Printer Friendly

Minerals of the prospect intrusion.

The prospect intrusion is a small, differentiated dolerite laccolith containing shrinkage cavities and miarolitic cavities lined with a variety of interesting minerals including prehnite, pectolite, analcime, albite and pyroxene.

HISTORY

The first mention made of Prospect Hill was in 1789 when Captain Tench observed the barrier of the Blue Mountains to the west from its summit. The area was attractive to farming due to the richness of the soil and a two-square-kilometer grant was taken up by Lt. William Lawson in the late 18th century.

The outcropping chilled margin of basalt was noted by Charles Darwin in 1836 and it was mentioned in his writings on the voyage of the Beagle. The columnar nature of the basalt exposed in a small quarry was pointed out to J. D. Dana by geologist Rev. W. B. Clarke when the American Fleet moored in Sydney Cove in 1839 (Dana, 1849).

Road surfacing material has been obtained from Prospect Hill since the early 1830's, long before Government Geologist C. S. Wilkinson reported on the economic potential of the deposit in 1879. Prospect quarry itself was opened as the Emu quarry in 1883 by Sperring and Partner, who were bought out by Emu and Prospect Gravel and Road Metal company in 1900. The latter company was in turn taken over in 1919 by New South Wales Blue Metal and the name changed to New South Wales Associated Blue Metal Quarries. Blue Metal Industries was formed in 1954 and this company was recently bought out by Boral Resources (N.S.W.) Pty. Ltd., who continue to operate the Prospect quarry on a large scale (Clark, 1976).

GEOLOGY

The intrusion forms a prominent landmark which rises to 60 m above the surrounding country and 122 m above sea level. It is 2.4 km long and 1.4 km wide. The periphery of the intrusion has been exposed by erosion and this, together with its proximity to the rapidly expanding city of Sydney, 29 km to the east, led to its early exploitation as a source of construction material.

The Prospect Intrusion is a Jurassic doleritic laccolith emplaced concordantly between Triassic deltaic Hawkesbury Sandstone and the overlying lacustrine Ashfield Shale member of the Wiannamatta Group, also of Triassic age. Thin remnants of the Ashfield Shale underlie the intrusion in some areas.

The igneous body was intruded at shallow depth, as indicated by the prominent chilled margins of basalt produced by high heat differential between the relatively cool (and probably wet) sediments and the magma. Maximum contact temperature appears to have been around 500-600 [degrees] C (Nashar, 1967). Despite the initial high temperature of the magma indicated by the mineralogy of the intrusion (900-1000 [degrees] C), the shales above and below have reached only very low levels of metamorphism and show superficial alteration to a fine quartz hornfels with virtually no increase in grain size. The development of any significant metamorphic aureole was prevented by minimal escape of heat and volatiles beyond the insulating chilled margins. The retained heat also allowed very slow cooling, during which differentiation of the basaltic magma occurred, producing a layered intrusion composed of at least seven distinct rock types, with analcime dolerite (teschenite) and picrite forming the bulk of the intrusion.

The abundance of analcime in the rocks toward the top of the intrusion indicates significant retention of magmatic water during crystallization. During the final stages of magma crystallization, residual hydrothermal fluids become concentrated adjacent to the upper chilled margin and were localized in horizontal lenticular shrinkage fissures, often extending laterally for tens of meters within the coarse analcime dolerite. Here the abundance of free water, high temperatures and resultant ease of element diffusion promoted the formation of pegmatite bodies composed of anhedral to subhedral pyroxene, hornblende, plagioclase and analcime crystals to over 2 cm in diameter, analcime being the last of the minerals to form. Other shrinkage cavities deeper within the coarse analcime dolerite were occupied by crystallizing residual magmas which produced microdolerite and syenite lenses. These features are clearly visible in the working faces of all the Prospect Hill quarries.

It is principally along secondary shrinkage fissures within the microdolerite and pegmatite lenses (schlieren of Joplin, 1964) that the wide range of minerals found at Prospect crystallized from residual hydrothermal fluids during the final stages of cooling.

