S.J. Turner
April, 2003
DEFINITIONS
* a ‘diatreme’ breccia is the vent zone of a maar-type or
hydro-magmatic volcanism, and results from a phreatohydro-magmatic
eruption.
* a phreatomagmatic breccia MUST have evidence for a juvenile
magmatic component = juvenile clasts and matrix.
* a phreatic or hydrothermal breccia is formed by over-pressured
fluids but with no direct magmatic component.
* phreatomagmatic and phreatic breccias commonly occur within
the same breccia complex.
Relationship of Mineralization to Breccia Complexes
• the formation of breccia complexes is simply a highly effective
PROCESS for mineralization; as a fluid conduit, as a mechanism to
fracture and brecciate surrounding rocks, and as a focussed zone of
pressure and temperature gradients, and fluid mixing.
• the presence of a breccia complex (diatreme) is not necessarily an
indication of a mineralized system. Many, maybe even most,
phreatomagmatic breccias are not associated with any mineralization.
Similarly, many mineralized breccia complexes may not have
significant or economic mineralization.
• Vicuña (Chile/Argentina), La Carolina (Argentina) and (Bald
Mountain, Australia) are examples of weakly mineralized breccia
complexes. There are many others.
Major Deposits Associated with Breccia Complexes
Deposit
Endowment
Deposit Type
(MM oz Au)
Yanacocha Complex, Peru
+41
high-sulfidationPierina, Peru
8
high-sulfidationPascua / Lama, Chile
22.9
high-sulfidationVeladero, Argentina
20
high-sulfidationPueblo Viejo, Dominican Republic
36.6
high-sulfidationKelian, Indonesia
5.8
carbonate - base metal - AuRosia Montana, Romania
13
carbonate - base metal – AuPenasquito, Mexico
27
carbonate - base metal – Au Camino Rojo, MexicoSari Gunay, Iran
+3
epithermal, disseminatedCripple Creek, USA
28
alkalicRattlesnake Hills, Wyoming
~2
alkalicFormation of Phreatomagmatic Breccias
•
phreatomagmatic breccias form where a rising magma intersects an aquifer at a sufficiently shallow level to erupt.• an eruption will occur ONLY when Pfluid > Plithostatic, which is at quite shallow levels (. • the diatreme grows by ‘drilling’ downward with time, due to a decreased Plithostatic
directly over the breccia column (eg Valley of Ten Thousand Smokes, Alaska).
• the diatreme will continue to form as long as there is a continuing (periodic) supply of magma and continued input of shallow meteoric water.
• if the magma supply shuts off then the diatreme becomes quiescent and water-saturated until the next magma pulse.
• if the meteoric water supply is exhausted then the magma will continue to rise in the breccia column or margins as dikes or domes, which explains the common association of diatremes and flow dome fields.
The Setting of Breccia Pipes
from Corbett, The Ishihara Symposium: Granites and Associated Metallogenesis
Textures and Characteristics of Phreatomagmatic Breccias
* rock flour matrix
* presence of ‘basement’ clasts * accretionary lapilli
* funnel-shape with upward / outward flaring margin * polymictic; including multi-stage breccias
* altered / mineralized clasts eg. vuggy silica * tuff ring (if preserved)
* slump blocks of tuff ring material * bedded fallback breccias
* interbedded lacustrine sediments and eruption breccias * matrix to clast-support
* milling and fluidized matrices * reaction rims on juvenile clasts
* ‘hypogene exfoliation’ of juvenile clasts
* deformed / very irregular-shaped, ‘wispy-textured’ juvenile clasts * clay overprint by magmatic volatiles
* intruded by endogenous / exogenous domes and dikes
* peripheral fracturing and crackle-type brecciation in competent units * peripheral hydrothermal breccia and pebble dikes
La Zanja, PERU
Unmineralized quartz -
tourmaline altered
diatreme breccia with
slightly deformed juvenile
clasts and strong reaction
rims in clasts and in the
matrix.
Textures:
Deformed, basaltic juvenile clasts with
chilled, reaction margins in k-feldspar
altered phreatomagmatic breccia.
Sapucai,
PARAGUAY
alkalic gold
prospect
Textures:
Juvenile Clasts
Santa Barbara, Puno, PERU
Weakly clay altered phreatomagmatic
breccia with chilled margins on
basaltic juvenile clasts.
Textures:
wispy-textured dacitic clasts : juvenile component
of clay-altered
phreatomagmatic breccia
wispy-textured juvenile quartz - porphyry clasts in
clay-altered, polymict phreatomagmatic breccia
Martabe, INDONESIA
Kelian, INDONESIA
Textures:
Juvenile Clasts
Lienetz Pit Juvenile clasts ? Accretionary lapilli Vein clasts 1 cm
Breccia facies of the Ladolam alkalic
epithermal gold deposit,
Lihir Island, Papua New Guinea
Jacqueline Blackwell, Jocelyn McPhie,
David R. Cooke, John Robinson
2007 JCU Breccia Symposium
Textures: Clasts
Ladolam, Lihir Island, PNG
Martabe, Indonesia
Clast with accretionary lapilli in
clay-pyrite altered,
Argillized phreatomagmatic breccia (barren) with basement
shale clasts in the Carachugo Norte pit.
