Stage C alteration at Cerro la Mina consists dominantly of kandite group minerals halloysite 10Å (Al2Si2O5(OH)4 · 2H2O), halloysite 7Å (Al2Si2O5(OH)4), kaolinite (Al2Si2O5(OH)4 ) and dickite (Al2Si2O5(OH)4). Stage C alteration is the most abundant alteration at Cerro la Mina mainly altering early potassium feldspar in the breccia pipe and changing the rocks to grey-white. The surrounding volcanic rocks have weak Stage C alteration.The alteration is largely pervasive and kandite + pyrite + marcasite cemented breccias are common. Individual kandite alteration clay minerals could not be identified by visual inspection and were identified using the combination of SWIR instruments (TerraSpec), quantitative X-ray diffraction and imaging with a scanning electron microscope. Stage C alteration comprises two distinct and coeval alteration sub-types. The first consists of quartz + dickite ± pyrophyllite ± alunite ± kaolinite (Stage CQD) and the second of halloysite + kaolinite
(Stage CHK).
5.3.3.1 Quartz-Dickite Alteration (Stage CQD)
Stage CQD quartz-dickite alteration consists of quartz + dickite ± pyrophyllite ± alunite ± kaolinite.
The strong pervasive nature of Stage CQD alters mainly the breccia pipe rock to grey obscuring most
of the primary texture of the rock. Dickite + pyrite cemented breccias are common with mosaic, jigsaw-fit, and chaotic textures (Figs. 5.7A-C). The clasts of the breccias are also quartz + dickite altered. Stage CQD can be pervasive or limited to faults/structures. Quantitative XRD analysis shows
that Stage CQD alteration comprises 34-36% quartz, 28-37% dickite, 3-8% pyrophyllite and 1-18% alu-
nite. The quartz is greater than all other alteration stages and least altered volcanic wall rocks (<1%). Stage CQD alteration zones are close to current surface and mostly occurs in the oxide zone. No vuggy
quartz is observed at the Cerro la Mina prospect. Occasionally a texture similar to vuggy quartz is observed in drill core; however, it was formed by the flushing out of clay minerals by fluids when the drill core quartz-clay altered porphyritic volcanic rock was cut for assay purposes.
The alunite at Cerro la Mina is fine-grained (< 150 µm) and cannot be observed with a hand lens and occurs in and 100 metres outwards from the oxide zone. To further study the origin of the alunite, six samples containing alunite collected from in and outside the oxide zone were observed with a SEM (Table 5.3; Appendix III.5). Alunite with various 1480 nm absorption peak positions were also selected to determine if there was an association of the 1480 nm absorption peak position with the oxide zone and/or size of alunite. The alunite was found to be euhedral, hexagonal, tabular crystals ranging from 5 to 150 µm (Fig. 5.7C). The largest alunite crystal size of 150 µm (IXCM08- 51 62.5) occurs in association with pyrophyllite. No relationship between the position of the 1480 nm absorption peak position and its location to the oxide zone or to size of the alunite was observed.
5.3.3.2 Halloysite-Kaolinite Alteration (Stage CHK)
Stage CQD halloysite-kaolinite alteration consists of halloysite + kaolinite with predominant halloysite.
Halloysite was first described by Berthier (1826) as a dioctahedral 1:1 clay mineral of the kandite group. Halloysite occurs widely in both weathered rocks and soils and it has been identified as having
Chapter 5: Wall Rock Alteration
formed by the alteration of a variety of rock types (Joussein et al., 2005). Halloysite may occur in partially hydrated forms as well as in a fully hydrated and a fully dehydrated form (Churchman and Carr, 1972). The terms halloysite (10Å) and halloysite (7Å) are used for the hydrated and dehydrated forms, respectively. Churchman and Carr (1972) reports that halloysite 10Å can easily dehydrate to 7Å under ambient conditions. Kaolinite and halloysite 7Å produce the same peak under XRD and the presence of halloysite 7Å is detected by the use of formamide intercalation which separates the XRD peaks (Joussein et al., 2007). Cerro la Mina halloysite was first identified with a TerraSpec in this study and Kyne (2009) confirmed it with XRD. Kyne (2009) reported the presence of both hydrated (10Å) and halloysite (7Å) whereas this study reports only the presence of halloysite (7Å). I will refer to halloysite 10Å and 7Å collectively as halloysite.
Stage CHKalteration is pervasive and mainly alters the breccia pipe rock to grey-white, similar
to the Stage CQD. Stage CHK cannot be distinguished without the aid of a SWIR instrument or XRD.
Stage CHKalso occurs as breccia cement (Figs. 5.8A and B). The halloysite + kaolinite + marcasite
+ pyrite cemented breccias are mosaic to jig-saw fit whose clasts are also Stage CHK altered. In hand
specimen Stage CHKalteration does not preserve much of the primary texture (Fig. 5.4); however in
thin section the primary texture can still be observed. Quantitative XRD indicates that the Stage CHK
alteration minerals form up to 17.5 percent of the rock. This study selected 10 samples in the Stage CHK alteration and used quantitative XRD to determine the relative abundances of halloysite and kao- linite. The samples showed varying abundances of halloysite to kaolinite (0.6 to 40), with kaolinite in greater abundance close to the Stage CQD alteration zone.
