4.5 Results
4.5.1 Major, minor, and trace elements
The major, minor and trace (excluding REE) element concentrations in the gangue carbonates from the main deposit and the studied occurrences in the San Vicente district are given in Table 4.1.
The hydrothermal dolomites (DRD, WSD, LFD) display a trend towards more stoichiometric Mg-concentration with advancing paragenetic stage (Fig. 4.3). The host very fine DRD (lvf) is enriched in calcium, with CaC03 ranging between 55.9 and 60.2 mole % (median= 58.1 mole%), and depleted in Mg (39.5 to 43.5 mole% MgC03). The altered DRD (If, Im), and the open-space filling WSD and the LFD are slightly more stoichiometric, with MgC03 medians of 42.7%, 43.1 %, 43.2%, and 43.4 mole % (Fig. 4.3). A similar trend was displayed by the MgC03/CaC03 ratio measured by electron microprobe analysis in selected samples of the ore-stage dolomites (Spangenberg, 1995).
The ranges of the trace element concentrations for the hydrothermal carbonates are shown in Fig. 4.4. The soluble fraction of the bituminous silty Uncush Limestone is poor in
Table 4.1. Geochemical composition of the acid soluble fraction from carbonates of the San Vicente district (including the San Vicente main deposit)
Sample Loc1 Mant2 Lith3 Carbo.4 CaC03 MgC03 Fe Mn Na Sr Zn Ba
Table 4.1. Continued
2 Manto name in the San Vicente main deposit (Alfon. =Alfonso; SVt =San Vicente tecbo)
3 Lithologies: AD= Alfonso Dolomite; UL =Bituminous silty Uncusb Limestone; SVD = San Vicente Dolomite;
SID= San Judas Dolomite
4 DRD = dark replacement dolomite ("lvf' very fme-grained, "If' fme-grained, "Im" medium-grained); WSD = white sparry dolomite (II); LFD = late filling dolomite (illd); LFC = late filling calcite (IDe); EPc = calcite replacing concentrations may indicate the contribution of the silicate minerals to the chemistry of the pore water (IR
=
21.7 to 39.8 wt.%).The content in Mn, Sr, Na, and Ba of the host dolomite (lvf) increases during advancing alteration: from values of 730 Jlg/g, 60 Jlg/g, 320 Jlg/g, and 2 Jlg/g in the DRD (Ivf) to values of 1720 Jlg/g, 100 J.lg/g, 400 J.lg/g, and 4 Jlg/g in the LFD (Fig. 4.4). The enrichment in Sr of the LFD relative to the unaltered DRD coincides with the trend towards higher Mg concentrations.
Iron and zinc in the hydrothennal dolomites display more complex distribution trends
0.84
0.80
('I)
0 (.)
<tS 0. 76
~
0(.)
0)
~
0.720.68
maximum 75
50 percentiles 25
minimum 0 outlier
*
extreme outlier 0n=4
DRD (lvf) DRD (If)
*
n=8 DRD (lm)
*
00 g
0
n=58
WSD (II) LFD (Ill) Figure 4.3. Ranges and medians of the MgC03/CaC03 ratio of the hydrothermal dolomites in San Vicente dolostones. DRD =dark replacement dolomite ("lvf' very fine-grained, "If' fme-fine-grained, "Im" medium-grained); WSD =white sparry dolomite (II); LFD
= late filling dolomite (IIId); n = number of analyzed samples.
(Fig. 4.4), which can be summarized as follows: (i) the partially altered host dolomite has higher Fe and Zn values (1800 ~gig, 700 ~g/g) than the host dolomite lvf (1630 ~gig, 130
~gig); (ii) the pervasive altered dolomite (lm) is poorer in Fe and Zn (1660 ~gig, 60 ~gig);
and (iii) the concentrations of Fe in the WSD and LFD are very low (WSD: 620 ~gig , LFD:
160 ~gig Zn). Some samples of the WSD have extremely high Fe (> 2500 ~gig, up to 5230
~gig) and Zn values(> 200 ~gig, up to 10650 ~gig) and are likely due to sphalerite leaching during the acid-digestion. Alternatively, the occurrence of Zn-dolomites, similar to those described by Kucha and Wieczorek (1984) in the Irish Navan Zn-Pb deposit, could explain the high values. However, this is considered as not possible because Zn-dolomites were not detected by XRD at San Vicente. Calcite pseudomorphs (EPc) display slightly higher Na and Ba (210 ~gig, 7 ~gig, up to 390 ~gig and 30 ~gig) compared to the late filling calcite (180
~gig, 3 ~gig). This may indicate a local supply in both cations by the replaced sulfate.
In addition, the variation of the Fe and Mn concentrations in the hydrothermal dolomites support the cathodoluminescence evidence of increasing Mn/Fe ratios during dissolution and reprecipitation of the host dolomite.