Carachugo Sur, Yanacocha, PERU
Pascua, Chile
Mineralized diatreme breccia:
alunite-pyrite-enargite ore
Diatremes: Internal Textures
Yanacocha, Peru
Hypogene exfoliation of
large juvenile clast of argillized
dacitic porphyry in diatreme = rapid
Minaspata, PERU
Alunite – clay – silica
altered phreatomagmatic breccia
(barren)
Diatremes: Internal Textures
Fluidized matrix to diatreme breccia with
altered clasts and pyritic margin to clasts
Pascua, Chile
‘APE’ Au-Ag mineralization in
crackle-brecciated granite
marginal to the Pascua diatreme
Diatremes: Marginal Features
Yanacocha, Peru
slump block of bedded breccias and
tuffaceous units on margin of diatreme
Diatremes: Marginal Features
Yanacocha, Peru
Crackle-brecciation in silicified
margin to a diatreme breccia.
Bald Mountain, Queensland,
Australia
Silica-hematite-altered
crackle-brecciated competent quartzite unit
Diatremes: Marginal Features
IS epithermal veins in cone fractures around the margin of the Santa Barbara diatreme breccia, Peru (Wasteneys, 1990)
Diatremes: Upper Facies
Upper facies in a maar diatreme breccia at Wau, PNG, including slide blocks along low-angle detachment faults, deformed lacustrine beds, hydrothermal eruption breccias, tuff rings, shallow hotsprings-style alteration with gold mineralization and endogenous domes (Sillitoe, 1984).
Pyritic, carbonized wood fragment in lacustrine beds over breccia.
Bedded hydrothermal eruption breccias.
Diatreme Breccias: Upper Facies
Bald Mountain, Queensland, Australia
Lake infilling diatreme crater
Marginal interleaved breccias and altered PE schist.
Pascua, Chile
Unmineralized eruption breccia
with native S + cinnabar.
Diatremes: Eruption Breccias / Tuff Ring
Pascua, Chile
Unmineralized, steam-heated
silica alteration in bedded
eruption breccias above deposit.
Diatremes: Eruption Breccias / Tuff Ring
Minaspata, Peru
Diatreme Breccias: gold (silver, base metal)
mineralization
Mineralization may occur within different trap-sites around and within diatreme breccias:
• within permeable facies within the breccia, typically where meteoric water influx was insufficient to alter the breccia matrix to clays eg. Pascua.
• within crackle-brecciated competent units around the margins of the diatreme eg. Yanacocha • in fractured competent units within the breccia columns such as dikes and domes eg. Rosia Montana.
• in permeable breccia facies below the flared out margin of a flow dome eg. Yanacocha • in breccia zones below and between slide blocks of relatively impermeable wallrocks eg. Peñasquito and Rattlesnake Hills
• in well-defined epithermal veins radial or concentric outwards of the breccia margins eg. Santa Barbera in Peru.
• mineralized clasts may signify the presence of deeper porphyry and skarn mineralization eg. Lepanto / Far South East in the Philippines and Rinti in Indonesia.
Diatreme Breccias: gold (silver, base metal)
mineralization
Pascua, Chile
The highly acidic alunite – pyrite – enargite ore within the Pascua diatreme indicates that this mineralization was dominated by
magmatic fluids, with virtually no meteoric component. Competent (granitic) wall rocks are also fractured and mineralized.
The lack of meteoric water limited clay formation which enhanced the permeability of the diatreme breccia column.
Yanacocha, Peru
Most of the diatreme breccias at Yanacocha are clay-altered due to the influx of large volumes of meteoric water following the cessation of magmatism, which caused the breccias to become relatively
impermeability and therefore barren. Gold mineralization is hosted within intensely fractured, competent massive silica around the margins of the diatreme. Weaker mineralization is hosted in less competent alteration such as alunite – pyrophyllite – clay – silica.
Diatreme Breccias: gold (silver, base
metal) mineralization
Rosia Montana,
Romania
A second stage phreatic breccia , termed the ‘Black Breccia’, between two dacitic domes, is thought to be
responsible for extensive fracturing and mineralization of the relatively competent intrusive rocks within a larger clay-altered, and mostly
barren diatreme breccia.
Diatreme Breccias: gold (silver, base
metal) mineralization
Rattlesnake Hills,
Wyoming
Gold mineralization focused on the margin of a diatreme breccia where slide blocks of Precambrian schist have partially slid back into the breccia and blocked upwards fluid flow on the margins of the breccia column.