Retention of magmatic water within the crystallizing magma by the immediate development of a thick, chilled, impervious margin was the critical factor in the development of the hydrated minerals at Prospect. Had venting of the hydrothermal fluids into the surrounding sediments occurred the wide range of unusual rock types and associated late-stage minerals would not have developed.

The intrusion itself is thought to have been an indirect result of tension fracturing of the continental crust down to the upper mantle during development of the rift divergence zone which preceded the separation of the Australian and Antarctic continental masses in the mid-Cenozoic. These fractures acted as conduits for basaltic magma from the mantle region and one or more of these may have acted as feeder dikes for the Prospect intrusion, with the magma rising to a zone of density equilibrium within the surface rocks. The feeder dike system at Prospect has not yet been exposed by quarrying nor located by diamond drilling.

MINERALS

Minerals of the Miarolitic Cavities

Small irregular cavities no larger than 10 cm in diameter formed by the expulsion of volatiles from the magma during crystallization are relatively common throughout the coarse analcime dolerite and are also present in the pegmatite schlieren. They appear to be genetically related to the larger shrinkage fissures containing the late-stage mineral suite, representing an earlier phase of volatile expulsion. These miarolitic cavities commonly contain well crystallized plagioclase and pyroxene, the principal components of the host dolerite. Crystals project directly into the cavities from the surrounding rock matrix.

Pyroxene [(Ca,Na,Mg,[Fe.sup.2+],Mn,[Fe.sup.3+],Al,Ti).sub.2][(Si,Al).sub.2][O.sub.6]

Simple black prismatic crystals of titaniferous diopsidic augite (Joplin, 1964) reaching 5 mm in length are occasionally observed in the miarolitic cavities associated with the pegmatite schlieren. Typical morphology of the crystals is shown in Figure 4, but skeletal pseudooctahedral crystal groups are also known.

[Figure 4 ILLUSTRATION OMITTED]

Plagioclase NaAl(Al,Si)[Si.sub.2][O.sub.8]

The occurrence of crystallized plagioclase is confined to the miarolitic cavities where it usually occurs alone, only rarely being associated with pyroxene crystals or the late-stage mineral suite. Crystals are white to colorless, lustrous, and vary in size from a few millimeters to over 2 centimeters. Most crystals are tabular on b{010} and show both repeated albite twinning and parallel growth. Scanning electron microscropy (SEM) and quantitative energy dispersive X-ray analysis (EDS) has shown this plagioclase to be pure albite.

Specimens showing groups of simple, prismatic, compositionally zoned crystals of plagioclase comprising the three triclinic pinacoids and reaching 1 cm in length were found in the early years of quarrying at Prospect.

Minerals of the Pegmatite and Microdolerite Schlieren

Within the large schlieren parallel to the upper margin of the intrusion there is considerable development of vugs which are lined with well formed crystals of a variety of late-stage minerals crystallized from residual hydrothermal solutions.

An observed paragenetic sequence of these minerals, based on the close examination of several hundred specimens in the collections of the Australian Museum, Albert Chapman, Barry Cole and George Dale (all of Sydney), is presented in Table 1.

[TABULAR DATA 1 NOT REPRODUCIBLE IN ASCII]

Analcime NaAl[Si.sub.2][O.sub.6] [multiplied by] [H.sub.2]O

Analcime is the only zeolite found in any abundance at Prospect and forms an essential component of the coarse analcime dolerite (teschenite) comprising the upper part of the intrusion. However, the analcime in the dolerite itself forms only small anhedral masses characteristic of late-stage deposition in cavities between the earlier crystallized species and it is only in vugs that specimens of interest to the collector occur.

Prospect analcime usually forms simple white trapezohedrons reaching a maximum diameter of 4 cm, although most are less than 1 cm. Crystals usually occur thickly encrusting vug walls or, more rarely, as scattered individuals on dolerite matrix. Since it was the first of the late-stage minerals to crystallize, analcime is rarely found alone in vugs. Crystals are often overgrown by later minerals (especially prehnite) and hence good specimens are relatively uncommon. However, specimens of analcime showing only minor subsequent crystallization of prehnite, natrolite, spherulitic siderite, or honey-yellow calcite have been found.

