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The Hilltop gold deposit is located some 30 km to the south-east of the town of Battle Mountain, 18 km NNW of the Gold Acres deposit and 30 km to the north-west of the Cortez orebody. It is within Lander County, in north-central Nevada, USA.
The Hilltop claims were acquired by the Cortez Joint Venture in 1989, but as of 1993 the deposit was sub-economic. The joint venture at the time of acquisition comprised Placer Dome US Inc, Kennecott and Vernon Taylor Jr. Placer Dome was the manager (Kirwin & Abrams, 1990). The deposit lies within the Hilltop Mining District, which up until 1952, when production within the district effectively ceased, had yielded 560 kg of Au, 11 t of Ag, 180 t of Cu, 245 t of Pb and 65 t of high grade Sb. The Hilltop deposit had been explored by a number of companies between 1973 and 1989. Some 140 drill holes were completed within the Main Zone over that period for around 15 000 m of drilling. This was considered sufficient to prepare an ore reserve estimation (Kirwin & Abrams, 1990).
Published resource figures include:
4.6 Mt @ 2.71 g/t Au = 12.5 t Au (Resource, 1983, Bagby & Berger, 1985).
5.2 Mt @ 2.7 g/t Au = 14 t Au (Resource, 1986, Lisle & Desrochers, 1988).
The Hilltop deposit is hosted by Ordovician siliceous sediments of the Western Allochthonous Assemblage above the Devono-Carboniferous Roberts Mountains Thrust. In this area the allochthon is believed to be in excess of 3000 m thick. The deposit lies within the late Tertiary North Nevada Rift, but is also near the eastern margin of the failed Tertiary Mt Lewis Cauldron, and approximately 20 km to the north of the Oligocene Caetano Caldera (Kirwin & Abrams, 1990).
Tertiary igneous activity in the Mt Lewis region commenced approximately 38 Ma with intrusion of stocks, dykes and sills of quartz-monzonite (adamellite) and granodiorite. Dacite and adamellite plugs and breccias were emplaced at 33 to 35 Ma and part of the Caetano Tuff was erupted at 31 to 33 Ma. The Hilltop deposit occurs adjacent to an apophysis of a granodiorite stock dated at 38.1 Ma (Lisle & Desrochers, 1988).
For more detail of the setting see the Battle Mountain - Eureka Trend Geology record.
The gold mineralisation at Hilltop is hosted within a sequence of thinly bedded cherts of the Ordovician Valmy Formation. Within the district the Valmy Formation is composed of chert, shale, quartzite, greenstone and limestone. In the immediate vicinity of the gold deposit the Valmy Formation comprises the following sequence of units, from the structural base (Kirwin & Abrams, 1990; Lisle & Desrochers, 1988):
- A lower, thick, pure quartzite which is medium to dark grey, thick bedded to massive, vitreous, predominantly well sorted and composed of spherical quartz grains, commonly with authigenic overgrowths. Interstitial grains of sericite, barite and pyrite occur sparingly;
- A middle, 150 m thick, thinly bedded chert with minor shale interbeds. The cherts are grey to black and well bedded in 5 to 12 m layers with 3 to 25 mm shale partings. Nodular bedding surfaces are well developed;
- An argillite unit which is approximately 150 m thick. Shales are dark grey to black, thin bedded to fissile and composed of very fine quartz grains, sericite, illite, chlorite, carbonaceous matter and pyrite. Locally thin interbeds of siltstone and mudstone are present and there is a continuum from shale and cherty shale to interbedded shale and chert to chert with shale partings on bedding surfaces; and
- An upper complex sequence of cherts, argillites, greenstones, siltstones and rare limestones. Greenstones include pillow lavas, dykes, breccias and tuffs that are spatially associated with thin lenses of dark grey, medium grained, locally fossiliferous limestone. These mafic rocks have an alteration assemblage of chlorite, calcite, hematite quartz and albite, which is intensely developed near the Hilltop deposit.
