Predicted Sulfide and Silicate Mineralogy at the Sentinel Copper Mine, Zambia

Transcription

1 Mineralogical Patterns in Hydrothermal Systems. A seminar presented by; Predicted Sulfide and Silicate Mineralogy at the Sentinel Copper Mine, Zambia Scott Halley July 2016

2 Thanks to First Quantum for permission to present this case study.

3 Sentinel Geochemistry Approximately 50,000 samples assayed by ALS method ME-MS61 (4 acid digest, ICP-MS/AES). Initially systematic data was collected on 2 type sections. Later there were reanalyses of pulps; 1 in every 20 from the resource drill-out. Currently, systematic analyses of grade control drilling to aid with metallurgical classification. Quartz hematite (red bed) with nickeliferous talc replacing quartz, Enterprise.

4 Sentinel 1 billion 0.51% Cu Host rock is a carbonaceous, sulfidic (pyrrhotite) shale Gangue mineralogy quartz-muscovite-phlogopite-kyanite Hypogene ore chalcopyrite + pyrrhotite Supergene ore chalcocite + pyrite

5 Sentinel Organic Carbon Host rock is a carbonaceous, sulfidic (pyrrhotite) shale. There is a distinct organo-metallic suite present in the host rocks; Mo-V-U. Carbon floats with the sulfides and affects the smelting properties of the concentrate. Organic carbon is difficult and expensive to assay, but Mo can be used as a proxy.

6 Sentinel 1 billion 0.51% Cu Host rock is a carbonaceous, sulfidic (pyrrhotite) shale Gangue mineralogy quartz-muscovite-phlogopite-kyanite Hypogene ore chalcopyrite + pyrrhotite Supergene ore chalcocite + pyrite

7 Sentinel 1 billion 0.51% Cu Host rock is a carbonaceous, sulfidic (pyrrhotite) shale Gangue mineralogy quartz-muscovite-phlogopite-kyanite Hypogene ore chalcopyrite + pyrrhotite Supergene ore chalcocite + pyrite

8 Sentinel 1 billion 0.51% Cu Host rock is a carbonaceous, sulfidic (pyrrhotite) shale Gangue mineralogy quartz-muscovite-phlogopite-kyanite Hypogene ore chalcopyrite + pyrrhotite Supergene ore chalcocite + pyrite

9 Sentinel Regolith Profile Sentinel has an unusual weathering profile. Pyrrhotite oxidizes to form acid plus sulfate in solution. The carbon in the rock reduces the sulfate to H2S. Lower in the profile the H2S converts pyrrhotite to pyrite. The result is hypogene chalcopyrite + pyrrhotite reacts to supergene chalcocite + pyrite.

10 Sentinel Regolith Profile; Potassium Potassium blue<0.4% red>4%. Muscovite breaks down to kaolinite just in the top 2 or 3 meters of the weathering profile.

11 Sentinel Regolith Profile; Magnesium Magnesium blue<0.25% red>2.5% Phlogopite breaks down to kaolinite in the top 10 to 30 meters of the weathering profile.

12 Sentinel Regolith Profile; Sulfur Sulfur blue<0.2% red>2.2% Sulfides break down to goethite in the top 20 to 30 meters, and deeper down faults.

13 Sentinel Regolith Profile; Calcium Calcium blue<0.05% red>0.5% The trace amount of calcite in the host shales is leached to a depth of around 50 meters, and up to 150 meters down faults.

14 Sentinel, Classification of sulfide mineralogy. Plot Cu versus S. Select the points with <1000ppm Cu AND <0.5% S, and assign them to a category called Barren.

15 Sentinel, Classification of sulfide mineralogy. Plot Cu versus S. Select the points with >1000ppmCu AND <500ppm S, and assign them to a category called Oxide.

16 Sentinel, Classification of sulfide mineralogy. Next make a ternary plot of Cu-Fe-S. Note that there is a cluster of points near the projected composition of pyrite, and a big cluster near the projected composition of pyrrhotite. Draw a tie-line from chalcopyrite to pyrrhotite. All the points that plot on the sulfur rich side of this line must contain another sulfide mineral with a higher S:Fe ratio than pyrrhotite; that is, points plotting to the right of this line must contain pyrite. However this group won t contain all the points with pyrite; we cant distinguish mixtures of pyrite and biotite from those points with just pyrrhotite.

