Geology & alteration

Hematite-rich IOCG deposits require magmatic fluid components exposed to atmospheric oxidation – Evidence from Prominent Hill, Gawler craton Part 1 – Geology & alteration Tobias Schlegel and Christoph Heinrich, January 2018

foto by OZ Minerals

Applied Mineralogy and Economic Geology

Introduction

Geology & alteration

Sulfur source

IOCG fluids

Genetic model

2

Iron oxide-copper-gold (IOCG) deposits Great economic attraction for mineral explorers – high ore-grades Prominent Hill

0.1

modified from Groves et al. (2010); after Williams et al. (2005)

Introduction


Sulfur source

IOCG fluids

Genetic model

How can we explain iron oxide + copper + gold breccias ?

hematite + carbonate + chalcocite + bornite, porous hematite breccia

3

Introduction


Sulfur source

IOCG fluids

Genetic model

4

Genetic models of IOCG systems Magma-derived

Surface or basin derived

Metamorphic derived

Barton & Johnson (2004)

Introduction


Sulfur source

Objective

What is controlling the distribution of high-grade copper mineralization in hematitic IOCG deposits?

-Is it the distribution of different types of host rocks? -Is it the intensity of hydrothermal alteration?

IOCG fluids

Genetic model

5

Introduction


Sulfur source

IOCG fluids

Geology of the eastern Gawler craton

Hiltaba Granite Suite / mafic intrusion (1595-1575 Ma) Gawler Range Volcanics (ca. 1605-1583 Ma) Metagranites and mafic intrusions (ca. 1760 Ma)

Calcareous metasedimentary rocks and metavolc. (ca. 1760 Ma and older)

Metagranites (ca. 1850 Ma) Archean to Proterozoic metamorphic rocks and greenstones ( 2% Cu

Schlegel & Heinrich (2015)

Introduction


Sulfur source

IOCG fluids

Genetic model

10

Geopetal structures in orientated drill core from Prominent Hill … indicate that Cu-rich sulfide mineralization occurred after tilting of the host rocks to a overturned, steeply dipping position

> 2% Cu

modified from Schlegel & Heinrich (2015)

Introduction


Sulfur source

IOCG fluids

Genetic model

11

Hematite breccia vs. calcareous host rock breccia

Hematite breccias (left images)

formed by pervasive replacement of calcareous breccia matrix and clasts

carbonate

(right images)

not by violent hydrofracturing


Introduction


Sulfur source

IOCG fluids

Genetic model

12

Hematite (-quartz) alteration of andesite and sandstone Hematite-quartz alteration results in a) destruction of rock and mineral textures (top), and b) replacement of feldspars, chlorite and sericite by hematite and quartz

Variably altered andesite

(lower right)

Hematite replacing chlorite and sericite bearing sandstone matrix (lower left)


Introduction


Sulfur source

IOCG fluids

Genetic model

13

High (Fe+Si)/(Fe+Si+Al) ratios stand for hematite-quartz alteration Partial mobility of most or all elements (incl. Al) is a result of intense hematite-quartz alteration  (Fe+Si)/(Fe+Si+Al) ratio approaches unity

alteration

intense

weak


Introduction


Hematite-quartz alteration Hematite-quartz alteration

Sulfur source

≠

IOCG fluids

Genetic model

iron enrichment “Hematite alteration” Magnetite in hanging wall

14

Introduction


Sulfur source

IOCG fluids

Genetic model

15

High-grade Cu grades correlate with hydrothermal alteration (I) Copper mineralization exists outside a discordant hematite-quartz alteration front (left). Rocks with blue values have a good chance for high Cu grades despite comparatively “low” Fe grades (right).


Introduction


Sulfur source

IOCG fluids

Genetic model

16

High-grade Cu grades correlate with hydrothermal alteration (II) Distal from Cu-poor zones of intense hematite-quartz alteration (left), hematite-chloritesericite alteration with high Cu grades are mapped using molar K-Na-Al ratios (right + next slide) Hematite-quartz alteration

Hematite-chlorite-sericite alteration


Introduction


Sulfur source

IOCG fluids

Genetic model

17

High-grade Cu grades correlate with hydrothermal alteration (III) Alteration gradients in the stability field of white mica solid solutions are correlated with high Cu grades at Prominent Hill (and Olympic Dam) (e.g. 65 % of all rocks plotting in the purple area but only 12 % of rocks in the dark blue area have Cu grades > 0.1 wt %)