The Valmy Formation has been intruded by Tertiary (?) intermediate and felsic plugs and sills, and is locally overlain by volcani-clastics and flows. Breccia pipes of possible Tertiary age commonly crosscut the Valmy Formation (Kirwin & Abrams, 1990). Four groupings of intrusives are recognised, namely, i). dacite porphyry; ii). basic dacite porphyry; iii). rhyolite; and iv). breccia dykes. The dacite porphyry is texturally equivalent to the 38 Ma granodiorites of the district and occurs as plugs dykes and sills in the Hilltop area. Fragments of dacite porphyry are found in both the stratabound and discordant mineralised breccias. Basic dacite porphyry is rare, occurring as thin dykes and sills. Gold values are elevated adjacent to basic dacite porphyry. Rhyolite is found as a small poorly exposed plug and as thin dykes. The dykes are characteristically flow banded and carry small fragments of wallrocks. Rhyolite dykes cut the dacite porphyry and texturally resemble the 33 to 35 Ma dykes elsewhere in the district. The breccia dykes at Hilltop are 0.15 to 1 m wide, steeply dipping and variable in texture and composition. Fragments of quartzite, chert, shale and dacite porphyry vary from sub-angular to rounded and 1 to 8 cm in diameter. The fragments are un-altered to strongly altered and are set in a matrix of composed of rock flour, igneous rocks or chalcedonic quartz. At any elevation there is a mixture of both upward and downward fragment transport. The breccia dykes post date dacite porphyry intrusion, and both pre-date and post-date the concordant and discordant breccias (Lisle & Desrochers, 1988).
Linear, vein like, hydrothermal breccia pipes have developed along NNW striking, steeply dipping faults, which may be related to the North Nevada Rift. These breccia pipes host narrow quartz veins which contain 'gold grades', including some visible gold. A broad set of ENE trending, steeply dipping fractures cross-cut the NNW faults in the Main Zone. The development of significant gold mineralisation in the chemically un-reactive cherts appears to require the coincidence of the NNW and ENE steeply dipping structures within the megabreccia.
The majority of the disseminated gold mineralisation at Hilltop occurs as a tabular body confined between two thrusts, the lower Hilltop Mine Fault and the upper Independence Thrust Fault. These intra-allochthon thrusts are up to 200 m apart at the surface, but appear to coalesce down dip from the Main Zone, while the Hilltop Mine Fault may coalesce with the Roberts Mountains Thrust at greater depth (Kirwin & Abrams, 1990). The Hilltop Mine Fault dips at 20 to 30š to the west and strikes north-south. It cuts the stratigraphic succession at a low angle to bedding and varies from a plane with slickensides to a zone of gouge up to 15 m thick (Lisle & Desrochers, 1988).
A concordant, tectonic megabreccia has developed between the Hilltop Mine and Independence Thrusts. This megabreccia is the primary host to the gold mineralisation in the Main Zone and is composed of thrust slices and sub-rounded fragments. The thrust slices appear to average approximately 75 m in their longest axes, with relative dimensions in the ratio of 1:3:5. The sub-rounded fragments are usually less than 3 m across. Altered intermediate to felsic sills have locally intruded the Main Zone along brecciated lines of weakness between the two thrusts (Kirwin & Abrams, 1990). The Main concordant breccia zone is spatially limited to altered rocks in the hangingwall of the Hilltop Mine Fault and is from 2 to 60 m thick. It exhibits a complete gradation from fractured chert to fragment supported and then to matrix supported breccia. Angular fragments from 0.2 to 8 cm across consist of altered chert, sparse dacite porphyry and breccia dyke material. Fragments are set in a matrix of similar composition. Recurrent fault movement is indicated by the displacement of temporally distinct alteration and mineralisation features. Footwall shales and cherts are sheared and crushed forming a shattered zone up to 11 m thick immediately below the fault (Lisle & Desrochers, 1988).
The main discordant breccia, with small, coeval, peripheral breccia pods and narrow quartz veins, is restricted to the hangingwall of the Hilltop Mine Fault. Angular to sub-angular 0.01 to 1 m fragments of altered chert with minor dacite porphyry and breccia dyke material are set in a matrix containing abundant quartz and sulphide, and minor rock flour. The margins of the breccia are marked by gouge zones, although there is no evidence of transport of the clasts within the breccia (Lisle & Desrochers, 1988).
Alteration and Mineralisation
Two types of alteration are documented, one prior to gold mineralisation and the other contemporaneous with it. The two are mineralogically similar and can only be differentiated by zoning patterns and cross-cutting relationships. The widespread early alteration masks that associated with mineralisation (Lisle & Desrochers, 1988).
