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Marigold |
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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 Marigold gold deposit is located approximately 64 km south-east of Winnemucca and 20 km to the north-west of the town of Battle Mountain in south-east Humboldt County, north-central Nevada.
Details of the original discovery of gold mineralisation at Marigold are unknown. Production in the past was restricted to the immediate area of the old Marigold Mine. Several thousand tonnes of ore averaging 7 g/t Au were shipped from underground workings in the late 1930's. In 1982 the Marigold Development Co mined, crushed and heap leached 3200 t of ore from an open pit on the site of the old underground workings to produce 8.43 kg of gold at an estimated recovery rate of 75% (Graney & McGibbon, 1990).
Exploration prior to 1985 involved a range of geochemical, geological and geochemical techniques and approximately 200 drill holes. In 1985 Cordex, a partnership of Rayrock Mines Inc., Dome Exploration (US) Ltd and Lacana Gold Inc., leased a group of claims held, and initially tested by a local syndicate. These claims were over alluvium covered ground approximately 1.5 km to the north-east of the old Marigold Mine. In one of their three drill holes the syndicate had encountered 0.6 g/t Au from 220 to 233 m. An initial 4 hole follow-up drilling program by Cordex was aimed at obtaining off-sets of the intersections in the syndicates three holes. The first two holes were 'disappointing' while the third hole cut 2.4 g/t Au from 67 to 88 m and the fourth intersected 7.5 g/t Au from 95 to 120 m. In 1986 the initial claims were pooled with those of the Santa Fe Pacific Mining Co. to comprehensively explore the alluvium covered area. Later in the same year the partnership leased a further 15 sq. km of claims from Nevada North Resources and other claim holders. This area took in the old Marigold Mines and other prospective ground. Drilling proved to be the most efficient technique over the alluvium as geophysical and geochemical techniques had proved ineffective. The reserve was drilled out using reverse circulation on 30 m centres, with some detailed 15 m infill drilling. Drill intervals of 1.5 m were analysed by fire assay, while numerous CN leach/AAS determinations were carried out to assess the amenability of the ore to leaching (Graney & McGibbon, 1990).
Approximately 350 drill holes, totalling 46 000 m, were used to calculate the ore reserves in the three main areas of the 8 South, East Hill and Top Zones. Additional ore in the 5 North, 8 North and Red Rock Zones had not been fully delineated in 1990 and, except for the leach resource at Red Rock, are not included in the reserve figures below. The 8 South, 8 North and 5 North zones are covered by up to 90 m of alluvium, while the Top, Red Rock and East Hill zones are partially exposed (Graney & McGibbon, 1990).
Published reserves are as follows:
3.94 Mt @ 3.56 g/t Au = 14 t Au (Mill Reserve, 1989, Graney & McGibbon, 1990).
7.18Mt @ 0.93 g/t Au = 6.7 t Au (Leach Reserve, 1989, Graney & McGibbon, 1990).
14.6 Mt @ 1.2 g/t Au = 17.5 t Au (Proven+Probable Reserve, 1994, AME, 1995),
33.2 Mt @ 0.64 g/Au = 21.5 t Au (Proven+probable reserve, December, 2005, Barrick Gold, 2006).
20.3 Mt @ 0.61 g/Au = 12.4 t Au (Measured+indicated+inferred resource, Dec., 2005, Barrick, 2006).
The individual ore zones were as follows in 1989 (Graney & McGibbon, 1990):
8 South Mill - 2.62 Mt @ 4.32 g/t Au + Leach - 1.86 Mt@ 0.75 g/t Au;
Top Mill - 0.69 Mt @ 2.06 g/t Au + Leach - 1.90 Mt@ 0.86 g/t Au;
East Hill Mill - 0.63 Mt @ 2.13 g/t Au + Leach - 1.45 Mt @ 0.82 g/t Au;
Red Rock Mill - 1.97 Mt @ 1.17 g/t Au;
Totals Mill - 3.94 Mt@ 3.56 g/t Au + Leach - 7.18 Mt @ 0.93 g/t Au; Overall total 11.12 Mt @ 1.85 g/t Au
Cordex, who also operated the Pinson and Dee mines, constructed a 1250 tpd carbon in leach mill that will process ore with a grade of more than 1.4 g/t Au, and a heap leaching operation with a 1500 tpd capacity for lower grade material. The life of the mine was expected to be 9 years from the commencement of operation in August 1989 (Graney & McGibbon, 1990). Production in 1993 was 2.85 t Au from 2.12 Mt of ore (AME, 1995).
