de Jong, Geoffrey (1995) Post metamorphic alteration and mineralization in a highly deformed Proterozoic terrain: the eastern Selwyn Range, Cloncurry District, NW Queensland. PhD thesis, James Cook University of North Queensland.

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Abstract

Structurally controlled sodic-calcic alteration and associated brecciation affected at least 200 square kilometres of the Proterozoic Cloncurry-Selwyn terrain, of the eastern Mount Isa Inlier. Shear zones separate the Cloncurry-Selwyn terrain into different lithostratigraphic domains and metamorphic grade subzones. These shear zones are interpreted as reactivated early terrain boundary faults. The alteration overprints the contact between the Maranan supergroup in the east and the Doherty Formation in the west, and is spatially associated with a major NW-SE trending lineament, the Cloncurry fault. Several east draining river sections (e.g. Maramungee Creek and Fullarton River) give a good insight into the alteration and breccia complex.

Three regional deformation phases affected the Cloncurry-Selwyn terrain. with D2 as the main phase The dominant foliation is S2, developed as a penetrative N-S fabric. Overprinting younger deformations cause variations of this trend, from NW-SE to NE-SW. The peak of metamorphism coincided with D2 and reached the sillimanite - K feldspar grade. Late-post D2 ductile shear zones and brittle structures control the alteration and breccia complex.

Hot ( 400-550°C) saline ( 16-38 equivalent NaCl wt.%) fluids present post-peak metamorphism, based 011 fluid inclusion studies, have been responsible for the alteration. Rocks infiltrated by the hot saline fluids are reconstituted to albite-actinolite with destruction of primary textures and mineralogy Albitic plagioclase (An < 8%). with magnesium rich actinolite (Mg/Mg+Fe ~ 80-90%) ± magnetite (±Ti) ± titanite are the main alteration minerals. Isothermal pressure reduction and a low XCO2 controlled the local occurrence of diopside. Dilatational veins commonly contain titanite and ilmenite, while some magnetites are rimmed with titanite, indicating local mobility of Ti. Main elements depleted during this fluid infiltration, have been potassium and iron. Iron liberated by mica destruction precipitated as a separate magnetite phase, and in magnetite veins.

Chlorine bearing minerals, such as scapolites, are also indicators of the presence of high salinity fluids, and support the fluid inclusion observations. Scapolite alteration is abundant in the Corella Formation of the Mary Kathleen zone, but comparatively rare in the eastern part of the Cloncurry-Selwyn terrain. Scapolites in the eastern Cloncurry-Selwyn terrain arc of two distinct types, a metamorphic Ca-scapolite with approximately the same composition as the Mary Kathleen scapolite and an alteration Na-rich scapolite with a high Cl content. The latter forms selvages around oligoclase-quartz veins and occurs in strained metadolerites, located in the zones affected by the sodic-calcic alteration.

The fluids were channelized in zones of high strain, localized in reactivated faults. Unroofing resulted in a decrease in lithostatic pressure, and a relative increase in fluid pressure, evident from the abundance of veining overprinting the ductile shear zones. Two sets of veins occur, namely layer parallel and subhorizontal vein sets. The layer parallel veins are also axial planar to the refolded folds in the shear zones. These veins consist of actinolite-tremolite centres with albite selvages. Subhorizontal veins mainly occur adjacent to the shear zones, and consist of actinolite and diopside centres overprinted by microcline selvages.

Megabrecciation is a result of increased fluid pressures caused by continuous unroofing and infiltration of new fluids, flowing up temperature. High level granite intrusions ( 1510-1480 Ma) are due to unroofing and (partly) sinistral strike slip movement on the Cloncurry fault. Brecciation, with similar alteration mineral assemblages, locally affects the syn to post D3 granites, suggesting similar fluids been active for up to a maximum of 40 million years. The breccia matrix has a "pseudo-igneous" texture, with similar minerals as the altered clasts. The clast size varies from millimetre up to 10s of metres scale. Many clasts are totally altered. The clasts are subrounded due to reaction with the fluid rather than to transport. Some clasts still contain relict schist textures.

Unroofing also resulted in a telescoping effect of the alteration, with K-feldpar+quartz+hematite (300°C, 24 Eq.wt.% NaCl), and epidote+prehnite+calcite±quartz assemblages overprinting the earlier sodic-calcic alteration. Quartz and pyrite are part of a late stage (280°C. 25 Eq.wt.% NaCl. hydrothermal) alteration, controlled in fractures related to the Cloncurry fault.

A mineralized corridor east of the regional sodic-calcic alteration system is the location of the Pb-Ag±Zn and Cu-Au Maronan Prospect. This corridor is a high strain zone with various lithologies and mineralized quartz lenses. Five distinct mineralized zones occur in the Maronan Prospect, and consist of various peculiar horizons composed of magnetite, garnets, pyroxenoids and carbonates. These zones give the appearance of a Broken Hill deposit / exhalative type setting. However, overprinting relationships and elevated and flat REE patterns of garnet-rich layers, which are different to sea floor hydrothermal alteration REE patterns, suggest an epigenetic origin. Calcareous horizons and to a lesser extent the pyroxenoid layers are the main host rocks for the Pb-Ag mineralization, while Cu-Au mainly occurs in fractures and veins associated with silicified rocks. Gold shows affinities with HREE, Cu, Sb and As. Silver exhibits a strong positive correlation with Sb. Constant Pb/Cu and Au/Ag ratios in all mineralization affected rocks, and the occurrence of a high Au-Cu-As-low Ag association, might indicate two types of fluids.

Item ID:	90478
Item Type:	Thesis (PhD)
Copyright Information:	Copyright © 1995 Geoffrey de Jong
Date Deposited:	25 Feb 2026 02:56
FoR Codes:	37 EARTH SCIENCES > 3705 Geology > 370505 Mineralogy and crystallography @ 35% 37 EARTH SCIENCES > 3705 Geology > 370510 Stratigraphy (incl. biostratigraphy, sequence stratigraphy and basin analysis) @ 35% 37 EARTH SCIENCES > 3705 Geology > 370511 Structural geology and tectonics @ 30%
SEO Codes:	25 MINERAL RESOURCES (EXCL. ENERGY RESOURCES) > 2503 Mineral exploration > 250399 Mineral exploration not elsewhere classified @ 50% 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 50%
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