1) Roberts & Hudson (1983); 2) Tappert et al. (2013); Schlegel & Heinrich (2015)

Introduction


Sulfur source

IOCG fluids

Genetic model

18

Take home messages – Geology & alteration •

The economic IOCG deposit at Prominent Hill is within unmetamorphosed, overturned, and carbonate matrix-rich breccias, sedimentary rocks as well as the Gawler Range Volcanics

•

Hematite breccias formed by pervasive replacement

•

Geopetal markers show that economic mineralization occurred after tilting of the host rocks into the present orientation

•

The economic Cu mineralization is broadly strata-bound and exists outside a discordant hematite-quartz alteration front

•

The hematite-quartz alteration index [(Fe+Si)/(Fe+Si+Al) can be used to map the transition of strong hematite-quartz alteration to moderate hematite-chlorite-sericite alteration and provides a local mapping vector toward high-grade Cu ore

Acknowledgement ETH-Zurich, SUERC and OZ Minerals thanks to Calma Rochat Patrick Williams Hamish Freeman

Mark Allen Ian Anderson Ricardo Brigante

Mick Sawyer Paul Hehuwat Marcel VanEck

Stuart Bull Dave Lawie Markus Tomkinson

References • Barton MD, and Johnson DA, 2004, Footprints of Fe-oxide(-Cu-Au) systems. SEG 2004: Predictive Mineral Discovery Under Cover. Centre for Global Metallogeny, Spec. Pub. 33, The University of Western Australia, p. 112-116. • Belperio A, Flint R, and Freeman H, 2007, Prominent Hill: A hematite-dominated, iron oxide copper-gold system: Econ. Geol., v. 102 p. 1499-1510. • Bull S, Meffre S, Allen M, Freeman H, Tomkinson M, and Williams PJ, 2015, Volcanosedimentary and chronostratigraphic architecture of the host rock succession at Prominent Hill, South Australia: Abstracts to the SEG 2015 conference: World Class Ore Deposits: Discovery to Recovery, Hobart, TAS, Australia, Society of Economic Geologists, 1p. • Freeman H, and Tomkinson M, 2010, Geological setting of iron oxide related mineralisation in the southern Mount Woods domain, South Australia, in Porter, TM, ed., Hydrothermal iron oxide copper-gold & related deposits: A global perspective - Advantages in understanding of IOCG deposits, v. 3: Adelaide, PGC Publishing, p. 171-190. • Groves DI, Bierlein FP, Meinert DL and Hitzman MW, 2010, Iron oxide copper-gold (IOCG) deposits through Earth history: Implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits: Economic Geology, v. 105, p. 641-654.. • Roberts DE, and Hudson GRT, 1983, The Olympic Dam copper-uranium-gold deposit, Roxby Downs, South Australia: Economic Geology, v. 78, p. 799-822. • Tappert MC, Rivard B, Giles D, Tappert R, and Mauger A, 2013, The mineral chemistry, near-infrared, and mid-infrared reflectance spectroscopy of phengite from the Olympic Dam IOCG deposit, South Australia: Ore Geology Reviews, v. 53, p. 26-38. • Schlegel TU and Heinrich CA, 2015, Alteration and lithology control Cu mineralization at Prominent Hill iron oxide-copper-gold deposit, Gawler craton, South Australia: Economic Geology, v. 110, p. 1953-1994. • Schlegel TU, Wagner T, Boyce AJ, and Heinrich CA, 2017, A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic iron oxide copper-gold (IOCG) province of South Australia: Ore Geol. Rev., v. 89, p. 1058-1090. • Williams PJ, Barton MD, Johnson DA, Fontboté L, de Haller A, Mark G, Oliver NHS, and Marschik R, 2005, Iron oxide copper-gold deposits: geology, space-time distribution, and possible modes of origin: Economic Geology 100th Anniversary Volume, p. 371–405. • Williams PJ, Benavides J, Sadikin P, OZ Minerals Prominent Hill Geology Team, 2017, Metallogenic significance of altered volcanic rocks near the Prominent Hill IOCG deposit, South Australia: Proceedings of the 14th SGA Biennial Meeting, Quebéc, v. 3, p. 895–898.