The pre-gold alteration comprises pervasive quartz-sericite-pyrite alteration and occurs within a 750 x 1220 m elliptical area that is elongated in a north-westerly direction. In porphyritic rocks both phenocrysts and matrix plagioclase and mafic minerals are altered to sericite and chlorite, while quartz has replaced the matrix and developed as overgrowths on phenocrysts. Sedimentary rocks are bleached to a light grey or white colour where argillaceous material is replaced by sericite. Shales are silicified, while silica in cherts is recrystallised to a polygonal mosaic. Overgrowths are developed on quartz grains in quartzites. Cherts retain their original bedding, although altered shales loose their fissility and outcrop as massive blocky ledges. This alteration is accompanied by low grade, disseminated Cu-Mo mineralisation and quartz veining. Alteration intensity increases with the quartz-pyrite-molybdenite-chalcopyrite veinlet density, and both are zoned outward from an apophysis of dacite porphyry, which is immediately to the south and south-east of the Hilltop deposit (Lisle & Desrochers, 1988).
Alteration that is coeval with the gold mineralisation is characterised by pervasive silicification of fault breccia matrix and local silicification of breccia fragments. Cherts and shales in the footwall of the Hilltop Mine Fault are bleached and silicified adjacent to gold bearing quartz-barite-stibnite breccia. Silicification associated with gold mineralisation is distinguished from earlier alteration by the occurrence of sulphides with barite, and occasionally with carbonaceous matter and alunite (Lisle & Desrochers, 1988).
Pervasive bleaching is the most widespread alteration within the deposit. Moderate to strong bleaching is visible over an area of more than 2.5 sq. km. Within this area clay alteration of the intrusive and argillaceous rocks is common. Within the Main breccia zone itself, the megabreccia is commonly silicified, with the breccia matrix being filled with quartz and sulphides (Kirwin & Abrams, 1990).
Gold mineralisation occurs within discordant and concordant breccias and in narrow quartz veins. Evidence apparently suggests spatially overlapping mineralising events associated with recurrent fault movement and hydrothermal brecciation that repeatedly fractured previously mineralised breccias (Lisle & Desrochers, 1988).
Discordant breccia mineralisation occurs in the large discordant breccia and in small peripheral breccia pods and quartz veins. Discordant breccia mineralisation formed by filling of randomly distributed open spaces which originally constituted 5 to 45% of the breccia. Fragments contain minor pyrite and chalcopyrite as disseminated grains and thin veinlets associated with quartz-sericite-pyrite alteration. Within the breccia late coarsely crystalline quartz and sulphides were deposited on earlier chalcedonic quartz containing minor sulphides. Pyrite, chalcopyrite, sphalerite, galena, gold, minor barite and rare fluorite occur with the quartz (Lisle & Desrochers, 1988).
Quartz veins strike approximately north-south, dip steeply west and vary in thickness from 2.5 to 30 cm. Quartz crystals are up to 2.5 cm in length (Lisle & Desrochers, 1988).
Concordant breccia gold mineralisation occurs dominantly in the Hilltop Mine Fault zone and in the extensive hangingwall breccia. Gold mineralisation extends for up to 25 m into the footwall of the Hilltop Mine Fault where it is cross-cut by another fault, the Saddle Fault. The mineralised zone with grades in excess of 1.37 g/t Au varies from 2 to 59 m in thickness, has a strike length exceeding 500 m and extends down dip at least 670 m, the limit of drilling in 1986 (Lisle & Desrochers, 1988).
Three mineralogically and temporally distinct, but overlapping mineralising events are recognised, namely;
i). Quartz-carbonaceous matter-pyrite-arsenopyrite-tennantite/tetrahedrite-gold - the As rich phase. The quartz, carbonaceous matter, sulphides and gold of this phase form a black aphanitic mixture that coats fractures, fills breccia veinlets, replaces breccia matrix and constitutes the bulk of the concordant breccia mineralisation. The gradational boundary of this zone with the enclosing hangingwall altered cherts is defined by gold grade. The base of the mineralisation is at the contact with the un-altered footwall cherts and shales. Within the mineralised zone intense quartz replacement largely destroys the original fault breccia matrix, although gouge is locally preserved (Lisle & Desrochers, 1988).
Approximately half of the mineralised rock in the hangingwall has well developed breccia texture, with rotated abraded fragments. Individual breccia zones vary from 0.1 to 11 m in thickness and are separated by zones of fracturing which comprise the other half of the mineralised zone. The latter rocks are pervasively fractured, showing little displacement of fragments, and jigsaw fracture textures within the fragments themselves. Fractured chert is laced with quartz-carbonaceous matter-sulphide fracture coatings and breccia veinlets, and with pyrite-arsenopyrite veinlets. Thick breccia zones are semi-parallel to bedding, while breccia veinlets occur at all orientations (Lisle & Desrochers, 1988).
