Mineralogical and whole-rock geochemical characterization of cherts from Gebel EL-Khashab (Oligocene), East Cairo, Egypt: evidence for silica sources

Kamaleldin M. Hassan

doi:10.2478/mipo-2025-0012

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Mineralogical and whole-rock geochemical characterization of cherts from Gebel EL-Khashab (Oligocene), East Cairo, Egypt: evidence for silica sources

Mineralogia

Volume 56 (2025): Issue 1 (January 2025)

By: Kamaleldin M. Hassan

Open Access

|Dec 2025

Figures & Tables

(a) Location of the area of Gebel El-Khashab reserve (X); (b) Geological map of East Cairo, Egypt (after Salama & Mustoe, 2023); (c) Stratigraphic section of the reserve area; (d,e) Field pictures showing chert gravels associated with sands in the area. Note the size, shape, and surface texture of these gravels are highly variable, most likely due to the long journey they made before reaching to their present place.

Photographs of freshly broken chert gravels from the studied area: (a–h) Siliceous gravels exhibiting banding structures; (i–l) Non-banded siliceous gravels without cortices (Note geodes or cavities in K and L samples).

Optical photomicrographs showing mineral and textural characteristics of chert gravels from the studied area. (a) Cavity occupied by megaquartz, drusy texture; (b) Another cavity filled with chalcedonic quartz; (c) Prismatic, zoned-detrital zircon; (d) Monocrystalline, detrital quartz; (e) Rhombic calcite microcrystals; (f) One complete section of an echinoid spine; (g) Fossil of a crinoid columnal; (h) Fragment of an echinoid plate. CPL, cross-polarized light; PPL, plane-polarized light; Qz_chc, chalcedoic quartz; Qz_detrital, detrital quartz; Qz_mega, megaquartz; Qz_micro, microquartz; Zr_detrital, detrital zircon.

Characteristic OM textures and morphologies in the studied chert: (a) Branching organic textures, diffused and clotted OM, tiny organic spheres, and organic morphotypes; (b) Various forms of organic morphotypes (c) Approximately 200 μm diameter sphere filled with OM and having spongy texture; (d) Organic sphere filled with microcrystalline quartz. CPL, cross-polarized light; OM, organic matter; PPL, plane-polarized light.

Table 1.

LOI values and major element contents (wt.%) of chert and its associated sand from the studied area.