Microcrystals of analcime to 50 [micro] m in diameter have also been found deposited on prehnite and, when present on the b{010} faces, these crystals appear to have been preferentially nucleated along incipient {001} cleavages.

Apophyllite A[Ca.sub.4][Si.sub.8][O.sub.20](Z) [multiplied by] 8[H.sub.2]O

Apophyllite is relatively uncommon in the Prospect intrusion. It is usually associated with pectolite as crystals showing at least two distinctly different habits and is often very pale pink in color.

Aragonite CaC[O.sub.3]

Aragonite is the least common of the carbonate species in the Prospect intrusion. Specimens in the Australian Museum collection show small, violet, columnar masses of strontian aragonite infilling prehnite-lined vugs, or deposited on analcime. No crystals have been observed.

Barite BaS[O.sub.4]

Barite was the last of the minerals to crystallize in the vugs. It is very rare and specimens are only found very occasionally. It has been observed as tabular white crystals in parallel groups to I cm across on drusy siderite (Australian Museum specimen D35330), rosettes of pale brown, transparent, tabular crystals to 4 mm across on white calcite (Australian Museum specimen D38535) and similar rosettes on drusy marcasite (George Dale collection) from the sheared gabbroic dolerite exposed between the Widemere and Prospect quarries.

Calcite CaC[O.sub.3]

Apart from prehnite, calcite is the most common of the late-stage minerals in the Prospect intrusion. It is common as vein and joint fillings which occasionally extend into and even beyond the chilled basalt margin. However, calcite is most spectacular as crystallizations on earlier minerals in vugs in the schlieren, where both habit and color vary considerably.

In the vugs calcite is most abundant as simple rhombohedra of the form r{10[[bar]1]1}, to 1 cm in diameter, often with slightly convex faces and varying in color from white to colorless and occasionally pale yellow. the latter usually associated with analcime. Roughly surfaced, cream-colored rhombohedra to 3 cm, showing internal color-zoning parallel to the crystal faces, have been found implanted on prehnite. These are among the largest calcite crystals known from Prospect.

More specifically associated with prehnite are white rhombohedra of the form e{01[[bar]1]2} reaching 1.5 cm in diameter. These crystals characteristically show incipient spherulitic development, as indicated by rounding of the edges of the crystal and/or development of smaller peripheral crystals in near-parallel growth, producing an attractive serrated fringe around the main crystal. Spherulitic development in calcite is produced by splitting of the rhombohedral crystals along the r{10[[bar]1]1} cleavage directions during growth due to sectorial enrichment in magnesium and iron carbonates along these planes (England, 1984). As a result, each crystal is composed of several individuals in near parallel growth and the corners of the rhombohedra curl alternately up and down about the c-axis, to a degree which depends on the amount of carbonate impurity present.

Small, equant, colorless crystals showing the rhombohedron e{01[[bar]1]2} and rough pseudo-prism faces formed by growth oscillation between positive and negative rhombohedra have been found in association with natrolite on prehnite.

Very occasionally. specimens have been found in which small white rhombohedra of the form e{01[[bar]1]2} have formed groups in parallel growth, retaining the trigonal symmetry of calcite and resembling oriental pagodas to 3 cm in length.

Groups of small, dark brown, lusterous rhombohedra resembling M{40[[bar]4]1}, to a maximum of 5 mm in diameter, occurred in the Widemere quarry, partially overgrown by larger white to colorless rhombohedra of the more common r{10[[bar]1]1} form.

Masses of white or pale yellow transparent calcite commonly completely fill vugs lined with analcime and/or prehnite. In specimens of this type, removal of the calcite by judicial use of dilute acetic or hydrochloric acid often reveals superb undamaged crystallizations of the earlier secondary minerals. In fact, many of the finest specimens of prehnite in major collections have been revealed in this way. Unfortunately however, prolonged acid treatment results in gelatinization of the analcime and hence the use of this technique with heavily coated specimens is usually unsuccessful.

Chabazite Ca[Al.sub.2][Si.sub.4][O.sub.12] [multiplied by] 6[H.sub.2]O

Chabazite is very rare but has been observed as small (less than 1 mm) white rhombohedra on botryoidal prehnite. Interpenetrant twins with the c-axis as the twin axis are common.

