Breccia For orma mati tion on in Ore Or e-Bearing Hy dr drot othe herm rma al Sy s t em s by Alberto Lobo-Guerrero S. Geologist, M.Sc. MinEx Ph.D. Candidate Economic Geology Research Institute University of the Witwatersrand Johannesburg, South Africa, 2002
CONTENTS Introduction Key Ref efer ere ence ces s World-Class Deposits associated with bxs Deposits where I have studied bxs What hydrothermal bxs are Seco cond nd boi oililin ng Frac Fr actu turi ring ng pr proc oces esse ses s Environments for hydrothermal bxn Examples of mineralized bxs
CONTENTS Introduction Key Ref efer ere ence ces s World-Class Deposits associated with bxs Deposits where I have studied bxs What hydrothermal bxs are Seco cond nd boi oililin ng Frac Fr actu turi ring ng pr proc oces esse ses s Environments for hydrothermal bxn Examples of mineralized bxs
“No one sees what he doesn’t know.” Goethe
1. HYDROTHERMAL BRECCIAS Most hydrothermal metallic ore deposits in magmatic arc and rift environments are genetically associated with breccioid rocks.
HYDROTHERMAL BRECCIAS There is an overlap of processes: -intrusive -volcanic -hydrothermal
2. KEY REFERENCES Vol 80 No. 6 of Economic Geology Burnham, W., 1997, in Geochemistry of Hydrothermal Ore Deposits, John Wiley & Sons Laznicka, P., 1988, Breccias and Coarse Fragmentites – Petrology, Environments, Asociations, Ores, Elsevier. Jébrak, M., 1997, “Hydrothermal Breccias in Vein-Type Ore Deposits: A Review of Mechanisms, Morphology and Size Distribution”, in Ore Geology Reviews, 12: 111-134. Jébrak, M., 2002, “Magmatic-Hydrothermal Breccias in Proterozoic Cu-Au Fe-Oxide Deposits: Processes and Exploration Significance”, in Memoirs of IAGOD Symposium, Windhoek.
3. BX- ASSOCIATED DEPOSITS DEPOSITS Olympic Dam, Australia (its own type, Cu, Au, REE, U) El Teniente, Chile (porphyry Cu) Yanacocha, Yanac ocha, Peru (high (high sulfid sulfidation ation epith epithermal ermal,, Au) Llallagua, Bolivia (Sn, Ag) Panguna, Pangu na, PNG PNG (Cu (Cu in irregu irregular lar brecci breccia a pipe) Toquepala, Peru (Porphyry Cu, Mo) Crippl Cri pple e Cree Creek, k, USA USA (dis (diss s Au in in brecc breccia ia pip pipes es and and vein veins) s) Kidston, Kidst on, Austra Australia lia (brec (breccia cia body, Au, Cu)
4. BRECCIAS STUDIED AT USA Cripple Creek (Au); Round Mountain (Au), McLaughlin (Au) CHILE Los Bro Bronce nces-R s-Riob ioblan lanco co (Cu (Cu); ); El El Tenie Teniente nte (Cu (Cu); ); Candelari Cand elaria a (Cu,A (Cu,Au); u); Cerro Cerro Colorado Colorado (Cu); (Cu); Epitherma Epith ermall tourmaline tourmaline brecc breccia ia pipe pipes s (Au) PERU Yanacocha Yanaco cha (Au (Au); ); Jul Julcan canii (Ag (Ag); ); Ant Antap apite ite (Au (Au); ); High High sul sulfid fidati ation on Cu-Au prospects, Bolivia-Peru border COLOMBIA Nevado Nevad o de dell Ruiz Ruiz (Au (Au); ); Maa Maarr de Sa San n Dieg Diego o (Au) (Au);; Marm Marmato ato (Au (Au); ); Nata Na taga gaim imaa-Ro Rovi vira ra (C (Cu, u, Au); Au); Payu Payuco co br brec ecci cia a pi pipe pes s (Au) (Au)
Maar in th the e Col olomb ombia ian n Ande An des s with signif significa icant nt gold g old d minations tions signi ficant go ld disse diissemina ssemina rela re late ted d to t o int i ntru rusi sive ve bre ecci ccia as. ntrus rusiv ive e br breccias. breccia
Caldera lake in the same maar . Northernmost volcano of Andean Cordillera. Disseminated Au in breccias of volcanic edifice.
HYDROTHERMAL BRECCIAS •wide variety of breccias •intimately related w/ one another •change radically when mapped in field Field identification → important tool for exploration geologist
HYDROTHERMAL BRECCIAS part of: -magmatic hydrothermal bxs -hydromagmatic (hydro-volc) bxs -phreatomagmatic bxs -phreatic bxs
HYDROTHERMAL BRECCIAS •Breccias →
best host rocks for low-grade hydrothermal ore deposits
•occur at hydrothermal 2nd boiling point •mineralizing fluids precipitate after sudden ΔP produced by pressure relief during rock explosive fragmentation
2nd BOILING Hydrous minerals in magma: hornblende & biotite Solubility of H2O in shallow-seated melts is strongly pressure dependent Explosive decompression process follows fracture failure of magma chamber roof rocks
From Burnham, 1997
From Burnham, 1997
2nd BOILING During 2nd boiling, a magma exsolves all of its initial H2O content, except that left in hidrous minerals. Great increase in volume (may be over 50%) Large energy output 8.4 x1010ergs/kg of magma!
