M I N E R A L O G I C A L M A G A Z I N E , SEPTEMBER I 9 7 4 , VOL. 39, PP. 8I I--1 5The mineralogy and paragenesis of a fluorspar flat
at Masson Hill, Matlock, Derbyshire
ROBERT A. IXER
Department of Geological Sciences, University of Aston in Birmingham
SUMMARY. Investigation of the mineralogy and paragenesis of the fluorspar flat at Masson Hill,
Matlock, Derbyshire, shows the mineralization to be a combination of void infilling and metasomatic replacement. The main paragenesis is of the non-sulphides, fluorite, baryte, and calcite accompanied by a sequence of sulphides, bravoite, pyrite, marcasite, chalcopyrite, sphalerite, and galena.
A wide range of secondary minerals occur in the oxidation zone.
THE presence of an intermittent fluorspar flat, within the bioclastic Matlock Lower
Limestones, along the summit of Masson Hill has long been known, and the presenceof fluorite, baryte, quartz, and galena recorded (Dunham, I952; Smith et al., I967;
Ford, I967, I969). The distribution of the mineralization was largely controlled byminor litbological variations, and by small-scale faulting and jointing, both within thelimestones and the largely barren dolomites that overlay it.
Surface and underground mapping of the central and north-eastern area of the flat
showed that the main paragenetic sequence was most clearly developed at three
localities: Masson opencast (SK 2845 5913), Jughole mine (SK 2793 597I), andOxclose mine (SK 2753 5994)- Many of the specimens from the last locality werecollected from spoil-heaps.
From the field evidence the mineralization is believed to have formed during one
extended phase of migrating fluids followed by oxidation of some of the primaryphases. Although small-scale fracturing and repetition of any phase are seen, it is solimited that it is best interpreted as localized movements during mineralization. Elsewhere in Derbyshire, notably at Long Rake (SK 216 653), there is evidence of polyphase mineralization; brecciation and fracturing of earlier mineralization are seen tohave accompanied later influxes of mineralizing fluids (Ineson and A1-Kufaishi, I97o).
This difference may reflect the greater sensitivity of rake deposits to minor earth
movements.

Textural classification
Using both petrographic and field evidence two distinct ore-types can be recognized(figs. I and 2):
a. A mixed mineral rock resulting from metasomatic replacement and showing
relict sedimentary calcite (to 40 /zm), dolomite rhombs (I2o /~m), and prismaticquartz 02o/~m). At Masson and Jughole the ore has a restricted occurrence being
confined to the junctions between limestone and dolomite. It consists of fluorite and9 Copyright the Mineralogical Society.

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quartz, with subordinate baryte, calcite, and galena. Locally a mixed sulphide layer
occurs, containing galena, bravoite, pyrite, marcasite, plus traces of chalcopyrite andsphalerite. At Oxclose the mineralization is of bravoite, pyrite, marcasite, chalcopyrite,sphalerite, and galena within fluorite orsmithsonite. The ore is friable andsecondary copper and zinc minerals arecommon.
b. A rhythmic layered ore showing
euhedral upper faces. The lack of any
relict sedimentary fabric and its occurrence within joints and premineralization solution cavities suggests the resultof void infilling. At Masson and Jughole
FIGs. I and z : FIG. I (left). Type a ore. Subhedral totheore consists of fluorite, baryte, andanhedral bravoite (B) showing alteration to goethitegalena,whilst at Oxclose the layers(Go) and surrounded by pyrite (P). Associatedchalcopyrite (Cp) with alteration along cleavage and comprise marcasite, sphalerite, galena,margins, galena (G), marcasite (M), and sphalerite(Sph) occur in a mixed fluorite and smithsonite baryte, and fluorite. Alteration of thematrix. FIG. 2 (right). Type b ore. Banded baryte (B), primary minerals is rare.
marcasite (M) with an oxidized base, sphalerite (S),Field evidence indicates that the twoand fluorite (F) indicating euhedral upper faces.
types
are contemporaneous and often
Zones of bravoite inclusions occur in the fluorite.
grade into each other.
OXCLOSEMINEJUGHOLE M I N E