Chlorite Group [A.sub.4-6][Z.sub.4][O.sub.10][(OH,O).sub.8]

Chlorite is relatively common, usually forming small moss-green coralloidal masses to 5 mm on analcime or occurring as small, dense, lenticular bodies enclosed by prehnite adjacent to the walls of the vugs.

Occasionally, microcoralloidal masses to several centimeters across and containing radiating acicular cavities after pectolite are found on and partially enclosed by prehnite.

Heulandite [(Na,Ca).sub.2-3][Al.sub.3][(Al,Si).sub.2][Si.sub.13][O.sub.36] [multiplied by] 12[H.sub.2]O

Pearly white microcrystals resembling heulandite have been found associated with chabazite, analcime and chlorite on the crystallized surface of botryoidal prehnite vug linings. The crystals show obvious monoclinic symmetry with faces approximating the heulandite forms c{001}, b{O10}, t{101} and s{10[[bar]1]} with occasional contact twinning on b{010}. Parallel growth is characteristic, and repeated splitting of crystals during growth has produced divergent and often stellate groups.

SEM/EDS revealed major calcium, aluminum and silicon, with minor potassium and magnesium and only a trace of sodium. This suggests partial substitution of [Mg.sup.2+] for [Ca.sup.2+] and almost complete substitution of [K.sup.+] for [Na.sup.+] in the heulandite lattice. Insufficient sample was available for the identity of specimens to be confirmed by X-ray powder diffraction (XRD).

Laumontite Ca[Al.sub.2][Si.sub.4][O.sub.12] [multiplied by] 4[H.sub.2]O

No specimens of laumontite were located during the present study. However a single specimen in the collection of the Australian Museum shows well formed prismatic crystals of laumontite with the forms m{110} and e{[[bar]2]01}, completely replaced by prehnite.

Marcasite Fe[S.sub.2]

A large remnant of gabbroic dolerite between the Widemere and Prospect quarries contains a significant proportion of sulfides as veins and segregations in which marcasite is the principal species. The outcrop is made conspicuous by brightly colored sulfide weathering products, but these are of little interest to the collector. This local abundance of sulfides may be the result of a final pulse of sulfide-rich magmatic water which penetrated fractures in a prominent shear zone and partially altered the host rocks (Branagan and Packham. 1967) or low-temperature groundwater alteration of pyrite originally present in the gabbroic dolerite.

Within the shear zone angular cavities to several centimeters across are lined with crusts of crystallized marcasite, often colloform in appearance. Crystals typically show orthorhombic bipyramids and rarely exceed 2 mm in diameter. Unfortunately, the material is very unstable and few specimens have survived.

Montmorillonite [(Na,Ca).sub.0.33][(Al,Mg).sub.2][Si.sub.4][O.sub.10][(OH).sub.2] [multiplied by] n[H.sub.2]O

In the Widemere and old Emu quarries large masses of tan montmorillonite replacing dense radiating pectolite were common. Prehnite masses replaced by white to buff montmorillonite and associated with unaltered pale yellow calcite rhombohedra have also been found.

Natrolite [Na.sub.2][Al.sub.2][Si.sub.3][O.sub.10] [multiplied by] 2[H.sub.2]O

In the old Emu quarry spectacular specimens of radiating terminated crystals to 2 cm long were found lining vugs to 15 cm in diameter within the schlieren. Natrolite also occurred as jackstraw masses of thin prismatic crystals to 9 cm across. Crystals show the forms a{100} and b{010}, terminated by what appear to be the dome forms D{101} and e{011}.

Occasionally groups of radiating needles to 1 cm in length are found partially overgrown by a thin crust of pale green prehnite. Natrolite has also been found associated with analcime and white calcite.

Opal Si[O.sub.2] [multiplied by] n[H.sub.2]O

Large masses of white to blue-gray common opal infilling pectolite-lined vugs to 20 cm in diameter have been found in both Style's and Widemere quarries.

Pectolite Na[Ca.sub.2][Si.sub.3][O.sub.8](OH)

While spectacular specimens have been found in the past, pectolite is relatively uncommon in the Prospect intrusion. It usually occurs as large, compact masses of radiating fibrous crystals to 12 cm in length completely filling vugs in the schlieren. Sprays of individual terminated crystals are rare but may reach 10 cm in length. Apophyllite is a common associate.

