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Mt Hope
Nevada, USA
Main commodities: Mo

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The Mt Hope porphyry molybdenum deposit is located at the southern end of the NW trending Battle Mountain-Eureka mineral belt, 35 km to the north of Eureka in central Nevada, USA.

The geology of Central Nevada is characterised by two Palaeozoic sequences, namely the: i). Western Assemblage of deep water siliceous and volcanic sequences, particularly carbonaceous shale, mudstone, chert and volcanic rocks; and ii). Eastern Assemblage rock consisting of thick shelf sequences composed of carbonates and lesser clastic rocks.

During the Devono-Carboniferous Antler Orogeny the Western Assemblage was thrust over the Eastern Assemblage on the Roberts Mountain Thrust Zone. Mount Hope is located on the leading edge of this thrust zone.

In the vicinity of Mt Hope molybdenum mineralisation is characteristically hosted within the Ordovician Vinini Formation of the Western Assemblage, while Eastern Assemblage carbonate rocks outcrop to the east of Mount Hope. The Permian Garden Valley Formation, which hosts the Zn-Pb-Ag-Cd section of the Mt Hope mineralised system, overlaps both Assemblages and is the local representative of the post-thrust Overlap Sequence.

Mt Hope is located within the 400 km long Battle Mountain-Eureka mineral belt which represents a NW-SE trending belt of repeated (Cretaceous to Tertiary) faulting, dykes and related igneous activity and has a characteristic magnetic signature on its northern extension associated with Au, Ag, Cu and Mo mineralisation. This part of Nevada has also been subjected to basin and range deformation and tilting of 10 to 20°.

The Mt Hope igneous complex intrudes carbonaceous shale, siltstone, silty limestone, quartzite, calcareous quartzite and bedded chert of the Vinini Formation which has been contact metamorphosed to biotite hornfels and calc-silicate hornfels within 300 m of intrusive contacts, while irregular blocks of hornfels are also found within the complex. Brown biotite hornfels host molybdenum mineralisation on the southern margin of the igneous complex.

The Mount Hope igneous complex is expressed by a 1.5x2 km topographic high of igneous rock exposure which includes both extrusive and coeval but younger rhyolitic intrusive rocks occurring as quartz porphyry which invaded the lower levels of the volcanic system but did not vent.

Ash-flow tuffs, characterised by pumice, broken phenocrysts and lithic fragments, are exposed on the summit and eastern slopes of Mount Hope where they reach a maximum preserved thickness of 450 m. The volcanic sequence was extensively altered by the mineralising event, but us is not a good molybdenum host, nor is it structurally favourable for the development of stockwork veining.

The intrusive quartz porphyry, which is the principal host to molybdenum, occurs as an irregular shaped rhyolitic stock underlying much of the Mount Hope complex. It is exposed to the south and east of the summit of Mount Hope and contains conspicuous phenocrysts of quartz and potassium feldspar and has a well developed quartz stockwork in the deposit area.

Two dome shaped stocks of aplitic quartz porphyry, which are important centres of molybdenum mineralisation, are concealed at depths of 300 m below the surface, enclosed within the quartz porphyry mass. Each of these domes is about 500 m in diameter, and are 700 m apart along a WNW axis. Each is surrounded by quartz stockworks, potassic alteration and mineralisation in the intruded quartz porphyry. The mineralisation, which is symmetrically distributed around the paired intrusive centres, is differentiated into separate western and eastern mineral systems.

Dome shaped stocks and steep walled apophyses of coarse quartz porphyry intrude the aplitic quartz porphyry at deeper levels. The coarse porphyry, which truncates earlier alteration in the aplitic quartz porphyry, is distinct from the other quartz porphyries, and is characterised by larger quartz and feldspar phenocrysts, abundant biotite and a variable matrix grain size, including a fine-grained contact zone. However, these coarse porphyry stocks are also a significant source of mineralisation of moderate grade in the surrounding rock. The coarse stocks have pronounced alteration mainly near their margins, although relatively fresh, unaltered rock are locally found in their interiors.

Late dacite porphyry dykes which post-date the mineralised phases are found around the perimeter of the Mount Hope complex.

The overall outward alteration patterns comprise a core of biotite alteration, passing out to high silica, potassic and outer argillic zones, each defined by the interaction of the original rock composition, alteration mineral assemblage and the abundance and types of veins and their selvages.

Molybdenum mineralisation is principally present as molybdenite which is hosted within porphyritic rocks of the Mount Hope complex and in the Vinini hornfels adjacent to its southern margin.   It is found in two dome shaped mineralised quartz stockworks occurring as inverted bowl shaped shells symmetrically developed around and over the two aplitic quartz porphyry stocks.   These two stockwork shells, the eastern and western mineral systems, are each at least 1000 m in diameter, centred about 700 m apart along the WNW axis.

The western mineral system has a generally triangular shape with well defined grade zones with the best grades to the south and east. The centre of the system is directly above the western aplitic quartz porphyry intrusive stock.

The eastern mineral system occurs above the eastern aplitic quartz porphyry stock and has well developed mineral grade shells within the older quartz porphyry. The apex of the shell has been displaced by around 300 m to the east along the Mt Hope Fault and down-faulted to form a steep zone some 300 m below the surface. To the NW, the eastern system is continuous with the overlap zone which comprises a concentration of higher grade (averaging around 0.15% Mo) mineralisation which connects the eastern and western mineral systems.

The overlap zone, the top of which is 100 m below the surface, is approximately 400 m in diameter and persists to depths of from 100 to 300 m. This zone is interpreted to have been mineralised by both mineral systems in sequence, resulting in a higher intensity of stockwork veining enhanced Mo grades. Overlap mineralisation occurs beneath the Mount Hope Fault, with the upper and eastern edges truncated by the fault.

The open pit mining reserve has been quoted as: 920 Mt @ 0.069% Mo.

The Mt Hope deposit is being developed by Idaho General Minerals Inc in 2006.

The most recent source geological information used to prepare this decription was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
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