LIMESTONE &VOIDSaMASSON O ~ N C ~ T

Mineralogy

The mineralogy is discussed in orderof its deduced paragenetic sequencet:::::::z2:::m" ~BARyT~(Fig. 3). Modal proportions are also
CALCITEBRAVOITE
given for five specimens of type a ore
PYRITE
fromOxclose Mine (Table I), which
MARCASITE
PL
L
CHALCOPYRITE
illustrate the local importance o f
GALENASPNALERITE
bravoite.
OXIDATION ZONE
Fluorite is the most common and theCOVELLINE
D?
earliest mineral, and occurs throughout
CHALCOSINEEl
CERUSSITEEl
D
the primary mineralization. Normally
SMITHSONITEI~]
7
OUARTZ
it is colourless to pale yellow, but less
GREEN CARSONAT~
D
often it is blue, green, or purple, the last
GOETHITE/CUPRIT~[3nI]
colour being especially common at theEARLY
LATE sLAT EARLY
LATE
crystal edges or surfaces near to otherminerals, especially baryte and smithFJG. 3. The paragenesis for the mineralization sonite. Cubic crystals are c o m m o n ,between Oxclose Mine and Masson Hill Opencast.
often exhibiting {21o} faces.
Three types of inclusions occur: the relict sedimentary carbonates and quartz
within fluorites from type a ore; fluid inclusions occurring in fluorites from type b ore,and described by Mueller 0954a); and primary sulphides common to all fluorites.
The most common inclusions are of bravoite, pyrite, marcasite, and chalcopyrite.
DOLOMITIZATIONSlLICIFICATEONFLUORITEFLUORITE

ATMASSONHILL,DERBYSHIRE

813

Baryte occurs as white to orange incrustations on fluorite, and as cockscomb
crystals (I'5 cm). Although it is essentially an early phase it has a number of late stage
associations, notably with marcasite or acicular goethite. The grey mammilated
baryte described by Ford and Sarjeant (1964) forms one of these late associations.
Both quartz and calcite occur throughout the primary mineralization, and calcite
in the oxidation zone. Both carry sulphide inclusions and both are far less significant
at Oxclose than in the more central areas of the flat.

TABLE I. Modal proportions for type a ore from Oxclose Mine
OC1

Fluorite

OC2

OC3

6.1

OC544"2441-68

52'20

4I'72

CalciteBravoite
PyriteMarcasite
Chalcopyrite
GalenaSphalerite
Covelline
Cerussite
Smithsonite
Green carbonates
QuartzGoethite< o.oI
6.29
2'35
1.21o' I 2< o.oiI '33o.12< o-oi39"5oo.I2
o.26
4"46

~ o.oI
8.2o
3'34
I .o 9o'49< o.o1
1.22o'5o-~ o.oI
37"46o.I2
o'52
5" 38

-~ o . o i5"292-o50-32
o.86< o'oiI '84
o.22( o.oi33"440-27
o.I 6
3" 35

~ o.oi13"I34"28
1"17
1 "o5
< o.oi7"43o'94<( o.oI27"790"90
o.o8
I. 51

<TotalIO0.O0IO0.O0IO0.O0IO0.O0IO0.O0

21"85
o.oi
0'98
I "57o'98
o. 33< o.oio'45o.06<~ o . o i72"240.02
o'39
I. 13

Bravoite is the earliest hydrothermal sulphide, occurring as inclusions in all thenon-sulphide phases. The inclusions are generally lO-3O ~m in size showing a pentagon
dodecahedral habit. They show zoning and pyrite and marcasite overgrowths.
At Oxclose two types of bravoite occur: A brown euhedral to subhedral, faintly
zoned bravoite (5o to 3o0 t~m) with average reflectivity in air in plane polarized light of
41 "9 to 42.5 %. This is typical of type a ore. And a rarer, euhedral, lilac-brown, zoned
bravoite (2o t~m) with an average reflectivity of 38"o to 38"5 %. This bravoite occurs
within the centre of large pyrite masses (lOO to 2oo/zm) or as inclusions in fluorite.
These reflectivities are similar to those determined by Vaughan (1969) for bravoites
from Millclose Mine.
The bravoite inclusions within fluorite occur within 5oo/~m wide zones, both asindividual grains and as long stringers (4o to lOO t~m• 2/~m) lying perpendicular tothe crystal edges. Although the spectral reflectivity within a given zone is uniform,
adjacent zones show differing reflectivities indicating a change in the iron to nickel
ratio of the mineralizing fluids.
Microprobe analysis of bravoites from Oxclose Mine (Table II) show the nickel
content to be similar to the 'type 1' bravoite analysed by Hey (in Bannister, 194o)and to the bravoites analysed by Vaughan (1969), both from Millclose Mine. The
presence of a second type of bravoite containing 28% nickel found by Bannister at