Color varies from white to tan; partial to complete replacement by pale brown montmorillonite is common. Some early specimens of white radiating pectolite turned dull brown on the surface after a few months exposure (Hodge-Smith, 1943). Although the cause of this color change is unknown, investigation of a specimen of brown pectolite (D38667) in the Australian Museum collection by SEM/EDS revealed the presence of coatings and intergrowths of porous talc. Reports of a barian pectolite could not be confirmed by analysis of a number of specimens in the Australian Museum collection.

Leached, acicular to thin columnar, radiating molds in pale green prehnite masses were probably originally occupied by pectolite.

Phillipsite [(K,Na,Ca).sub.1-2][(Si,Al).sub.8][O.sub.16] [multiplied by] 6[H.sub.2]O

Phillipsite, like most of the other zeolite species at Prospect, is never conspicuous. It occurs as prismatic crystals to 2 mm in length scattered on the botryoidal surfaces of prehnite masses.

Prehnite [Ca.sub.2][Al.sub.2][Si.sub.3][O.sub.10][(OH).sub.2]

Prehnite is the most abundant and visually the most spectacular of the late-stage minerals in the Prospect intrusion. It forms mammillary to botryoidal crusts of compact, radiating platy crystals to almost 1 m in diameter. Color variation between vugs is striking, with white, buff, pale to apple-green, bright yellow-green, honey-yellow, reddish brown, deep golden brown and even black varieties being recorded. Deeply colored varieties display a diffuse color zonation, with a white or colorless zone adjacent to the dolerite and becoming darker toward the crystal terminations. Some indication of the range of color can be seen here in the color photographs.

Surfaces of the prehnite masses may be smooth and lustrous with no discernible crystal faces. However, more characteristic is the development of either randomly oriented or sub-parallel b{010} faces to 4 mm in length or globular masses of striated lenticular crystals to 4 cm in length, the latter predominating in the paler colored varieties. Distinct, well formed crystals are rare but occasionally blocky euhedral crystals to I cm protrude from peaks of the globular prehnite masses.

Prehnite always predominates in the vugs in which it occurs, but may also be associated with other late-stage minerals including analcime, chlorite, pyrite, calcite, and more rarely minerals of the zeolite group. One unusual specimen in the George Dale collection shows colorless to white calcite rhombohedra to 4 cm across coated with a pale green prehnite crust several millimeters thick.

Particularly abundant in the collection of the Australian Museum are curious masses of pale green prehnite containing closely spaced, radiating, acicular to thin columnar euhedral cavities. These are probably molds after pectolite sprays which crystallized prior to prehnite deposition and were subsequently removed, probably via hydrothermal alteration. In fact, prehnite is only rarely associated with unaltered pectolite, suggesting that dissolution of the pectolite may have provided at least some of the calcium and silicon required for crystallization of the prehnite overgrowths, with the sodium released into solution and absorbed in the subsequent crystallization of zeolite species either within the same vug or elsewhere.

A rare occurrence, represented by a single specimen in the Australian Museum collection, is prehnite pseudomorphs after well-formed prismatic crystals of laumontite to 3 cm in length. Prehnite has also been observed partially replacing pectolite crystals.

Pyrite Fe[S.sub.2]

Although the amount of pyrite present in the schlieren is very small, it is ubiquitous throughout the vugs. It usually forms very small, cubic crystals slightly modified by o{111} and rarely exceeding I mm across, scattered on prehnite surfaces and occasionally encrusting calcite. It has also been observed as microcrystals on albite in the miarolitic cavities. Spherulitic forms are also known.

Irregular masses of pyrite to several millimeters across are commonly scattered through the fabric of the analcime dolerite and occasionally it forms veins to a few millimeters in width.

A section of the Ashfield Shale exposed in the floor of the old Emu quarry (which later became the Prospect quarry) in the early 1900's produced roughly spherical groups of pyrite crystals to 7 cm in diameter. Specimens preserved in the George Dale collection show cuboctahedral crystals with concave and distorted faces reaching 4 cm on edge, arranged in sub-parallel groups.