Gold mineralisation spatially correlates with the quartz-carbonaceous matter-sulphide content of the breccia which varies from 10 to 50% in breccia and 3 to 15% in fractured chert. In detail there is not a direct correlation between the two. The sulphide paragenesis comprises pyrite pseudomorphing of marcasite, associated with arsenopyrite, barite and alunite, followed by tennantite/tetrahedrite. Gold appears to occur as micron to sub-micron inclusions in sulphides. Orpiment and minor realgar coat late fractures (Lisle & Desrochers, 1988).
ii). Quartz-barite-stibnite-gold-silver - the Sb rich phase. Chalcedonic quartz, euhedral barite and stibnite, and gold and silver of undetermined mineralogy form the matrix of a breccia containing mixed fragments of hangingwall altered chert, with quartz-carbonaceous matter-sulphide mineralisation and footwall un-altered chert. This breccia occurs as a 0.3 to 10 m thick envelope to the Hilltop Mine Fault, thickening adjacent to the Saddle Fault. The breccia is matrix supported with up to 60% matrix enclosing small clasts that have been silicified in-situ. At least two phases of brecciation and cementation are recognised. Thin barite-stibnite veins occur up to 150 m above the quartz-barite-stibnite breccia, but increase in density towards the breccia (Lisle & Desrochers, 1988).
iii). Marcasite-pyrite, the late barren phase. Marcasite, pyrite and barite cement breccia developed along the Hilltop Mine Fault, subsequent to the quartz-barite-stibnite mineralisation. The breccia is 0.05 to 2.5 m thick and contains angular fragments of both previously mineralised breccias and footwall cherts and shales. Matrix consists of thick crusts of rhythmically banded, radial, fibrous marcasite and coarsely crystalline pyrite. Vugs are lined by euhedral barite. The gold content of these breccias is uniformly low with mineralisation being confined to the clasts (Lisle & Desrochers, 1988).
Supergene mineralisation and oxidation is developed to a depth of 6 to 100 m, increasing with the intensity of fracturing. Variations in the supergene mineralogy reflect the composition and texture of the primary mineralisation. Disseminated Cu-Mo mineralisation exposed to the south of the Hilltop workings is characterised by turquoise coated fractures at the surface, underlain by a zone of Cu enrichment, with chalcocite, minor native copper and cuprite as coatings on chalcopyrite and pyrite. Sulphides in the discordant breccias and veins are wholly oxidised to iron oxides near the surface, while chalcocite, covellite and bronchantite coat sulphides at depth. Gold occurs in cavities from which sulphides have been leached. Oxidised concordant breccia is characterised by pervasive iron oxides in breccia matrix, scorodite coated fractures and stibnite partly altered to various Sb oxides, including stibiconite and kermisite. Barite is un-affected by oxidation (Lisle & Desrochers, 1988).
The principal sulphide minerals within the Main Zone include pyrite, with lesser arsenopyrite and occasional chalcopyrite and stibnite. These sulphides are present both as disseminations within the chert and in the matrix of the megabreccia. Within the breccia pipes pyrite and chalcopyrite are common, with lesser galena (Kirwin & Abrams, 1990).
A number of phases of metal deposition have been interpreted at Hilltop, according to Kirwin & Abrams, (1990). These are not necessarily consistent with that described by Lisle & Desrochers, (1988). Their phases include the following:
i). Au with lesser As, Sb and minor Ag; ii). Ag with lesser Pb, Sb and As; iii). Hg with minor As and Sb; iv). Mo with lesser Cu; v). Cu with Mo and lesser Pb and Zn.
These represent elemental associations, but are not a paragenetic sequence. The precious and toxic metals are apparently spatially distinct from, and adjacent to, the zone enriched in base metals (Kirwin & Abrams, 1990).
While the bulk of the disseminated gold occurs within the megabreccia, volumetrically less significant mineralisation is hosted by the breccia pipes, and minor gold has been encountered in bedded cherts above the Independence Thrust. Less than 20% of the gold occurs as free particles between 200 and 100 µm in diameter. The remainder occurs in complex associations with several minerals (Kirwin & Abrams, 1990).
The most recent source geological information used to prepare this summary was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
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Kelson C R, Crowe D E and Stein H J, 2008 - Geochemical and Geochronological Constraints on Mineralization within the Hilltop, Lewis, and Bullion Mining Districts, Battle Mountain-Eureka Trend, Nevada: in Econ. Geol. v103 pp 1483-1506|
Lisle R E and Desrochers G J, 1988 - Geology of the Hilltop gold deposit, Lander County, Nevada: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno, pp 101-117|
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