Tango Epithermal Gold and Porphyry Copper System, El Rosario Mining District, Sinaloa, Mexico
Summary
The 5541.6 Ha Tango Property, owned 100% by Minera Camargo, S.A. de C.V., is centered in Southern Sinaloa State, Mexico, in the municipality of Rosario near geographic co-ordinates 105º45’W and 23º12’ N (1:50 000 map sheet F13A47 and F13A48). The Property was acquired by staking in early 2003, at the bottom of a prolonged cycle of depressed metal prices. The Tango Property is located in western foothills of the Sierra Madre Occidental, one of the world’s largest silicic igneous provinces (Aranda-Gomez et al., 2003). It is located over the former “Viva Zapata” Mineral Reserve, a project that was staked and explored by the Servicio Geologico Mexicano in the 1980’s (Bon-Aguilar, 1987 and Rodriguez-Rodriguez et al., 1984). Later regional geochemical work by the SGM at the turn of the millennium highlighted the District as one of the largest contiguous anomalies for gold and base metals in southern Sinaloa and Northern Nayarit.
Geology
At least two major volcano-plutonic complexes defined by occur on the Tango Property (Ferrari et al., 2005), and there is at least one mineralizing event associated with each Complex. The lowermost Complex of Paleocene to Eocene mafic volcanics and rhyodacitic ignimbrite are intruded by monzodiorite porphyry of the Piaxtla Batholith of probable Early Eocene age. The intrusive rocks host a significant northeast-southwest trending, mainly northwesterly dipping, porphyry copper-molybdenum system centered on Sitios de Picacho that is at least 5 km long and 1.5 km wide. Similarly oriented epithermal quartz veins such as La Cocolmeca, San Agustin and El Pino occur in the volcanic rocks above and peripheral to the porphyry system, and might be of the same age as the porphyry.
Rhyolitic ignimbrites of probable Oligocene age (Complex 3) unconformably occur above Complex 3, and are mainly mineralized by northwest trending epithermal veins that can dip either southwesterly or northeasterly. El Placer and La Chorrera are the most important northwest-trending vein systems identified on the Tango Property.
Porphyry Prospects
The monzodiorite intrusion contains multiple zones of porphyry-copper style mineralization characterized by: (i) pervasive potassic alteration of original hornblende and plagioclase by biotite, magnetite, chalcopyrite and K-feldspar, (ii) quartz-tourmaline veining, and (iii) large zones of phyllic alteration. Pórfido La Guacamaya is the largest porphyry copper occurrence with average values of 3851 ppm Cu, 34 ppm Mo and 44 ppm Ag from 87 rock samples. The porphyry molybdenum deposit “El Cuervo” is characterized by cockscomb veins of quartz and molybdenite with abundant magnetite. The average result of 74 rock chip-channel samples from Pórfido el Cuervo is 5477 ppm Mo and 184 ppm Cu. The gold potential of the porphyry-style mineralization has not yet been evaluated.
Table 1. Summary of average analytical results from the porphyry system on the Tango Property. Geochemical determinations are done using a NITON GOLDD portable XRF for trace and major elements. Fire assays for Au-Ag are pending. N=Number of Samples. Mag Sus = Average magnetic susceptibility.
| ZONE | Area (Ha) |
N |
Alteration Style | Mag Sus | Ag (ppm) | Mo (ppm) | Zn (ppm) | Cu (ppm) |
| Pórfido El Cuervo* | 44 |
74 |
Potassic-Phyllic | 10.2 | 15 | 5477 | 339 | 184 |
| Pórfido La Aguila | 89 |
85 |
Phyllic-Potassic | 0.9 | 23 | 223 | 313 | 1745 |
| Pórfido La Guacamaya | 93 |
87 |
Potassic | 4.6 | 44 | 34 | 378 | 3851 |
| Pórfido Sepultado | 73 |
33 |
Potassic-Propylitic | 4.6 | 61 | 25 | 858 | 2455 |
| Pórfido El Halcón | 44 |
30 |
Phyllic | 0.9 | 9 | 24 | 389 | 1320 |
| Pórfido El Búho | 80 |
23 |
Potassic | 4 | 30 | 78 | 245 | 1946 |
| Zona Preciosa del Zorro** | 18 |
10 |
Argillic | 0.1 | 126 | 69 | 804 | 1034 |
*Geological inspection of these zones pending. **AVG Bi=200 ppm, W=123 ppm.
