Abstract
For low-sulfidation epithermal deposits, which are globally significant sources of Au and Ag, we propose a new multi-stage ore-deposition mechanism initiated by cold groundwater. This cold-water trap was verified in the Philippine Sirawai Au–Ag deposit through geological modeling using the data of ore minerals, X-ray diffraction-based alteration minerals, and fluid-inclusions, and the numerical simulation of fluid flow using TOUGH2. Gold grades and Ag/Au ratios were especially focused on because they were characterized by various Au ore-deposition mechanisms. Initially, neutral-pH hydrothermal fluids at ~ 270 °C ascended through veins and encountered a shallow aquifer. The cooling of rising fluids by cold, shallow groundwater resulted in early-stage mineralization (Au ~ 1 g/t, Ag/Au ratio 4–89). Hydrothermal alteration halos were also generated at this stage by fluid flow from the veins into surrounding permeable zones (i.e., a shallow aquifer). The greatly decreased permeability of these alteration halos formed mushroom-shaped low-permeability alteration halos (LPAHs) around the veins. Middle-stage mineralization (Au ~ 26 g/t, Ag/Au ratio ≤ 10) occurred by the boiling of fluids when temperatures increased due to low permeability and insulating properties of the LPAHs. As the permeability of the LPAHs decreased further, hydrothermal brecciation occurred preferentially in horizontal brittle zones (i.e., the caps of the LPAH in the shallow aquifer). Late-stage mineralization (Au ~ 2 g/t, Ag/Au ratio ~ 30) then occurred by the boiling of fluids as the pressure decreased. Consequently, the cold-water trap mechanism is an innovative approach that clarifies mineralization in low-sulfidation epithermal Au deposits.
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Acknowledgments
We would like to appreciate leader Mr. Brian Esber and the Sirawai exploration team. To ensure low-cost exploration, the Sirawai exploration laboratory performed accurately the chemical analyses of the drill cores. Our sincere gratitude is extended to mineralogist Yasuhiro Takai and Sachiko Toki, Enecom Co., Ltd., for their assistance in observing the boiling phenomena in fluid inclusions under the microscope, and the Dr. Jonathan Naden and two anonymous reviewers for essential and constructive comments and suggestions that improved clarity of this manuscript largely.
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Appendix
Appendix
Grades of Au, Ag, Cu, Pb, Zn, and Ag/Au and Pb/Cu ratios in the three mineralization stages at the 45 intersections with Au–Ag veins, obtained from the test pits and drill holes in the Sirawai deposit.
Early-Stage Ore
No | Number of intersection with vein | Elevation (m a.s.l.) | Width of ore (m) | Au (ppb) | Ag (ppb) | Ag/Au ratio | Cu (ppm) | Pb (ppm) | Zn (ppm) | Pb/Cu ratio | |
|---|---|---|---|---|---|---|---|---|---|---|---|
1 | V1 | −1 | 301 | 1.5 | 0.6 | 22.4 | 38 | 527 | 386 | 386 | 0.9 |
2 | −2 | 313 | 2.5 | 0.5 | 24.6 | 45 | 885 | 531 | 531 | 1.9 | |