2nd BOILING - ENERGY enough to lift an equivalent mass of magma 900 m in absence of friction > twice energy released at Mt. St. Helens 1080 eruption May lift 100-ton blocks >500m (Bisbee, Arizona)
2nd BOILING If explosion does not take place: gasses form miarolitic cavities If explosion does not reach surface: brecciation & stockwork formation If it reaches surface: volcanic eruption
2002 eruption of Reventador volcano, Ecuadorian Andes. A fourth of the energy released by second boiling.
HYDROTHERMAL BRECCIAS Periods of explosive boiling propagate fractures in the rock. = open space for breccias.
HYDROTHERMAL BRECCIAS Any type of brecciation generates open spaces There is an inherent volume increase (> Δ V) Explosive hydrothermal breccias = prime candidates for infiltration of hydroth. solutions
HYDROTHERMAL BRECCIAS
HYDROTHERMAL BRECCIAS
HYDROTHERMAL BRECCIAS Breccias = “sponges” where mineralizing fluids drop their charge of minerals after being absorbed
HYDROTHERMAL BRECCIAS Hydrothermal bxs = recurrent in precious metal deposits of hydrothermal origin BRECCIA = “ clastic rock composed of fragments held together by a matrix and containing cavities filled by postbrecciation hydrothermal minerals” .
HYDROTHERMAL BRECCIAS INTRUSIVE BXS. = “heterogeneous mixture of angular to rounded fragments in a matrix of clastic material, which has been mobilized and intruded into its present position along pre-existing structures”. They generally display hydrothermal alteration.
HYDROTHERMAL BRECCIAS Fragments or fractured rock become progressively milled with increased brecciation.
HYDROTHERMAL BRECCIAS Matrix = fine rock material between fragments; depending on degree of milling, may grade into fragments. •Bxs may be matrix- or fragment-supported •Most of the mineralized component is introduced as hydrothermal fluid and occurs in the matrix •Mineral cement holds fragments & matrix together
HYDROTHERMAL BRECCIAS Cavities develop during bx formation -filled w/ hydrothermal minerals including metallic mineralization -constitute integral part of brecciation & mineralization processes.
HYDROTHERMAL BRECCIAS Bxs associated w/ porphyritic systems •form at greater depths •more likely generated by hydraulic fracturing than by explosion In P environments, Bx fragments tend not to be displaced nor rotated.
From Jébrak , 1997
From Jébrak , 1997
From Jébrak , 1997
From Jébrak , 1997
JIGSAW BRECCIA or ”mosaic breccia” (synonim) that where fragments can ca n be “f “fit itte ted” d” ba back ck toge to geth ther er by re remo mova vall of the matrix
JIGSAW BRECCIA Made by fragments that “fit” together as a result of •< ex expa pans nsio ion n w/ w/o o sign signif ific ican antt ro rota tattio ion n •reactive re replacement along network of frac fr acttur ures es (a (als lso o ‘c ‘cra rack ckle le br brec ecci cia’ a’)) •mechanical re-packing of angular fragments
BRECCIA-STOCKWORK stockworks = variety of jigsaw bxs produ pr oduced ced by re react activ ive e re repla placem cement ent in ‘c ‘cra rack ckle le br brec ecci cia. a.’. ’. Without exotic fragments
JIGSAW BRECCIA have -very little rotation -very little fragment rounding Matrix tends to be derived from local comminuted material; mainly composed of introduced hydrothermal substances.
HYDROTHERMAL BRECCIAS maxim = Description of breccias must be objective. Descriptive terms + useful than genetic ones to qualify bxs. Especially true in preliminary field mapping.
HYDROTHERMAL BRECCIAS Hypotheses on genesis of a particular bx should be kept in field notes and/or reports. Description should always be as objective as possible, to allow for future interpretations.
HYDROTHERMAL BRECCIAS Apply word by word the method of multiple working hypotheses, described by T.C. Chamberlin CHAMBERLIN, TC [1897] ”The Method of the Multiple Working Hypothesis”, Journal of Geology, v. V, pp. 837-848, reprinted in Mather, KF & Mason, SL, A Source Book in Geology 1400-1900, Harvard University Press, Cambridge, Massachussetts, pp. 604-812.