Epithermal Gold Veins
San Agustin, La Cocolmeca and El Pino occur in the volcanic rocks above and peripheral to the porphyry system, are oriented similarly to the porphyries (about 210/80 NW) and might be of the same age. These early veins are cross-cut and offset by northwesterly trending veins that extend deep into the Complex 2 volcano-plutonic rocks. The largest northwesterly rending vein systems are El Placer and La Chorrera. Northerly trending epithermal quartz vein systems occur at La Botica, Las Chiveras, El Cobre, and west of Sitios de Picacho in the porphyry system. The northerly trending veins cross-cut and offset the northwesterly trending veins. Because there are at least three mineralization events, the oldest ENE trending veins may have complex histories of activation and re-activation, and record multiple pulses of mineralization. Geochemical analyses for several veins are below.
Table 2. Results for selected epithermal gold veins on the Tango Property.
| SAMPLE | Showing | Width |
Strike |
Dip |
Ag (g/t) |
Au (g/t) |
| 19815 | Guayabo | 0.1 |
135 |
45 |
25 |
111.5 |
| 23758 | Los Colgados | 0.3 |
140 |
65 |
24 |
64.1 |
| 15990 | Carmen | 0.4 |
335 |
90 |
100 |
53.8 |
| 19856 | San Antonio | 0.8 |
245 |
85 |
47 |
44.1 |
| MX285 | Mina San Agustin | 0.8 |
210 |
85 |
>100 |
36.8 |
| 19691 | Palodismo | 1.3 |
327 |
78 |
4 |
27.6 |
| 15994 | La Gloria | 0.3 |
162 |
75 |
16 |
26.7 |
| 19664 | La Botica | 1.1 |
182 |
90 |
5 |
26.5 |
| 23762 | Urrea | 1.2 |
125 |
54 |
6 |
23.2 |
| 15926 | El Pino | 3.5 |
285 |
85 |
99 |
21.9 |
| 15988 | La Urrea | 0.7 |
135 |
60 |
6 |
11.8 |
| 23877 | Las Chiveras | 1 |
320 |
90 |
7 |
8.9 |
| 15924 | Carboneras | 1 |
160 |
90 |
9 |
8.7 |
| 19891 | Mina de Riolita | 1.7 |
150 |
75 |
3 |
8.6 |
| 18511 | La Flauta | 2 |
315 |
75 |
178 |
8.2 |
| 23791 | Las Chiveras | 0.15 |
360 |
90 |
33 |
7.7 |
| 15921 | Los Tajos | 1 |
105 |
85 |
38 |
7.5 |
| 15523 | Ojo del Agua de Garavato | 0.5 |
180 |
55 |
356 |
6.6 |
| 15626 | Urrea #5 Tunnel | 0.6 |
151 |
83 |
6 |
6.6 |
| 23788 | Las Chiveras | 0.8 |
360 |
90 |
10 |
6.4 |
| 19890 | La Cocolmeca | 1 |
315 |
77 |
4 |
6.2 |
| 15789 | Tres Hermanos | 1 |
140 |
90 |
17 |
5.6 |
| 19141 | Zona Preciosa del Zorro | 1 |
220 |
80 |
142 |
5.3 |
| 15791 | Mina El Copalquin | 0.5 |
135 |
80 |
12 |
5.1 |
San Agustin
San Agustin is a gold-rich quartz-calcite vein located about 2 km southeast of Sitio de Picacho between 640 m and 720 m elevation. On surface, the vein is marked by several small prospects and open cut mines over a strike-length of 340 m. In the 1990’s, Minas de Picacho S.A. de C.V. drove a 177 meter long tunnel 3 m high by 3 m wide under the historic workings at the 640 meter elevation. The tunnel intercepted the historic stopes right under the surface cuts. In 2010, Minera Camargo pumped the water out of the old stopes. The average result of 3 chip-channel samples across the San Agustin Vein at the 640 m elevation is 15.4 g/t Au and 70 g/t Ag across 0.8 m (samples 25816-25818). Ten meters below the level of the adit, a fourth chip channel sample yielded results of 36.8 g/t Au and > 100 g/t Ag across 0.8 m (sample MX285).