	SiO₂	TiO₂	Al₂O₃	Fe₂O_3T	MnO	CaO	MgO	Na₂O	K₂O	LOI
Chert
C-1	97.05	bdl	0.04	0.41	bdl	bdl	0.01	0.005	0.01	0.80
C-2	98.12	bdl	0.14	0.36	0.003	0.01	nm	nm	nm	nm
C-3	96.96	bdl	0.12	0.45	0.004	0.17	nm	nm	nm	nm
C-5	97.40	0.04	0.05	0.46	bdl	bdl	0.02	0.01	0.01	1.28
C-6	96.40	0.01	0.09	0.68	bdl	0.02	0.03	0.01	0.02	1.50
C-8	93.10	0.03	0.09	3.86	0.31	0.64	0.04	0.06	0.01	1.53
C-9	94.38	bdl	0.05	2.70	0.20	0.29	nm	nm	nm	nm
C-10	91.27	0.04	0.05	6.10	bdl	bdl	0.02	0.01	0.02	1.75
C-12	96.10	0.03	0.11	0.89	0.01	0.32	0.05	0.06	0.01	2.03
C-13	97.22	bdl	0.07	1.70	0.01	0.04	nm	nm	nm	nm
C-14	97.58	bdl	0.07	1.18	0.06	0.13	nm	nm	nm	nm
C-15	94.20	bdl	bdl	1.44	0.04	2.96	0.01	0.04	0.01	1.19
C-17	96.60	bdl	bdl	0.47	0.004	1.55	bdl	0.03	0.01	0.58
C-19	95.33	0.04	0.26	0.58	bdl	0.03	0.02	0.05	0.03	1.87
C-20	96.73	0.01	0.05	0.80	bdl	0.12	0.01	0.005	0.01	0.40
C-22	96.31	0.03	0.12	0.27	bdl	0.56	0.03	0.004	0.02	1.27
C-23	90.42	0.02	0.05	5.07	bdl	1.44	0.05	0.01	0.02	1.80
C-24	90.42	0.01	0.08	5.78	bdl	0.05	0.02	0.005	0.02	1.49
C-25	89.44	bdl	0.13	4.74	0.38	0.61	0.07	0.04	bdl	2.30
C-26	88.18	0.02	0.05	5.59	bdl	1.35	0.02	0.01	0.02	1.90
C-27	94.08	bdl	0.08	1.26	0.17	0.14	nm	nm	nm	nm
C-28	nm	bdl	0.08	3.95	0.39	0.48	nm	nm	nm	nm
C-29	98.18	bdl	0.04	0.37	0.007	0.11	nm	nm	nm	nm
C-30	92.34	bdl	0.04	2.10	0.19	0.33	nm	nm	nm	nm
C-31	98.26	bdl	0.06	0.20	0.04	0.11	nm	nm	nm	nm
C-32	94.00	bdl	0.13	0.29	0.007	1.19	nm	nm	nm	nm
Av.	94.80	0.025	0.085	2.00	0.11	0.55	0.028	0.023	0.016	1.45
Sand
S1	95.00	0.15	0.67	0.64	0.011	0.66	0.15	0.05	0.22	1.57
S2	92.50	0.59	1.47	1.17	0.024	1.21	0.25	0.16	0.45	1.83
S3	94.35	0.22	1.14	0.41	0.01	1.73	0.07	0.12	0.38	1.30
S4^*	94.80	0.25	1.20	0.46	0.04	2.34	0.10	0.23	0.39	nm
S5^*	94.85	0.41	1.59	0.75	0.07	1.12	0.11	0.13	0.48	nm
Av.	94.3	0.32	1.21	0.69	0.03	1.41	0.14	0.14	0.38	1.57

* Analysis from Hassan (2017).

Bdl, below the limit of detection; LOI, loss on ignition; nm, not measured.

Table 2.

Calculated ratios of some elements for samples listed in Table 1.

	Al/(Al + Fe + Mn)	Al₂O₃/TiO₂	Al₂O₃/(Al₂O₃ + Fe₂O_3T)	(Fe + Mn)/Ti	Fe/Ti	Fe₂O_3T/TiO₂
Chert
C-1	0.07	-	0.09	-	-	-
C-2	0.23	-	0.28	-	-	-
C-3	0.17	-	0.21	-	-	-
C-5	0.07	1.2	0.10	13.40	13.40	11.50
C-6	0.09	9.0	0.12	79.30	79.30	68.00
C-8	0.02	3.0	0.02	163.0	150.0	128.7
C-9	0.01	-	0.02	-	-	-
C-10	0.01	1.2	0.01	178.0	178.0	152.5
C-12	0.02	3.7	0.11	34.60	34.60	29.70
C-13	0.03	-	0.04	-	-	-
C-14	0.04	-	0.09	-	-	-
C-15	-	-	-	-	-	-
C-17	-	-	-	-	-	-
C-19	0.25	6.5	0.30	16.90	16.90	14.60
C-20	0.04	5.0	0.06	93.20	93.30	80.00
C-22	0.25	3.6	0.30	10.50	10.50	9.000
C-23	0.01	2.5	0.01	295.0	295.0	253.5
C-24	0.01	8.0	0.01	673.0	673.0	578.0
C-25	0.02	-	0.03	-	-	-
C-26	0.01	2.5	0.01	326.0	325.0	279.5
C-27	0.04	-	0.06	-	-	-
C-28	0.01	-	0.02	-	-	-
C-29	0.07	-	0.10	-	-	-
C-30	0.01	-	0.02	-	-	-
C-31	0.18	-	0.23	-	-	-
C-32	0.25	-	0.31	-	-	-
Av.	0.08	4.20	0.11	171.0	170.0	145.9
Sand
S1	0.43	4.47	0.51	5.0	4.97	4.27
S2	0.48	2.49	0.56	2.4	2.33	1.98
S3	0.67	5.18	0.74	2.3	2.20	1.86
S4^*	0.65	4.80	0.72	2.3	2.13	1.84
S5^*	0.60	3.88	0.68	2.3	2.13	1.83
Av.	0.57	4.16	0.64	2.86	2.75	2.36

* analysis from Hassan (2017).

