Source:
p. 20, 35, 37
First Majestic acquired the San Dimas mine in May 2018 with the acquisition of Primero Mining Corp.
The San Dimas Silver Mine is owned and operated by the First Majestic Silver Corp. wholly-owned indirect subsidiary, Primero Empresa Minera S.A. de C.V.
Deposit Type
- Epithermal
- Vein / narrow vein
Summary:
The deposits of the San Dimas district are high grade, silver-gold-epithermal vein deposits characterized by low sulphidation and adularia-sericitic alteration. They were formed during the final stages of igneous and hydrothermal activity from quartz-monzonitic and andesitic intrusions.
Typical of epithermal systems, the gold and silver mineralization at the San Dimas mine exhibits a vertical zonation with a distinct top and bottom that the prior owner of the mine termed the "Favourable Zone". At the time of deposition, this Favourable Zone was deposited in a horizontal position paralleling the erosional surface of the LVG on which the UVG was extruded.
This favourable, or productive, zone at San Dimas varies from 300 metres to 600 metres in vertical extent and can be correlated, based both on stratigraphic and geochronologic relationships, from vein system to vein system and from fault block to fault block.
The mineralization is typical of epithermal vein structures with banded and drusy textures. Within the San Dimas district, the veins occupy east-west trending fractures except in the southern part of Tayoltita where they strike mainly northeast and in the Santa Rita mine where they strike north- northwest. The veins were formed in two different systems. The east-west striking veins were the first system developed, followed by a second system of north-northeast striking veins. Veins pinch and swell and commonly exhibit bifurcation, horse-tailing and sigmoidal structures. The veins vary from a fraction of a centimetre in width to 8 metres, but average 1.5 metres. They have been followed underground from a few metres in strike length to more than 1,500 metres.
Three major stages of mineralization have been recognized in the district: (1) early stage; (2) ore forming stage; and (3) late stage quartz. Three distinct sub-stages of the ore forming stage also have been identified, each characterized by distinctive mineral assemblages with ore grade mineralization always occurring in the three sub-stages: (1) quartz-chlorite-adularia; (2) quartz- rhodonite; and (3) quartz-calcite.
The minerals characteristic of the ore forming stage are composed mainly of white, to light grey, medium to coarse grained crystalline quartz with intergrowths of base metal sulphides (sphalerite, chalcopyrite and galena) as well as pyrite, argentite, polybasite, stromeyerite, native silver and electrum.
The ore shoots within the veins have variable strike lengths (5 to 600 metres); however, most average 150 metres in strike length. Down-dip extensions of ore shoots are up to 200 metres but are generally less than the strike length.
Mining Methods
- Mechanized Cut & Fill
- Longhole stoping
Summary:
The San Dimas Mine operation includes four underground gold and silver mining areas: the West Block (San Antonio mine); the Sinaloa Graben Block; the Central Block and the Arana Block (the Santa Rita mine). Vein thickness varies from 0.1 metres to 8 metres with the average approximately 1.9 metres. Some veins have a strike length of more than 1,500 metres. Vein dips vary from about 35 o to sub-vertical, the latter being decidedly more prevalent. The general mining recovery factor is about 95%, while that for sill mining is about 75%.
Typical mining of the vein systems is by mechanized cut-and-fill and long-hole, using drill jumbos or jacklegs, pneumatic long-hole drills and load-haul-dump machines, with primary access provided by adits and internal ramps from an extensive tunnel system under the steep, mountainous terrain.
The basis for ore haulage at San Dimas is load-haul-dump (“LHD”) equipment feeding trucks for haulage to the portal and then plant stockpiles. In the Tayoltita mine, the LHD equipment load rail wagons for haulage to the Tayoltita mill crushing plant. Development waste is generally moved to stopes as fill.
Crusher / Mill Type | Model | Size | Power | Quantity |
Ball mill
|
|
12' x 14'
|
|
3
|
Summary:
The Mill presently employs three-stage crushing and three ball mills (12' x 14') that can operate simultaneously or separately to achieve 70% to 75% passing 200 mesh.
At San Dimas, the Company plans to install a new 3,000 tpd HIG mill circuit to further improve recoveries and reduce operating costs.Furthermore, milling at San Dimas will be converted into a single AG grinding mill thereby eliminating the use of the three standard ball mills currently in operation. This AG conversion is expected to reduce energy consumption, eliminate steel grinding media, improve metallurgical recoveries and streamline maintenance and mill practices.