5.3.4 Oxide Zone
At the Cerro la Mina prospect the upper 1 to 3 metres of the oxide zone is a soil horizon formed by the intense weathering of the bedrock to smectite, halloysite and gibbsite. In bedrock, the oxide zone continues on average to 50 metres below the surface with a maximum depth of 175 metres where opal (XRD), goethite, hematite and barite occur without sulfides. Weaker oxidation, where minor sulfides occur, extends to depths of 5.6 metres below the surface in structures that are less than 0.5 metres
Sample Location Crystal form Alunite size (µm) 1480 position SWIR Ident
NJ08IX-61 Oxide Eu 30 1485 Dik-Alu-Kao
NJ08IX-5.8 Oxide Eu 30 1489 Ill-Alu
IXCM06-09 22.8 Oxide S 25 - 90 1478 Alu
IXCM07-25.23.5 Oxide n/a n/a 1481 Alu
IXCM08-51 62.5 30 m below oxide Eu 100 - 150 1478 Alu-Pyr
IXCM06-10 140.3 50 m below oxide Eu 5 - 40 1479 Alu
Table 5.3 - Summary for SEM imaging of alunite. Abbreviations: Alu - alunite, Dik - dickite, Eu - euhedral, Kao - kaolinite, Pyr - pyrophyllite, S - subhedral.
Chapter 5: Wall Rock Alteration
Kandite Alteration
Quartz-dickite (Stage CQD)
Alteration description
Minerals: Quartz + dickite ± pyrophyllite ± alunite ± kaolinite
Colour: Grey Texture: Massive
Hydrothermal breccias: Dickite + pyrite cement and quartz-dickite altered clasts. Mosaic, jig-saw fit and chaotic breccias.
Distribution: Small pod at the surface and in brec- cia pipe. Often oxidized due to close proximity to the surface. Sometimes limited to fault/structures. Intensity: Strong texturally destructive Timing: High sulfidation overprint (Stage C)
Distinguishing Features: Massive grey and can only be distinguished from halloysite-kaolinite with SWIR/ XRD. 0.5 cm 1 cm 1 cm A B C D Breccia Pipe Volcanic Wall Rocks
Quartz + dickite ± pyrophyllite ± alunite ± kaolinite (Stage CQD) Halloysite + kaolinite (Stage CHK)
Hydrothermal Breccia
White: dickite + pyrite cemented
Black: halloysite + kaolinite + pyrite + marcasite
Stage C Alteration A-D
Figure 5.7 Characteristics of quartz-dickite alteration (Stage CQD). A. Mosaic dickite + pyrite cemented hydrother- mal breccia. B. Jigsaw fit dickite + pyrite cemented hydrothermal breccia. C. Chaotic dickite + goethite (after pyrite)
cemented hydrothermal breccia. D. SEM image of abundant hexagonal and tabular alunite around a larger feldspar.
Legend for geology in Figure 4.14. Abbreviations: Alu - alunite, Cem - cement, Dik - dickite, Goe - goethite, Kfsp
- potassium feldspar, Py - pyrite. Sample ID: A. IXCM06-09 100.7, B. IXCM08-47 238.4, C. IXCM07-44 26.9, D.
IXCM06-10 140.3.
Alu Kfsp
5 µm
Schematic not to scale
Dik + Py Cem
Dik + Py Cem
Dik + Goe Cem
Chapter 5: Wall Rock Alteration 1 cm 1 cm Kandite Alteration Halloysite-Kaolinite (Stage CHK) Alteration description Minerals: Halloysite + kaolinite Colour: Grey-white
Texture: Infill in veins.
Hydrothermal Breccias: Halloysite+ kaolinite + marca- site + pyrite cement and halloysite + kaolinite altered clasts. Mosaic and jig-saw fit breccias.
Distribution: From surface to at least 800 metres depth in breccia pipe. Hydrothermal breccias occur at depth Intensity: Moderate to strong texture destructive Timing: High sulfidation overprint, Stage C
Distinguishing Features: Massive grey-white and can only be distinguished from dickite with SWIR/XRD
A B 1 µm C Breccia Pipe Volcanic Wall Rocks
Quartz + dickite ± pyrophyllite ± alunite ± kaolinite (Stage CQD) Halloysite + kaolinite (Stage CHK)
Hydrothermal Breccia
White: dickite + pyrite cemented
Black: halloysite + kaolinite + pyrite + marcasite
Stage C Alteration C A
B
Figure 5.8 Characteristics of halloysite-kaolinite (Stage CHK) alteration. A. Halloysite-kaolinite vein. B. Jigsaw fit hal-
loysite + kaolinite + pyrite + marcasite cemented hydrothermal breccia. C. SEM image of halloysite cylindrical tubes.
Legend for geology in Figure 4.14. Abbreviations: Cem - cement, Hall - halloysite, Kao - kaolinite, Mc - marcasite, Vn
- vein. Sample ID: A. IXCM08-51 109.5, B. IXCM08-51 531.5, C. IXMC08-51 151.0.
Schematic not to scale
Hall Hall/Kao + Py Vn
Chapter 5: Wall Rock Alteration
wide and close to the northwest fault. Stage CHK does not occur in the oxide zone but occurs in zones
of weak oxidation with pyrite and with pyrite below the oxide zone. On the contrary sericitic altera- tion (Stage B) occurs in the oxide zone creating a resistant opal and sericite-rich zone at the surface and also at depth with pyrite (Fig. 5.9).