8. ENVIRONMENTS FOR BXS Sorry, but no detailed description of different geological environments where breccias & related ore deposits form Brief overview of main environments -Granitoid-related -Volcanic-related -Alkaline rock-related
From Lasnicka, 1988
BRECCIAS AND ORE DEPOSITS IN GRANITIC ENVIRONMENTS
From Lasnicka, 1988
DIATREMES AND DEBRIS MOVEMENT DURING THEIR LIFETIME
From Lasnicka, 1988
ANATOMY OF A DIATREME OR “ BRECCIA PIPE”
BRECCIAS AND ORE DEPOSITS IN VOLCANIC ENVIRONMENTS
From Lasnicka, 1988
9. CASE STUDIES EXAMPLES OF MINERALIZED BRECCIAS MAGMATIC ARC ENVIRONMENT A. High sulfidation mineralization case study Northern Andes volcano* B. Payuco epithermal breccias, Cauca district, Colombia RIFT ENVIRONMENT C. FeOx Cu-Au breccias in Namibia D. FeOx Cu-Au breccias in Zambia Extra: Au-Mineralized Tourmaline Bxs, Copiapo, Chile
N Andes volcano*, Case Study Active volcano in the northern Andes Lateral explosion opened large window of mineralized breccia pipes and epithermal mineralization hosted by interlayered pyroclastics and lavas Au-rich high sulfidation epithermal deposit Diatremes contain microcrystalline pyrite +Au
Northern Andes Volcano*
Lateral explosion opened large window of mineralized breccia pipes & epithermal mineralization hosted by interlayered pyroclastics & lavas
Aspects, exposed diatreme
Northern Andes Volcano*
Previous photo
Various diatremes within pyroclastic layer
Northern Andes Volcano*
Diatreme 1
Diatreme 2
Various diatremes within pyroclastic layer
Stockworks in pyroclastic sequences. 3-D stereoscopic image. Goethite around fractures enhanced by weathering
Stockwork. Most of rock was clay altered
Moving out in same stockwork.
Stockworking over a diatreme. +4 discrete Au-mnzg events.
Northern Andes Volcano*
Moving out in same stwk. All surface here is mineralized.
Northern Andes Volcano*
Moving away still more. 1.6km vertical exposure.
Payuco Breccias, Case Study Large district in Cauca valley, Colombian Andes Miocene subvolcanic porphyritic rock intrudes Cretaceous siliciclastic-calcareous sequence. Porphyritic rocks are intensely clay-altered (white). Tourmaline-rich breccia body (black) intrudes porphyry and produces epithermal Au mineralization.
Payuco breccias, Caldas , Colombia
Close-up of previous photo
Detail of matrix composition
Matrix grades to ‘ flour ’
1
2
Intrusive aspects, Payuco breccia
1
Variation from in situ rock jigsaw bx transported bx
Forced inyection of bxs 2
Breccia gradation - host rock “ sheeted veins ” , bx , argillized porph .
Sampling aspects in hydrothermal breccias squares or “panels”
From Lasnicka, 1988
BRECCIAS IN ALKALINE AND CARBONATITIC ENVIRONMENTS -RIFTS-
FeOx-Cu-Au Bxs, Namibia On-going part of Ph.D. research project in granitoid rocks of Zambia and Namibia. Diatremes emplaced in NeoProterozoic rift (?) environments of northern Namibia. Bxs cemented by massive magnetite. Abundant sulfides. Possible large copper and gold mineralization. Features similar to Olympic Dam.
BRECCIA PIPE IN CARBONATITIC ENVIRONMENT -RIFT (?)-
Polymictic hydrothermal bx cemented by magnetite with vugs after sulfides. Part of a FeOx-Cu-Au mineralized system. Namibia
Note roundness (corrosion) and haloes of alteration
Polymictic hydrothermal bx cemented by magnetite. From FeOx-Cu-Au mineralized system. Note concentric alteration.
Mineralized stockwork, sample L-736. On-going Ph.D. project, Namibia.
Black hematite stockwork. Sample L-669 from on-going project, Namibia
FeOx-Cu-Au Bxs in Zambia On-going part of Ph.D. research project in granitoid rocks of Zambia and Namibia. Round-pebble polymictic hydrothermal bxs around subvolcanic porphyritic intrusives. Sometimes host to iron oxides with sulfides and copper-gold mineralization. Reactive emplacement of iron oxides and Bxs cemented by massive magnetite/hematite.
ROUND-PEBBLE HYDROTHERMAL BRECCIAS ASSOCIATED TO FeOx-Cu-Au MINERALIZED SYSTEMS MB-34 BOREHOLE, ZAMBIA
MULTIPLE BRECCIATION IN ROUND-PEBBLE HYDROTHERMAL BRECCIAS ASSOCIATED TO FeOx-Cu-Au MINERALIZED SYSTEMS MB-34 BOREHOLE, ZAMBIA
PROGRESSIVE HEMATITIZATION IN ROUND-PEBBLE HYDROTHERMAL BX MB-34, ZAMBIA
Time for questions
