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East Tintic |
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Utah, USA |
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Au Ag Cu Pb Zn
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Super Porphyry Cu and Au
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The East Tintic sub-district lies within the Tintic district of Utah which is located near the town of Eureka, approximately 90 km to the SSW of Salt Lake City in north-central Utah. For background information and geological setting see the Tintic District record.
The ores of the East Tintic field are mineralogically similar to those of the Main Tintic mines which are centred some 4 km to the south-west. The orebodies of the East Tintic sub-district however, are more obviously localised by structural features and do not generally form the extensive, linear ore runs that characterise the Main Tintic field (Shepard et al., 1968).
The mineralisation at East Tintic has been sub-divided into two styles, namely: i). massive "replacement" orebodies that are rich in Ag, Pb, Zn and Mn; and ii). issure ores that are valuable primarily for their Au, Cu and Ag (Shepard et al., 1968).
Most of the ore discovered to date occurs in the lower part of the stratigraphy, in the lower Cambrian Tintic Quartzite and the limy members of the middle Cambrian Ophir Formation. However, in the 1960's additional, but low grade mineralisation was discovered in the Devonian Victoria Formation. The ore bearing Ophir Formation is subdivided into three members, namely:
i). the 53 m thick Lower Shale Member which has a middle 3 m thick limestone marker;
ii). the 44 m thick Middle Limestone Member which contains several limestone beds, interlayered with lenses and beds of green to blue-green shale; and
iii). the 20 to 7 m thick Upper Shale Member which is composed of light greenish-grey fissile shale.
The stratigraphic position of the Ophir Formation, between the massive underlying Tintic Quartzite and the massive limestone and dolomite section above, has made that formation the focus of deformation in the East Tintic district (Shepard et al., 1968).
The "replacement" orebodies are localised chiefly in the Middle Limestone Member of the Ophir Formation where it has been thrust-faulted against older or younger rocks and cut by north-east trending mineralised fissure zones. The fissure veins are primarily productive only in the underlying lower Cambrian Tintic Quartzite.
The "replacement" ores within the Middle Ophir Member carbonates are composed principally of argentiferous galena and sphalerite, with some silver which is probably present in blebs of argentite and tennantite in the galena and in small amounts of other silver minerals such as proustite, pearceite and polybasite. Barite, rhodochrosite, mangano-siderite, calcite and ubiquitous quartz, jasperoid and pyrite are the main gangue minerals. The auriferous copper veins hosted by the Tintic Quartzite contain native gold, enargite and tetrahedrite, while the gold-telluride veins in the same unit are characterised by various gold and silver tellurides. Both sets of veins have a gangue of quartz, pyrite, barite and clay minerals. The other primary minerals found in the East Tintic mines, generally in minor quantities, include altaite, bismuthinite, bornite, chalcopyrite, chalcocite, jamesonite, marcasite and tetrahedrite (Shepard et al., 1968).
The ores have been further classified into the following groupings: i). silver ore; ii). silver-lead ore; iii). siliceous silver ore; iv). lead-silver ore; v). lead-zinc ore; vi). gold-silver-copper ore; and vii). zinc ore.
Approximately 80% of the past production came from the silver-lead ore class with an average grade of 870 g/t Ag, 20% Pb, 0.3% Cu and 1 g/t Au, and the siliceous silver ore with a grade that averaged 620 g/t Ag, 4% Pb, 0.5% Cu and 1.4 g/t Au. The third most important type are the siliceous gold-silver-copper ores, typified by the 0.375 Mt @ 290 g/t Ag, 1.5% Pb, 0.4% Zn, 4% Cu and 21 g/t Au from the Eureka Standard mine (Shepard et al., 1968).
The paragenesis of the ore and gangue mineralisation commenced with hydrothermal dolomite of the early barren stage, followed by clay minerals and then the "sanded" dolomite of the mid-barren stage, and the quartz, or jasperoid of the late barren-stage. These were followed by pyrite of the early productive and productive stages that was commonly formed in open spaces in the fractured and vuggy jasperoid. Minor amounts of gold, enargite, sphalerite and galena were emplaced in the early productive stage, with late major deposition of sphalerite, galena, enargite, tetrahedrite, proustite, hessite, gold and both primary and secondary chalcocite during the productive stage and weathering respectively. At the Burgin mine there is evidence for two stages of galena emplacement with an intervening episode of sphalerite. At Burgin there appears to be several stages of manganese carbonate, with intervening galena. As in the Main Tintic district there is a substantial oxide zone (Shepard et al., 1968).