17 Sentinel, Classification of sulfide mineralogy. The previous plot didn t get the sulfide mineralogy quite right because not all the Fe is in sulfides. The most common Fe silicate is phlogopite. I have assumed that all the Mg is in phlogopite. Also I have estimated that the phlogopite has Mg:Fe of 4:1. Subtract this amount of Fe from the total iron, and replot the Cu-Fe-S ternary. The Cu:Fe:S ternary now looks like this. There is a very distinct node at the composition of pyrrhotite and another node at pyrite.

18 Sentinel, Classification of sulfide mineralogy. Draw a tie-line from the Fe apex to chalcopyrite. All the points that plot on the copper rich side of this line must contain another copper mineral other than chalcopyrite; these could be a mixture of Cu-Fe sulfides and malachite, but more likely these are chalcocite-bearing samples. The dark blue group won t contain ALL the points with chalcocite; we cant distinguish mixtures of pyrite and chalcocite from those points with just chalcopyrite. We can only see those that have a high proportion of chalcocite. The red, yellow and brown groups are divided up on a Cu:S ratio.

19 Sentinel, Origin of Pyrite. The cumulative frequency plot for Sulfur split by sulfide group shows that the chalcopyrite and pyrrhotite bearing rocks have very consistent Sulfur contents, but the pyrite-bearing rocks show a supergene enrichment in Sulfur by a factor of around 50%. VERY interesting that the chalcopyrite-rich ore has the same S content as the pyrrhotite bearing shale.

20 Errors in the Sulfide Classification One of the uncertainties in the model is how to classify the points that plot on the sulfur-poor side of the chalcopyrite to pyrrhotite trend; ie to the left of the dashed line.

21 Errors in the Sulfide Classification One of the uncertainties in the model is how to classify the points that plot on the sulfur-poor side of the chalcopyrite to pyrrhotite trend; ie to the left of the dashed line. In the current version of the model these are classified more or less on the Cu:S ratio. However most of these points are S depleted via partial oxidation.

22 Errors in the Sulfide Classification Plot a Cu-Fe-S ternary plot. If those S-depleted points are reclassified as chalcocite, it is still not entirely correct, but it is a much more realistic interpretation.

23 Sulfide Classification 3D slice of the classified sulfide mineralogy.

24 Silicate Mineralogy K/Al versus Na/Al molar ratio plot for all samples with >0.1% Cu. Surprisingly, the K/Al ratio is constant at 1:3. Constant ratio implies that kyanite and phlogopite are always present in the same proportions. Need a plot that includes Mg Phlogopite; KMg 3 AlSi 3 O 10 (OH) 2 Phlogopite-rich rocks should plot here, with K:Al = 1:1 Kyanite; Al 2 Si 3 O 5 Kyanite-rich rocks should plot here, with K:Al = 0

25 Sentinel Gangue Mineralogy K-Al-Mg ternary plot. This is a subset of the data showing just those samples with >0.1% Cu. Although the K/Al ratio is constant, the Mg content is highly variable. The muscovite/kyanite/phlogopite ratio can be predicted from where the points plot within this triangle.

26 Sentinel Gangue Mineralogy K-Al-Mg ternary plot. This is a subset of the data showing just those samples with >0.1% Cu. We could describe the mineralogy in terms of categories, ie dominantly muscovite, or kyanite-phlogopite, or the variability in gangue could be described with a numeric value; Mg/(Mg+Al+K). The hydrothermal reaction can be written as: KAl 3 Si 3 O 10 (OH) 2 + SiO 2 + 3Mg 2+ KMg 3 AlSi 3 O 10 (OH) 2 + Al 2 SiO 5 + 6H + Muscovite + Quartz +Mg 2+ (aq) Phlogopite + Kyanite + acid

27 Sentinel Gangue Mineralogy The ratio of Mg/(Mg+Al+K) is a numeric proxy that describes the variability in the gangue mineralogy. How does the variability in gangue mineralogy affect crushing and grinding properties, power consumption, mill throughput rates, etc? Ideally, we should select samples for test work that reflect the variability of gangue mineralogy. Pick samples for comminution tests that reflect the range of variability in the gangue mineralogy x x x x x x

28 Silicate Classification 3D slice of the classified silicate mineralogy.

29 Conclusions Multielement chemistry clearly maps out sequential weathering fronts. There are clearly defined hypogene chalcopyritepyrrhotite and supergene chalcocite-pyrite domains; need to know the flotation characteristics of each. Mg/(Mg+Al+K) is a numeric value that describes the variability in the gangue mineralogy. Can this be related to comminution properties? Mo is an adequate proxy for organic Carbon content.