Quartz Si[O.sub.2]

The plagioclase of the analcime dolerite adjacent to the vugs in the schlieren commonly shows partial replacement by analcime, which occurred during the final phase of magma crystallization (Joplin, 1964). The small amount of silica released by this reaction was probably the source of the occasional thin coatings of drusy colorless quartz found on analcime crystals in vugs. Similar coatings have also been observed on prehnite, but only where zeolite species are absent. Quartz also occurs as colorless microcrystals lining thin veins in the analcime dolerite.

A single specimen of heulandite microcrystals associated with earlier prehnite from the Chris Parkinson collection was found, on close examination, to contain late-stage spherulites of unknown fibrous crystals to 3 mm in diameter. Qualitative EDS revealed the presence of major silicon, with only trace amounts of calcium and aluminum (probably from the underlying prehnite). XRD was attempted but there was insufficient material available to obtain a conclusive result. The morphology and composition suggest [Alpha]-quartz.

Siderite [Fe.sup.2+] C[O.sub.3]

Siderite is considerably less abundant than calcite, but is conspicuous because of its lustrous, tan to brown microcrystals and bright iridescent surface tarnish caused by incipient alteration to hematite. It occurs as simple rhombohedral crystals to 5 mm in diameter, as spherulites, or as small, complex, parallel groupings scattered over the surface of analcime or prehnite crusts. Small masses of siderite containing trigonal molds after calcite were occasionally found in the Emu quarry.

CONCLUDING REMARKS

Although quarrying at Prospect is continuing on a large scale, operations have reached their maximum lateral extent as governed by lease boundaries. As a result, the coarse analcime dolerite and associated schlieren containing the vugs lined with secondary minerals have virtually been mined out. Extraction is continuing on unmineralized gabbroic dolerite and picrite.

However, although the likelihood of further specimen discoveries in the quarries themselves appears limited, there is a wealth of fine material in both public and private collections.

ACKNOWLEDGMENTS

Appreciation is expressed to the management of B.H.P. Research, Newcastle Laboratories, for the use of the equipment and resources of the laboratories. Final prints of the author's photographs were produced by Murray McKean of the photography group.

The Australian Museum, Sydney, gave the author access to the extensive range of Prospect specimens in the collection. Ross Pogson of the Earth Science Department provided extensive and invaluable assistance. Photographs of specimens in the collection were made by former Museum Photographer John Fields. Copyright on these photographs is retained by the Trustees of the Australian Museum.

Appreciation is also expressed to Barry Cole, Albert Chapman, Chris Parkinson and the late George Dale (all of the Sydney region) who gave the author access to their extensive collections of Prospect minerals and permission to use selected specimens for illustrative purposes.

[Figures 1-3, 5-21 ILLUSTRATION OMITTED]

REFERENCES

BRANAGAN, D. F., and PACKHAM, G. H. (1967) Field Geology of New South Wales. Science Press, Sydney.

CLARK, B. (1976) The Prospect Intrusion. Mineralogical News, 12, 8-11.

DANA, J. D. (1849) United States Exploring Expedition during the years 1838, 1839, 1840, 1841, 1842. Geology. C. Sherman (Philadelphia).

ENGLAND, B. M. (1984) The paragenesis of contrasting habits of calcite and aragonite-calcite associations from Kulnura, New South Wales, Australia. Mineralogical Magazine, 48, 519-527.

HODGE-SMITH, T. (1943) Mineralogical notes No. VI. Records of the Australian Museum, 21 (4), 251.

JOPLIN, G. A. (1964) A petrography of Australian igneous rocks. Angus and Robertson.

NASHAR, B. (1967) Geology of the Sydney Basin. Jacaranda Press.

Brian M. England BHP Research Newcastle Laboratories P.O. Box 188 Wallsend, N.S.W., Australia 2287
COPYRIGHT 1994 The Mineralogical, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1994 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:England, Brian M.
Publication:The Mineralogical Record
Date:May 1, 1994
Words:4203
Previous Article:The Jaguaracu pegmatite.
Next Article:Copper roses from the Rose mine.
Topics:

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters |