Dr. W. k. Jones—Topaz and Cassiterite in Malaya. 257 
sometimes extremely irregular even in the same vein, but is greatest, 
in general, where the rock contains a network of small fissures filled 
with ore. There are, however, exceptions to this, for at the Chendai 
and Menglembu lode-veins in Kinta, Malaya, where felspar crystals 
-are cut by minute cassiterite-bearing fissures, the felspars, as has 
been previously pointed out by Mr. Scrivenor,'are quite fresh. The 
writer saw an interesting case at Serendah, Malaya, where the 
absence of alteration in the country rock of a lode appeared to be due 
to its coalescence with another lode, exposed later, and through which 
the mineralizing vapours found an easier conduit. 
The dissimilarity between the vein rock and the adjacent alteration 
rock is only what would be expected, for the rock intruded as veins 
in the consolidating granite was the more acid part of the differentiated 
magma and formed, before being subsequently acted upon by an 
abundance of fluoriferous mineralizers, an aplite or pegmatite vein 
rich in quartz, felspar, and little or much mica, such as are so very 
frequent in this neighbourhood. It is not the rule, but rather the 
exception, to find the peripheries of cassiterite-bearing veins similar 
in mineral content to the central parts of the vein, especially where 
the veins are thick. The footwalls of tin-lodes are, in general, richer 
in ore than the hanging walls and very considerably richer than in 
the body of the lode; and where wolfram, for example, occurs 
associated with tin-ore the former frequently occurs sporadically as 
coarse crystals along the walls. At Zinnwald? quartz and zinnwaldite 
occur in a lode as layers in such a way that the individual layers are 
parallel to the walls, and one of these may occur to the almost 
complete exclusion of the other. 
The presence in these ‘ quartz-topaz’ veins of an iron-rich 
zinnwaldite seems to support, rather than contradict, the secondary 
origin of the topaz and the cassiterite, for zinnwaldite is also 
abundant in the veins in the Erzgebirge tin-field. At Zinnwald in 
this district ‘‘the chief gangue minerals are quartz and zinnwaldite’’.* 
It will also be shown later that the addition of iron takes place in the 
process of greisenization in districts where chemical analyses of the 
greisen and the unaltered rock are available. 
Emphasis is given to the marked difference between the ore-bodies 
occurring at Gunong Bakau, admitted to be of secondary origin, and 
the ore-bodies in the veins where the topaz and cassiterite are 
presumed to be of primary origin. The ore-bodies worked at Gunong 
Bakau occur mainly in four types of lodes: (1) The rock at Hemy’s 
Lode, which has been described elsewhere* by the writer, is not 
sufficiently coarse-grained for a typical pegmatite, and although more 
acid than ordinary granite, is best described as an altered medium- 
grained granite, very rich in quartz, mica (some being secondary), 
tourmaline, and cassiterite, relatively poor in felspar, and containing 
1 J. B. Scrivenor, The Geology and Mining Industry of Kinta District, 
Federated Malay States (Kuala Lampur), 1918, p. 62. 
2 J.T. Singewald, jun., ‘‘ The Erzgebirge Tin Deposits’’: Hconomic Geology, 
vol. v, p. 173, 1910. 
> Thid. 
4 W. RB. Jones, ‘‘ Mineralization in Malaya’’: Min. Mag., vol. xiii, No. 4, 
p. 198, Oct. 1915. 
DECADE VI.—VOL. III.—NO. VI. 17 
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