Geology
Gold mineralisation at Marigold occurs in coarse to fine grained clastics of the upper Carboniferous (Pennsylvanian) to Permian Antler Sequence, and in quartzite and argillite of the Ordovician Valmy Formation (Graney & McGibbon, 1990).
The Antler Sequence rocks belong to the Overlap Assemblage. This assemblage was deposited after the Valmy Formation of the Western, Siliceous Assemblage was thrust eastward over the Roberts Mountains Thrust during the Devono-Carboniferous Antler Orogeny. As such the Antler Sequence unconformably overlies the Valmy Formation and was derived locally from the Antler Highland produced by the Roberts Mountains Thrust imbrication. The siliciclastics and greenstones of the Carboniferous to Permian Havallah Sequence was overthrust onto both sequences over the Golconda Thrust during the Permo-Triassic Sonoma Orogeny. This thrusting was also directed towards the east. Tertiary Basin and Range faulting has produced north trending horsts and grabens within these Palaeozoic rocks (Graney & McGibbon, 1990). For details of the geological setting see the 'Battle Mountain - Eureka' Trend - Geology record.
The stratigraphy within the mine area is as follows, from the structural base:
Ordovician, Valmy Formation - This is a folded and faulted sequence of quartzite, argillite and chert. Most of the deformation was the result of the Antler Orogeny. Structural complexity has rendered correlation and thickness determinations very difficult. The Valmy Formation is the host to the Top Zone and a major host of the East Hill Orebody (Graney & McGibbon, 1990).
Unconformity - separating the Allochthonous Valmy Formation from the alluvial to marginal marine Antler Sequence (Graney & McGibbon, 1990).
Upper Carboniferous (Pennsylvanian) to Permian Antler Sequence, variable but near 60 m thick - The inferred alluvial to marginal marine Antler Sequence resulted in a complex lateral and vertical interfingering of rocks with both marine and terrestrial affinities. The Antler Sequence dips at 15 to 20° westward and locally carries slump structures. The upper sections of the Antler Sequence are the dominant ore hosts at Marigold. The sequence comprises, from the base (Graney & McGibbon, 1990):
-Coarse conglomerate and sandstone - the conglomerate consists of clasts derived from the Valmy Formation chert and quartzite set in a siliceous, sandy, hematitic matrix. This unit grades conformably upwards into,
- Limy mudstone.
- Intercalated sandstones, siltstones and shales,
- Marker shale unit,
- Calcareous siltstone - containing lenses of conglomerate, sandstone and fossil hash which was deposited conformably on top of the shale. Lenticular beds of manganiferous, dirty sandstone were intermittently deposited with the calcareous siltstone. Beds of limestone interfinger with the calcareous siltstone,
- Calcareous-matrix conglomerate - which overlies the limestone beds,
- Brown siltstone at the top of the Antler Sequence interfinger with the calcareous matrix conglomerate.
The Antler Sequence is the major host to mineralisation at the 8 South, Red Rock. 8 North, 5 North, East Hill Zones and the old Marigold Mine (Graney & McGibbon, 1990).
Permo-Triassic, Golconda Thrust - This structure separates the Antler Sequence from the overlying allochthonous Havallah Sequence, and is the principal structural feature in the mine area. The sense of movement of the allochthon has been towards the east (Graney & McGibbon, 1990).
Carboniferous to Permian, Havallah Sequence, variable from >180 to 900 m thick - comprising the undifferentiated Havallah and Pumpernickel Formations, described elsewhere. The sequence contains chert, argillite, silty limestone, shale, sandstone and mafic volcanic rocks that constitute a complex structural package of thrust repeated slices. They are highly deformed and generally dip westward, with the trace of the thrust trending northwards (Graney & McGibbon, 1990).
Tertiary, Feldspar Porphyry Dykes - are found locally and are aligned along steep north and north-west trending structures. No dykes have been encountered in the mine area, although altered and mineralised dykes have been mapped several kilometres to the south of the mine area. Tertiary tuff and basalt are also known several kilometres to the south (Graney & McGibbon, 1990).
Tertiary Basin and Range faulting produced north trending horsts and grabens controlled by north-south striking normal faults, the dominant structures in the area. North-east, north-west and east-west striking faults are also present. North and north-west faults with limited displacement correlate with the mineralisation (Graney & McGibbon, 1990).
Deep erosion and weathering formed a pediment surface that is now covered by Tertiary and Quaternary alluvium in the mine area. Alluvial thicknesses of >150 m are known, although the average is 30 to 45 m. Several distinct alluvial layers can be mapped (Graney & McGibbon, 1990).
