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The Platosa silver-lead-zinc deposit is located in the northeastern corner of the State of Durango, north-central Mexico, ~45 km north of the city of Torreón, and ~5 km north of the village of Bermejillo.
(#Location: 25° 55' 44"N, 103° 39' 23"W).
For a brief overview of the distribution and character of the deposits in the carbonate replacement and related vein Pb-Zn-Ag belt in Mexico and the western United States, and links to other deposits of that belt, see the Regional Setting section of the Fresnillo record.
Platosa is located in the Oaxaquia terrane which has a basement of Proterozoic gneiss, uncomformably overlain by Palaeozoic terrestrial siliciclastic and metamorphosed submarine volcanic arc rocks, which are, in turn, overlain by Triassic siliciclastic rocks. The overlying Late Jurassic to Late Cretaceous supracrustal assemblage comprises carbonate and calcareous siliciclastic rocks, which are intruded by Late Jurassic rift-related rhyolite-andesitic continental magmatic rocks of the Nazas arc. The Nazas arc coincides with the opening of the Atlantic Ocean throughout the Oaxaquia Terrane's north-south elongated axis.
The Platosa mine is located near a major NW trending fault structure, on the western margin of the Coahuila Platform, and is part of a NW-trending line of major carbonate replacement deposits.
The Platosa deposit is located within the Sierra Bermejillo, which coincides with a NW trending anticline-syncline pair that has folded in Mesozoic sedimentary rocks. The Sierra Bermejillo Anticline is a relatively open, SE plunging fold. The paired Saltillera Syncline is to the west of the anticline and is a doubly plunging structure. A set of north- to NW striking, steeply dipping fractures and faults cut the folded sequence. In the western part of the Sierra Bermejillo, these structures are intruded by Tertiary felsic to intermediate dykes and plutons.
The main fault system in the deposit area is a 250 to 1500 m wide, NW trending zone of fracturing and shearing known as the Platosa Structural Zone, which cuts the eastern margin of the Sierra Bermejillo. The structural zone, which has been traced over a strike length of 5 km both NW and SE of the deposit, includes a series of fault planes that strike NNW and dip steeply to the east (Megaw 2002). It is characterised by brecciated, crushed, and dolomitised limestone, slickenside fracture surfaces, iron and manganese oxides. Some of the breccias are travertine-filled, whilst coarsely crystalline selenite veins, locally up to 5 m thick are present. The faulted rocks form topographic lows as they weather recessively.
The stratigraphic sequence and magmatic history at Platosa may be summarised as follows (after Cox et al., 2015) from the base:
• Acatita Formation which comprises a gypsum-rich suite that outcrops 8 km north of Platosa and is the lowermost unit in the local sequence.
• Treviño-Cuesta del Cura Formation, representing platformal and deeper marine-facies calcareous sedimentary rocks that have been variably hornfelsed, dolomitised, and mineralised.
• Lower Limestone which marks the top of this sequence, and has been strongly metamorphosed to marble.
• Lower Hornfels, which overlies the Lower Limestone and comprises an altered and hornfelsed shale-sandstone unit of unknown thickness. A number of endoskarn altered dykes within this unit host lead-zinc-molybdenum-bearing veinlets.
• Black Limestone, a 50 to 80 m thick sequence of shallow marine, thinly bedded to laminated calcareous mudstone that overlies the Lower Hornfels, and is, in turn, overlain by the Black Sandstone, a thin, black, organic-rich, pyritic sandstone.
• Grey Sandstone, which is ~30 m thick.
• Fragmental Limestone, or 'Heterolithic Fragmental Limestone', which overlies the Grey Sandstone unit and is the main host to mineralisation. It is typically 50 to 120 m thick and comprises a variably dolomitised sedimentary breccia containing angular limestone and dolomite clasts that vary from <1 to >50 cm and are set in a sandy carbonate matrix. It also contain a number of fossiliferous horizons and has been widely subjected to post-lithification dolomitisation, creating a highly permeable rock susceptible to dissolution and mineralization. To the NW of the deposit, the contact with the overlying Upper Limestone is gradational over a few metres.
• Upper Limestone, comprising ~200 m of medium-bedded calcareous mudstones. It has been locally dolomitised or recrystallised to marble in the district surrounding Platosa.
• Indidura/Caracol Formation, composed of basal shales, calcareous shales, calcarenites and limestones, grading upwards into siliceous shales, sandstones and conglomerates.
Intrusive rocks are rare in the deposit area but have been encountered in drilling in several locations on both the west and east sides of the Sierra Bermejillo. A large magnetic anomaly and extensive thermal metamorphism of the Mesozoic sedimentary rocks suggest that intrusive rocks are widespread in the district. The largest exposure of intrusive rocks to the west is the Tertiary Pozo Porphyry, a medium-grained feldspar porphyry thought to be a quartz monzonite (Megaw, 2002), occurring as 1 to 3 m thick felsic dykes in working 4 km SW of Platosa, whilst a multi-phase granite porphyry has been intersected in drilling further to the south. At the Platosa mine, 1 to 10 m wide endoskarn altered felsic dykes with associated sulphide-bearing veinlets have been intersected in deep drillholes into the Lower Hornfels. Some 6 km NW of Platosa, a fine-grained sub-volcanic neck or flow dome of felsic rocks is exposed accompanied by minor gossan and ferruginous jasperoid that contain anomalous concentrations of arsenic, zinc, silver, and lead.
The bulk of mineralisation at Platosa occurs as more than 11 shallow to steeply dipping massive carbonate-replacement bodies that are either discrete pods or mantos containing sulphide concentrations. The surface projection of the Platosa manto system covers an area of ~400 x 700 m, with individual mantos ranging from 200 x 100 m to 400 x 150 m and up to 40 m thick. The mantos dip east parallel to the stratigraphy where a series of late extensional features down-drop the mineralisation so that its depth ranges from 60 m on the west side of the mine to ~320 metres below surface on the east side of the deposit.
The principal ore minerals are galena, sphalerite and acanthite and lesser proustite. Acanthite predominates, whilst proustite is evident where grades exceed the average of the mineralised body.
Drilling has also occasionally intersected anomalous gold and copper mineralization, which is believed to indicate a hotter source or hotter mineralising fluid pathways within the system. These anomalous values are seldom of economic value, but are regarded as important vectors to ore.
NI 43-101 compliant Mineral Resources as at 31 March 2018 at a cut-off of 375 g/t Ag equivalent was (SRK Consulting, Canada, 2018):
Indicated Resources - 0.485 Mt @ 549 g/t Ag, 5.6% Pb, 5.9% Zn;
Inferred Resources - 0.013 Mt @ 516 g/t Ag, 4.7% Pb, 6.5% Zn for a total of 273 t of contained silver.
This summary is drawn from "Maynard, R., Marino, R., Dhadwar, R., Stafford-Coyte, M., Hasan, N., Ford, C. and Cahill, B., 2018 - Technical Report for the Platosa Silver-Lead-Zinc Mine, Mexico; an NI 43-101 Technical Report prepared by SRK Consulting (Canada) for Excellon Resources Inc., 150p."
The most recent source geological information used to prepare this summary was dated: 2018.
This description is a summary from published sources, the chief of which are listed below.
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