3 | −3 | 309 | 1.5 | 0.6 | 27.2 | 45 | 607 | 652 | 652 | 1.5 | |
4 | V2 | −1 | 293 | 2.0 | 0.5 | 15.5 | 30 | 1499 | 564 | 564 | 1.6 |
5 | −2 | 298 | 3.5 | 0.9 | 44.8 | 47 | 4715 | 669 | 669 | 1.2 | |
6 | −3 | 320 | 1.5 | 0.6 | 39.0 | 66 | 1196 | 2829 | 2829 | 3.5 | |
7 | −4 | 300 | 1.5 | 0.7 | 23.4 | 33 | 8320 | 3162 | 3162 | 0.2 | |
8 | −5 | 308 | 2.6 | 0.9 | 37.7 | 43 | 3795 | 14,501 | 14,501 | 1.7 | |
9 | −6 | 308 | 3.0 | 0.8 | 53.0 | 63 | 4715 | 5108 | 5108 | 1.3 | |
10 | −7 | 308 | 1.5 | 0.6 | 45.0 | 78 | 1939 | 631 | 631 | 6.5 | |
11 | V3 | −1 | 291 | 1.7 | 0.6 | 22.5 | 39 | 2654 | 2498 | 2498 | 1.0 |
12 | −2 | 323 | 3.5 | 0.7 | 40.6 | 62 | 1010 | 778 | 778 | 1.7 | |
13 | −3 | 323 | 3.0 | 0.8 | 49.8 | 60 | 517 | 2022 | 2022 | 4.7 | |
14 | −4 | 314 | 3.4 | 1.4 | 99.0 | 72 | 3148 | 1785 | 1785 | 1.5 | |
15 | −5 | 301 | 2.0 | 1.6 | 62.0 | 40 | 2683 | 8445 | 8445 | 1.9 | |
16 | −6 | 284 | 1.5 | 1.1 | 20.4 | 19 | 708 | 403 | 403 | 0.7 | |
17 | V4 | −1 | 284 | 1.0 | 0.8 | 28.6 | 36 | 1911 | 1034 | 1034 | 1.0 |
18 | −2 | 292 | 1.5 | 1.0 | 33.3 | 33 | 2766 | 1323 | 1323 | 0.8 | |
19 | −3 | 284 | 1.8 | 1.4 | 49.4 | 36 | 3131 | 7605 | 7605 | 0.8 | |
20 | −4 | 323 | 2.5 | 1.1 | 76.0 | 69 | 1481 | 1329 | 1329 | 1.4 | |
21 | −5 | 305 | 0.5 | 0.6 | 28.0 | 45 | 2284 | 648 | 648 | 1.1 | |
22 | −6 | 318 | 1.5 | 0.5 | 32.8 | 62 | 1882 | 1259 | 1259 | 1.5 | |
23 | V5 | −1 | 278 | 1.7 | 0.9 | 16.0 | 18 | 2641 | 1829 | 1829 | 0.6 |
24 | −2 | 292 | 1.7 | 0.7 | 7.5 | 11 | 4267 | 4019 | 4019 | 0.7 | |
25 | −3 | 325 | 2.5 | 0.6 | 52.6 | 89 | 2980 | 2800 | 2800 | 1.7 | |
26 | −4 | 310 | 2.5 | 1.1 | 46.9 | 44 | 3324 | 13,567 | 13,567 | 7.0 | |
27 | −5 | 298 | 1.0 | 1.0 | 28.5 | 29 | 442 | 592 | 592 | 0.7 | |
28 | −6 | 295 | 1.7 | 0.8 | 16.0 | 19 | 2100 | 1965 | 1965 | 0.8 | |
29 | −7 | 296 | 3.4 | 1.4 | 30.3 | 21 | 9735 | 1900 | 4961 | 0.2 | |
30 | V5 | −8 | 301 | 1.0 | 0.6 | 13.2 | 21 | 7306 | 3887 | 5201 | 0.5 |
31 | −9 | 285 | 1.0 | 0.8 | 11.6 | 14 | 1888 | 1018 | 2369 | 0.5 | |
32 | −10 | 295 | 1.7 | 1.4 | 44.5 | 31 | 5612 | 7129 | 5321 | 1.3 | |
33 | −11 | 302 | 1.0 | 0.5 | 27.0 | 50 | 543 | 876 | 654 | 1.6 | |
34 | V6 | −1 | 325 | 1.0 | 0.8 | 59.0 | 72 | 972 | 1463 | 1216 | 1.5 |
35 | −2 | 326 | 3.0 | 0.7 | 47.1 | 71 | 2248 | 3329 | 1180 | 1.5 | |
36 | −3 | 318 | 1.5 | 0.7 | 45.5 | 64 | 1293 | 6773 | 5069 | 5.2 | |
37 | −4 | 313 | 3.0 | 1.8 | 134.8 | 76 | 872 | 5985 | 1103 | 6.9 | |
38 | −5 | 271 | 1.0 | 1.1 | 5.0 | 4 | 374 | 416 | 524 | 1.1 | |
39 | −6 | 298 | 1.0 | 0.6 | 35.1 | 57 | 650 | 790 | 754 | 1.2 | |
40 | −7 | 295 | 1.0 | 0.6 | 21.9 | 37 | 1006 | 1148 | 594 | 1.1 | |
41 | V7 | −1 | 322 | 1.0 | 0.5 | 30.7 | 63 | 1845 | 2830 | 2313 | 1.5 |
42 | −2 | 323 | 3.0 | 0.7 | 48.3 | 73 | 480 | 5408 | 1684 | 11.3 | |
43 | −3 | 309 | 1.0 | 0.4 | 16.2 | 46 | 16,935 | 1221 | 2294 | 0.1 | |
44 | −4 | 303 | 4.0 | 1.0 | 35.3 | 34 | 277 | 564 | 241 | 2.0 | |
45 | −5 | 285 | 2.0 | 1.1 | 18.7 | 17 | 2191 | 1109 | 1227 | 0.5 | |
Middle-Stage Ore
No | Number of intersection with vein | Elevation (m a.s.l.) | Width of ore (m) | Au (ppb) | Ag (ppb) | Ag/Au ratio | Cu (ppm) | Pb (ppm) | Zn (ppm) | Pb/Cu ratio | |