Mineralisation and Alteration
The reserves quoted above are contained within four separate deposits, the 8 South, East Hill, Top and Red Rock Zones. Two others, 8 North and 5 North have additional resources, while minor production has been derived from the old Marigold Mine. These seven deposits are distributed over an area of approximately 5 x 1 km. Additional mineralisation has extended the mineralised interval to around 12 m.
Both structural and stratigraphic controls were important in localising gold mineralisation. The quartzites to limestones of the upper Antler Sequence are the dominant ore host, particularly beds of calcareous-matrix conglomerates, although sections of both the Valmy Formation and the lower Antler Sequence also contain some ore. Changes in porosity and permeability appear to have been important factors in the stratigraphic localisation of mineralisation. Partial removal of carbonate from original calcareous rocks apparently preceded gold mineralisation and resulted in an increased porosity and permeability. In the 8 South ore zone (described below) siltstones in the overlying Havallah Sequence, and argillites in the Valmy Formation below, have been intensely altered to clays forming impervious barriers on either side of the ore bearing decalcified rocks of the Antler Sequence. This chemical preparation of host and wallrock has been interpreted as an important factor in ore zone localisation and in controlling associated barite crystallisation and iron oxide development (Graney & McGibbon, 1990).
Steeply dipping, north-south and north-west striking structures, with minor or no offset appear to be the most important structural influences on the occurrence and localisation of ore. The intersection of north-south striking faults with less obvious north-west striking structures and zones of fracturing seem to control the better grades. Other structural controls include imbricate thrusts within the Valmy Formation, and the Golconda Thrust. The Golconda Thrust is not an important ore control, other than that it may have been responsible for the pre-ore emplacement of a cap of less permeable rocks above the ore hosts (Graney & McGibbon, 1990).
Most of the gold occurs as very fine, generally <5 µm blebs in iron oxide pseudomorphs after pyrite, or as similar sized particles in carbonate veins or jarosite stained quartz matrix material. The ore zones are completely oxidised. The gold was probably co-precipitated with pyrite and later liberated by supergene oxidation (Graney & McGibbon, 1990).
The individual zones have the following characteristics:
Old Marigold Mine- Ore from this deposit occurred as pod like bodies in silicified basal Antler sequence rocks over a down dip length of 45 m, strike interval of 100 m and thickness of 1 to 5 m. The ore contained white barite as breccia filling, cross cutting calcite veinlets and locally intense hematite, limonite and jarosite staining and was best developed in a conglomerate directly overlying the Valmy Formation. The underlying Valmy Formation was also intensely altered with the quartzite having been leached of its cement leaving a soft friable mass of rounded grains of quartz sand. Argillite beds of the Valmy Formation were altered to a soft white clay with remnant bedding (Graney & McGibbon, 1990).
8 South Zone - The 8 South Zone, which is the largest outlined to date, is located under alluvial cover about 1500 m to the north-east of both the old Marigold Mine and the Top Zone. The ore zone has plan dimensions of approximately 650 x 350 m and is up to 50 m thick. It is elongated in a north-south direction along a set of north-striking faults that produced horst and graben structures within the ore zone. The bedrock surface was offset by reactivation of these faults after mineralisation. North-west striking faults or fractures with little displacement also appear to have exerted some control on ore. The ore zone appears to be concentrated at the intersection of a set of these north-west structures and the north-south trending faults and fractures. The ore was concealed below 45 to 90 m of cover (Graney & McGibbon, 1990).
The dominant host rocks were originally calcareous-matrix conglomerates and siltstones of the uppermost Antler sequence. A brown siltstone which interfingers with the conglomerate also hosts ore. Other adjacent units, including the overlying Havallah Sequence and the Valmy Formation below also contain ore, but only in close proximity of faults (Graney & McGibbon, 1990).
Two discrete zones of better grade are present in the orebody. The main zone has undergone total carbonate removal in its northern half, but incomplete removal in the southern half. Silicification of the conglomerates and siltstones is widespread, but irregular in this zone. Gold correlates well with the intensity of iron staining in the matrix. Dark red iron oxide and jarosite usually indicate high gold, whereas yellow iron oxide is associated with lower grades. Clay alteration is present in the north-west extremities of the zone but is not widespread. The other zone is located to the north-east of the main zone. In it, carbonate removal is nearly complete throughout, and structural control appears to be more important than in the main zone. More clay is present, usually associated with spotty silicification. The intensity of iron oxide staining is less than in the main zone and gold seems to be associated with orange iron oxide. Both zones carry abundant barite, including i). massive, grey-white, crystalline bedding replacements; ii). euhedral, coarse white or translucent barite crystals in silicified matrix material; and iii). white, finely crystalline barite veinlets and fracture fillings. The first two types are spatially associated with gold mineralisation, although the fine barite veinlets and fracture fillings cross-cut silicification and mineralisation (Graney & McGibbon, 1990).
