcanic rocks.

 

Panned gold flake samples were collected from five sites, which correspond to samples from Upper Moore Creek

(A), Six Pup (B), Nevada Gulch (C), Moore Creek (D) and Lower Moore Creek (E) below the Willow Creek

confluence. Selected results are tabulated below. Probing of two Moore Creek gold nuggets has confirmed Hg

contents in gold with values as high as 5.5 wt% Hg.

 

Table 1: Summary of Au and Hg composition values of detrital gold grains from Moore Creek.

Site

Size

(microns)

Shape

#

grains

Fineness

(max)

Fineness

(mean)

Fineness

(min)

Hg wt%

(max)

Hg wt%

(mean)

Hg wt%

(min)

A

100-400

s-r

15

762

749

727

15

5.5

1.2

B

200

s-a

13

766

748

705

9.2

3.9

0.8

C

100-300

s-a

15

764

737

681

7.3

4

1.9

D

300-500

R

15

767

733

650

17

6.6

0.6

E

200

R, f

20

844

764

721

9.2

3.4

0.5

 

Shape: s-r (sub-rounded), s-a (sub-angular), r (rounded), f (flattened)

Fineness is the Au fineness expressed as (wt% Au/(wt% Au + wt% Ag))*1000.

 

In that there are differences between the sample sites in both shape and composition, the data conform to a model

indicating that the gold at Moore Creek was locally derived. It is suggested by the data that both Nevada Gulch and

Six Pup contain similar sourced low-fineness Au with moderate Hg contents. Upper and Moore Creek samples are

similar in having both low fineness gold and the presence of extremely high Hg contents. Finally Lower Moore

Creek shows to typical Ag-leaching/increasing Au fineness expected moving away from the source area.

 

The extremely high Hg contents recorded in these gold grains can not be the result of anthropogenic sources, as the

values were measured using a microprobe on locations within polished grains. The only other area in the

Kuskokwim Basin the authors are aware of with similarly high Hg in Au values are from placers draining the giant

Donlin Creek deposit located 80 km to the southwest where values ranging from 11.8 to 2.7 wt% Hg (average 5.7).

Hg values of greater than 6 wt% in gold are rare in the published literature.

 

Explanations for the high Hg in gold values fit with a crustal-derived source of metals retorted from the

Kuskokwim basin during a regional heating event (Goldfarb and others, 2005). Reducing conditions likely favor

incorporation of Hg into the gold lattice, especially at lower temperatures. Variations in fluid chemistry, PT

conditions are plausible explanations for the range in observed values. In support of this model, samples A and D

with the highest Hg in gold values sampled drainage basins hosting high level volcanic rocks. Samples B and C

were collected from drainages with plutonic rocks contained lower Hg in gold values.

 

The compositional data presented in this paper, together with the shapes of fine crystalline gold nuggets recovered

by prospectors, all indicate a local source for the high-grade gold veins at Moore Creek. The likely location being

the foothills immediately northwest of the placer workings, likely associated with structures related to the Nixon-

Iditarod fault system, which would have acted as conduits for deep crustal derived fluids.

 

References:

Bundtzen, T.K., Miller, M.L., and Hawley, C.C. 2003. ARDF-Iditarod Quadrangle. USGS OFR 2003-1311

Goldfarb, R.J., Baker, T., Dubé B., Groves D.I., Hart C.J.R. and Gosselin P. 2005 Distribution, character, and

genesis of gold deposits in metamorphic terranes. Economic Geology 100th anniversary volume, pp 407-450.