La Cocolmeca
The Colcomeca Vein trends east-northeast and dips about 80 degrees to the northwest. It has been followed almost continuously for 1650 meters from Guayabo through to San Antonio, Gavilan and Gavilan North. The Cocolmeca Vein consists of gemmy clear quartz with chalcocite, chalcopyrite, and chrysocolla (Figs. 8.23 to 8.26). The average result of 55 chip-channel samples cut across surface exposures and shallow underground workings is 4.2 g/t Au, 37 g/t Ag, 0.7% Cu, 0.3% Pb and 0.5% Zn across about 1.3 meters. This includes a result of 44.09 g/t Au, 47 g/t Ag, 0.4% Cu, 0.2% Pb, 0.7% Zn and 6.7% Fe across the San Antonio ore shoot (sample 19856), and 347 g/t Ag, 5.2% Cu and 18.6% Fe across 0.7 m at the Guayabo ore shoot (sample 15953). Near Guayabo, the Cocolmeca structure is cut by northwest-trending cockscomb quartz veinlets that carry visible gold. A sample across one of these yielded 111.5 g/t Au and 25 g/t Ag across 0.1 m (sample 19815). Both the San Antonio and Guayabo ore shoots are localized near the intersection of the Cocolmeca structure with the northwest trending Palodismo and Copalquin structures, respectively.
La Chorrera
In general terms, “La Chorerra” refers a swarm of NNW trending veins in the 1 kilometer wide area between Mina de Salva and Prospecto del Bote that are exposed on the steep northwesterly facing slope of a major cliff that bisects the Tango Property. The veins are hosted in dacitic volcaniclastic rocks and in the overlying rhyolite ignimbrites, as well as in younger rhyolite dikes. Mineralization characteristics are similar to those described for El Placer, with higher gold grades occurring in quartz-specularite veins with oxidized ore minerals, chlorite or stilpnomelane envelopes, and pervasive kaolinite alteration in the wall-rocks. Distal to the veins, the wall rocks are pervasively altered to smectite. The largest known historic working in this area is Mina La Gloria, a cross-cut about 103 meters long that provides access to a northerly trending stope more than 100 meters long aligned along a shear. A chip-channel sample across the stope north of the cross-cut yields results of 21.1 g/t Au, 0.5% Zn, 0.8% Pb and 0.4% Cu.
El Placer
“El Placer” is a large northwesterly trending quartz vein system 100 to 300 meters wide that has been mapped for 4.5 kilometers along strike. The veins are hosted both: (i) in dacitic volcaniclastic rocks with intercalated basaltic flows, and (ii) in the overlying quartz porphyritic rhyolite ignimbrites. The fault system at El Placer is intruded by finely crystalline diorite dikes as well as felsic dikes. Gold occurs in cockscomb quartz veins and veinlets that contain specularite, but rarely pyrite. Sample 23749 from La Flauta contains apatite gangue. Sulfides such as sphalerite, chalcopyrite and galena have been observed in a few samples from the Flauta. Oxidized minerals such as cerussite, chrysocolla, chalcocite, brochantite and mimetite are more common in near surface prospects. Quartz vein selvedges are commonly rimmed by Zn-chlorite or stilpnomelane. Proximal to higher grade gold veins, argillic alteration minerals such as kaolinite are usually present in the wall rocks beyond the vein selvedges. Distal to the veins, montmorillonite smectite and/or sauconite smectite pervasively altered the tuffaceous host rocks, and preferentially replaces volcanic glass.