Table 3.

Trace element compositions (ppm) of chert and its associated sand from the studied area, along with average compositions of the chondrite and PAAS (see text for references).

C-1	C-5	C-8	C-10	C-19	C-20	C-22	C-26	Av.	S3
Sc	0.043	0.042	0.090	0.042	0.238	0.194	0.107	0.087	0.10	0.640	nr	nr
Y	0.839	0.126	1.125	0.336	1.301	1.237	2.204	1.561	1.09	3.040	2.100	27
La	0.472	0.153	0.815	0.262	1.405	0.871	1.308	0.818	0.76	4.952	0.367	38
Ce	0.796	0.514	0.890	0.634	1.813	3.432	1.134	0.867	1.26	8.817	0.957	80
Pr	0.098	0.041	0.133	0.050	0.295	0.160	0.210	0.131	0.14	1.351	0.137	8.9
Nd	0.296	0.077	0.365	0.131	0.809	0.449	0.586	0.373	0.39	3.556	0.711	32
Sm	0.065	0.020	0.082	0.031	0.162	0.092	0.132	0.083	0.08	0.738	0.231	5.6
Eu	0.026	0.013	0.061	0.022	0.056	0.051	0.049	0.037	0.04	0.200	0.087	1.1
Gd	0.092	0.026	0.111	0.041	0.173	0.134	0.188	0.114	0.11	0.661	0.306	4.7
Tb	0.017	0.005	0.019	0.007	0.035	0.023	0.034	0.022	0.02	0.133	0.058	0.8
Dy	0.073	0.020	0.098	0.033	0.149	0.120	0.175	0.113	0.10	0.577	0.381	4.4
Ho	0.024	0.006	0.032	0.010	0.046	0.039	0.057	0.041	0.03	0.159	0.085	1.0
Er	0.055	0.013	0.078	0.024	0.107	0.103	0.139	0.094	0.08	0.358	0.249	2.9
Tm	0.010	0.003	0.014	0.005	0.019	0.018	0.026	0.017	0.01	0.064	0.036	0.4
Yb	0.069	0.018	0.079	0.033	0.110	0.106	0.148	0.097	0.08	0.381	0.248	2.8
Lu	0.012	0.003	0.014	0.005	0.018	0.018	0.024	0.018	0.01	0.067	0.038	0.4
ΣREE	2.100	0.912	2.791	1.288	5.197	5.616	4.210	2.825	3.11	22.01	3.891	183
LREE	1.753	0.818	2.346	1.13	4.54	5.055	3.419	2.309	2.67	19.61	2.49	165.6
HREE	0.352	0.094	0.445	0.158	0.657	0.561	0.791	0.516	0.45	2.40	1.401	17.4
LRRE/HREE	4.98	8.70	5.27	7.15	6.91	9.01	4.32	4.47	6.35	8.17	1.78	9.52
Y/Ho	34.9	21.0	35.0	33.6	28.3	31.7	35.4	38.10	32.3	21.4	24.7	27.0
PAAS normalized
Ce/Ce^*	0.85	1.49	0.61	1.26	0.65	2.10	0.49	0.60	1.01	0.78	-	-
Eu/Eu^*	1.55	4.02	2.99	2.90	1.56	2.07	1.45	1.77	2.29	1.34	-	-

[i] nr, not reported, PAAS, Post Archean Australian Shale; REE, rare earth elements. Eu/Eu^* = (EuN)/[(SmN × GdN)½], Ce/Ce^* 2CeN/(LaN + PrN), where the subscript N denotes to the normalization of the REE to PAAS.