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Millclose could not be confirmed at Oxclose Mine although the primary mineralization is similar in both mines.
Fracturing of bravoite occurs; early fractures are cemented by fluorite, later onesby smithsonite. Oxidation of bravoite is widespread, commonly with goethite pseudomorphs that still show the original zoning.
Bravoites with essentially similar parageneses have also been recognized in Derbyshire at Ball Eye Mine (SK 286 574), Hucklow Rake (SK r75o 779o), and Dirtlow Rake,Castleton (SK 15oo 819o) suggesting that it may occur throughout the pyrite-fluorite
zone (Mueller, I945b) and not just in the Matlock area (Ford and Ineson, I970.
TABLE II. Microprobe analyses of bravoitesfrom Oxclose MineFeNiSCo

28"52
I6"4752"44
1.22

32"9II2"I352"370"83

34"6t
13"o352'78
o'92

27'20
I8"92
53"36
1.22

Total

98'65

98'24

lot "34

IOO'70Pyrite and marcasite generally occur as overgrowths on bravoite inclusions, characteristically on the faces away from the direction of growth of the host mineral. In
addition a late-stage marcasite occurs as bladed crystals associated with baryte andcalcite.
Small anhedra of chalcopyrite occur, often showing extensive and variable alteration.
Successive rims of covelline, chalcosine, cuprite, and malachite are common, due tolater oxidation. Some chalcopyrite occurs as exsolved lamella (Ioo• 2o/~m) within
sphalerite.
Large subhedral to euhedral cubes of galena are typical throughout the flat, occurring in both ore types. Occasionally, octahedral galena is associated with calcite
scalenohedra at Masson Opencast. Oxidation to cerussite is very common.
Sphalerite. Semi-quantitative microprobe analysis and single crystal diffraction
confirmed the sphalerite as an iron-poor honey-blende with an average 4"3 mol % Fe.
In addition to the typical internal reflections of this type of sphalerite, deep-purple
reflections are common, often along twins and growth zones. The colour is not due
to the inclusion of purple fluorite but its origin is undetermined. Alteration to smithsonite is extensive, accompanied locally by rosasite and aurichalcite. The latter twominerals are known from elsewhere in the flat (Braithwaite and Ryback, I963).
Conclusions
The mode of occurrence and the mineralogy throughout the flat is consistent witha low-temperature hydrothermal origin. The textural evidence is more consistent witha single long-term primary mineralization event followed by extensive oxidation bymeteoric waters. The occurrence of bravoite within the flat and elsewhere suggest it
may be a characteristic accessory sulphide of this type of mineralization.

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815

Acknowledgements. This paper forms part of a Ph.D. thesis submitted to the University ofManchester. Thanks are due to Dr. A. C. Dunham who suggested the research topic andsupervised the work. Financial support from Laporte Chemicals Limited is gratefully
acknowledged, as is help from their Divisional Geologist, Dr. J. E. Mason.

REFERENCESBANNISTER(F. A.), 194o. Min. Mag. 25, 6o9-14.
BRAITHWAITE(R. S. W.) and RYBACK((3.), I963. Ibid. 33, 44I-9.
DUNHAM (K. C.), I952. Geological Survey on the Mineral Resources of Great Britain, 4. Fluorspar,
97-1oi. London (H.M.S.O.).
FORD (R. D.), 1967. Some Mineral Deposits in the Carboniferous Limestone of Derbyshire, 53-75, inGeological excursions in the Sheffield Region. University of Sheffield.
1969. The Stratiform Ore Deposits of Derbyshire, 73-96, in Sedimentary Ores Ancient and Modern.
University of Leicester.
- - and INESON(P. R.), 197I. Trans. Inst. Min. Met. 80, BI86-zlo.
and SARJEANT(W. A. S.), I964. Proc. York. Geol. Soc. 34, 371-86.
INESON(P. R.) and AL-KUFAISHI(F. A. M.), 197o. Mercian Geol. 3, 337-51.
IXER (R. A.), I972. Unpublished Ph.D. Thesis, University of Manchester.
MUELLER(G.), I954a. Mining Mag. 90, 148-54 and 2o8-17.
- - I954b. X I X Int. Geol. Congr. Alger. Fasc. 15, 532-9.
SMITH (E. G.), RHYS ((3. H.), and EDEN (R. A.), 1967. Geology of the Country around Chesterfield,
Matlock and Mansfield. Mere. Geol. Surv. Gt. Britain.
VAOGHAN(O. J.), 1969. Amer. Min. 54, IO75-83.
[Manuscript received 3 July 1973; revised I4 January 1974.]