Overall, structural features appear to be more important than specific lithologies in the localisation of ore at East Tintic, with ore being found associated with structural dislocations in a variety of lithologies. The productive section of the East Tintic sub-district is divided into structural blocks by a series of faults. The largest share of production has come from a block bounded by the East Tintic Thrust to the east, the Eureka-Lilly fault on the west, the Tintic Standard fault to the north and by the Apex Standard fault to the south [see the accompanying plan]. Within this approximately 2.5 x 1.5 km block, north-east trending faults have produced a series of horsts and grabens. Ore is intimately related to minor folds and crenulations within the larger grossly deformed structural unit (Shepard et al., 1968).
A 750 x 200 to 450 m stock of middle to upper Eocene quartz-monzonite, the North Lily monzonite porphyry, cuts the Palaeozoic sequence immediately north-west of the mine workings. The ore deposits at East Tintic lie below up to 200 m of Tertiary volcanics which are usually only poorly mineralised (Shepard et al., 1968).
The following description of the main orebodies at East Tintic illustrates the distribution and character of the mineralisation exploited.
Tintic Standard - the largest of the mines in the field, with a recorded production of 2 Mt of high grade silver-lead ore from the Tintic Standard "pot hole", a unique structural node that localised massive replacement orebodies. The ore deposits of this mine are associated with the low angle Tintic Standard thrust which occurs on the west side of, but near the crest of, the East Tintic anticline. In this area the thrust plane was localised within the shales and shaly limestones of the Ophir Formation, above the lower Cambrian Tintic Quartzite, but below the overlying Cambrian and younger limestones. The Middle Ophir Member limestones of the upper plate were thrust east over the lower member shales to be juxtaposed above the Tintic Quartzite. In locations where cross-cutting NNE trending fissures cut the underlying quartzite and pass directly into the Middle Ophir Member limestones, large "replacement" silver-lead orebodies are formed in the limestones. Some of these orebodies lie in direct contact with the quartzite footwall (Shepard et al., 1968).
The Tintic Standard structure has been variously interpreted as either, a). a folded thrust, b). a down-faulted structure, bounded by a series of normal faults to form the "pot hole" structure, or c). a thrust fault folded into the complex structure by a tear fault along its southern side. Regardless of its origin, the "pot hole" structure appears to have created a large volume of prepared ground amenable to mineralisation. A number of orebodies formed in this setting. In the Central orebody, for instance, the Middle Ophir limestones were mineralised over a length of 200 m from the South Fault to the Tintic Standard thrust. These beds, and the mineralisation, dip gently to the east into the narrowing "pot hole". Down-dip they are intersected by the Tintic Standard thrust, which was also mineralised, upwards for a distance of as much as 60 m. Over much of this interval a large part of the limestone was mineralised sufficiently to form ore. The gently dipping beds were mineralised to form bands of high grade silver-lead ore and low grade siliceous silver ore. Upwards from the Central orebody there are other "replacement" orebodies in fault blocks, both within and adjacent to the brecciated "pot hole" structure. These form upwards directed pipe-like fingers that are mineralogically similar to the Central orebody (Shepard et al., 1968).
Above the 365 m level, the ore was largely oxidised to form an earthy mixture of altered "sanded" dolomite, iron oxides, vuggy quartz and jasperoid, with cerussite, argentojarosite and other oxidised silver and lead minerals. The "sanded" dolomite above the orebodies is iron stained [due to oxidation of primary disseminated pyrite] and brecciated [due to the oxidation of disseminated pyrite and slumping of the rocks above the oxidised ore] (Shepard et al., 1968).
On the lower levels of the Tintic Standard mine the larger pebble dyke fissure veins within the Tintic Quartzite were mined for their gold-copper-silver content, where they were of sufficient thickness and grade to constitute ore (Shepard et al., 1968).
