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Dee |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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The Dee gold deposit is located in western Elko County, north-eastern Nevada, some 48 km to the NNW of the town of Carlin, USA. It falls within and towards the northern end of the Carlin Trend and is approximately 2.5 km to the NNW of the Bootstrap/Capstone deposits, near the margin of the Bootstrap Window which lies to the north of the Lynn and Carlin windows.
Published reserve figures for the Dee orebody include:
3.9 Mt @ 1.65 g/t Au = 6.4 t Au (Proven+probable reserve, 31 Dec. 1992, Christensen, 1993)
2.4 Mt @ 3.94 g/t Au (Original high grade reserve, 1983, Ellis, 1987)
1.8 Mt @ 0.86 g/t Au (Original low grade reserve, 1983, Ellis, 1987)
14.0 Mt @ 2.1 g/t Au = 29.27 t Au (Production+reserves+resources, 2002, Theodore, et al., 2003)
~10.3 Mt @ 3 g/t Au = 31 t Au (Pre-mining resource, Jory, 2002)
The original discovery at Dee was in Boulder Creek, in 1980, by a prospector, Whit (Dee) DeLaMare who was working for Cordex Exploration Co.. Cordex was a partnership between Dome Exploration (US) Ltd, Lacana Gold Inc and Rayrock Mines Inc, each with 29.33% interest, and J S Livermore with 12% (Ellis, 1987). Cordex also operated the Pinson Mine.
Following the original discovery (the literature does not mention what this discovery entailed), some 247 exploration holes were drilled for a cumulative length of 36 600 m. This resulted in two bodies being delineated, the Main and the Ridge. A feasibility study was commenced in late 1982, and a decision to mine taken in April 1983. The feasibility study envisaged a capital expenditure of $US 24 m, $US 2.4 m of which was on the mill. Production officially began on 15th October, 1985 with an 815 tpd cyanide leaching mill with a carbon in pulp circuit to recover gold. In addition there was a 0.45 Mt pa heap leach facility (Ellis, 1987).
Heap leaching of 0.5 to 1.4 g/t Au ore on pads of from 45 000 to 90 000 t each yielded 55% recovery of gold in the first six month period. The mining rate in 1985 was 24 000 tpd of ore + waste with a maximum haul distance of 1500 m. The total mining cost in 1985 was $US 6.60/t, and the all up cost of production $US 200/oz ($US 65/kg) of Au, for the 42 000 oz (1.3 t) mined, 10% of which was heap leach production (Ellis, 1987).
Geology
Within the Bootstrap Window, silty limestones of the Siluro-Devonian Roberts Mountains Formation underlie relatively high energy carbonate facies of the Devonian Bootstrap Formation, an equivalent of the Popovich Formation which is found in the Lynn Window to the south. Thinly interbedded siliceous and calcareous sediments of the Upper Devonian Rodeo Creek Unit overlie, or are in high angle fault contact with the carbonates. Gold mineralisation is localised along north-trending high angle faults, and primarily occurs within Rodeo Creek lithologies peripheral to the faults. Altered Cretaceous (?) dykes of intermediate composition fill some faults and are commonly mineralised. Silicification is the dominant alteration type. Gold is associated with moderate to very dense silicification in the form of quartz stockworks and silica flooding. In places argillised siltstones and dykes, as well as hematitic fault gouge, host gold. The Roberts Mountains Formation and Bootstrap limestones are un-altered, except for the presence of jasperoid breccias along structures (Flaherty & King, 1991).
For more detail of the setting see the Carlin Trend Ð Geology and Carlin Trend Ð Mineralisation records.
The stratigraphy at Dee is as follows (after Ellis, 1987), from the structurally lowest unit:
Devonian, Bootstrap Formation - This unit is an equivalent of the Popovich Formation within the Lynn Window, and the Devonian Limestones of the Carlin Window. It is the main host to mineralisation at Dee. On the basis of drill intersections at Dee it may be subdivided into three members and four units, as follows:
- Massive limestone at the base, comprising a limy mudstone and an oolitic limestone unit;
- The middle Adit Member, composed of intercalated limestone and siltstone; and,
- The upper siltstone member made up of calcareous siltstone.