Figure 4.4. Ranges and medians of the minor- and trace elements in the hydrothermal carbonates of San Vicente district. UL =bituminous silty Uncush limestone, n=7; DRD = dark replacement dolomite ("lvf' very fine-grained, n=4; "If' fine-grained, n=8; "Im"
medium-grained, n=8); WSD = white sparry dolomite (II), n=58; LFD = late filling dolomite (IIId), n=2; LFC = late filling calcite (lllc), n=ll; EPc = calcite replacing evaporitic sulfate, n=3. a) iron; b) manganese; c) strontium; d) sodium; e) barium; f) zinc.
a) .
The distribution of Fe, Mn, Na, Sr, Zn and Ba in hydrothermal dolomites at district scale is summarized in Table 4.2 and Fig. 4.5. The following features can be recognized:
(1) Mn and Fe are enriched in the localities between San Vicente and "Quebrada Uncush";
(2) the highest values of Mn (up to 2360 ~gig) and Sr (up to 110 ~g/g) were found in the localities south from "Chilpes", and show a significant areal correlation with major tectonic lineaments (e.g. localities "Yanachuro Sur", "Rondayacu") and the basement highs (e.g.
Paleozoic rocks near "Quebrada Pifi6n"); (3) the locality of "Machuyacu" is characterized by anomalous values of Sr (up to 100 ~gig) and Na (up to 660 ~gig) and low values for Mn and Fe (820 ~gig and 220 ~gig); (4) the distribution of Na and Sr correlates roughly, and in contrast to Fe and Mn, do not show any clear trend. This may reflect that sodium (e.g., Bein and Land, 1983; Shukla, 1988) and likely Sr are enriched in hydrothermal carbonates as solid or fluid inclusions.
Table 4.2. Minor and trace element ranges and median values in the hydrothermal dolomites1 of the San Vicente district
Locality2 Fe Mn Na Sr Zn Ba
~nl ~1!&&2 ~uwg~ ~l!&sl ~!;!g/gl ~l!&sl ~!;!gig}
QP 475 to 1283 1674 to 2358 436 to 551 74 to 93 3to9
(2) (879) (2016) (493) (84) (7)
RO 185 to 628 982 to 2022 302 to 398 72 to 113 24 to 41 5 to 6
(3) (262) (1617) (375) (87) (37) (6)
YS 326 to 534 850 to 1275 283 to 357 66 to 91 13to60 4 to 6
(2) (430) (1062) (320) (79) (36) (5)
ur
375 to 555 535 to 805 327 to 384 50 to 60 8to9 5 to 6(2) (465) (670) (356) (55) (8) (6)
QU 620 to 1801 961 to 1247 320 to 397 37 to 107 25 2 to 7
(2) (1104) (1105) (364) (72) (4)
AY 398 to 2522 982 to 1557 322 to 516 19 to 164 19 to 164 6 to 70
(7) (988) (1213) (378) (68) (46) (12)
CH 234 to 3367 970 to 2291 274 to 542 39 to 76 6 to 18 2 to 19
(9) (2103) (1230) (377) (50) (12) (4)
AC 2532 to 3899 1017 to 1080 322 to 421 34 to 163 144 to 1779
(3) (3758) (1069) (340) (34) (1476)
us
65 to 435 766 to 1095 378 to 476 48 to 49(3) (205) (799) (455) (49)
San Vicente 30 to 5226 527 to 3022 205 to 483 35 to 153 4 to 2919 1 to 9
(28) (1325) (1313) (369) (56) (352) (3)
AR 230 to 2700 619 to 1115 270 to 650 52 to 69 5 to 14 2 to 5
(4) (698) (865) (425) (58) (6) (3)
MA 149 to 87 722to909 531 to 660 58 to 100 4 4 to 5
(2) (218) (816) (596) (79) (5)
PA 327 to 533 635 to 1266 457 to 520 35 to 41 250 1 to 4
(3) (405) (763) (494) (37) (3)
PJ 279 to 3506 1249 to 1677 436 to 479 36 to 46 3
(2) (1892) (1463) (458) (41)
SP 418 768 579 53 7
1
1 Hydrothermal dolomites= altered dark replacement dolomite ("lm" medium-grained); white sparry dolomite (ll) and late filling dolomite (illd)
2 Locality: abbreviations as in Fig. 4.1
n = number of analyzed samples; the median values are given in parentheses
Figure 4.5. Distribution of the minor- and trace elements in the ore-stage hydrothermal dolomites (DRD If, DRD Im, WSD) of the San Vicente district. a) iron; b) manganese; c) strontium; d) sodium. Zinc plot was not included because the high concentrations in this metal are probably due to contamination during the analysis
Fe
(~g/g)• > 2000
• 900 to 2000 0 < 900
- - -
5kmSr
(~gig)• >75
• 50 to 75 0 <50
5km
/
Mn
(~g/g)• > 1500
• 1000 to 1500 0 < 1000
-
5km-Na
(~gig)• >460
• 350 to 460 0 < 350
5km
/
"
MA
/