Diatreme Breccias: gold (silver, base
metal) mineralization
Diatreme Breccias: gold (silver, base
metal) mineralization
Corimayo, Yanacocha, Peru
High-grade gold mineralization (eg 70 m @ 16 g/t Au) hosted in
massive silica trapped below less permeable, clay and alunite-silica-clay altered flow dome rocks. This mineralization was blind below clay altered and unaltered barren rocks.
3600m
3400m
70m 16 g/t
Kelian, Indonesia
Carbonate-base metal – gold
mineralization hosted in diatreme breccia with fluid flow focussed along low-angle ‘detachment’ faults which formed large-scale slump blocks sliding back into the breccia column.
Diatreme Breccias: gold (silver, base
metal) mineralization
Rinti, Indonesia: mineralized porphyry clasts within
diatreme breccia.
The porphyry remains undrilled.
Diatreme Breccias: gold (silver, base
metal) mineralization
0 M 300 0 500 100 200 300 400 600 m RL Zone of abundant mineralized B Soil-Au (ppb) 100 200 300 Soil-Cu (ppm) V Diatreme Breccia Dacite Porphyry Tonalite Porphyry Andesite Volcanics Unaltered Illite Clay Chlorite - Epidote Pyrophyllite - Alunite High Sulfidation SilicaChlorite -magnetite Biotite -magnetite Fault
Clast of Chalcopyrite - Bornite mineralized Tonalite 10 20 30 V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V B’ 0 500 100 200 300 400 600 m RL Proposed Drill Hole Zone of abundant mineralized fragments fragments
Gold in Soils
PURNAMA PELANGI BASKARA KEJORA GERHANA < 10 ppb 100 ppb > 300 ppb # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 0 1 kmDiatreme Breccias:
Geochemical Response
Gold-in-soils geochemistry
showing the principal gold
mineralization focused in
competent silica-altered rocks
around the SW margin of the
Purnama diatreme breccia. The
diatreme is clay-altered and
barren with a late-mineral,
weakly altered felsic dome.
12/7/2011
Diatreme Breccias: Geochemical Response
A’ Diatreme Breccia Diatreme Breccia 1 Km 1 Km 0 0 PRD-03 PRD-01 PRD-02 PRD-04 PRD-05 PRD-06 PRD-07 PRD-08 Proposed Drill Hole
Core Holes Y2000
0 500 1000 0.4
0.8
ppb Au
West Rinti, Indonesia: gold-in-soil response within competent silicified
rocks around a diatreme
Peñasquito, Mexico
The Peñasquito diatreme
breccias have are characterized by distinct gravity lows within a broader gravity and magnetic high.
The magnetic and gravity data were assessed as good quality, the CSAMT conductivity data are dubious.
Diatreme Breccias: Geophysical Response
Residual Bouguer Gravity
RTP ground magnetics
CSAMT:1500 m
Important Factors (1)
• in many districts diatreme breccias were mapped as conglomerates or volcanic breccias in the early stages of exploration eg. Veladero in Argentina, Rosia Montana in Romania.
• in many cases diatreme breccias are barren due to the presence of relatively impermeable clays. This clay alteration also causes recessive weathering.
• competent rock units on the margin of diatreme breccias are highly favorable targets.
• upper parts of diatremes (maar volcanoes) are very complex
• lower parts of diatreme breccia may be invaded by felsic intrusions (porphyries), which may also be mineralized eg. Peñasquito in Mexico, Spring Valley in Nevada and Yanacocha in Peru.
• the presence of pervasive crackle-type brecciation, arcute hydrothermal breccia dikes, bedded eruption breccias and / or interbedded lacustrine sediments, mineralization associated with circular patterns in aerial photography or satellite imagery may all signify the presence of a diatreme breccia.
• some breccia complexes may have been mis-interpreted as diatreme breccias eg. Olympic Dam, where new data indicate the breccias are tectonic, forming a local sedimentary basin. The jury is still out on Pueblo Viejo, which may comprise a series of smaller diatreme breccias within a larger shale basin.
YANACOCHA
Characteristics of Diatreme Breccias
* dominantly clay-altered with dacitic clasts as the juvenile component.
* gold mineralization in fractured and brecciated silica-altered wall rocks flaring out away from the diatreme margins
* gold mineralization is post-diatreme but not hosted in most of the diatreme due to the impermeability of the clays
* diatreme hosts dacitic to rhyodacitic domes, and at depth young porphyry intrusions with associated late breccia phases.
* minor bedded, eruptive facies were recognized at the surface
* hypogene exfoliated dacitic clasts and fluidal textures are present
* marginal silicified rocks are strongly crackle-brecciated with hydrothermal (phreatic) breccia and pebble dikes.