Production:
Commodity | Product | Units | 2021 | 2020 | 2019 | 2018 | 2017 | 2016 | 2015 |
Silver
|
Metal in doré
|
koz
| ...... ^ | ......  | ......  | 3,622 | 3,960 | 5,320 | 8,300 |
Gold
|
Metal in doré
|
koz
| ...... ^ | ......  | ......  | ......  | ......  | ......  | ......  |
Silver Equivalent
|
Metal in doré
|
koz
| ...... ^ | ......  | ......  | ......  | | | |
Silver
|
Payable metal
|
koz
| | ......  | ......  | 3,618 | | | |
Silver Equivalent
|
Payable metal
|
koz
| | ......  | ......  | | | | |
Gold Equivalent
|
Metal in doré
|
oz
| | | | | ......  | ......  | ......  |
^ Guidance / Forecast.
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Operational Metrics:
Metrics | 2020 | 2019 | 2018 | 2017 | 2016 | 2015 |
Tonnes milled
| ......  | ......  | ......  | 444,711 t | 759,087 t | 993,093 t |
Daily milling capacity
| ......  | ......  | ......  | | | |
Ore tonnes mined
| ......  | ......  | ......  | 437,918 t | 762,167 t | 988,168 t |
Daily milling rate
| ......  | ......  | ......  | 1,555 t | 2,074 t | 2,721 t |
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Reserves at December 31, 2019:
A two-step constraining approach has been implemented to estimate reserves for each mining method in use: A General Cut-Off Grade (GC) was used to delimit new mining areas that will require development of access and infrastructure and all sustaining costs. A second Incremental Cut-Off Grade (IC) was considered to include laterally adjacent mineralized material which recoverable value paysfor the following costs: variable cost of mining and processing, indirect costs, treatment costs, administration costs and plant sustaining costs, but excludes: fixed costs of mining and processing, and development sustaining costs. GC for Longhole: 340 g/t Ag-Eq, IC for Longhole: 250 g/t Ag-Eq, GC for Cut&Fill: 320 g/t Ag-Eq, IC for Cut&Fill: 240 g/t Ag-Eq these cut-off grades are based on actual and budgeted operating and sustaining costs, metallurgical recoveries and payable terms.
Cut-off grade considered to constraint resources assuming an underground operation was 250 g/t Ag-Eq, and was based
Category | Tonnage | Commodity | Grade | Contained Metal |
Proven
|
1,918 kt
|
Silver
|
313 g/t
|
19,270 koz
|
Proven
|
1,918 kt
|
Gold
|
4.38 g/t
|
270.2 koz
|
Proven
|
1,918 kt
|
Silver Equivalent
|
671 g/t
|
41,350 koz
|
Probable
|
3,199 kt
|
Silver
|
327 g/t
|
33,660 koz
|
Probable
|
3,199 kt
|
Gold
|
3.12 g/t
|
321.6 koz
|
Probable
|
3,199 kt
|
Silver Equivalent
|
583 g/t
|
59,910 koz
|
Proven & Probable
|
5,117 kt
|
Silver
|
321 g/t
|
52,930 koz
|
Proven & Probable
|
5,117 kt
|
Gold
|
3.6 g/t
|
591.8 koz
|
Proven & Probable
|
5,117 kt
|
Silver Equivalent
|
615 g/t
|
101,260 koz
|
Measured
|
1,860 kt
|
Silver
|
487 g/t
|
29,110 koz
|
Measured
|
1,860 kt
|
Gold
|
6.99 g/t
|
418 koz
|
Measured
|
1,860 kt
|
Silver Equivalent
|
1050 g/t
|
62,810 koz
|
Indicated
|
2,957 kt
|
Silver
|
438 g/t
|
41,620 koz
|
Indicated
|
2,957 kt
|
Gold
|
4.26 g/t
|
405 koz
|
Indicated
|
2,957 kt
|
Silver Equivalent
|
782 g/t
|
74,290 koz
|
Measured & Indicated
|
4,816 kt
|
Silver
|
457 g/t
|
70,730 koz
|
Measured & Indicated
|
4,816 kt
|
Gold
|
5.32 g/t
|
823 koz
|
Measured & Indicated
|
4,816 kt
|
Silver Equivalent
|
885 g/t
|
137,100 koz
|
Inferred
|
5,871 kt
|
Silver
|
341 g/t
|
64,350 koz
|
Inferred
|
5,871 kt
|
Gold
|
3.58 g/t
|
676 koz
|
Inferred
|
5,871 kt
|
Silver Equivalent
|
630 g/t
|
118,840 koz
|
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