Devono-Carboniferous, Roberts Mountains Thrust Package - represented by at least three major thrusts which are recognised in the Boulder Creek district.
Ordovician to Silurian - Western Assemblage silici-clastics which have been thrust eastward over the Roberts Mountains Thrust package to overlie the Bootstrap Formation. Sediments mapped in this assemblage may include equivalents of the Devonian Rodeo Creek Unit recognised in the Lynn and Carlin Windows.
Cretaceous, Intrusives - generally of quartz-dioritic composition, present as dykes. These may be either fresh or altered, with the latter most common along faults and shear zones.
Tertiary, Cover - which conceals large areas around the Boulder Creek area, in places exceeding 150 m in thickness. The predominant lithologies are clayey chert pebble conglomerate and a thick sequence of tuffaceous volcanic sediments.
Structure
Following the Devono-Carboniferous Antler Orogeny which produced the major eastward thrusting over the Roberts Mountains Thrust, normal faulting was associated with Mesozoic doming. Some of these faults host the Cretaceous dykes. Post-ore Basin and Range block faulting has preserved deep Tertiary basins in graben like structures, with some evidence of reactivation of normal faults in the ore zone at Dee (Ellis, 1978). Normal faulting is both north-south and NNE to NE trending (Stewart & Carlson, 1978).
Mineralisation and Alteration
The ore mineralisation at Dee is controlled by both structure and stratigraphy. In the Main ore zone the upper Boulder Thrust forms the hangingwall, while the Central Thrust forms the footwall. Wide zones of shearing and brecciation have provided favourable sites to host mineralisation and by inference have acted as conduits for ore forming media. Within these structurally favourable zones grade and continuity are determined by the nature of the sediment. Siltstones that were originally muddy, calcareous and possibly carbonaceous have hosted mineralisation and have been mostly silicified and clay altered (Ellis, 1987).
The limestones at Boulder Creek are not carbonaceous and leaching of carbonate and replacement by silica, gold and other metals has resulted in large volumes of silicified mineralised limestone throughout the deposit (Ellis, 1987).
Silicification is the most prominent alteration, apparently leaching the carbonate host rock and replacing it with silica, especially in and near the main structural zones of broken and sheared rocks. Argillisation, or clay alteration of the muddy members of the host sediments can either enhance or inhibit gold accumulation. The altered clays are believed to be important micro-sites of gold accumulation. Oxidation extends to a depth of some 180 m, although studies of the un-oxidised ore indicate that finely disseminated pyrite is the source of the bulk of the gold in the orebody (Ellis, 1987).
The geochemical associations include Hg which is present in geochemical levels; Sb which is commonly seen as stibnite and as yellow oxides; Ag which forms an erratic halo with assays of up to 250 g/t; and Ba which is commonly seen as barite. As forms a negative anomaly, with values in ore being 25% of the surrounding background (Ellis, 1987).
Drilling indicates that the Main ore zone is an irregularly tabular body either dipping steeply, or displaced by a series of faults, to the west. It has maximum dimensions of 300 x 220 m in plan with a maximum thickness of 55 m. More than 55% of the ore is in rock units containing siltstone. Exploration to 1986 had indicated potential for extending this orebody at depth. The smaller Ridge zone is located 180 m east of the Main ore zone and comprises two small bodies, the upper and the lower. The upper zone is stratabound while the lower is structurally controlled by the Ridge Thrust. The Main and Ridge ore zones are separated by a deep, steep walled depression filled by Tertiary sediments with a surface extent of 1.6 ha. This has been interpreted as a collapsed karstic feature (Ellis, 1987).
The most recent source geological information used to prepare this decription was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Ellis R, 1987 - The Dee gold mine: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 285-288
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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