North Lily Mine - which has a very similar setting to that of the Tintic Standard mine, and is hence not discussed below. At this mine a keel of pyritic-gold ore extends down into the Tintic Quartzite along fissures below the upper part of the orebody, as shown on the accompanying plan (Shepard et al., 1968).
Burgin Mine - the last mine to be operated on an appreciable scale on the East Tintic Field. The principal orebody discovered is a complex replacement deposit that is localised in the immediate hangingwall of the East Tintic Thrust where it is intersected by the north-east striking Eureka-Standard and Apex Standard faults. Fault displacement on the thrust has juxtaposed rocks of the middle Cambrian Middle Ophir Member over beds in the middle and lower parts of the lower Ordovician Opohongo Formation in the mine area. Total displacement along the fault is calculated to be approximately 1.5 km (Shepard et al., 1968).
In the mine area the footwall plate of the thrust overlies rocks that include the early Ordovician Opohongo Formation, to the Devonian to Mississippian Pinyon Creek Formation. These rocks of the footwall range from gently dipping to the east, to completely overturned and nearly flat lying. The hangingwall rocks are completely broken by faulting, with the majority of the ore being found in brecciated hangingwall rocks that lie directly on the footwall of the East Tintic thrust (Shepard et al., 1968).
The unoxidised ore is an intimate mixture of lead and zinc sulphides in various amounts, accompanied by argentite, in a gangue of rhodochrosite, barite, jasperoid and quartz. The oxidised portions of the orebody contain cerussite, anglesite, smithsonite, cerargyrite, pyrolusite, chalcophanite and unreplaced masses of primary sulphides (Shepard et al., 1968). Sangameshwar & Barnes (1983) further divide the zone of oxidation into i). an upper residual ore zone , composed predominantly of pyrolusite [MnO2], groutite [MnOOH], goethite [FeOOH] and barite with minor amounts of cryptomelane and other Mn oxides, the latter containing minor amounts of Pb, Zn and Cu; ii). an underlying zone of oxidised Zn-Pb-Ag ores which consist dominantly of cerussite, anglesite, smithsonite, cerargyrite, native silver, goethite, barite and rhodochrosite; grading downwards into mixed and then primary ore. The zone of oxidation persists to the water table which is at a depth of around 330 m (Sangameshwar & Barnes, 1983).
Sections of the orebody are composed of highly brecciated limestone beds of the Middle Ophir Member that have been the focus of pre-mineral cavern development. These caverns were apparently filled with finely bedded sediments which also sifted into the interstices between the breccia of limestone blocks. "Replacement" mineralisation was developed within these sediments and breccia, with grades that averaged 250 g/t Ag, 10% Pb, 5.5% Zn. However the largest proportion of the ore is composed of replacement of the highly brecciate Ophir Formation immediately above the East Tintic thrust. Mineralisation can apparently be traced along the Eureka-Standard and other north-east trending faults, as well as upwards along the thrust zone, before spreading laterally into the brecciated rocks of the hangingwall (Shepard et al., 1968).
Weak mineralisation was also encountered in development drives which cut the Devonian Victorian and Bluebell Formations in the footwall of the East Tintic thrust (Sangameshwar & Barnes, 1983).
Exploration - Most of the orebodies are concealed by 150 to 180 m of Tertiary volcanics, with much of the exploration being based on drilling and underground development. The principal guides to ore include: i). the intersection of north-east trending fissures and cross-cutting faults that brecciated the carbonate rocks; ii). the presence in the overlying lavas of zones of late stage pyrite, calcitic and sericitic alteration, weakly mineralised fissures and pebble dyke intrusion breccias; iii). primary geochemical anomalies in the altered rocks; iv). mineral zoning patterns of the known orebodies, comprising an outward progression from a Pb-Ag zone, to Pb-Zn-Ag, to Pb-Zn-Ag-Mn zones. Cu-Au ores are restricted to tabular deposits in quartzite, sometimes located in close proximity to "replacement" ore (Shepard et al., 1968).
For detail also consult the reference(s) listed below.
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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