|---|---|---|---|---|---|---|---|---|---|---|---|
1 | V1 | −1 | 299 | 1.5 | 23.9 | 89.2 | 4 | 1261 | 1218 | 1320 | 1.0 |
2 | −3 | 311 | 1.5 | 2.8 | 16.7 | 6 | 625 | 770 | 799 | 1.2 | |
3 | V2 | −6 | 309 | 0.5 | 21.4 | 222.9 | 10 | 3797 | 7170 | 6211 | 1.9 |
4 | V3 | −3 | 325 | 1.5 | 3.2 | 20.9 | 7 | 1853 | 3845 | 2500 | 2.1 |
5 | −4 | 315 | 0.5 | 3.2 | 24.7 | 8 | 2880 | 2545 | 936 | 0.9 | |
6 | V4 | −1 | 292 | 0.5 | 20.2 | 411.1 | 20 | 10,120 | 40,940 | 61,220 | 4.0 |
7 | −4 | 326 | 1.0 | 36.5 | 98.7 | 3 | 2601 | 7580 | 6198 | 2.9 | |
8 | −5 | 304 | 0.5 | 5.0 | 15.0 | 3 | 1525 | 1993 | 1220 | 1.3 | |
9 | V5 | −1 | 277 | 0.9 | 22.2 | 123.0 | 6 | 3048 | 3591 | 1434 | 1.2 |
10 | −2 | 290 | 1.7 | 14.7 | 379.2 | 26 | 6501 | 31,905 | 69,775 | 4.9 | |
11 | −3 | 328 | 0.5 | 2.0 | 13.0 | 6 | 2446 | 2283 | 1102 | 0.9 | |
12 | −5 | 296 | 1.5 | 54.9 | 2370.4 | 43 | 1811 | 15,348 | 3406 | 8.5 | |
13 | −8 | 300 | 0.5 | 1.8 | 53.1 | 29 | 3507 | 8088 | 7535 | 2.3 | |
14 | −9 | 286 | 0.5 | 5.4 | 100.6 | 19 | 6867 | 31,760 | 34,955 | 4.6 | |
15 | −10 | 296 | 0.9 | 183.3 | 169.0 | 1 | 6374 | 5811 | 2533 | 0.9 | |
16 | V6 | −3 | 315 | 0.5 | 2.3 | 22.0 | 9 | 1232 | 3207 | 3178 | 2.6 |
17 | −5 | 273 | 1.0 | 25.4 | 26.5 | 1 | 2490 | 1066 | 1156 | 0.4 | |
18 | V7 | −4 | 305 | 2.0 | 6.3 | 33.0 | 5 | 974 | 1331 | 807 | 1.4 |
Late-Stage Ore
No | Number of intersection with vein | Elevation (m a.s.l.) | Width of ore (m) | Au (ppb) | Ag (ppb) | Ag/Au ratio | Cu (ppm) | Pb (ppm) | Zn (ppm) | Pb/Cu ratio | |
|---|---|---|---|---|---|---|---|---|---|---|---|
1 | V2 | −5 | 307 | 0.9 | 1.3 | 43.0 | 33 | 5582 | 11,500 | 8460 | 2.1 |
2 | V4 | −1 | 286 | 1.5 | 1.8 | 59.5 | 34 | 5146 | 7785 | 9691 | 1.5 |
3 | −2 | 293 | 0.5 | 1.8 | 44.2 | 24 | 5559 | 16,200 | 3098 | 2.9 | |
4 | −3 | 285 | 1.7 | 3.3 | 103.0 | 31 | 8397 | 4489 | 13,580 | 0.5 | |
5 | −4 | 327 | 0.5 | 3.5 | 85.3 | 24 | 1685 | 2633 | 1132 | 1.6 | |
6 | V5 | −1 | 276 | 0.9 | 0.7 | 13.7 | 20 | 5483 | 1342 | 1880 | 0.2 |
7 | −2 | 286 | 0.9 | 0.8 | 23.6 | 30 | 1095 | 1896 | 14,875 | 1.7 | |
8 | −6 | 292 | 0.9 | 6.4 | 180.0 | 28 | 15,420 | 20,650 | 22,655 | 1.3 | |
9 | −7 | 293 | 1.7 | 3.0 | 62.8 | 21 | 7698 | 2620 | 2111 | 0.3 | |
10 | −8 | 295 | 0.5 | 1.3 | 23.1 | 18 | 2322 | 2728 | 6040 | 1.2 | |
11 | V6 | −2 | 323 | 1.5 | 0.9 | 20.3 | 23 | 3272 | 5759 | 2535 | 1.8 |
12 | V7 | −1 | 320 | 0.5 | 3.2 | 60.3 | 19 | 11,870 | 3403 | 3476 | 0.3 |
13 | −2 | 320 | 2.0 | 1.8 | 58.6 | 32 | 1885 | 3494 | 12,679 | 1.9 | |
14 | −5 | 283 | 1.0 | 1.9 | 53.9 | 28 | 3218 | 1311 | 816 | 0.4 | |
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Takahashi, H., Tomita, S.A., Koike, K. et al. Geological Modeling and Numerical Simulation of Cold-Water Trap in the Low-Sulfidation Epithermal Sirawai Au–Ag Deposit, Mindanao, Philippines. Nat Resour Res 31, 67–98 (2022). https://doi.org/10.1007/s11053-021-09978-3
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DOI: https://doi.org/10.1007/s11053-021-09978-3