East Hill Zone - The East Hill Zone is 500 m to the east of the Red Rock Zone and 250 m to the SSE of the Top Zone. It has plan dimensions of the order of 400 x 150 m and comprises multiple lenses. Mineralisation is hosted by limy siltstone and basal siliceous conglomerate of the lower Antler Sequence, and by underlying Valmy Formation quartzite. Several, closely spaced, east dipping, north striking normal faults apparently control the mineralisation. The ore within the Antler Sequence rocks is best developed in intensely iron stained limy siltstones that are under thin limestone beds and in close proximity to faults. The mineralisation hosted by the Valmy Formation occurs in shattered, iron stained quartzite and light coloured argillite near fault zones, although it also spreads laterally along distinctive dark, graphitic quartzite beds. This dark, gold bearing quartzite commonly has a vuggy, leached appearance (Graney & McGibbon, 1990).
Top Zone - The Top Zone is found 250 m to the east of the old Marigold Mine and has a plan area of roughly 400 x 150 m. It was partially exposed in outcrop and trenches and trends north-west with a shallow west dip. Ore is developed in brecciated, iron stained and locally silicified, interbedded quartzite and argillite of the Valmy Formation. The ore occurs as several flat lying zones, possibly bounded by thrust faults. The grade of mineralisation tends to correlate with iron oxide intensity. An old adit driven under the zone also indicates the presence of mineralisation within near vertical north to north-west fractures. Locally the argillites have been altered to white clay while the mineralisation is concentrated in quartzite between clay rich horizons. The altered argillite rarely contains high grade mineralisation (Graney & McGibbon, 1990).
Red Rock Zone - This zone is around 200 m to the south of the Old Marigold Mine and is partially exposed at the surface. Gold mineralisation is present as a narrow, steeply dipping, north-south strip some 600 m long within the Antler Sequence along a northerly striking, westward dipping normal fault. The hosts are predominantly silicified limestone and siltstone. The silicification has in places converted the limestone to a dense red jasperoid, while brecciation in the fault zone has allowed the formation of abundant hematite and jarosite as fracture fillings and coatings. Some gold occurs in silty limestone beds away from the faulting, especially to the south, indicating local lateral dissemination along bedding (Graney & McGibbon, 1990).
8 North Zone - The 8 North ore zone also occurs under alluvial cover, approximately 250 m to the north of the 8 South Zone, following the same north-south structural zone. It is elongated north-south and has plan dimensions of approximately 300 x 150 m. North striking faults with some north-west splays seem to provide an important structural control. A north-striking, west dipping, normal fault has preserved the down thrown western segment of the ore zone, while the eastern section appears to have been eroded. The ore is hosted by iron stained and partially decalcified conglomerates and siltstones of the Antler Sequence, similar to the hosts at 8 South, as described above. Havallah Sequence rocks form a cap over the ore zone, while thick limestone beds define the lower vertical limit of mineralisation within the Antler Sequence (Graney & McGibbon, 1990).
5 North Zone - This is the northern most ore zone, located some 1500 m to the north of the 8 North deposit, and 4 km to the NNE of the old Marigold Mine. Gold mineralisation occurs within calcareous conglomerate and siltstones of the Antler Sequence, similar to the hosts at the 8 South and 8 North deposits. The ore is concealed by both alluvium and un-mineralised Havallah Sequence rocks. A major north-west striking structure cuts across the sequence in the vicinity of the ore zone and contains anomalous gold values for at least 750 m. North-south splays from this structure seem to control the better grade mineralisation (Graney & McGibbon, 1990).
Arsenic, antimony and mercury appear to be associated with gold, while silver is essentially absent and base metal contents are low. This association is similar to that of the orebodies on the Carlin Trend, but contrasts with the Ag, Bi and base metals of the Battle Mountain skarn deposits. The presence of barite, spatially related to gold, and the association of certain types of iron oxide with ore grade mineralisation, implies that hypogene oxidation may also have taken place (Graney & McGibbon, 1990).
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.
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